I just bought a 91 z28. I got a crazy good deal on the car. Anyway it does have a few problems. The car is a little hard to start but not a real big deal. However, once it starts it will only idle when the rear defrost is on. And if you try to give it just a little gas, it stumbles and dies.

A little background on the car. It sat in a garage for about four years when the owner was in Iraq, when he got back he discovered the problem. He replaced the fuel pump and filter, new plugs, wires, rotor and cap, new o2 sensor, still the same problem. I cleaned the thottle body, iac and checked the tps. Put new gas and a fuel system additve and let it idle for a while. The ses finally came and showed a 42 and 44.

The car is has a 5.7 tpi, rear disk, dual cats, red leather, hard top and factory radio.

I will post some pics later.

 

Here are some pics. Oh ya, I only bought it for $1800.

 

DTC 42 is a ignition module fault
Code 42- This code will be set when the engine is running at 600 rpm or greater, and there is no pulse on the white wire from the ecm to the ignition module for more than 200ms (milliseconds). Common causes for this code include bad connections, frayed wiring, or broken wires such as the tan/black (ecm pin D5) purple/white( ecm pin B5) or white wire (ecm pin D4). Check your wiring carefully along with the connections back to the ecm.

44 is a O2 sensor fault.
If the module is playing up it would throw the engine out of wack so the new O2 sensor sees a wrong fuel condition
Time to start checking the wiring

 

So where should i start? Try a new ignition module? And why the high idle when i turn the defrost on? When i turn it off it almost dies at idle.

 

Sounds like you have a wiring problem
This is where the factory service manual comes in handy.
It has diagnostic proceedures and tests to follow through so you don't just replace parts at random ($$$) until the problem is fixed.

Well worth the outlay if you plan on keeping the car
http://cgi.ebay.com/ebaymotors/1991-...sQ5fLiterature

Even the Cd version is better than nothing
http://cgi.ebay.com/ebaymotors/1991-...sQ5fLiterature

 

Code 42: Electronic Spark Timing- Poor electrical connentions, open or short in circuit; defective ignition module; defective ECM.

Code 44: Lean Exhaust- Lean condition caused by malfunctioning carburetor/fuel injector, vacuum leak, low fuel pressure, etc; poor electircal connection, open or short in circuit; defective MAP sensor; defective oxygen sensor; defective ECM.

You said that "sat in a garage for about four years when the owner was in Iraq, when he got back he discovered the problem. sat in a garage for about four years when the owner was in Iraq, when he got back he discovered the problem."

I would look at the wiring first, cause mice could have got to the wiring and chewed it . I've seen this happen many times before!!!

 

Yeah I checked all the wiring and nothing seemed chewed up but it doesnt do that all the time. When it idles by itself, w/o the defrost on, it will only get to about 2500 rpms. Does that have anything to do with the "limp mode" or maybe it has a bad ground in the system?

 

Could try changing out the o2 sensors, if nuthing, might have a bad ecm. there cheap too, and easy to install. If nuthin, could be a bad tps.

 

No its not "Limp Mode", instead of ECM keeping RPM's at 600-650. "Limp Mode" would be running your car at 650-800 rpms. Limp mode just dumps a little more gas then what is needed to keep your car running.

Take a look at Post #2 again.....ignition modules are cheap and should be easy to do....Replace it, and see if there's any difference.

 

It Really sounds like to me that you DO have some bad wiring somewhere!!! Your rpms range depending on your defroster being ON or OFF..... TPS is not your problem here. I think it could be a bad ground connection somewhere!......BUT WHERE????

 

I tried everything and no luck so I finally replaced the ECM and it fixed the problem. The only thing now is that it a little sluggish off idle, the timing might be out. I know its been a while since this was first posted but I thought I should say something just in case anyone else runs into the same problems.

 

Good deal on the car man. 1800 is pretty cheap.