Questions and Answers

Pontiac Grand Prix DTCs 75, 76, 77 Q. Dear Mr. Ciulla, Over the last several months I have been chasing trouble code problems with my 1994 Pontiac Grand Prix. The engine is a 3100 L throttle body. It has an automatic transaxle, air conditioning, ABS and fuel injection. It has 169,000 miles. The Service Engine Soon (SES) light came on.

I replaced the O2 sensor cleared the codes and went on only to have the SES light come on shortly after driving it. I surrendered to temptation to change any other item and took it to a mechanic. He charged me $307.00 and replaced the EGR solenoid.

He then cleared the codes and off I went only to have the SES light come back on. The codes indicated that there was a problem with all three solenoids with the new EGR valve. He then told me that diagnostics indicated that the computer was sending signals to the solenoids.

He also indicated that the engine was pinging according to the computer although I’ve never heard it ping myself. He told me that he had cleaned the ports that led from the EGR valve into the plenum.

He indicated that the map sensor voltage was not normal and that would be what needed to be changed. Well I changed it myself cleared the codes and to my total frustration the SES light came back on. He then wanted to replace the intake manifold gasket at around $500.00.

I couldn’t take it any more and took the car to a local auto hobby shop. I removed the EGR valve and looked at the ports. I didn’t remove the plenum but they didn’t appear to be blocked and I flushed them with throttle body cleaner and blew them out with compressed air. I cleaned the throttle body. This car runs great.

There is no foamy residue in the radiator. No obvious foam around the intake areas that would make me believe the gasket is bad. The car does use a quart of oil every 600 to 700 miles. The SES light is not on all the time. The car has to be running for awhile.

The codes that come up are 75, 76, and 77 which indicates the 3 solenoids on the EGR valve are what triggered the SES light. When I was looking at the EGR system I noticed that the vacuum line rubber seal that went into the throttle body was brittle and needed replacing.

GM dealers have a vacuum line replacement kit that sells for $25.00 or $30.00 and I plan to purchase it put it on the car and then have the car rechecked. I would like any and all advice that you could give me.

Thank you for your time,
Michael

A. You need to make sure the wires from the PCM to the EGR are all okay. I've seen a partially clogged catalytic convertor cause these codes. What happens is that the back pressure build up causes the EGR to open off their seats when they aren't supposed to.