3.1 V6 misfire

[sky81]

I have a 94 Century with the 3.1 V6 in it.
The problem I'm having is that it hesitates on acceleration.
It hesitates, then feels like a cylinder is missing the first few seconds after I start off from a dead stop.
Then the misfire stops and acceleration is normal.

I have checked for vacuum leaks, I can't find any. I checked things like hoses, and sprayed carb cleaner around the intake manifold.

I checked the ignition system over. I pulled all of the plugs out of it. Cylinder #2 had a fouled plug in it (greyish deposits) so I put in a new spark plug. The ignition wires are new. I took a new ignition coil and replaced each coil one at a time to see if it was one of the ignition coils.

None of this has changed the hesitation/misfire.

I am not sure what to check next. I would like to know 1. if anyone has any ideas 2. what are the connections that hook up to the base of the 3 coils? There are three connections. Two of the connectors have two wires, one of them has 4 wires.

Can anyone tell me what these do?
Thanks.

[Duke George V]

Might be the ignition module. Go to a junkyard and get the newest one you can find. Any 94+ 3100 or 3400 module will work, and should only be like $20-30.

[6000 SSEi]

What type of plugs and wires? AC Delco are known for carbon tracing. I gave up on their plugs and wire sets after tossing so many in the garbage. Does it work good wide open throttle? Change the fuel filter lately?

x2 on the module. common problem.

[sky81]

The spark plugs are AC Delco R44LMST6. They have a long center electrode and a long side electrode. The gap is 0.060". What sgould I use instead? Champion? The spark plug wires are made my Autolite. I don't usually gun it wide open, but I could try. It seems like if I give it less gas at first and then put my foot further into it, it hesitates/misfires less. The harder I stomp on it, the worse it seems to be. It's also worse when the engine is cold.

The ignition module is the piece the coils plug into, isn't it? (Sorry, I know the older cars better.) I could try to get another one and see if that changes things. The fuel filter has been changed within the last 2 1/2 years, but I suppose it couldn't hurt to change it again and see if that helps it. The ignition module is a good call. I was thinking maybe it was the crank sensor, but it's buried in the back of the block and under the exhaust pipe so it's not that easy to get to. This is why I am wondering what the other wires that plug into the base of the 3 coils do (which is what I believe is the ignition module based on what you're telling me.) Thanks.

[Duke George V]

Crank sensor is doubtful. When those go they kill the engine immediately, and you can't restart it until the sensor cools off. If you replace the ICM and nothing changes, I'm leaning toward fuel pump.

[sky81]

OK. The crank sensor is a pain to get to anyway. Years ago I had a crank sensor that got a crack in it on a Ciera. It kept stalling over and over. I found the crank sensor had the crack and replaced it- then it was fine.
I'll try the ignition module for sure. I checked online and they are over $250 at the auto parts store- so I think a trip to the salvage yard is in order! The only reason I don't suspect the fuel pump is because I replaced it about 4 years ago. But the fuel filter certainly could be getting blocked and it's worth replacing it just to see if that's it.
On another thread someone posted something about testing the resistance of the fuel injectors. Apparently if they are below 11.9 ohms then they are bad. I have no idea how to test the resistance of a fuel injector. Or would that not be a possible cause of this particular symptom? Thanks again.

[Prospeeder]

The AC plugs are exactly what you want to run. I wont say you need to stay with AC wires tho. So ALL the plugs are new and so are the wires? Fuel injector could be bad, or a wireing problem to it. Sounds like you covered ignition, what about fuel?

[sky81]

The AC plugs are exactly what you want to run. I wont say you need to stay with AC wires tho. So ALL the plugs are new and so are the wires? Fuel injector could be bad, or a wireing problem to it. Sounds like you covered ignition, what about fuel?

Yes, new plugs, new wires. Tested each ignition coil by taking a new coil and replacing each one at a time with the new one, then test driving the car - no change.
Checked for vacuum leaks from cracked hoses (none) and by spraying carb cleaner along the intake manifold- no change.

I plan to try a different ignition module if I can get over to the junkyard this weekend. That will be the next step.
Will also try a new fuel filter. The fuel pump was replaced 4 years ago.

However I am not well versed in newer cars (i.e. post 1990) with multi-port fuel injected engines. I understand carbureted engines better and have some experience with TBI, but not MPFI.
I am not even sure what I would test, though I saw someone posted on another thread that if a fuel injector has less than 11.9 ohms then it is considered to be bad.

I did pull a vacuum on the fuel pressure regulator and confirmed the diaphragm is not leaking on it.

[6000 SSEi]

Id run an an autolite copper plug equal to the temp range of your delco part number. I wouldn't run ac anything in any of my ignition systems ever again. Ive sat there arguing with parts counter people about how junk the AC products are in ignition systems... And Ive tried their stuff on my 3.1 and my modded 3800 SC. I even started experimenting with brisk in my SS Silverado.

[Prospeeder]

oh Please, I have had 5+ gm cars and my buddy has had 10+ GM trucks (yes hes a hillbilly with 8 rigs at a time) ALL only ran with AC plugs with no problems, not sure what your issue is but its not AC plugs. If they sucked GM would not use them from the factory, shit NGK one of the best plug manufactures produce AC plugs. Autolites and Champions SUCK, i wont touch them, if you wanna say a plug sucks in GMs, its BOCSH plugs, absolute junk. Anyway the plugs are NOT your issue. You may have a failed injector, if you have only a 1 cylinder misfire its not a coil or module, it fires in pairs, so if a coil was bad, you would have 2 dead cylinders. Try swapping a pair of injectors and see if it changes the miss. Does the 94 OBD 1.5 have a missfire moniter like is the Check engine light on? And does the code say "missfire on cyl bla bla" or no? If it only misses under load it would seem like the fuel injector cant flow enough for the demand and causes a lean miss in that cylinder.

[sky81]

No, the check engine light is not on. No codes. I have a scanner and when I read the values I'm getting with the car running (at least the ones I understand- I don't know what all of them are) everything looks OK.
You may be right on the fuel injector- but honestly I'm not familiar with multi-port fuel injection so I suppose if I want to move the injectors around, I need to pull off the intake manifold to get to them, right?

I'm on my way to the salvage yard now to see if I can pick up an ignition module...

[Prospeeder]

Yea you must remove the upper plenum to gain access to them, then 4 bolts on the fuel rail and they pull out. Try ohm checking them first, try and get the engine hot, and then quickly remove everything to get a warm injector reading. 12 ohms or higher is good. Remove the fuel pump fuse or unplug is and try starting the car to releive fuel line pressure, and remove the gas cap to releive any tank pressure before starting. You dont need to remove the injectors or the rail to just OHM check them. That would be the next place i would start. Also check for any obvious wiring issues under the plenum while your there. To do it quick, dont remove the cable linkages or the coolant hoses on the throttle body, just pull the plenum up and user a bungie cord to keep it out of your way, saves alot of time and no fluid loss

[Century7667]

oh Please, I have had 5+ gm cars and my buddy has had 10+ GM trucks (yes hes a hillbilly with 8 rigs at a time) ALL only ran with AC plugs with no problems, not sure what your issue is but its not AC plugs. If they sucked GM would not use them from the factory, shit NGK one of the best plug manufactures produce AC plugs. Autolites and Champions SUCK, i wont touch them, if you wanna say a plug sucks in GMs, its BOCSH plugs, absolute junk. Anyway the plugs are NOT your issue. You may have a failed injector, if you have only a 1 cylinder misfire its not a coil or module, it fires in pairs, so if a coil was bad, you would have 2 dead cylinders. Try swapping a pair of injectors and see if it changes the miss. Does the 94 OBD 1.5 have a missfire moniter like is the Check engine light on? And does the code say "missfire on cyl bla bla" or no? If it only misses under load it would seem like the fuel injector cant flow enough for the demand and causes a lean miss in that cylinder.

Ditto. I have the original AC plugs in all three of my vehicles with 93k, 136k, and 141k miles. I don't know if the '95 3100 had platinums or not, but the '96 did.

Ken T.

[sky81]

Sorry to not have responded sooner, but I haven't had time to work on the car until tonight.
The car seems to be running better, and there seems to be several reasons for this.
1. It has gotten MUCH warmer here, that seems to have helped
2. I took the car on a good long highway trip and that seems to have helped somewhat.
3. I did get another ignition control module and put it on the car.

So things are a bit better, but not really completely.
What I notice now is when I hit the gas, the engine diesels for a second and then goes.
When I take off from a dead stop, I put my foot into it without changing the position of my foot- but the car accelerates with a certain amount of power for 2 or 3 seconds, then there seems to be more power after that- as if I pushed down harder on the gas, when in fact I did not. I can repeat this same situation each time I take off from a dead stop. It's mre noticeable the harder I hit the gas.
Also, when idling, I wouldn't say it's really misfiring now, but I do still feel some pulsating fromthe engine.

Besides putting the new ignition control module onto the car, I was also able to test 3 of the fuel injectors with the car warm to hot.
The 3 I tested were 12.6, 12.6, and 13.0 ohms- so those are normal.

You ask why did I check only three- because I cannot get the connectors off the other three!
Has anyone else had this problem? I push the little clip in but still cannot unplug the connector from the fuel injector.
Finally the clip pinches my finger so badly that I can't keep pressing on it.
I also notice I cannot press the connector back on so that the clip snaps into place.
Anyone else have this problem?

Well, at least I can rule out the ignition control module and three of the fuel injectors.

What I would like to know is, how is the timing advance controlled on the 3100 SFI engine?
What are the inputs for advancing the timing?
I think it is the MAP sensor on most GM port fuel injected engines, but I don't think the 3100 SFI engine has one.
It has to be something tha indicates the load on the engine.

I'm womdering if the delay I'm feeling has something to do with the timing not advancing as soon as the gas is depressed.

[Duke George V]

I know you said you replaced the fuel pump, but that really sounds like the problem now. If you've run low or empty a few times, that will kill it. Gasoline acts as the pump's coolant, so the more there is in the tank, the better. I knew a guy about eight years ago that bought a brand new Stratus and delivered pizzas in it on 1/8th tank. He went through four pumps in six months. Chrysler eventually said they wouldn't warranty the pump again.

[sky81]

Wow. OK.
Well, you may well be right.
I replaced it a few years ago.
I've definitely run it low on gas.
I don't think I ever ran out, but I ran it low.
That will be the next thing I check.
THANKS!

[6000 SSEi]

oh Please, I have had 5+ gm cars and my buddy has had 10+ GM trucks (yes hes a hillbilly with 8 rigs at a time) ALL only ran with AC plugs with no problems, not sure what your issue is but its not AC plugs. If they sucked GM would not use them from the factory, shit NGK one of the best plug manufactures produce AC plugs. Autolites and Champions SUCK, i wont touch them, if you wanna say a plug sucks in GMs, its BOCSH plugs, absolute junk. Anyway the plugs are NOT your issue. You may have a failed injector, if you have only a 1 cylinder misfire its not a coil or module, it fires in pairs, so if a coil was bad, you would have 2 dead cylinders. Try swapping a pair of injectors and see if it changes the miss. Does the 94 OBD 1.5 have a missfire moniter like is the Check engine light on? And does the code say "missfire on cyl bla bla" or no? If it only misses under load it would seem like the fuel injector cant flow enough for the demand and causes a lean miss in that cylinder.

No one I know uses delco plugs. Not in my 3800SC, not in my 6.0 HO SSS, next to no one on the GP forum... Why? Junk. You wanna run them, help yourself. Never will you find one in anything I own.

[sky81]

I had another thought on this.
Could it be a fuel injector that is leaky, i.e. leaks fuel when it should not?
How would I test ofr that?
Or would the ohm test check for that (I would think it wouldn't, it would just test for whether it is electrically OK or not).
I've put fuel injector cleaner in the tank before, didn't seem to change things.
Thanks again.

[Casper]

You may be able to check the fuel pressure on the rail with a gauge. If so, do that. You should have 41-47 psi (according to a 95 Chevy Lumina book) Give or take a little. I'd check fuel psi before I mess with injectors. But that's just me.

[Prospeeder]

Definatly, and also! Leave it hooked up and if it reaches tape to the windsheild and drive it, May be good pressure at idle, but under demand the worn out pump cant provide engough fuel. If you say you replaced it a few years ago, if you used some shitty 50 dollar autozone pump iv seen those die in as little as a year

[sky81]

OK, that's a great idea... but I looked for a fuel pressure gauge a while back because I was wondering if that is what it was, but I couldn't find one.
I could only find a fuel pressure gauge for a throttle body injected car, not for port fuel injection.
I tried Sears, Advance Auto, Auto Zone. No one had one. Where would I get one?

I bought the fuel pump at CarQuest. For parts like that I only go to CarQuest or NAPA.
I don't buy stuff like that at AutoZone because I don't like redoing things like this...
That's not to say that it couldn't be going bad though.

Thanks for the continued help.