Mass Air Flow (MAF) Sensor

[85_Ciera_Rebuild]

My 88 Beretta has been down for preventive maintence for about two weeks. After installing new timing set and water pump, I fired it up and let it run for several plus minutes while topping off anti-freeze. Then I went back to turn it off and noticed SES (Service Engine Soon) light was on hard....Code 33....MAF Sensor Circuit.

Question - I'm going to check a few things Friday, but has anyone had this happen before where the vehicle sets for several weeks, and then the code is set?

Shop Manual suggests prolong idling will set it, if other conditions exist ( TPS less than 8%, Air Flow greater than 20 grams per second).

[CieraSL92]

Does your manual indicate if this code means a total lack of continuity or just a reading outside spec?

[85_Ciera_Rebuild]

Does your manual indicate if this code means a total lack of continuity or just a reading outside spec?


1988 Beretta: 2.8 V6: Code 33 (too high frequency)

Code 33 will set if:

* Ignition "ON" and air flow exceeds 20gm/sec

OR

All of the below are met for 2 seconds:

1. Engine is running less than 1300 RPM
2. TPS is 8% or less
3. Air flow greater than 20 grams per second
&&&&&&&&&&END&&&&&&

When I drained the radiator, I had to remove the battery and I opened up the Air Filter Container, which exposed the MAF to direct air for about two weeks.


Code 33 (Too High Frequency) means that MAF is detecting a fast movement of air going across it....I'm going to have to see if I screwed the Air Filter Container down right...it was getting dark when I screwed it down...

I'll know more Friday

[85_Ciera_Rebuild]

I took it out for a Test-Drive...and with the pedal pressed, the SES Light goes away...but as soon as the pedal is released, the SES Light comes on.

I have a digital mpg gauge, and it too follows SES Light...when light if off, fuel mpg goes back to normal...when SES Light stays on, fuel mpg sucks, at around 18 mpg.

Something to do with TPS, in some manner...I don't know yet...have to get a reading on TPS sensor via relative of mine...bummer.

[85_Ciera_Rebuild]

Update - I took it on a 160 mile cruise today...same symptoms, SES light goes off under throttle usage. I bought a can of MAF sensor cleaner...no cigars.

M.P.G is about the same or better with this condition...maybe a pinch better...but I must assume computer is using O2 sensor to control fuel burn. I have digital display which has mpg displayed. Initially, mpg sucked, but with more miles, mpg went up...so, I must conclude O2 sensor was determining the fuel burn. (battery was disconnected for several weeks, so computer had to relearn via O2 sensor).

Most likely, I will buy another MAF sensor....I have this feeling my fuel mpg will drop when I do.

I was driving on interstate roadway and was getting about 35 mpg at 60 mph, with one heck of a wind behind my vehicle....cheatin...

[Tonglebeak]

I don't use a MAF. K&N cone filter straight to the throttle body. I've gotten 34 on the highway. Is your car updated with speed density? If so, ditch the maf.

EDIT: Just realized the ecm probably wouldn't throw a maf code if it's running off the map...

I actually remember having a tps voltage issue before, and the ses light was on w/no throttle, but went away w/throttle. But that voltage was tricky. Even if I adjusted it so the light was gone allt he time, the computer would behave wildly. For instance, when I had I believe .56v at no throttle, there was no ses light, but on startup the engine would idle very high. So high it sounded like it was towards the redline. Once it was put in drive though, it drove beautifully and idle was fine. Lockup would also immediately engage when the throttle didn't move for more than .3 seconds, no matter what. I actually liked how it ran with high voltage, all except for that obnoxiously high idle which is NOT a good thing for a car to do when it's just started up with the engine not fully lubed yet!

[Pontiac6ksteawd]

Start looking around in the junkyards for a 88 model Beretta, that has the speed density software installed. It will have a sticker saying this on one of the strut towers. If the motor and tranny are the same as your car, take the EEPROM out of the computer and try installing it in your car. If this clears the SES light, then you can remove the MAF sensor completely, and wire in a IAT sensor into the air canister. Your MPG will go up, and you will get a slight power increase. I did this on my 87 Pontiac 6000 LE, using a chip from a 88 Chevy Cavalier Z24 Automatic. Speedo stayed correct, no check engine light, and no more MAF. Fuel milage went up 3-4 MPG.

Or you could go to the dealer and ask them how much the Speed Density software upgrade is. Might be easier than hunting down a speed density chip.

[85_Ciera_Rebuild]

I don't use a MAF...Is your car updated with speed density?
....remember having a tps voltage issue before, and the ses light was on w/no throttle, but went away w/throttle.

After cleaning the MAF Sensor yesterday, I forgot to plug it back in....duh...and drove it some 120 miles.

Today, I plugged it in, and these Extra Codes came up:

23 - Manifold Air Temperature (MAT) Sensor Cirucit
33 - Mass Air Flow (MAF) Sensor Circuit
45 - Oxygen Sensor Circuit

So, I just now unplugged the battery and MAF Sensor...maybe later today I will drive it to town.

It worked fine without MAF (once 02 sensor had enough data points in computer).

Speed Density Software - Yes it has it, I just now Googled it, and found this story:


1988 Chevrolet 2.8 is a strange bird. (http://www.misterfixit.com/beretta.htm)

They began manufacturing "speed density software" to update these vehicles. This speed density software consisted of a new prom, a new idle speed control motor, and a label to attach to the ECM (computer) that the vehicle had been updated with the speed density software. I am unsure of exactly what this corrected, but it told the ECM to ignore all signals from the MAF sensor. The MAF (mass air flow) sensor was still there, it just didn't do anything anymore. It presents problems for a repair technician if he is unaware of whether the vehicle has been updated or not. A scanner plugged into the ALDL link will give erroneous data and trouble codes if the vehicle has been updated and the scanner is not made aware of it!
...
...End...

TPS Voltage:


Even if I adjusted it so the light was gone allt he time, the computer would behave wildly.

On cold startup...everything is fine....when motor is warmed up, then at startup, motor will go to about 2,000--3,000 RPM and come back to around 1,000 in neutral.

I'll have to check voltage and see where TPS is now...everything was "normal" before I worked on it, but it got colder around here, and voltages change with colder air. I know someone has messed with this vehicle...so like the article says,


Even the technicians at the large Chevrolet dealership near our shop tell us that they try to run from them (1988 Chevrolet Beretta with a 2.8 V-6 engine) whenever they come in.

[Tonglebeak]

The MAF has a built in IAT sensor. The computer should still be reading at least that much from the MAF, actually.

Also, for my car the speed density sticker is in the glovebox compartment, as well as a couple of other places.

But I recommending doing what I did, and that's ditching the MAF (ditch the entire air intake altogether if you can, at the least the ciera's had a huge bottleneck right from the beginning).

You will need an IAT sensor. As you know, all I did was get one of those intakes off ebay, which is just a short pipe with a k&n cone filter on it, and a hole to seat the IAT sensor. Splice it in and you're good to go (I believe it's the pink and brown wires. Don't quote me on that yet, I need to go doublecheck).

[85_Ciera_Rebuild]

Start looking around...has the speed density software installed....

According to this article I found, this mechanic notes:


..Immediately, GM had some sort of problem with this arrangement. They began manufacturing "speed density software" to update these vehicles. This speed density software consisted of a new prom, a new idle speed control motor, and a label to attach to the ECM (computer) that the vehicle had been updated with the speed density software.


Hence, you would also need a revised idle speed control motor.

This vehicle has the speed density sticker on it...so I must assume it was upgraded.

thanks to both of you mentioning this....

[85_Ciera_Rebuild]

EDIT: Just realized the ecm probably wouldn't throw a maf code if it's running off the map...


I would have to do a series of test since when I initially checked Code, it was after I did the repairs, and then just started the vehicle up to let it idle, and it threw a code....later I drove it around, but never checked it until some 150 miles later. So, different conditions...



tps voltage issue before...

Vehicle's wiring shows TPS and MAF (temperature side of MAF) use the same ground path back to the comptuer...which means one needs to check all terminals first before messing with it.

[Tonglebeak]

Hmm, I didn't think they did that. Hmm...

BTW, when you adjust the tps voltage, make sure it's at .5 volts.

And another thing: you can rest assured that even though everything else might be going wrong, YOUR ENGINE AND TRANNY IS FINE! :)

[85_Ciera_Rebuild]

BTW, when you adjust the tps voltage, make sure it's at .5 volts.


Have to get another manual for this matter: which I just bought.

1987-1988 GM-Wide Driveability and Emissions Diagnosis & Trouble-Code Manual for 2.8L VIN W Speed Density Engine #75841 (http://www.factoryautomanuals.com/product.php?productid=75841)

GM's update, Speed Density Software, is what a person needs to know here first before messing with it.


YOUR ENGINE AND TRANNY IS FINE!


Well, let's see, the previous owner was attempting to sell this vehicle to the Salvage Dealer, so I bought it for its transmission without seeing it for $200.00....hmmm....and now I've spent about $800.00 in parts (+ 2 tires...plus my cheap labor)....hmmm

But, since I checked a rod/main bearing for clearance, it should run another 100,000 miles...only need to change transmission's CV seals and maybe front wheel bearings now...then it should be a good ride without mechanical problems....;):eek:

[86euro]

I am unaware of a revised idle air control motor. It could be just a better "improved" design internally or it could have a different sized plunger which could help the computer adjust the idle a little quicker, maybe. Was it stated what was different about it?

[85_Ciera_Rebuild]

Was it stated what was different about it (IAC)?

I'll come back to this topic when I receive the manual I ordered today....I'm sure all the details are spelled out in details there...

This mechanic seems to think so...

[85_Ciera_Rebuild]

The computer should still be reading at least that much from the MAF, actually.


Actually, since I had the MAF unplugged, I was still getting temperature readings on my digital instrument panel....it was outside air temperature, but how the computer was getting this temp reading is beyond me, since MAF was unplugged.

[85_Ciera_Rebuild]

The MAF has a built in IAT sensor. The computer should still be reading at least that much from the MAF, actually


Upgrading your 1988 Beretta to Speed Density (Eliminating the MAF)
October 27, 2005 (http://www.berettaspeed.com/information/view_article.php?id=16&action=print)

Anyways, to upgrade your car to be 'Speed Density' which will delete the MAF and use only the MAP sensor, you will need to do the following:

1. Locate and install a PROM (memcal/chip) from a '89-'90 Beretta (to avoid issues with your EGR - auto or manual shouldn't make a difference)
2. Remove your MAF sensor (it was robbing power anyway)
3. Splice in an IAT sensor from most any other year Beretta model (they're pretty much universal in the GM world) in place of your MAF sensor.
...
...
Ratings from GM show a 5hp increase in 2.8L equipped vehicles from 1988 to 1989 -- most would say that the MAF delete is the reason.

[Pontiac6ksteawd]

As I mentioned in my earlier post, that is all that is required. The other sensor that the dealer mentions is NOT required. Think about it this way. Not all cars got the speed density upgrade, so that sensor would have to be made for the home mechanic, from their local parts store, for both a car that has the software, and one that doesnt..

But ANYWAYS, since we know his car HAS the software, this is the problem I see going on. First and foremost, you need to take off the MAF, and splice in a IAT, then put in a air snorkel and whatever that allows for this sensor, and reset your computer. You ran your car with the sensors unplugged. And when you plugged it back in, it thru a trouble code. The computer was going off the readings it had befor the engine was last shut off. So when you plugged it in, it was so drasticly diferent, that the computer went WTF, and set a trouble code for it...

At least that would be a start, and then we can come back to this once we know the computer has been reset, and the sensors are in line for it. Dont forget, to be sure the software has been installed, you must check the underhood relays, located on the drivers side strut tower, under that plastic cover. There should be a plug there that does not have a relay. If there is a relay, one of them needs to be unplugged, but I dont remember which.

[85_Ciera_Rebuild]

First and foremost, you need to take off the MAF, and splice in a IAT, then put in a air snorkel and whatever that allows for this sensor, and reset your computer.

I have ordered the Service Manual which explains this Speed Density upgrade...when it comes in, I will read thru it and determine if mine was fully upgraded or not. After driving it around, the fuel mpg appears to be OK...so I must assume the O2 sensor with computer was setting data points for fuel/air.

Thanks for thoughts...but I have to read the service manual first...I will report back this info...

[85_Ciera_Rebuild]

I am unaware of a revised idle air control motor...a different sized plunger...

I will attempt to find out the specifics....but as you would expect, if you replace your IAC, make sure the new one has same/equivalent part number.

As one GM book says, "Different shaped pintles are used for different IAC valve applications.

[CieraSL92]

If you got rid of your old prom, and this one replaces the MAF with IAT readings, why would it throw a code for a MAF sensor at all? I've run 3100's with no IAT plugged in it all, it I get no code for an IAT. I know its different, but..

[CieraSL92]

Also, I would think splicing would reduce the contact surface, thereby alterating the resistance a bit... True or no? I generally wrap wires and stick heatshrink over them... Seems like the best way to keep full conductivity.

[Pontiac6ksteawd]

I would normally agree with sensors that are fully active. But the IAT on the older chevy engines dont really do alot. My old Celeb, the SES light would turn on for a faulty IAT or O2 because as the elevation changed, the readings it had when started arent the same as the elevation climbs. I took it into a dealer, and the service writer said to not even bother bringing the car in for service. next time as I am climbing the elevation, and the SES light turns on. Pull over, shut the motor off, countr to 20, and start it. Solved my problem entirely.

The older GMs arent really a active engine management system like the newer cars. In fact, I would just call them management systems. They dont actively monitor the change in temp, or barom.

So basicly what I am saying is normally I would agree with you for systems that are fully active. But on the Gen 1.5 and 2 60* V-6's, its not active, it reads one pulse off the IAT, at startup, and becomes redundant.

[Tonglebeak]

I would normally agree with sensors that are fully active. But the IAT on the older chevy engines dont really do alot. My old Celeb, the SES light would turn on for a faulty IAT or O2 because as the elevation changed, the readings it had when started arent the same as the elevation climbs. I took it into a dealer, and the service writer said to not even bother bringing the car in for service. next time as I am climbing the elevation, and the SES light turns on. Pull over, shut the motor off, countr to 20, and start it. Solved my problem entirely.

The older GMs arent really a active engine management system like the newer cars. In fact, I would just call them management systems. They dont actively monitor the change in temp, or barom.

So basicly what I am saying is normally I would agree with you for systems that are fully active. But on the Gen 1.5 and 2 60* V-6's, its not active, it reads one pulse off the IAT, at startup, and becomes redundant.

So...you're saying the IAT can be ditched?

Makes perfect sense to me. Air density changes with temps, so if the computer is reading the MAP, then it has no use for temp/ All it cares about is how much air is in the intake and runners. Temp won't mean a thing in this case.

[mickstan_VR]

No. Yes.....Both! You got the same motor as mine. Look under the relay cover on the drivers side strut tower. Is one of the relays unplugged and have tape wrapped around it? Thats the relay for the MAF. If it does, it already has the speed density update.
The IAT is part of the MAF sensor. The computer is not using the MAF signal, but it is still using the IAT signal. Get rid of the MAF and you loose the IAT at the same time.
Sooooo......go get a non-MAF tube and air filter canister (with the IAT sensor in the side, plug and pigtail) from an '89 GM whatever with the same engine. Put it in in place of yours. Plug the black wire from the pigtail into the purple wire of the old MAF plug on the wiring harness. Plug the tan wire into the other tan wire on the plug. Put an air filter in it and yer' done.
No SES lites. Car runs great. Big improvement.

[85_Ciera_Rebuild]

The IAT is part of the MAF sensor.

100% Correct; there are two separate circuits in the MAF, one for temp, and the other for AirFlow.


The computer is not using the MAF signal, but it is still using the IAT signal.

Correct, 100%


go get a non-MAF tube and air filter canister

I had driven the vehilce for about a month without SES light going on/off.....hmmm....I will check TPS voltage before jerry-rigging it. I know the IAT is working via digital dash reading.

Again, I'll read up on it, once I get the book...

[mickstan_VR]

Its not really a jerry-rig. You're just doing what GM was too lazy to fix or change on the assembly line. The speed density update was available in '87. But they didnt change the parts 'till '89. It looks and behaves totally factory.
Check under the hood and see if that relay is already unplugged.

[85_Ciera_Rebuild]

Check under the hood and see if that relay is already unplugged.

Where described (Left Shock Tower Area), it don't exist....and never did there. There is a Fuel Pump Test lead....nothing else, over there.

88 Beretta Service Manual Supplement does not show a Relay under Component Locations 2.8; none exist.

I will check Electrical Manual later....but so far, it don't exist...nor any wires

[85_Ciera_Rebuild]

I am unaware of a revised idle air control motor.


I have received the 1987-1988 manual on Speed Density Engine (Vin W: 2.8). and reading todate only indicates this tidbit:

Idle Learn Procedure:

1. Install Tech 1 "Scan" tool
2. Ignition "ON," engine "OFF."
3. Select "IAC SYSTEM," then "IDLE LEARN" in MISC TEST" mode
4. Proceed with idle learn as directed.

This procedure allows the ECM memory to be updated with the correct IAC valve pintle position for the vehicle and provides for a stable idle speed.

The book further notes elsewhere that there is a idle range for IAC of about 700 to 1500 RPM....but, if you can't get it to idle at lower end, you have a problem.

Hence, the computer can compensate to a point, with different IACs....but, whenever possible, get the correct replacement.

I have this feeling one trouble code is related to a vacuum leak...I spayed ether and found leak at EGR's push-rod, and firewall-side of top intake manifold...didn't check PCV valve/tube, but suspect there is small leak there too.

23 - Manifold Air Temperature (MAT) Sensor Cirucit
33 - Mass Air Flow (MAF) Sensor Circuit
45 - Oxygen Sensor Circuit

I ran a Snap-On OBD scanner, and MAF/MAT show good numbers....but MAF is not needed...

Hence, I have no idea who did the speed density change-out, but my gut feeling is that someone took a short-cut...but specifics are not explained in this booklet.

[85_Ciera_Rebuild]

Check under the hood and see if that relay is already unplugged.

I wish I could find it....still looking for it...Berreta book so far does not show it...and MAF sensor is still giving data....so I must assume complete circuit is working.

[Pontiac6ksteawd]

JEFF!!!! You know where this relay is, help these guys out!!

[mickstan_VR]

They're trippin' Brian. Am I gonna have to get a picture? lol...

[85_Ciera_Rebuild]

Am I gonna have to get a picture? lol...

Yea...send it to the mechanic who put the Speed Update in...he couldn't find it;)

Let's see the pic....and on 88 Beretta, it ain't over there by shock tower...only a fuel pump test lead.

[mickstan_VR]

Are we talking about an '88 Beretta?

[mickstan_VR]

These are from my '88 Celebrity, Beretta will be different, Ciera and 6000 will be the same. This car does NOT have the Speed Density Software Update sticker, but my '87 did. Since the relay was already unplugged from the factory, that means it has the updated ECM. I never checked under the relay cover in the '87 to see if the relay was unplugged, although it must have been or it would have lit the SES light. RarecarVR could check, as he has the carcass of my '87.

[mickstan_VR]

wtf? wheres the pictures? Heres the link to them... http://picasaweb.google.com/jray66/VR/photo#5142851110340943362 http://picasaweb.google.com/jray66/VR/photo#5142851131815779858

[mickstan_VR]

Woopsie! I guess we ARE talking about a Beretta here! I screwed up! Just re-read the first few posts. Truthfully, I don't know dick about Berettas. Hope I could be of some help though...oh well. :(

[85_Ciera_Rebuild]

Are we talking about an '88 Beretta?

Yes Sir...now in this photo here (http://picasaweb.google.com/jray66/VR/photo#5142851131815779858), where it says MAF Relay, do you have a clean picture of this relay?

I think there is a relay like that, but its located in Center of Firewall area, behind the plastic cover for other relays.

thx

[mickstan_VR]

That stupid relay looks just like several other relays on the car. They all look the same!

[85_Ciera_Rebuild]

That stupid relay looks just like several other relays on the car.

There is an oversized relay there...when the cold lessens, I'll pop the cover off, and look at color coding on wires.

[85_Ciera_Rebuild]

I am unaware of a revised idle air control motor.

So far, I have not seen this topic mentioned....but, when vehicle was at OEM specs, they mated IAC size/movement to computer's coding.

I have not got my ALDL cable to work yet, but my hunch is when you turn on key with warm/cold start, and wait several seconds, the IAC moves to a set position. It's logical to assume this is the case, and if true, if the IAC's plunger-movement/airflow-amount are not same as OEM specs, then stuff will happen.

I do know via SnapOn Scanner, that IAC is making adjustments when driving down the roadway...I assume to fine tune fuel/air requirements via O2 sensor reference.

My 88 Beretta with 2.8 will start up fine in cold weather ( 3 °F ), but with a warm start, you have to slap the peddle once/twice to maintain idle...my guess is IAC is related...but without scan tool...can't say for sure.

[jinxtigr]

I'm just fussing with this sensor myself and wondering if I'm missing anything.

I understand that if the MAF sensor is dirty, it's not good. I got my MAF sensor out and cleaned it with a Q-tip and circuit-board cleaner, very gently, and got a bit of crud off it.

I've got it back in there, and the engine seems a little bit happier- it seems to run quieter and stronger, though I might be imagining it.

What's this about removing the sensor entirely and using a different chip? I think it might be moot for my engine, as mine is a 3300. However, I already have a very modified air intake already and the only thing stopping me from experimenting further is, one of the MAF-body screws is frozen, and I'm not going to destroy it getting it out unless I'm really sure I don't have to put it back.

I also need to figure out where my O2 sensor is and what it needs. Everything is functioning, I'm just pursuing small incremental increases in power and MPG in a methodical way, and paying attention to parts that might wear out one day is good.

Can/should you remove the MAF sensor on a 3300 engine, or does that make no sense? I can see that it's a sensor that tells the computer whether or not there's a lot of air coming in- which there is, in my car, an awful lot- which means, what? If you have excessive air does the MAF sensor just get happy and go 'all clear!' or is it causing other issues?

[CieraSL92]

The 02 sensor has about zero effect on the power output, but a massive affect on MPG. I believe there supposed to be replaced every 60k miles, old ones generally won't set a code. It may or may not illuminate the SES light, depending on condition.

The maf sensor typically does get kinda crappy.. I cleaned mine and it got rid of a hesitation I was having and it seemed to be stronger off the line.

The reason they replaced the MAF with speed density software was because the MAF's for the 2.8 were very problematic. This doesn't seem to be the case with the 3300's maf. The function of the MAF is to inform the ECU off the airflow, which assist's the computer in determining how long the fuel injectors should stay open. Obviously, this is to keep the proper air to fuel ratio.

What the speed density software did, I _think_ is assume values for the MAF and use the Throttle position sensor in place of it, as in, okay we have 10% throttle so we should have x grams of air.

Be careful about the MAF sensor too, It's a 240ish dollar part. And I am not aware of any updated EPROMS for the 3300 for the purpose of eliminating the mass airflow sensor.

[85_Ciera_Rebuild]

got my MAF sensor out and cleaned it with a Q-tip and circuit-board cleaner, very gently, and got a bit of crud off it.

They have a cleaner for this...I would suggest using it.



O2 sensor is and what it needs.


Most likely on back side (firewall) of block....I would strongly suggest putting in a heated O2 sensor....this will have to be wired up...but if you do city stop/go traffic, I would highly suggest doing this.

[jinxtigr]

Thanks guys- my experience working with things like surface-mount electronic parts helped with the MAF sensor, I sure wasn't about to wreck it. The wires the sensors ride on are actually real sturdy, but of course the traces winding around the little sensor are way more delicate. I'm pretty sure I didn't hurt them, and the cleaner I used is a form of alcohol that shouldn't dissolve wires. I knew it was an electrical part, after all- not to be scrubbed with a toothbrush or steel wool ;) just because the wires it rides on are heavy doesn't mean anything!

I'll look into the O2 sensor- I'm sure it's never, ever been replaced in 100K miles. I would certainly like to get better MPG- already doing pretty well at around 30-33mpg. The ram-air duct and air intake I made are a huge help on the highway, so maybe better than 33 mpg is a reasonable goal...

Probably the next thing I'm looking into is, somebody was saying they were getting about 15K on a set of spark plug wires, and when they were bad the engine would bog down not at 0% or 100% power but at around 30%. This is happening to my engine so it's got me wondering, since I know my spark plug wires have 100K miles on them.

[CieraSL92]

Follow the exhaust until its right about level with the firewall. O2 sensor is there. If it has never been removed, soak it in some PB blaster and expect some trouble unscrewing it. Autozone will *rent* you a special socket to remove this. I think you pay X amount and you get it back when you return whatever it is you borrowed.

Yeah, my wires actually don't last much longer than 13 or 14k then the engine starts to miss under light loads.

[Zaloryan]

I didn't read every reply to this thread, but in case it hasn't been said: One way to check your MAF: When the engine is running, lightly tap the MAF. If the engine stutters or responds in some way, the MAF is bad.