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Thread: 93 Century 3300, misfire when hot

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    Default 93 Century 3300, misfire when hot

    Long time no see guys! I haven't had an a-body since 2010 when my rear trailing arm bracket on the Ciera rotted off the body..and that was the end of that

    Anyway, I picked up a 93 Century. Only has 111k miles, 3T40 tranny. To start off with: I changed the plug wires, and plugs..Used AC-Delco plugs and Napa Belden wires. Helped a little bit. OHM tested the new wires (I've seen em bad out of the box) and they tested okay..I don't see any arcing at night either.

    Runs well when its cold for the most part. Starts right up..After 20-30 minutes of driving things go down hill quick. It will eventually miss intermittently at idle (you can feel a skip randomly in park, drive.) When it's acting like this anymore than 1/4 a throttle and it will buck and miss..If you put it to the floor it sounds and feels like it drops two cylinders at the very least. Way down on power. If you stay at WOT, once the RPMs get up over 3000 it feels like these cylinders start to fire again and it feels normal. Shudders when its in lockup if you try to do more than cruise-I'm pretty sure this is the misfire not the tranny. Gets real bad taking off from stop signs unless you baby it...on a hot restart, it takes 4-5 seconds of cranking to start. When it does start, it will miss bad, then clear up after a few seconds and go back to the random miss. So, I changed the wires.. Didn't help, but a few were bad. Plugs looked normal. Checked the resistance of all the injectors and they were all 14.5-14.6ohms.. Checked them hot. I realize this doesn't mean that they're not clogged internally, but I feel like this is an ignition issue. Fuel pressure is within spec. Cleaned MAF sensor. No change. Tried running it w/o 02 sensor, didn't make a difference.

    Having said that, I haven't tested for spark on this. It's kinda tough since it needs to be heat soaked for it to act up.e I read it wasn't good for the coils to just disconnect the wires from the tower and let them arc-is there a preferred way to test for spark quality? I also want to ohm out the coils, but I do not know the specs. Any insight? I'm hoping this is some simple like a coil failing when hot..or the ignition module...

    Edit: Haven't done a compression check, but given that it runs fine when it's cold (sometimes when hot too) I think I can safely rule that out. Engine uses no oil, either.



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    Senior Member fixitman333's Avatar
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    sounds like a bad coil pack.

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    I'm hoping that's all it is. Checked for vac leaks today just for laughs..Couldn't find any. It runs just fine until it's been at temp for awhile then this misfire crap starts. Does anyone have any resistance specs for the ignition coils? I'd like to get it hot then pull them and check resistance before I start throwing parts at it. The u-pull-its are about an hour away from me and I refuse to pay someone to pull a part for me. I parted out a 95 Skylark w/the 3100 in July. Wish I had took the coils/ignition module from it now..

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    Senior Member Zaloryan's Avatar
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    Not too long ago I submitted an assignment on the theory and operation of the distributor-less ignitions that our A-body cars came with; specifically, the 3300. I used a lot of information from my 1990 Oldsmobile Cutlass Ciera Service Manual. I'll copy/paste it here. Hope it helps you out.






    Engine Ignition System Assignment
    AT615 Engine Performance
    October 12, 2013

    Waste Spark Ignition Theory:
    In 1990, General Motors equipped their A-body vehicles with a Computer Controlled Coil Ignition System. My personal Oldsmobile Cutlass Ciera with a 3.3 liter V6 received such a setup. To be specific, the system is of the waste spark variety. Waste spark systems fire two plugs simultaneously, one cylinder on the compression stroke, the other on the exhaust stroke. Since the cylinder on the exhaust stroke is under little to no pressure, minimal voltage is required to fire the plug. As a result, one coil can fire two cylinders. This decreases emissions by burning off excess fuel that would otherwise exit via the tailpipe. The spark plug on the compression stroke receives the initial voltage and the electrical charge carries through the engine block to the opposite plug on the exhaust stroke. This plug fires, then electrical energy carries back to the coil, reaching electrical equilibrium. Each cylinder is paired together with the opposite cylinder in the firing order. The 3300 engine firing order is 1-6-5-4-3-2. The cylinders paired together in the ignition system are 1/4, 2/5, and 3/6.

    Computer Controlled Coil Ignition Synopsis:
    GMs CCC Ignition System utilizes three coils, an ignition module, reluctor rings, and a crankshaft position sensor. The ignition module is located underneath all three coils and serves as a mounting plate for each coil. It serves to time the ignition of the fuel/air mixture based on inputs when the engine is under 400 RPM. Above 400 RPM, the ECM applies 5 volts to a bypass circuit which causes the ICM to defer to the ECM and timing is controlled by the ECM. Reluctor rings which signal the crank position sensor are mounted on the harmonic balancer and the crankshaft position sensor is mounted at the three-o=clock position of the harmonic balancer, bolted to the engine block. Two reluctor rings are used and the crank position sensor is made up of two hall-effect switches. The crankshaft position sensor is adjustable for precise placement according to OEM gap (0.025 of an inch). Secondary ignition wires transfer current from the coils to the six spark plugs mounted in the two cylinder heads of the engine.

    Of the ignition system, the following are outputs: ECM, Ignition Control Module, coils, secondary ignition wires, and spark plugs. The ignition control module receives input from the crankshaft position sensor, coolant temperature sensor, tachometer, and mass air flow sensor to calculate proper ignition timing under 400 RPM. Above 400 RPM, the ECM controls ignition timing based on input from the crank position sensor, tachometer, engine coolant sensor, mass air flow sensor, throttle position sensor, knock sensor, vehicle speed sensor, and park/neutral state switch.

    No Start Diagnosis Procedure:
    Begin diagnosis by checking for spark on cylinders 1, 3, and 5. Crank engine to command ICM to trigger spark. If no spark is found on all cylinders, the fault is a loose/intermittent connection at the ICM or a faulty ICM. If no spark is only on one or two cylinders, check resistance of spark plug wires. Resistance should be less than 30,000 ohms. If not, secondary wire is at fault. Assuming the secondary wires resistance is acceptable, remove suspect coil and connect a test light at the two exposed terminals. If the test light does not blink during cranking, the ICM is faulty. If ICM is OK, check the suspect coils primary resistance (0.5 0.9 ohms). If within specification, install a back-probe on Pin M of the ICM connector. Connect the positive lead of the voltmeter to the back-probe and the negative lead to a chassis ground. With Ignition ON, voltmeter should read battery voltage. If battery voltage is not found, a short to ground has occurred in wire #839 (Pink w/Black tracer). At this point, the crank sensor is the next thing to check. Back-probe the crankshaft sensor connector at pins C & D. Measure voltage between terminals C & D of the crank sensor with Ignition ON. Voltmeter should read between 10 to 12 volts. If not, an open or short to ground has occurred in Connector #644 or #645. If the harness is OK, the ICM is at fault. If voltage is within specification, the crankshaft position sensor has improper gap or the crankshaft position sensor is faulty. Check the sensor for signs of the reluctor rings hitting it.

    Service Information:
    General Motors engineers specified a particular spark plug to this system; multiple aftermarket equivalents will also be listed. AC Delco #R44LTS6 is a resistor spark plug with a long tapered shell calling for a gap of 0.60 thousands of an inch. Thread size is 14 millimeters with a heat range rating of 4. Accel Ignition, a performance aftermarket supplier, offers an alternative spark plug with a silver core for increased conductivity and lowered voltage requirements. Part number is 526S. NGK, another spark plug manufacturer offers an OEM plug equivalent: Part number is TR55. NGK specifies a gap of 0.59 thousands of an inch for their TR55 plug. Factory tightening specification is 20 lbs. ft. due to the spark plug having a tapered seat. Spark plugs with gaskets included would require a higher tightening specification. NGK recommends 10.8 lbs. ft. 18.0 lbs. ft. for tapered seat spark plugs installed in cast iron engines. However, NGK specifies to defer to OEM torque specifications when available.
    When testing or diagnosing ignition system, never crank the engine with the coil towers not connected to spark plug wires. Irreparable damage to the ignition control module or coil may occur. In addition, crank position sensor to reluctor ring gap is critical! Factory specification is 0.025 of an inch.
    What is this & what does pulling it out do?

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    Senior Member Zaloryan's Avatar
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    In all likelihood, I suspect your Ignition Control Module to be faulty.
    What is this & what does pulling it out do?

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    Thank you! Good read. Will check coil resistance when it gets above 30 degrees here in the next day or so. If it checks out I'll just grab another module from the j/y and go from there.

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    Hey, I have a 90 3300 parts car... if they are the same for your 93, I can pull the idle contol module and the coils ( I was told coils were new), and send them to you, perhaps with anything else you need... I want to use as much as possible before she goes to the junkyard. Just pm me if your interested... pretty much shipping is all I care about.
    Roads.... Where we're going, we don't need roads.

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    Senior Member Zaloryan's Avatar
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    IIRC, a different Ignition Control Module was used for the 1992-1993 models opposed to the 1989-1991. Double check part numbers to be sure.
    What is this & what does pulling it out do?

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    Quote Originally Posted by thesam1984 View Post
    Hey, I have a 90 3300 parts car... if they are the same for your 93, I can pull the idle contol module and the coils ( I was told coils were new), and send them to you, perhaps with anything else you need... I want to use as much as possible before she goes to the junkyard. Just pm me if your interested... pretty much shipping is all I care about.
    Thank you! I haven't checked the coils out yet. I'm interested though. If the part numbers are the same, I'll send you a PM about shipping. I'm in the middle of replacing the transmission on my Grand Marquis so this just got a little more urgent since I'm driving this to work and the misfire is beyond annoying.

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    Senior Member thesam1984's Avatar
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    Quote Originally Posted by Zaloryan View Post
    IIRC, a different Ignition Control Module was used for the 1992-1993 models opposed to the 1989-1991. Double check part numbers to be sure.
    he's right, its a different part number for the Ignition Control Module, but the coils are the same (I checked on Autozones website) I myself am in the process of replacing the motor in my wifes Beetle. Its stressful to be down a car, even though I have an extra one.
    Roads.... Where we're going, we don't need roads.

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    Welp! I got all the bellhousing bolts back in, the transmission crossmember and mount are back in..I'll throw in the driveshaft and starter in tomorrow and hopefully the Grand Marquis will move assuming the junkyard didn't screw me. Got away without taking off either of the downpipes on this car, really pleased with it.

    I swapped the ICM (I checked pns, didn't have time to update this thread-there was a change somewhere) with a junkyard one from a 1995 LeSabre 3800. The 'new' icm had one less pin than the original off my century, but it doesn't seem to make a difference. Likely unused on the century. I haven't fully tested it in stop and go, but I did let it run for about 20 minutes. Has about 10x more power and doesn't misfire YET..This was always a heatsoak problem so I can't rule anything out but it *appears* fixed. Previously when I stomped on it from a stop it would miss bad. Tried that today and its running well enough it will even spin the tires (its wet out haha)

    I will update this thread after some more driving. Sam: If this hasn't fixed it, I may still be interested in the coils. You didn't mention what car the 90 3300 was, is it a Century? There are some interior parts I may be interested in.

    Happy Holidays!

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    Senior Member thesam1984's Avatar
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    Yes its a century custom sedan. Car is baby blue with navy blue interior, in pretty good shape... the taillight is perfect too. It has locking wire hubcaps. The parts car is identical to my Roxanne in every way except she is silver/gray cloth and has no rear defroster (originally from Georgia I was told)

    Here's a photo of the parts car before I pulled the parts off mine... I toyed with the notion of driving this one and parting out mine, but mine is very clean all around and this one (Named her Raggedy Anne) was well worn and it needs all new brakes. Tested out a rat rod paint scheme that turned out pretty well. If I had more space I would cut her in half and make a trailer from the rear (thats the other reason shes named Anne) but its not happening right now.

    Last edited by thesam1984; 12-06-2013 at 05:11 AM.
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