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1991 Chev Cavalier Won't Start - Fuel System
Question: I have a 3.1l v6 in a convertible. It quit one day. It has a new igniton module, crankshaft sensor, knock sensor, oil pressure switch, ECM, Prom, plugs, plug wires, fuel pump, fuel pressure regulator, Timing has been checked, chain and gears in good shape and lined with the marks, It will spark but the injectors will not fire. I have checked the wires all are continuos with no shorts. fusible links are good, grounds are good, battery is good. turns over fine but the injectors will not fire. I can manually fire the injectors by grounding the injector signal wires from the ecm. ???
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Technican: Hello, Since you have spark but no fuel you have reason to believe the ECM is not firing the injectors.
Why was the ECM replaced? Did you transfer the Prom from the old ECM or replace the Prom? Have you pulled the Prom and looked for bent pins?
Please advise.
Owner: I purchased a new ECM because It looked like the wiring was correct and the only thing left to make them fire would be the ecm. when this didn't work I bought a new prom. the prom was ordered from a dealer using the vin number and it has the same markings as the old prom. I am an electrical engineer and can verify that the prom was installed correctly with no bent pins
Technican: Is the ECM you purchased in fact new or a remanufactured one?
Technican: Perhaps this will help.
The Camshaft Position sensor (CMP) is used to correlate crankshaft-to-camshaft position so that the PCM can determine which cylinder is ready to be fueled by the injector.
Have you checked your car's CMP?
Owner: I have replaced the CMP. It reads 948 ohms which is in th range of the haynes manual. But, the manual says when cranking it should put out more than .1 volts (100 millivolts) but it doestn't seem to put out more than 5 millivolt. I thought maybe it was because I was using my analog meter and it wouldn't respond fast enough so I tried my digital meter and 5 millivolt was the max. While I'm typing I thought, should it be AC volts or DC volts?
Technican: AC milivolts. Hall Effect Switches generate an AC pulse.
The starter won't spin the engine fast enough for a higher end of the scale reading but the point is does it generate a signal? Removing all the spark plugs will allow for a faster cranking speed but should not be needed.
Can the signal be verified to be reaching the PCM?
Owner: I verified the signal at the harness where it plugs into the ignition module. Can I verfy the signal on the purple/white wire on the ECM harness with the harness unplugged from the ECM?
Technican: It appears you can. Try monitoring Connector 1 pins C8 White & D8 Ppl/White. Or perhaps just the Ppl/White C1 D8 and ground.
Interesting, the wiring diagram I'm looking at does not show the CMP....just the Cranshaft sensor.
Owner: O.K. I misread your reply. This motor does not have a camshaft sensor, only a crankshaft sensor. it reports to the ignition module and then the ignition module sends a reference signal to the ECM. Would that signal be the same ac voltage signal or would it be a pulse?
Technican: It should be the same AC milivolt reference. The ICM does not rectify the voltage to DC that I am aware of. It should just pass through.
Owner: I will check that tonight after work and let you know the results.
thanks,
James
Technican: I'll look forward to the results.
Owner: I checked the purple/white wire on terminal D8 against the red/black ground on D9 but I am not getting a signal at all. I need to remove the ignition wire and inspect the pins as well as the wires. I checked the Purple/white wire for continuity the other day but maybe when you bend it to plug it back up it loses? I will try to work on it somemore tonight.
Owner: I verified all of the wiring is correct. The crankshaft position sensor is giving the correct signal to the ignition module. I get little to no signal on the purple/white wire that is supposed to signal the ECM. I went through the diagnostic tests and checked all the key-on voltage signals to the ECM connectors. According to the diagnostic list the 2nd and 3rd gear input signals on terminals D4 and D15 should be B+ or battery voltage but they only read .08 and .09 volts each. Could this stop it from injecting?
Technican: OK, I assume your check engine light turns on when the key is on and that the check engine light will flash code 12 when the key is on and the diagnostic connector is shorted between pin A & B. If this is OK, then check that the TPS voltage is below 2.5V when backprobed. If OK, check your tachometer to see if it flickering when cranking the engine. It should move a bit. If OK, we need to check the purple/white wire more closely. That wire is the signal from the module to the computer. It should have a square wave signal, generating a duty cycle on your digital meter.
The gear input signals will not cause your fault.
Sterlingfixer
Owner: Thanks for the reply. I will check the TPS AGAIN tonight after work and let you know how it looks. T will also check the tach and try the Purple/white wire again. I did buy another ignition module with coils to try tonight also.
Technican: Double check the service engine light and code 12 as well. Are there any other codes?
Owner: I will try again tonight but so far three computers and one new prom and only get code 12.
Technican: If the check engine light is working and you can get a code 12, then the power and ground to the computer are OK and the computer is booting up. Lets check that TPS, tach and purple/white wire more closely.
Owner: What kind of a voltage reading should I get on the Purple/white wire?
Technican: Should be 5V, 50% duty cycle. Make sure you use a high impedance meter when backprobing.
Owner: AC or DC? Do I check against ground or the black/red wire?
Technican: Check against ground. It should be 5VAC, 50% duty cycle. If not close to that, unplug the computer and retest at the module. If still bad, ohm the wire from the purple/white wire from the computer to the module to make sure it is not grounded or shorted to power, or open circuit. If that tests out, the problem is somewhere on the module side of things.
Owner: I have checked with the wire unplugged and I get 0-.05 VAC. My digital multimeter has an input impedance > 1 Megaohm. I have checked the purple/white wire for continuity and against ground and it is good. The diagnostic table I have says it should be near 0 with the key on and 2.3volts when running. When you say 5VAC at 50% duty cycle do you mean when cranking?
Technican: Yes, that is cranking specs. 2.3v is dc spec when cranking or running. 5vac, and 50% duty cycle when cranking.
Owner: I will try that tonight.
James
Owner: I checked the TPS and it reads .59 volts. The tach moves when cranking. i read the p/w wire and got .05-.19 volts dc while cranking but no volts ac when cranking. I changed the ignition module and coil packs and the voltage readings were the same but the injectors fired occasionaly. i am going to reassemble the intake and fuel rails tonight and see if it will run. anymore ideas?
Technican: Since you have intermittent injection, I would ohm each fuel injector. They all should measure between 12 and 12.4 ohms. If on is lower than that, it can shut down the injector driver in the computer.
You can check for injection pulse with a test light between pin A & D with the 4pin injector connector harness disconnected. The light should flash when cranking. Also check between B & C. C & D are power and A & B are from the computer.
I am not pleased with your results on the purple/white wire. However, I always check this wire with an oscilloscope, so I am not positive on the results you will see with a meter. The purple/white wire sends the signal to the computer regarding engine RPM. The computer adjusts it and sends it back on the white wire to adjust the timing.
If you would have a scanner, you could look for engine RPM while cranking. This would confirm the purple/white wire, the crank sensor, coils and module.
Owner: I also noticed that one of the injectors is leaking very bad so I will replace it with a new one and try again tonight.
Technican: Make sure you compare the resistance readings as well.
Owner: I didn't get to work on it last night. I will try tonight and let you know.
Owner: I checked the resistance and had to buy another injector. I found the purple wire was not connecting when plugged in so I worked on the connector and now have the 4.75-5 volt AC signal when cranking. The injectors are now firing. I have spark. Still no start. I guess I should recheck the timing?
Technican: OK, cranks but no start with fuel and spark. Check to make sure you get spark on all 3 plug wires on the front.
Does the fuel pump run when cranking? Do you have at least 32-47 psi fuel pressure?
If both OK, check the condition of the plugs, that they are not flooded. I should mention that the spark required needs to be strong, as in, able to jump 3/4 inch.
Owner: I have 46 psi and spark on all plugs. I checked the plugs at 1/2" and got good spark. I will check again tonight at 3/4".
Owner: I have checked and I do get a spark at 3/4" on all plugs.
Technican: Since you have pressure, spark, fuel, then we nust look at the spark plugs themselves (flooded?) and the engine compression/timing. You have checked the chain already and they are NOT prone to failure. Compression should be 130psi+. Are the spark plug wires on the right plugs?
Owner: Can the compression be checked on a cold engine? I have never gotten over 95 psi compression at any time I checked it.
Technican: Cold compression is acceptable. Try adding about 1oz engine oil in each cylinder to see if that boosts compression. If so, then the cylinder walls may have been washed down with fuel, or you may have worn rings.
If compression does not improve you may have a valve problem, or a timing chain problem. Do your measurements with the throttle wide open. Here's How To Get Guaranteed Solutions In Minutes
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