Georgia Fiero Club Forum
All Things Fiero => Tech Tips, Tech Questions => Topic started by: GTRS Fiero on June 07, 2021, 06:41:49 pm
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When my Fiero gets warm, it starts missing. When this occurs, it seems that neither the injectors nor the plugs fire. Once the Fiero cools off (5-10 minutes), it runs OK again.
New plugs, new plug wires, new ICM. Coils should not affect the injectors.
So, replaced the crank sensor. Again, because it was leaking oil through the plug. No change in the misfiring.
Scratched head. Lost several more hairs. Problem still not solved. Wrinkled brow. Problem still not solved.
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What kind of plugs? I tried some "fancy" plugs in my yellow car once. It ran just fine when cold, but lousy when it warmed up.
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Stock 3.4 plugs. But, the missing signal is at the ICM.
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Not much time, lately, but still struggling with this miss issue. Everything runs fine--when the engine is cold. Once it warms up, the car can barely move under its own power.
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ICM's can break down and cause misfires while still keeping the engine running. When I bought my 87 in Hickory, NC, it began to misfire on the way home. Having already owned a Fiero for 7 years, I replaced the ICM and the problem went away. I suspect the same can happen with a coil pack ICM.
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Not that new ICMs can't be bad, but we replaced it. No change.
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When my Fiero gets warm, it starts missing. When this occurs, it seems that neither the injectors nor the plugs fire.
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If the ECM doesn't see ignition pulses, it turns off the fuel.
(Not sure if it kills the fuel pump or the injectors, but whatever it does, it does.)
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Not that new ICMs csn't be bad, but we replaced it. No change.
Check the pickup coil. It should read a high resistance. In the thousands of ohms, I think. (Sorry. Not sure. Been a while.)
It'll probably either be good, or "open". Never saw one short out.
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When my Fiero gets warm, it starts missing. When this occurs, it seems that neither the injectors nor the plugs fire.
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If the ECM doesn't see ignition pulses, it turns off the fuel.
(Not sure if it kills the fuel pump or the injectors, but whatever it does, it does.)
That function is certainly working. We replaced the crank position sensor. No change. We checked the wiring betweek the CPS and the ICM, good.
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Not that new ICMs csn't be bad, but we replaced it. No change.
Check the pickup coil. It should read a high resistance. In the thousands of ohms, I think. (Sorry. Not sure. Been a while.)
It'll probably either be good, or "open". Never saw one short out.
Not sure that I still have a separate pickup coil.
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Not sure that I still have a separate pickup coil.
Sorry. I forgot that you're running a 7730, now. (Right?)
CPS ought to cover that function, I would think. Not sure, though. Never messed with one.
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Yes.
So, I checked the RPMs, as reported by the ECU. These are all over the place. 6K at idle, when it's barely able to turn over, zero when it's at idle, and everything in between.
I've been considering that it's a bad ECU.
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The new CPS died. Replaced the CPS. Again. No change.
Re-worded for clarity.
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The engine runs fine, for 10-20 minutes. Maybe a bit down on power, but smoothly and quietly. No problem getting up to highway speed. Then it starts cutting out, and has no power. Shut it off, and 3 minutes later, it runs normally, again.
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I had the same sort of problem. I would run fine and then start to miss and stall. After letting it sit for a while it would be ok........
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What did you do?
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I mean, how did you fix it?
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I had it Mikes shop and we were discussing the problem.... and one of the guys sort of went and felt the exhaust pipe exhaust and said it seemed like the muffler was getting pluged. Mike got rid of the muffler and I haven't had the problem since. Not to say that that might be your problem
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It was a clogged cat. I'm not sure, but some of the "cat guts" may have escaped and clogged the muffler, too. I believe you replaced it with an Ocelot system. Maybe?
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Mike said that whenever he handled the muffler, there was nothing coming out of it. When the ends of my muffler started separating, I was about to order an Ocelot from TFS. Mike told me about Bob's muffler, so I got it, installed it, and it's running just great.
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Why would the exhaust make the ignition pulse be erratic?
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Your right it was Ocelot Performance Cat. Not cheap but problem is gone along with the muffler
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Why would the exhaust make the ignition pulse be erratic?
Probably wouldn't. That got overlooked. Often we, and that includes me, on this forum and PFF fail to read the whole thread for previous comments and suggestions, especially when it's a multi-page thread. The symptoms were probably similar, both resulting in the loss of power. I think in Slyp's case, none or very little of the converter guts got down into the muffler. It seems to have clogged at the outlet of the converter. I think the converter was temporarily changed to a test pipe.
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My cat is clean. My muffler is also good.
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I read that and I could think was the scene from one of the poltergeist “this house is clean!”
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Just going back, this issue seems to have started shortly after adding the heat shield below the ICM.
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I solved some and created others when adding my shield.
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Maybe a grounding issue?
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I solved some and created others when adding my shield.
That's a bit vague.
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Well, I've sortof hit a snag. My Fiero drives fine--for 4-5 miles, then starts missing. Everything to the ICM is good, but garbage coming out, at this point. If I let the Fiero sit for a few minutes, it runs fine--for another 4-5 miles. If I cool the ICM, the Fiero runs fine.
It seems that the ICM is overheating.
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It seems that the ICM is overheating.
What kind of heat sink compound did you use?
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Wells.
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Often wondered why the caps are not ventilated. Some sort of hole or holes with a screen or foam filter. Could even have a small fan in there.
Do you have an IR temperature gun? very useful for these types of issues.
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Larry, the Fiero V6 distributor does have 2 screened vents in the base plate. But GTRS is running a 7730 with the coil pack. The ICM is mounted underneath the coil pack.
GTRS, ignition coils can work well when cold, then break down as they warm up. Of course, the same holds true for the ICM, too. It does seem to be one of those two items, though, unless it only happens as soon as the ECM goes into closed loop.
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The coils have also been replaced. No change.
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I solved some and created others when adding my shield.
That's a bit vague.
Well I made a shield and replaced some other stuff. Ends up I got a bad crank sensor. But I still have the same problem. I haven’t pulled the new scanner out of the box yet. So only time will tell.
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I'm going to replace the coils again, and see what happens. I don't know how to test the coils separately from the ICM.
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Well I made a shield and replaced some other stuff. Ends up I got a bad crank sensor. But I still have the same problem. I haven’t pulled the new scanner out of the box yet. So only time will tell.
The same problem you had, or the same problem I have.
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Same problem I had but similar to you but it’s a spark misfire instead of injector issue.
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Here are the facts:
1. The engine appears to run fine when cold, for about 15 minutes.
2. The ICM and coild have been replaced.
3. The heat shield is in place, but the symptoms remain when removed.
4. When the issue is hapoening, the signal from the crank position sensor to the ICM seems to be good.
5. When the missing is happening, the signal from the ICM to the ECU seems to be bad.
Here are the symptoms:
1. The engine will be running fine, then start missing, after about 15 minutes of running.
2. Idling, city driving, highway driving all appear to be the same.
3. The only constant in this occurring is time.
4. If, after the problem occurs, the vehicle sits for a few minutes, the engine will again run fine. The longer it sits, the longer it will again run fine, up to about 15 minutes.
5. When the missing occurs, if I try to keep driving, the engine will die. On the other hand, if I just let it idle, it idles fine.
6. When it misses, the tach jumps all over the place.
7. If I stop the vehicle, place it in Park, and gently blip the throttle while idling, the tach may show max RPM, or 0 RPM.
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Once an ICM has failed once, is it bad? A new ICM behaved exactly the same.
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Same problem I had but similar to you but it’s a spark misfire instead of injector issue.
My injectors weren't firing, because there was no signal from the ICM to the ECU.
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I replaced the coils and ICM with an old set, and the car ran for about 40 minutes before stumbling. I shut it off, but when I tried to start it about 5 minutes later, the engine did not want to start. I let it set for another 3 minutes or so, and it started right up.
To my knowledge, the crank position sensor gets its input mechanically, but provides electrical output. The signal from the crank position sensor to the ICM is good, but from the ICM to the ECU is eventually garbage.
For the last few years, i've had an exhaust leak at the EGR, right by the ICM. Sometimes, I could drive for several hundred miles, with no issues, but then the gasket would blow again. No issues with stumbling, though. There are no current exhaust leaks that I can detect. I see no reason to believe that it is suddenly hotter in this area.
The issues seemed to start right after the heat shield was installed. I've tried with and without the heat shield. The stumbling still occurs.
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The crank position sensor uses the "Hall effect" to produce electrical pulses. In other words, the crank position sensor is basically a magnet with a coil of wire wrapped around it. It is positioned close to a wheel on the crankshaft that has notches in it. Each time a notch passes by the sensor as the crank rotates, it disturbs the field of the magnet, which causes a current to be generated in the coil of wire.
The crank position sensor is a simple device, but it can go bad. If you haven't, try replacing it.
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Well, on the 6th one. The first 2 were just bad. Then there were several that had issues. The previous one had oil leaking up through it.
I think the problem is the ICM itself is overheating. I relocated it inside the trunk, with a ground to the engine. Same issue. I suspect it's much cooler in the trunk than by the crossover pipe. In any case, the exhaust shouldn't have been causing issues with excessive heat in the trunk.
Is there anything that could happen to cause the ICM to generate more heat?
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Any time there is high resistance in an electric circuit, heat increases. Larger gauge wire, if possible, might work, though the harness plug should have the correct gauge.
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Tried changing plug wires. Hmmm. There were some issues with the wiring on one of the harnesses to the ICM.
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Well, new, expensive thermal transfer grease installed. Found a bolt in one tire, so had to fix that. When the rain stops, I'll take the Fiero for a drive.
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There have been some storms, power outages, and cottonwoods falling. Considering that the cottonwood blocking the road is laying on the ground, and has a diameter greater than the height of my Fiero, it is likely that I won't be going anywhere for a bit.
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I'm wondering if the issue could be resistance in the wiring.
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Any ideas?
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Do you have a megger? Weak insulation can be a big problem. I’ve megged out a lot of old wiring and found problems a lot faster. I actually did it today on a tow behind turbine blower.
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Megger?
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https://smile.amazon.com/dp/B07BGSPH8P/ref=cm_sw_r_cp_awdb_imm_HD7SVNZN0Q7T1JYX25Q1?_encoding=UTF8&psc=1
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I don't have one of those. I do have a multi-meter that tests for resistance. What's the diff?
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This test for failed insulation too. If the insulation gets thin on two wires they can arc or cause interference. It’s also a good tool to check for shorts to ground. I normally use this for hvac compressors or motor windings. But the applications are endless.
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I don't have one of those. I do have a multi-meter that tests for resistance. What's the diff?
Though I don't have one, I would imagine that the insulation test involves a resistance test using a higher voltage than a regular ohm meter.
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It puts a high DC voltage through the line and looks for it on other lines. So yes everything has to be unplugged.
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Hmmm. I was going to just unplug one wire at a time, both ends, to test. Am I missing something?
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It’s just for when you are getting picky. These wires are so small compared to what I’m testing daily. Normally I’m checking for high voltage jumps but I can show me where one wires signal can be affected by another. A simple pin out test may be all you need. Hopefully next week I can tackle my problem.
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I get to replace the plug wires. 3rd time this year. Ugh.
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Is you do a thing for 1000 hours your considered an expert. Just 994 more hours of wire replacement to go.
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More like, 6 more hours to go. My expertise comes much more slowly, so probably something like 600,000 hours.
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The MSD wires currently in use are rated at 50 Ohms/ft. I suspect something happened to the wires, but I am seeing everything from that resistance to 350 Ohms/ft. Lower resistance would seem to be better, but what is right?
I do notice that these MSD wires seem to reduce power.