This post is long but I wanted to give as much informationon the problem I am having with my repeated ICM failures as possible. I haveread a lot of information on this forum and other places about this commonproblem, I may be missing it but I don’t seem to be finding a common solution. Myspecific questions are at the bottom. My ICM saga is in the middle. I reallydon’t know what to do next. I really have no confidence that 6 ICMs and 4 Coilpacks later that my problem is properly fixed. HELP!!! Please??





I just had my car into a national tire chain for the fourthtime since May to have my ICM replaced. I keep going back because they have a “1year Fixed Right First Guarantee”. Afterpaying $400+ (talked them down from the $500+ they wanted to charge me) whyshould I pay for the same repair again?


I first had the ICM replaced by my regular mechanic in 2013.It suddenly started running rough and threw a P0300 random misfire code. ICMreplaced car ran fine for two years no problem. In May same thing happenedagain. The only symptom I felt before the failure was a slight hesitation inthe engine while waiting at an intersection. The RPM would drop for a splitsecond like the car was going to stall but just as you felt it thing would goback to normal, light would change and everything ran the way it should. I tookit to the tire chain in May because they were open on the weekend and I hadavailable credit to pay for the repair. They changed the ICM and gave me thecar back. When I picked it up it ran well getting it home. went out a few hourslater, it started right up then stalled backing out of the driveway. Irestarted the car with no problem and drove to the store. The car ran finegetting to the store but when I tried to leave the car wouldn’t start. It wouldcrank but wouldn’t start. I had to have it towed back to the tire chain (approximately1000’ down the street from the tire store). They changed the ICM and got thecar running. They told me that when they took it for a test drive the IMC “fried”(their words) when they were pulling into the parking lot. They put another ICMin and decided to change the coils. After getting the car back I replaced theplugs and wires with Ac Delco parts (they wanted $380 to do them I did them for$80). The car was running better than it had in years until one night about amonth later it just died in the middle of the road with no warning. Once againit would crank but wouldn’t start. Had to have it towed again back to the tirestore, this time about a mile away. By this time I had done some research andhad found this forum and some YouTube videos. I spoke to the manager and toldhim I wasn’t happy and I wanted him to find and fix the underlying problem andnot to just replace the ICM. Got the car back with another new ICM and one newcoil pack. The car ran ok (had a 6th cylinder misfire turned out tobe a burned spark plug wire) until a couple of weeks ago. I started feelinglike the car was a little sluggish on acceleration, started getting ahesitation to start once in a while when turning the key. Then the day afterthanksgiving it suddenly started running rough and threw the p0300 code again. Iwas right near one of the tire chain’* other stores so I stopped in. Iexplained the saga to the store manager, he looked up the records and said itwould be about an hour before they could look at it. I waited, when the managercame back to me he said he had pulled my car into the bay and felt the misfirehimself but by the time his mechanic got to it the problem had gone away andthe car was running fine. I insisted that they take it out for a test drive tosee if it would come back for them. It didn’t!!?? it ran fine for a couple moredays them the random misfire was back. This time I took it to the original storeand had it out with the manager. I told him I need the problem fixed once andfor all. I insisted that if the ICM and Coils needed to be replaced again thatthey use the AC Delco parts this time but I wanted them to find the cause forall of these ICM failures. He won’t let me talk directly to his mechanic. Thismakes me think that they know what the cause is but because of their warranteethey would have to fix it for free.
Based on some of the information I have read onthe forum I suggested they try changing the crank sensor and/or cam sensor. WhenI bring up possible causes of the problem, the manager says “Or the Computer”. Inall the information I have read from others who have had repeated ICM failuresI have not run across anyone that said they have had to replace the ECM to makethe problem go away. If it was an ECM problem wouldn’t I be having othersymptoms? Is there a way a dealer could test the ECM to see if it is theproblem? Would the test be able to pick up a problem if it is only intermittent?Approximately how much would such a test cost me? I am going to try this weekendto check the ICM, Cam, and Crank connectors for any signs of corrosion, arcing,or loose wires. I will probably spray each of them with electrical contactcleaner before putting them back together. I pulled the ICM mounting bracketoff a while back to get better access to the radiator hose clamp and noticed aground connection on one of the bolts. Not sure if this is the ground for theICM but I will pull the bracket off again and wire brush everything down tomake sure it is making a good connection. I plan to also do a visual check onthe wires to see if I can find any damage. Beyond those things I don’t knowwhat else to do until it breaks down again and I throw an epic fit in themiddle of the tire store. Any suggestions??


by the way the car is a 2002 Bonneville SE, non supercharged

Unfortunately its random. That P0300 could be fuel related. Meaning, if fuel pressure gets weak, you develop a misfire on ALL cylinders. Even a drop in voltage at the pump can do this.

What other maintenance was done?

I thought of the possibility of a fuel issue also. Before taking it to the tire store last week I borrowed a fuel pressure tester from AutoZone. With the engine misfiring I got a steady reading of 44 psi. when I pulled the vacuum line from the pressure regulator the reading jumped up to about 55 psi. After taking that reading I found that one of my throttle body nuts was missing. I replaced the nut, the engine was still misfiring and I haven't had a chance to take another reading yet.


I replaced the original fuel filter with a Fram about six months ago when I was changing the plugs and wires.


I replaced the lower intake gaskets and replaced the upper plenum in August. I also had to replace the catalytic converter. The insides had turned to dust and random chunks.


each time the ICM has gone bad they put a new one in and the car runs fine. the problem is for how long? one was a couple of years, the next one was for a couple of hours, one was a few minutes, the one I just had changed lasted about 4 months.


when the car is misfiring it feels like in the old days when the timing was off a degree or two. it doesn't feel like it is starving for fuel. I realize an engine can misfire from being starved for fuel, but would that really take out the ICM??


tomorrow (Wednesday) I am going to retake the fuel pressure reading. if it is still low (book says it should be 47 to 55 psi) I am going to replace the fuel filter to see if that has any effect.


I don't believe this is related but I am also chasing an Evap code p0440. I've tried changing the purge valve when I did the intakes, I changed the gas cap, and replaced the vacuum line from the throttle to the purge valve (dry rotted at the elbow by the purge valve). I am replacing the rear shocks (blew the seal on the right one this spring after hitting a pot hole had to wait till I had the money to change them) While I am doing the shocks I am planning to change the vent solenoid and take a closer look at the canister and tubing in the rear(engine compartment and under the front of the car looked fine)


It is frustrating getting the car back from the shop knowing that the problem isn't really fixed and not knowing when or were you are going to break down next.

I found a good post on reddit that may help add some insight, it seems like continuous failing of the ICM would mean something is frying it, sounds like bad grounds, and or cracked coil packs can cause issues where the charge shorts and makers its way back to the ICM which can fry it.
Hopefully this read helps a little.
https://www.reddit.com/r/MechanicAdv...oing_bad_2005/

After reading that I'd probably get a multi-meter, set it to ohms, and check the ohms of each coil pack, the resistance should be close to the same for each, there is a spec resistance mentioned, but it was in my manual, and I lost it.. If you have a manual you may be able to find it in it.
The reason I suggest that is from what I have read, a failing coil pack can kill the ICM, that is a new one on me, but others say it has happened to them.

Thanks for the tip on the article. I will try to read itthis evening.
Before I decided to take it back to the tire store and haveit fixed under warrantee I printed out instructions on how to test the ICM andthe coils from the forum(I don’t have the links handy. Type ICM testing or coiltesting in the search box and they will come up). Both were written by DanThures (not sure I spelled the last name right). Both articles are well writtenand easy to follow and best of all have pictures. He included the range ofreadings you should get during the tests.
I wrote a post the other night listing all the possiblecauses of repeated ICM failures that I have found and why I didn’t think theyapply to my situation. Unfortunately I wrote it while logged in on the site. SinceI type slow the system timed me out so when I hit post the system told me Iwasn’t logged in and everything I had typed was lost. I have now learned totype offline and paste it into the post box online.
I don’t know about a bad coil taking out the ICM? Wouldn’tit be the other way around? Several times the ICM has been replaced withoutreplacing any of the coils and my misfire has gone away. One of those times Igot two years out of the ICM. One of the times all three coils were replacedalong with the ICM and I only got a month out of that one.
I haven’t ruled out electrical (bad ground, connector issue,short, weak sensor signal) yet. I am starting to wonder if it could bemechanical. Is it possible I may need a new timing chain?? If mechanical timingis off the ICM (and ECM) would work to correct the issue. If it requires substantiallymore work on the part of the ICM to keep the timing wouldn’t that increase theheat generated at the ICM?? This could be an explanation why the ICM is frying.
Please correct me if I am wrong but don’t the cam and cranksensors (timing) send their signal only to the ICM? And in turn the ECM getsits information on timing directly from the ICM?
How would one test this theory? Is there a wayto test the condition of the mechanical timing? Is there anyway to see if thetiming chain needs to be replaced short of a visual inspection??

Did you ever find out what is causing the ICM failures?

not really. I had the last one replaced December 7th and so far so good. I insisted that the tire company install an AC Delco ICM. I personally don't believe this fixed the problem but with the quality of Delco parts it may run a little longer before blowing again.

the car is actually going to the mechanic tomorrow for a p0420 cat efficiency code. Had this problem last summer. The mechanic replaced the catalytic Converter and attributed the destruction to the ICM misfire issue. I hope its not the converter again. I still owe him $70 on the last one.

I have a 3 coil pack and module that I got from the pick and pull that I intend to upgrade on my 1989 Buick Electra. I have did this swap before to a Lesabre because of the green goo oozing out the single Motorola unit. I never used the heat sink compound in the past and had no problems, but after further reading, it is advised to use a thermal compound between the module and the CLEAN mounting plate to dissipate the heat. The coils and module get very hot. I bought a small "vial" of compound from a computer repair shop. Something to think about and stick with GM electronics if possible.

A P0420 is usually the converter. It sounds like you have been running rich due to vacuum leaks(nut missing from throttle body) for a while and that is why your converter is fouled. The P0440 code could be as simple as a bad gas cap. I replaced my gas cap and never had that code again! Instead of taking it to the mechanic why don't you check for vacuum leaks with some carb cleaner? Just spray it around the intake and throttle body while the engine is idling
and if you hear the engine speed up then you have found your leak. I had 2 leaks that I found this way....cracked vacuum lines and a hole in my upper intake manifold directly above the EGR stovepipe.