JimRockford

90 Camaro,305 tbi with 700r4. My Camaro started “bucking” on the freeway about four days or so ago when it would get into 3rd and 4th. Then it devoloped a rough idle. Today it had to be towed. It wouldn’t go more than 10-15 mph without “bucking” and would miss and stall if you tried to accelerate further.
I can smell gas in the exhaust now when it ran last then it idled rougher and now won’t start at all.
I can hear the fuel pump running when I turn the key. It sounds like it always did. I am assuming this may be a throttle body issue? There’s a YouTube video by a member called “third gen guy” who claims 90 per cent of the running issues these cars have is caused by throttle bodies that need to be rebuilt.
I’ve heard the seals go bad and they will leak too much fuel.

I figured I would ask fellow tbi owners what it sounds like to them.

Schurkey

Check fuel pressure. Checking at key-on is better than nothing. Checking at idle is better. Testing at WFO, max RPM is best.
Connect a scan tool. Look at the data stream.
Verify that the "usual" tune-up items are serviceable. Cap, rotor, plug wires, plugs, idle air control, decent spray from injectors, adequate cranking compression, adequate rocker-arm motion. A weak ignition coil may allow the engine to idle, but not run right under heavier load. I had a failing pickup coil in the distributor that caused a high-speed miss--which deteriorated into low-speed miss, and eventually stalling at idle.

JimRockford

The tbi cars don’t have a schrader valve to check fuel pressure like the tpi cars. There is no check engine light so would there be any data for a scan tool to read?

vinny R

Sounds to me like the ICM that is in the bottom of the dizzy. It controls your spark and timing. If your timing is off it will cause a whole host of issue that you will think is something else. I am speaking from experience on this. You can have them tested at the auto parts store I think if you key on and the pump runs you will be able to see the injectors leaking if you shine a flashlight down there. I don't think the TBI is your issue though

JimRockford

Thanks Vinny,I am assuming this is the ignition control module,it looks like a transformer you’d see on audio amps from the old days. I’ve attached a photo. Is this the part?
like I said,this whole thing started about a week ago when the car would buck on the freeway only in third and fourth. I initially attributed it to the transmission, until it started doing it worse recently when it wasn’t even in gear.

i sure hope that’s all it is

JimRockford

Can someone here show me a photo of the location and the icm? What I’m seeing on the autozone website doesn’t look like what I am looking at. I

Schurkey

True. You will need one of two styles of adapter. One goes near the TBI where the hoses enter the casting. This is my favorite type, but can't be used on some applications due to clearance problems. The other kind installs in place of the fuel filter, so any restriction IN the filter, or farther forward of the filter can't be accounted for.


Of course. Sensor inputs, control solenoid outputs, and memory items like fuel trim numbers are not dependent on having "codes" stored.

JimRockford

Ok,well I pulled the ICM since Vinny said it sounded like that is the issue. Supposedly autozone can test it for me on their machine and let me know if it’s bad before I throw $30 some dollars away.
the module is the original to the car,it had a gm abel saying “warranty void if module removed.”

the bottom side side of the module is coated with a sticky white grease that reminded me of that marshmallow creme that comes in a glass jar. What exact type of grease is this so I can buy some more

Tuned Performance

iTrader: (92)

Schurkey

The computer (ECM) controls timing. The module triggers the coil, and supplies pickup-coil information to the ECM.

Not very reliably. Lots of false negatives, lots of false positives.

No photo on my computer. If it looks like a "transformer" you're probably looking at an ignition coil.

ICM is screwed to the distributor. It's what the two wire harnesses plug into on the side of the dizzy. A TBI vehicle should have an 8-pin module.


...although I wouldn't buy this particular brand.

You're very silly to replace the ignition module without testing fuel pressure and looking at the data stream.

JimRockford

Thanks Schurkey,yes,that is the piece I’ve removed. The first thing I saw was the coil. The icm was under the cap.
Are you saying it’s a waste of time having the parts store test it? How can it be “reliably tested then? I really don’t want to toss money away on something that’s not bad.

JimRockford

The car wouldn’t even start anymore today. It just cranks

JimRockford

I tell you,the more I deal with this 80’s hi tech crap,the more I miss the simplicity of 50’s,60’s and early 70’s cars.

Schurkey

Yes. You can't trust the results. They tell you it's good...you don't know for sure. They tell you it's bad...you don't know for sure.

They CAN'T be reliably tested. Check everything else, if nothing else fixes it, try a module. Or have it tested, believe the results, and see what happens. Maybe you get lucky.

The older modules could be tested with "adequate" certainty. Not so much the computer-controlled ignition modules.

Are the spark plugs fouled?
Is there any spark TO the plugs?
Is there fuel pressure during cranking?

JimRockford

Thanks Shurkey ,I appreciate your advice
Just curious,but why are these so hard to test reliably? You’d figure in this day and age there would be a way to test this kind of stuff so mechanics just aren’t guessing at things? I have to go to the parts store anyway for a cap and rotor,if they test mine and it says it’s bad and then I have them test the new one and it says good,would that be a fairly positive sign mines bad?

yes,there was fuel to the injectors yesterday when it was doing this. there was spark since the car ran,but rough. It had been getting increasingly rougher the last three days.

havent pulled the plugs. I doubt the plugs would have gone bad in three days

What else should I test first? I don’t have a scanner so that rules that out. This is my only “new car”. I stay away from this stuff normally.

Schurkey

1. I would LOOK at the cap 'n' rotor. 'Bout the only things I see wrong with TBI caps 'n' rotors is a destroyed/worn-out carbon button in the center, or corroded terminals. So, sure, it could be bad in some other way...but I bet it's fine.

2. Ignition modules are about like old-time tubes in radios and TVs. Tube testers that the public has access to are designed to sell tubes. Module testers are designed to sell modules. I don't trust any of them. There are functional tests you can do, but that doesn't test the pass-through portion of the circuitry, where the pickup-coil signal is sent to the ECM, and returns as ECM-derived timing signal.

Fuel pressure. Spark. Exhaust pressure. Data stream. Plugs fouled? Cranking compression. In other words, re-read post #2.

You're doing D-I-Y repairs on a computer-controlled, feedback-fuel-injected vehicle. You need to get a scan tool. There are dozens of options, borrow one, or maybe you and five friends can go together and buy one as a group. Without a scan tool (NOT just a p-o-s "code reader") you've unlikely to get anywhere with ECM diagnostics, ABS diagnostics, Body Computer diagnostics, etc.

It is INFINITELY easier to diagnose with a scan tool and access to the data stream, than to test individual components and the wire harness with voltmeters and ohmmeters. Yeah, it can be done...with such tremendous difficulty that I refuse to do it.

JimRockford

I obviously can’t measure exhaust pressure. I’m not a professional mechanic with a shop.
the compression is fine. A mechanic friend did that when I bought the car less than a year ago and the thing only has 90 some thousand miles on it.
it was doing 80 on the freeway yesterday. I seriously doubt it’s anything major.

I am looking for the most likely things that would cause it. Even If the testers don’t test every part of the module wouldn’t it still have something bad if it read bad on the machine? I own a tube tester and usually it’s right,I’ve only had about 3 times so far when there was an issue the tube tester missed

wouldnt the icm going bad cut spark to the engine? I believe that was what Vinny said. I have a tube tester,it’s helped on a lot of occasions with TVs and amplifiers.

Is the likelihood HIGH that the icm is bad?

cap is clean inside,rotor shows wear in center. I am going to replace them both.

JimRockford

Update. I bought a new cap and rotor as the rotor had wear in the center where it makes contact with the cap.I took the Ignition control module down to AutoZone to have it tested. They tested it twice and it passed all the tests and had no faults.

I reinstalled my icm,installed the new cap and rotor,I turned the key and it fired right up and idled and I could rev it and it didn’t stumble or buck or die,but it still has that periodic “hiccup” . im sure it would benefit from plugs and wires.
my concern is that if the issue was the ICM it could be one of those heat related gremlins that will reoccur out of the blue.

Obviously though this wasn’t compression or exhaust flow related or any of the far less likely things that could cause this. Those things don’t magically fix themselves.

Do those icms usually just quit,or do they act intermittently?

vinny R

Sorry Jim I did not respond back to your questions on this, Schurkey is doing a good job. Yes these can be intermittent and fail when hot absolutely. When mine was on it's way out my car would just start running bad and die. I would let it sit and it would fire back up and run again. sometimes for a week without an issue. At the end it would not advance my timing at all. If that is the original ICM I would just spend the money and buy a new one for just the piece of mind alone. You also could have had an issue with the little spring loaded contactor in the top of the cap. That could have caused the car not to fire at all. I had that also when I removed the cap to change the ICM.
I would do the wires and plugs also as you stated and then you know all that is new and if the wires are original also it is probably time.

What's wrong with 80's tech!

JimRockford

Thanks Vinny,you were right. It was the ICM,it did it again and I replaced it with a new one and it’s running fine now. No stalling out of the blue for no reason or idling fine one day and then acting screwy the next.

Schurkey

Yep. Your module tested good. Engine runs good. For awhile. Then it doesn't. You replace the module, and problem vanishes.

Module testers can't be trusted. False positives, false negatives.

That's not the way I'd have diagnosed this, but what you did worked. Congrats.

vinny R

Glad to be able to help you out Jim!