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94 Cavalier Z24 Crank - No Start

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7 years 1 month ago #5435 by CoreyK90
Feel like I am chasing my tail hopefully you guys can give me some insight.

1994 Cavalier 3.1L

I have spark and fuel pressure at the rail, I also checked for codes and there were none besides code 12 which just tells me it's in service mode from what i've read. I pulled the upper intake manifold and ohm'd out all 6 injectors and all are at 11.8 ohm. I checked voltages at the connectors and have 12v and 5v at all 6 injector connectors.

Now I know these GM 3.1s have injector shorting issues but showing about 12 ohms of resistance doesn't tell me they are shorted, correct? I have poked around on the internet for some help with no luck.

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7 years 1 month ago #5439 by Tyler
Hey there, welcome to the forums! Moved this to the repair section. ;)

I really like the testing you've done so far. You're clearly getting power to the injectors, but we need to know about the ground control from the PCM. I know most of the injectors are covered by the intake on this one, so we'll have to get to the control wires at a different location.

Here's the injector diagram, for reference:



I'd be interested to know if the dark blue and dark green wires will pulse a test light (connected to B+) at C351, or at the ECM, if that's easier. If the test light pulses, then we know the injector driver is working. If not, then we may still be dealing with shorted injectors.

Now I know these GM 3.1s have injector shorting issues but showing about 12 ohms of resistance doesn't tell me they are shorted, correct?


Not completely. :( It's a good static measurement, but it doesn't prove that they're not drawing too much current with an electrical load applied.

If you get no pulsing of the test light at those two control wires, then I'd suggest disconnecting C351 and rechecking. With the injectors out of the picture, the ECM should easily be able to pulse a test light on both wires. Does this make sense?

For anyone else following along, this video is great study material:

Attachments:
The following user(s) said Thank You: ScannerDanner

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7 years 1 month ago #5441 by CoreyK90
Yes, that definitely makes sense. I'll grab my test light and try testing from both locations, C351 and the ECU. I will get post back as soon as I can get the results.

Thank you for the wiring diagram as well.

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7 years 1 month ago #5451 by ScannerDanner
I like Tylers suggestion. Do that. Then also check the purple/white reference wire from the ICM (ignition control module). This is your RPM reference signal. The crank sensor feeds an AC sign wave into the ICM and the ICM converts it to a 0/5v square wave that is send to the ECM for injection control (and timing). It is common for the ICM to fail and not deliver this signal. It is weird to say your ICM can be faulty with good spark, but it happens often on this design.
If there is no signal there, we are not worried about the crank signal because you have spark, and we are not worried about the ICM power and ground circuits either (because you have spark). It would be a faulty ICM. Confirm injection pulse by turning the key on and take a test light to battery positive and touch on/off the purple/white reference wire, which will make the ECM fire the injectors.
Like I did in this video.

Don't be a parts changer!

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7 years 1 month ago #5528 by CoreyK90
Sorry about the late response. Just got back into town.

I had no pulse to the injectors or from the ecu. I checked the purple/white wire as suggested, the purple/white does run from the ICM to the ECU correct? If so I found a tested the purple/white wire at the ECU. The location of the ICM is awful for testing. During cranking the test light did not light. (Only reason I ask if that is correct is because when I turned the key on for a second the test light lit then went out and then would not light during cranking). It was the only purple/white wire I found at the ECU location. So if I tested correctly I then it showing that the ICM is not sending a signal to the ECU for injection pulse.

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7 years 1 month ago #5599 by Tyler

CoreyK90 wrote: Sorry about the late response. Just got back into town.

I had no pulse to the injectors or from the ecu. I checked the purple/white wire as suggested, the purple/white does run from the ICM to the ECU correct? If so I found a tested the purple/white wire at the ECU. The location of the ICM is awful for testing. During cranking the test light did not light. (Only reason I ask if that is correct is because when I turned the key on for a second the test light lit then went out and then would not light during cranking). It was the only purple/white wire I found at the ECU location. So if I tested correctly I then it showing that the ICM is not sending a signal to the ECU for injection pulse.


Alright, so no injection pulse as suspected. Did you have a chance to disconnect the injectors and recheck for pulse? That would help to possibly rule out shorted injectors.

About the ICM and the purple/white wire, there may have been some misunderstanding. :unsure: We really weren't looking for the test light to light on the purple/white wire, as much as use the test light to simulate the ICM's signal to the ECM. I dunno why the light lit up at all, to be honest. :blink:

For reference, here's the ICM wiring diagram:



I'm not sure if it's the ONLY purple/white at the ECM, but it should have a black/white and a red/black next to it on the same connector.
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7 years 1 month ago #5703 by CoreyK90
I must have tested at the wrong wire, that's my fault. I jumped the gun and replaced the ICM. Still no start.

Rechecked fuel pressure about 40psi when the pump primes and about 43-44psi during cranking and the needle on the gauge slightly bounces, I assume that to be injectors pulsing. I was able to check injector pulse at one injector with the upper intake manifold on (injector number 6) it had a pulse a fast pulse but I assume that is due to batch firing. I had an extra similar style injector laying around and had enough room to disconnect injector 6 and hook it to another injector and also helped me confirm pulse. Double checked and I have spark on all 3 coils and all 6 plug wires. Every now an again it will pop or back fire through the exhaust during cranking. Pulled plugs and they were wet after some time of cranking, cleaned and dried them all and did some more cranking and pulled them again and they were not wet but her a very faint smell of fuel, although that could be from when they were wet prior.

Before replacing the ICM if I were to crank on it for a period of time there was no fuel smell and after replacing it there is now occasionally fuel smell after cranking. So maybe a compound problem? I will say first crank after replacing the ICM the RPM and MPH on the cluster shot way up (about 4k RPM and about 50mph) but hasn't duplicated it self since first crank after replacing the ICM. It also sounded like it was about to make an effort to start but again this only happened first crank after replacing the ICM.

Going out to the garage here in a few minutes to do more diagnosis.

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7 years 1 month ago #5704 by ScannerDanner
I don't like the backfiring in the exhaust. Time for a compression test. I'm worried about the timing chain now.

Don't be a parts changer!

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