I have a problem that is threatening to make me bald very prematurely:

Basically I have a 1985 Z-28 drivetrain (5.0 TPI, automatic) and wiring inside a 1990 RS body. The complete wiring harness was switched over and some basic wiring changes were made to use the 1990 cluster. This car has no developed a problem where it will not run smoothly below 3500RPM and it will idle rougher and rougher then quickly die. At the same time it smells very rich. TPS voltage checked as well as MAF output (yes it is a analog MAF in a 1985with the piggyback controller) fuel pump is very new and pressure is at 40psi. No trouble codes are being set. I accidentally stumbled on to the fact that if the MAF is disconnected the car will run better but not still not as good as it did or should (code 33 will set with the maf disconnected) I'm fast running out of ideas. I should add that the basic tuneup items are all done, spark plugs, plug wires, cap and rotor, fuel filter, air filters. Isuspect an air leak but haven't found a leak anywhere on the intake.

looking for some new ideas
anyone?

 

Possible MAF issue.

Possible injectors as well.

U say its only below 3500 but it will do WOT fine.

Secondary ignition problem could cause this also.

Check the grounds for the ecm on the back of the cylinder heads?

later
Jeremy

 

Also,does this car have a sensor in the drivers side exhaust manifold. If so, make sure no burnt wires and you might just replace it. They are cheap at Autozone. If it is not working properly, then it could be telling the ecm its running lean.

 

are there subtle MAF problems that would not cause a code to set?

i haven't tried diconnecting injectors one by one yet

The grounds are an interesting thought as they may have been moved when we installed the engine or left off if they slipped down out of sight


Your talking about the o2 sensor? Yes it is installed, would the o2 sensor cause this dramatic of a problem? (the only other one I had fail was in my L69 84Z and the only symptom i remember noticing was the torque convertor not locking anymore probly because it was in open loop)

Does this sound right: The car runs OK (not great) with the mass airflow disconnected because it is in limp home mode and disregarding whatever erroneous input the ECM is getting. Once the MAF is reconnected it tries to run the engine based on sensor data and some of that data is screwed. As a way to verify that, what other sensor being disconnected would cause the ECM to switch to limp home mode? (so i can prove that the switch to limp home mode is whats causing the car to run better, not removing possibly bad MAF data)

How can I test the o2 sensor? Can I measure voltage at the input to the ECM?

thanks for the help so far!! This problem has been driving me nutty for about 3 weeks now and i really appreciate the help and advise from this sight!

 

Check all the basics.

Make sure the MAF burnoff and power relyas are working(open them up and check for smoke/pitting).

U have an 85 setup so you have that stupid MAF setup right? With the extra module?

Check everything dumb. Verify ign components and all easily doable quick tests, like ohming the injectors and checking cold start vavle operation.

Get a scanner and make sure all sensor readings jive when the car is cold/medium/hot.

Dont forget about vacuum leaks and the dum things like disconnected lines or bad grounds.

later
Jeremy

 

yeah i have the seperate maf module. I dont think the maf is causing any problems though as the output measured at the ecm seems to be normal.

new o2 sensor make no difference at all. No vacuum leaks that i can find. didn't check the grounds on the cylinder head yet.

my local parts store loans out a scanner but it was gone yesterday and today so hopefully i can use it in the next couple days

 

just discovered the free OBD-1 diagnostic software for your computer. I'm gonna make a cable to run from the ALDL port to my laptop serial port and see how it works then solve this *$%%# problem