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what are signs of a bad ECM?

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Old 07-28-2006, 11:57 AM   #1
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what are signs of a bad ECM?

my car has an interment problem. it sometimes will act up really bad. tach all over the place, poor idle, jerky operation. mind you this car only acts up when it wants to. this whole thing has me stumped. i already changed the MAP sensor(used one off of an s10 blazer, known working unit), EGR valve bought a new one at autozone(also had a code 32),and a new coolant temp sensor. how do i diagnose this thing. it drives me nuts. someone said my ECM could be bad. i doubt that becuase 90% of the time the car runs great. how about my coil or ignition module? i dont know what to do with this car to make it run right. i dont want to replace both injectors and all underhood electronics.i could spend a small fortune and still not fix the problem. everytime it acts up i feel hopeless becuase it runs fine a few minutes later. i cant diagnose something very easily if its not acting up when im looking at it. ive been working on older GM vechiles for many years now but this one's driving me nuts.
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Old 07-28-2006, 03:52 PM   #2
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The jumpy tach is a sign there is something wrong with the ignition system. The ECM has (almost) nothing to do with the tach. It is driven by the negative side of the coil (white wire).

Look for leaky or bad plug wires, bad coil, bad pickup coil, cap/rotor, ignition module, bad spark plugs.

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Old 07-29-2006, 12:05 AM   #3
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My ECM went bad once on my 86. Car would run perfect and the engine light would be off but as soon as the light came on(and it came on completely randomly) the car would go berserk and idle crappy, hesitate, backfire, you name it. But it only did it once in a while, and the light would sudenly go off and it would run perfect again. You should get a code too telling you the ECM is bad......I did when mine went bad.
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Old 07-29-2006, 12:42 AM   #4
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would that jumpy tach be combined with very poor performance? and would it act up at random. my car doesnt consitantly run like crap. hit or miss. sometimes when it act up if you let up on the gas for a while, the car will "snap out of it" and smooth back out, and run good again.im at a loss i dont know what to replace next. i thought it was a fuel problem, car running too rich. coil or module? if i attack iginition next which one should i replace?
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Old 07-29-2006, 01:43 AM   #5
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Hey, Vacum leak!
Have you checked all of your vacum hoses??? Belive me my car was acting the same and it was a vacum leak close to the map/maf sensor (not sure which). Try vacum first before you go replacing things.
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Old 08-08-2006, 09:53 PM   #6
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I second that vacuum leak.
For over a year, my car exhibited all sorts of computer-related problems.
Drove me nuts.
Replaced/ rebuilt almost everything, including all hoses.

THEN I FOUND A LEAK IN THE VACUUM STORAGE CANISTER.

Seth
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Old 08-22-2006, 04:23 PM   #7
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HEY, mine is runnin like crap. I have not checked the vac BUT I do know my cruise controll stopped working. SO I guess it could be a sign of vaccum not holding..thus leaking?

Thanks,
Sam
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Old 08-22-2006, 08:20 PM   #8
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WOW
I went under the car and noticed the vacuum line that goes from the cruise control unit to the chamber located under the battery was no longer attached causing a leak= my problem for the past 3 weeks !!!

FINALLY FIXED heehe

I also changed all plug wires since i had purchased them before locating the leak.

BUT the cruise control is not working... does it have a fuse? anything to check?

THanks,

Sam
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Old 08-22-2006, 09:57 PM   #9
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On my '83, the cruise control is [was] connected to a split in the speedometer cable. The lower cable broke, and that led me to a faulty cruise unit. I simply got a cable for a 'Bird w/o cruise. Never liked cruise control anyway. It is a stupid gadget, that has been on cars for 40 years.

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Old 10-05-2006, 02:15 PM   #10
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Go to a junk yard and pull a coule of turn signal switches with cruise on them and plug it into your harness. Eliminate the possibility of a bad switch first.
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Old 02-26-2007, 07:20 PM   #11
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Quote:
Originally Posted by ad356 View Post
my car has an interment problem. it sometimes will act up really bad. tach all over the place, poor idle, jerky operation. mind you this car only acts up when it wants to. this whole thing has me stumped. i already changed the MAP sensor(used one off of an s10 blazer, known working unit), EGR valve bought a new one at autozone(also had a code 32),and a new coolant temp sensor. how do i diagnose this thing. it drives me nuts. someone said my ECM could be bad. i doubt that becuase 90% of the time the car runs great. how about my coil or ignition module? i dont know what to do with this car to make it run right. i dont want to replace both injectors and all underhood electronics.i could spend a small fortune and still not fix the problem. everytime it acts up i feel hopeless becuase it runs fine a few minutes later. i cant diagnose something very easily if its not acting up when im looking at it. ive been working on older GM vechiles for many years now but this one's driving me nuts.
Hey, what is this.my maro started doing the same as you guys about 2 wks ago.it do drive u nuts.it only last for a short while,but it **** me off everytime.my car is in the shop now coundn't take it anymore,punishment.wish i waited one day,or atleast till i read this post,now its going to cost me. damn rip-off ''man-can-it'' fuuuuuuuuuuuuuuuk
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Old 02-26-2007, 07:40 PM   #12
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Aren't these intermittent things a real bitch?
My 'Bird has a noise that comes and goes. If it were an engine bearing, the thing would be long dead. Could be the tranny pump, could be a ghost or gremlin. Actually, Gremlin was a cool little car from AMC.
Just had to respond to your post, when I got an email from this board. Need to remain intouch with my helpful, generous buddies, here.

Can't wait for winter to leave, so I can tinker once again.

Seth
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Old 03-13-2007, 01:38 PM   #13
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I've seen a problem like ad356's on a 91 L03 car, it was a bad ground wire either in the ECM harness or in the ignition system, I don't remember exactly where. I'm pretty sure it was the ECM harness. I remember splicing a wire and hooking it to the negative battery terminal and the car ran like a top after that.
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Old 09-03-2007, 11:55 PM   #14
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Re: what are signs of a bad ECM?

Hey, I'm having sortta the same problem... did you ever get it fixed?
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Old 09-15-2007, 01:15 AM   #15
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Re: what are signs of a bad ECM?

Don't forget the EST timing control module, I ran with bad wiring and blew a few modules (over a period of weeks and 2 modules) after installing a CD ignition. My 88 LB9 Camaro would run for a while...poor mpg, tacho failure, surging idle, the odd miss and then a no start, never did fail during a drive.
I believe the tacho drive to the ECM comes off the EST so this could be the problem?
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Old 09-18-2007, 07:33 PM   #16
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Re: what are signs of a bad ECM?

Quote:
Originally Posted by RBob View Post
The jumpy tach is a sign there is something wrong with the ignition system. The ECM has (almost) nothing to do with the tach. It is driven by the negative side of the coil (white wire).

Look for leaky or bad plug wires, bad coil, bad pickup coil, cap/rotor, ignition module, bad spark plugs.

RBob.
I concur this was the problem on mine, did the same exact thing. The coil wire would work it's way off of the distributor and the retainer that held the wires that snapped onto the coil was broke. The problem would be intermittant and was most noticable at steady speed when it first started. It then got progressivly worse.
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Old 11-18-2007, 09:30 PM   #17
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Re: what are signs of a bad ECM?

Quote:
Originally Posted by ad356 View Post
my car has an interment problem. it sometimes will act up really bad. tach all over the place, poor idle, jerky operation. mind you this car only acts up when it wants to. this whole thing has me stumped. i already changed the MAP sensor(used one off of an s10 blazer, known working unit), EGR valve bought a new one at autozone(also had a code 32),and a new coolant temp sensor. how do i diagnose this thing. it drives me nuts. someone said my ECM could be bad. i doubt that becuase 90% of the time the car runs great. how about my coil or ignition module? i dont know what to do with this car to make it run right. i dont want to replace both injectors and all underhood electronics.i could spend a small fortune and still not fix the problem. everytime it acts up i feel hopeless becuase it runs fine a few minutes later. i cant diagnose something very easily if its not acting up when im looking at it. ive been working on older GM vechiles for many years now but this one's driving me nuts.
I had he same symptoms with my IROC... It seems the ECU was bad and was reading the engine temp at 240 degrees all the time... (An infrared gun read at 180 all day long) this of course made the computer think it was running to lean and richened the mixture. That is the cause for the missing - too much fuel. Resolution, get a new ECU and plug in my original ECM. I did try a remanufactured unit from NAPA - it ended up doing the same thing a week later - then got one from the dealer no problems since going on 2 months. The remans sometimes have bad soldering or so I am told. All it took was an OBDI reader and scrolling trought the readings from the computer to find the problem, 240 degrees jumped right off the screen and screamed at me!
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Old 02-25-2010, 10:10 AM   #18
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Re: what are signs of a bad ECM?

Did this problem ever get fixed? mine is doing the samething!
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Old 04-19-2011, 03:19 PM   #19
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Re: what are signs of a bad ECM?

Well my 87 IROC vert is doing this "bucking and limiting" thing, don't know if its the same problem, but used to do it at speeds "over" the limit, like a speed limiter would, but know it is doing it at slow speeds, under heavy load usually. It jumps and bucks like it is cutting itself off for some reason. Brian, from TPICHIPS.com suggested that I change out a very problematic pick up coil in the distributor. I have a new one and module, cap and rotor. I will be rebuilding the distributor this weekend and will post the results. Maybe this is the same problem. Just my 2 cents.
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Old 04-20-2011, 02:20 PM   #20
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Re: what are signs of a bad ECM?

That pick up coil is very robust and I have never seen one intermittent. Doesn't mean it can't happen just not likely. Now the EST module it feeds! Now there is a intermittent known problem sensor!
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Old 04-20-2011, 02:24 PM   #21
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Re: what are signs of a bad ECM?

Quote:
Originally Posted by dboersma69 View Post
I had he same symptoms with my IROC... It seems the ECU was bad and was reading the engine temp at 240 degrees all the time... (An infrared gun read at 180 all day long) this of course made the computer think it was running to lean and richened the mixture. That is the cause for the missing - too much fuel. Resolution, get a new ECU and plug in my original ECM. I did try a remanufactured unit from NAPA - it ended up doing the same thing a week later - then got one from the dealer no problems since going on 2 months. The remans sometimes have bad soldering or so I am told. All it took was an OBDI reader and scrolling trought the readings from the computer to find the problem, 240 degrees jumped right off the screen and screamed at me!
I know this is an old post but since someone brought it back from the dead figured I would comment on this. First glad you solved your problem. Second I have never seen this happen, doen't mean it can't just not likely. Now the CTS, that happens all the time and gives the ECM a reading the other extreme of -40 degrees... funny how the ECM went bad on that circut only and went bad the other way, HOT!
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Old 04-20-2011, 03:16 PM   #22
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Re: what are signs of a bad ECM?

Well please tell me more. I am not happy with my car acting up, after being a good reliable ride for the past 24 years. Where is the EST located. Hate to just replace part after part after part as I'm sure others have and I don't mind spending 100 bucks for a well needed tune up. If the EST needs to be checked, how would you go about that. Thanks EagleMark!!!
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Old 04-20-2011, 03:38 PM   #23
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Re: what are signs of a bad ECM?

Well you said your rebuilding the distributor so you will find it in there. It's the only sensor in there and the pick up coil you have to take gear off and pull the shaft to get the pick up coil out.
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Old 08-04-2011, 06:08 PM   #24
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Re: what are signs of a bad ECM?

I have a 91 camaro rs 305 tbi auto trans.I plugged up a code reader and it say that its unable to read the codes and there is no power going to the crank fuse could the ECM be bad The car will not crank but it has power to the gauges and radio and when i try to turn it over the gauges jump ever where any advice on the computer being bad
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Old 10-26-2012, 11:41 PM   #25
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Re: what are signs of a bad ECM?

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Old 11-18-2012, 07:34 PM   #26
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Re: what are signs of a bad ECM?

my 88 Z has this same intermitent problem,cant figure it out for the life of me. i think there is a 3rd gen ghost...its my only explanation.
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Old 11-18-2012, 07:58 PM   #27
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Re: what are signs of a bad ECM?

check your grounds. Engine ground and grounds behind the heads.
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Old 11-20-2012, 03:05 PM   #28
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Re: what are signs of a bad ECM?

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Old 01-02-2013, 09:06 AM   #29
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Re: what are signs of a bad ECM?

My 83 L69 has the same problem; intermittent rough running with the tach jumping around. No check engine light either. Tried several troubleshooting options. Next, I did swapped in an 84 Firebird ECM. Immediately got a check light, so I removed it thinking that the ECM wasn't compatible.

Reinstalled old ECM, didn't have the problem for months. Started doing it again later and I removed and reinstalled the ECM connectors. Problem went away again. Did it a third time. Running fine for the last couple of months.

I'm thinking of just replacing the ECM and see what happens!

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Old 01-02-2013, 11:31 AM   #30
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Re: what are signs of a bad ECM?

Did you check the ECU ground? Maybe run a new fresh dedicated ground. After all it is getting up in age. I ran a new ground for my fuel pump install
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Old 02-16-2013, 07:24 PM   #31
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Re: what are signs of a bad ECM?

I have a 92 firebird 3.1 manual trans. It idles rough and when I drive it it often acts up by cutting out. The speedometer and tachometer go to zero and back and the car cuts out. Then it will run okay for a while but die at idle. I'm not sure if I should replace the pickup coil or the ECM. Any suggestions?
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Old 02-16-2013, 08:43 PM   #32
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Re: what are signs of a bad ECM?

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Old 04-01-2015, 09:36 PM   #33
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Re: what are signs of a bad ECM?

Hey can a bad ECM prom cause the car to not turn on?
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Old 04-01-2015, 10:50 PM   #34
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Re: what are signs of a bad ECM?

Wow. I am still subscribed to this very old thread. Have not had a Third Gen since June 2009. You guys are lucky to have each other for support. I have a 2004 GTO [from Australia] and most of the guys at the largest and most-informed internet-based support group are dicks. I always found the members at ThirdGen to be just the opposite. Miss you guys.
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Old 04-02-2015, 11:09 PM   #35
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Re: what are signs of a bad ECM?

What years your car joker86
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Old 04-09-2015, 09:12 PM   #36
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Re: what are signs of a bad ECM?

Im having what i think is an ECM or PROM problem. (83 L69) The car does not want to idle. It will run with my foot on the gas but take my foot off and it dies. I have checked everything and cant figure it out. I was getting trouble code 41 which is not signal from distributor. The car has almost everything new....Im at a loss
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Old 04-09-2015, 09:30 PM   #37
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Re: what are signs of a bad ECM?

Do you know what year proms and ecms are compatible with your car because I just went last week to the junkyard and took a couple of prims and im pretty sure I pulled one out of an 84 or 85 jus dont know engine size it was from

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Old 04-09-2015, 09:33 PM   #38
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Re: what are signs of a bad ECM?

I think only proms that will work for my car are Qjet computers with a 5spd. other than that im not sure. I really dont know much about the ECM on the QJet cars.
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Old 04-11-2015, 01:26 AM   #39
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Re: what are signs of a bad ECM?

Travas, your car should idle in open or closed loop, meaning with or without input from the ECM. The carburetor is designed to allow feedback fuel control from the ECM, but since the O2 sensor isn't heated, the system often idles in open loop. In open loop, it functions much the same as a non feedback carb.
You need to figure out what is missing. Is spark KV what it should be? How about compression and timing? Is it going lean at idle. Check for air leaks. Check the EGR for sticking open. Does it run normal when driving?
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Old 04-11-2015, 01:23 PM   #40
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Join Date: Sep 2010
Location: Paradise, CA
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Car: 1983 Camaro Z28HO
Engine: 305ci 5.0L L69
Transmission: Borg/Warner T5
Axle/Gears: 9 Bolt 3.73 gears

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Re: what are signs of a bad ECM?

When i first got the problem i was driving (aggressively) and went to slow down and for a stopped car and i usually use the clutch to slow down in conjunction with the brakes. When i did it acted almost like it stumbled on itself. When i engaged the clutch the car wanted to die. I saved it from driving and kept it running enough to get it home and then i died on my road and i had to coast onto my driveway and that was the last time i could get the car to start. Only with me playing on the gas will it start. will not idle on its own. almost everything is new as i just rebuilt the engine (about 1000 miles on it). Only original parts are the block/crankshaft/distributor/smog equipment. I will start going down the list for fuel/spark. The car ran like a beast before this.
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Old 04-28-2015, 06:20 PM   #41
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Engine: 3.1
Transmission: 700r4
Axle/Gears: stock

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Re: what are signs of a bad ECM?

Would a prom that goes in the ecm from I think a 1986 5 speed lg4 work with your car?
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Old 04-29-2015, 06:11 PM   #42
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Join Date: Apr 2010
Location: Aurora, OR
Posts: 3,799
Car: 87 IROC Z28
Engine: 355 cid TPI
Transmission: Pro Built Automatic w/3,500 stall
Axle/Gears: OE stock 3.27 Limited slip

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Re: what are signs of a bad ECM?

Hey Travas, any news? From the sound of things, your engine should be mechanically sound. Most likely looking at a fuel or spark issue.

One thing that comes to mind, since this issue started after a hard decel, I have had a few cases, on various late model carbs, where the phenolic float sticks, causing the carb to flood with fuel. It happens because the float after years of sitting in fuel swells up and comes in contact with the inside of the float bowl. Initially, the symptoms seem about the same as no fuel. The engine loses power, stalls and won't restart. If you look at plugs, you'd see them fuel soaked, even though spark KV checks out okay. If the engine runs long enough you'll get black smoke from the exhaust. May not be your issue at all, but I'ts something I've seen a few times, especially on Rochesters.

Something else too, if you don't get spark when testing, try unplugging the 4 pin connector coming from the distributor. This puts the ignition module in timing set mode where it operates without ECM input. If there is an ECM issue or a short in the EST circuit(s), this will allow the ignition to operate. It's a quick test to tell you if the no spark is due to something in the dist or coil, or if it's in the EST circuits. If you get spark this way, but not with the connector plugged in, then you need to look for faults in the wiring to the ECM or the ECM itself. ECM failure is rare.

If you get no spark either way, suspect the ICM or pick up coil in the dist. Of course, test the coil before you jump on the ICM.
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87 IROC Z28 355CID TPI - Car hand built from the ground up with numerous upgrades focused on speed and cornering. Brakes from Fly N' Bye performance. Engine prepped by Somers Machine.

13.4 @ 104MPH 2.2 60FT on BFG Comp TAs
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