tnts94z

working on a 94 z28 here and just trying to figure out some of the many issues with this car. Heres the basics, engine overhauled 20000 miles ago and car has 120000 miles on it. I bought it from a guy who has not driven it much in the past 2 years. When i bought the car, he said in the past 6 months he has replaced the battery, fuel pump, starter, alt. , and fuel filter. Car would not run when i bought it. I just put in a new coil, icm and ecm. Car sputters, misfires, backfires from idle to 3000 rpms, has a miss at idle, smells rich at exhaust. I have checked the plugs, they look good. Fuel pressure is 41 psi with key on, drops about 5 psi when you start and run it. Fuel pressure will gradually drop over a 30 minute span, about 10-12 psi, but jump right back up to 41 if you turn over or turn key on then on. Hooked car up to my laptop with Freescan program to get some codes and data. While I sure dont know how to diagnose the data, i did get 3 codes. 18, 34, and 48. My book says 18 isnt a valid code for my car, 34 is for map, and 48 for maf sensor. I did the tests on the maf connector and it all checks out, i also cleaned the sensor. the map sensor test i am not sure on. I checked reference voltage from computor to sensor, and it checks out fine. But when i do test for signal voltage, i get nothing at all. I thought maybe wire was broke, so checked it all the way back to ecm and its not. It appears there is nothing coming out of ecm, if i am doing test right. So, I am open for any suggestions. Thanks again and sorry for the long thread.

Injuneer

Use Shoebox's LT1-specific code list:

http://shbox.com/1/Dtcs.htm

DTC 18 = Injector Circuits

Check for 12V at the pink wire on each injector harness connector. Check the resistance across the two pins of each injector - should be 12-15ohms. If you have both within spec, put a 'noid light on it and verify the PCM is providing the ground to fire each injector.

DTC 34 = Manifold Absolute Pressure (MAP) sensor circuit (signal voltage low, high vacuum)

The MAP sensor is reading irrationally low pressure (impossibly high vacuum). What does the scanner data say its reading at idle? With a stockish engine, should be about 9-10"Hg or 30-35kPa. I believe anything below 14kPa sets a code. Do you have the 5V reference voltage between the gray and black wires? If so, that's all that "comes out" of the PCM. The signal is measured between the green and black wires, and the signal goes to the PCM. Almost always the sensor, because an engine can't produce MAP that low, excepts maybe a closed throttle decel from very high RPM. MAP sensor is used to set the ignition timing, since MAP is considered an measure of engine load. MAP also is used to determine which long term fuel correction cell the engine is operating in. The MAP sensor is also critical for fueling the engine if there is a problem with the MAF sensor, and the PCM defaults to speed-density control - see next code.

DTC 48 = Mass Air Flow (MAF) sensor circuit

Check for 12v between the pink wire and the black/white wire in the MAF sensor harness. If you have 12V, substitute a known good MAF sensor.

Any or all of those codes can be producing the problems you are having.

For some background on how to interpret the FreeScan data:

http://www.injuneer.com/ScanMast.html