1990 Coupe, Intermittent problem. Rough idle, dies, rich exhaust

[Larry]

1990 Coupe, 28,000 miles
Vehicle history – Plugs, wires and fuel injectors replaced several thousand miles ago, in 2007, I am not sure why. The rest is original.

Problem is intermittent and started last fall. Very rough idle and dies, rich exhaust. I read the ECM codes and then stored car for five months: 15 Coolant Temperature Sensor, 43 Electronic Spark Control (ESC) Circuit, 53 System over voltage, check charging system. I don’t have factory manual. I bought Haynes Repair Manual over the winter – wish I had that $20 back. Some of the info regarding the 1990 is just wrong.
Problem recurred upon reconnecting battery this spring. ECM codes: 43, 53. Problem is intermittent, does not occur on all start ups but occurs most times.
Alternator and battery checked OK
Coolant Temperature Sensor - replaced
Electronic Spark Control (ECS) Knock Sensor – replaced.
Throttle Position Sensor - O.K.. Voltage about 0.6 volts at fully closed and steadily increasing as throttle is manually activated.
Air Idle Control Valve - Replaced. Throttle body removed and cleaned. Ran great for about 5 minutes then the problem returned.
Distributor Cap and Rotor – Replaced. Inspected and looked worn, one prong bent & loose. Ran great for 10 minutes. I bought gas. Ran great 5 minutes more. Upon next start the problem recurred. Car sat for a couple hours and then restarted, same problem except Service Engine Soon light flashing - - - - .

I removed and reinstalled PROM in ECM. It was not loose.
I just took a ten mile drive without a problem. But I have no idea what will happen the time I turn the key.
I read through a bunch of posts but don’t know what to look for next. MAP sensor - Could this sensor cause an intermittent problem?
?Fuel injectors – Could one have gone bad? Injector wire problem? Why does it sometimes run fine?

 

[John Robinson]

One thing all those sensors have in common is the bundled ground were the wires all come together in the wiring harness. If you follow the wires back into the harness you will find a taped up group of black wires. Remove the tape and you will probable find the that there is gray corrosion on the joint were all the black ground wires are connected. If so you will have too cut that section out and reconnect the wires and solder them together. In some cases you may have to add a small piece of wire to each wire so you can join them together. Be sure to solder all connections and use heat shrink over the splice joints. I too started to replace every sensor that you have replaced until I found my problem was the wires and the grounds. If you cut the wires from the bundle take and check each wire for continuity from the ground bundle back to the plug that goes to the sensor. I found that the wires to the TPS were broken and that was another contributing factor for my intermittent problems.

 

[boomdriver]

One thing all those sensors have in common is the bundled ground were the wires all come together in the wiring harness. If you follow the wires back into the harness you will find a taped up group of black wires. Remove the tape and you will probable find the that there is gray corrosion on the joint were all the black ground wires are connected. If so you will have too cut that section out and reconnect the wires and solder them together. In some cases you may have to add a small piece of wire to each wire so you can join them together. Be sure to solder all connections and use heat shrink over the splice joints. I too started to replace every sensor that you have replaced until I found my problem was the wires and the grounds. If you cut the wires from the bundle take and check each wire for continuity from the ground bundle back to the plug that goes to the sensor. I found that the wires to the TPS were broken and that was another contributing factor for my intermittent problems.

Wire rot is the biggest reason for "phantom" electrical issues there is, on a C4.

I remember reading about issues with GM wire harnesses back in the 80's and early 90s. It seems there are SAE guidelines for minimum service life of wire and how much abuse it will take in a car over a period of time. These harnesses failed the base guidelines for resistance to typical automotive chemicals, oils and detergents.
This is the problem with a name brand product(Chevrolet-Corvette) subbing out portions of their project (to vendors) for the sake of economics and profit margins. Low bidder (cheapest product) wins the contract. The consumer (us) do the R&D and basically pay the price of testing that particular sub contracts quality control.

I can speak as the voice of experience on this, I've had to chop out sections (entire left inj harness section & most sensors) and solder in new wire because of internal rot, insulation that was dried out and cracking or simply falling apart and allowing signals to jump from one wire to another.

The solution is replacing the bad wire as its found and doing as John did, add heat shrink, tape them well and encase the mess in some of the conduit or plastic tubing made for automotive. Tie-strap that together and mount it or secure it. What you can't get sealed up can be covered and protected with some spiral wrap that keeps things from rubbing thru the rotten insulation.

 

[Larry]

Thanks for your inputs identifying wiring insulation breakdown as a likely contributor to my intermittent problem. The suggestions make a lot of sense. Plasticizers migrate out of PVC over time and the insulation loses flexibility. I guess they held up for twenty years. Unfortunately, I am probably exacerbating the problem by removing and installing sensors. :cry

 

[Larry]

I looked over wiring to the extent that I could. I did not find any obvious degradation.
I also checked the fuel injectors - all found to be about 16.2 Ohms.

I cleared codes and restarted, ECM code 43 again.
Knock sensor is new. I checked voltage between knock sensor signal wire connector and ground - 4.8 volts. Knock sensor resistance between sensor terminal and ground is 4000 Ohms.
Any other advice?

 

[John Robinson]

How did you check the ground for the knock sensor?

Did you use the actual ground in the wiring harness or did you take your meter and probe the ground terminal on the sensor to a ground point on the engine.

If you only checked the ground on the sensor then you still may have bad grounds were they all come together in the wiring harness. At this point it looks like you have proven the knock sensor is good. Remember codes only tell you somthing is wrong in the signal the ECM is receiving from that circuit it DOES NOT mean the actual sensor is bad. The ECM looks at each sensors signal and then determines if that system is within program perimeters. The ECM is not able to tell the difference between a bad sensor or a bad ground or broken wire.

I also got the code 43 but after I cleaned up the grounds on the body and the wiring harness it went away.

 

[Hib Halverson]

First thing to do is stop replacing parts based on guesses.

Second---if you're going to solve these problems DIY, get the factory service manual and use its diagnostic info to guide you to a solution. I get FSMs from Zip Products. They stock many of them.

Then, solve the codes which are resetting.

As for the running rich, misfire, etc, you mentioned that after a cold start, it runs ok for a few minutes then begins to act-up. It's possible the engine controls are having trouble controlling idle while in closed loop. Once you get the DTC 15 and 43 solved, then I'd be looking for a scan tester so you can see what the fuel trim values are doing along with looking at the oxygen sensor output.

You mentioned that the service engine light was flashing. Under what circumstances does that happen?

 

[Larry]

rough idle problem... solved

After checking the wiring adnauseam, cleaning and reinstalling grounds, the code 43, knock sensor circuit, continued to recur. I replaced the ECM, reinstalling the original PROM in the remanufactured ECM. I am approaching 100 miles without a problem. Previous to the ECM change, problem would crop up immediately or after up to as many as eight miles.

What I have learned through all this is that there are many things that can go bad and result in the same symptoms. Thanks for all the input. I ended up spending about $200 on parts and quite a bit of time learning some basics.

I will repost if the problem ever returns. Now, on to that persistent seat belt reminder light.