I'm highly frustrated with this situation and hope that someone can help point me in the right direction. The first thing is there are no codes in the car. When checking code 12 does flash. According to the Snap-on scanner that I used the ECM is running in "LIMP HOME MODE" and the O2 sensor is showing lean. The car runs and starts everyday with no problem. It will hot start with no problem as well. It just runs terrible. The car has a new distibutor and control module, cap, rotor, plugs, wires and MAF sensor. I've removed and checked the EGR valve. It seems to be in great working condition. I've also used a smoke tester and there are no vacuum leaks. The serial # on the ECM is 01227165.I'm electronically capable of doing what ever needs to be done with a multi-meter if there is anyone that can tell be what and where to start checking. Any HELP is GREATLY APPRICIATED.

 

Some basic tests: first disconnect everything from the ALDL connector.

Then do a key-on, engine-off and watch the SES light. The SES light should come on, blink-off, then come on solid. Did it do the blink off (for an instant)? Yes, good. No, bad, the ECM isn't running and there is a problem.

To repeat that test the igniton needs to be off for at least 10 seconds. Then the test may be repeated.

If the above is OK, then start the engine. Did the SES light go off? Then good. Still on, then there is a problem. With a solid SES an error code should be stored.

If the SES is off then use the scan tool and check the sensor values. Does the CTS look correct? IAT/MAT? TPS at .5 volts at idle and increases as the throttle is opened. Does the ECM go closed loop and the O2 sensor start to swing?

With the engine running tap on the ECM, did the engine stumble? Bad ECM if it did.

Tap on the MAF, engine stumble? If so then the MAF is bad.

Check the ground wires on the back of each cylinder head. Bolts tight and wires OK?

Check fuel pressure, plug wires, timing, . . .

RBob.

 

Thank you for the reply. How do I go about disconnecting everything from the ALDL connector? Also I've checked the grounds on the car. They are good. The ground at the computer is a little high at 1.1 ohms, but I'm sure this isn't enough resistence to cause problems.

 

I've done the ECM tap and MAF sensor tap. There is no change when tapping these items. The TPS sensor is set at .54 volts. The ECM never goes to closed loop because the computer is in LIMP HOME MODE and keeps it in open loop. How do I check the O2 sensor to see if it swings? Fuel pressure is 35lbs. Ok according to specs. All other ignition products are new and fine. When I say new, just replaced in last 2 weeks. Thanks for your time

 

Unplug any scan tools and remove any jumpers or such from the ALDL connector.

RBob.

 

Fuel pressure has to be checked with the vacuum line disconnected from the regulator (and plug the vac line). Spec is 41 to 47 psi.

RBob.

 

The basic test you had me do with the SES light came out OK. It does immediately flash and then stay on. It goes off when the engine is started. All is good there. My fuel pressure gauge isn't here so I can't check the pressure again at this time. What is the next step from here?

 

Snap-On scanners can put early GM vehicles into a test mode that will fix the timing and put it into open loop as well. Do you perhaps have the Limp Home Mode test selection selected as you can select the option to test limp home mode.