One of my friends has an 87 GTA, and just finished rebuilding the top end of the motor. The engine ran great before the rebuild, but after he had the heads redone, LPE 216 cam, and SLP runners, headers, and FMS 24 lb injectors, it is not running correctly. It does not idle that well, doesn't have good throttle response, and is overall not that powerful, less than when it was stock. He saidthe first time he took the car out it was running great, but the next time it started running bad and the burnoff relay went bad, and threw a code 36, but then the burnoff relay was replaced, and the code is fixed, the car also pings with timing at 6 degrees, ans sometimes diesels and is difficult to start sometimes. The things that we have checked so far:

Vacuum at idle is only 10, the guage says it should be about 20 for a stock engine, we disconnected all the vacuum lines one at a time and no difference. We even sprayed startting fluid around the runners and intake manifold to check for leaks.

We replaced his MAF with mine, and no difference.

We set timing at 6 degrees and tried advancing and retarding, no difference.

We set the TPS at .54 and it tops out above 4. We set the IAC and even moved it around more closed and more open to check for any changes.

The fuel pressure is at about 45 psi. We even tried richening it and leaning it out, with no real difference. His fuel pump seems to be keeping up, we held the motor at 3000 rpms and the pressure never dropped below 40 psi

His alternator also gets a little hot, and the battery is fully charged, not sure if this could be related.

Is there anything else we could check? Or anyone one thing that could be causing all these problems.

 

Something keeps telling me that it has to be a vacuum leak, but i can't find it, i am going over to his house to check again tonight, any other ideas???

 

We just rechecked the manifold vacuum. It was fluctuating rapidly between 8 and 12 in Hg. What could cause this. We individually took one vacuum line off the plenum or intake at a time and plugged the outlet and no difference. Even tried spraying carb cleaner around intake but no raise in rpm. I sprayed a little on the air cleaner and that made no difference either, just to see if the carb cleaner would raise the rpms, even tried spraying starting fluid around intake, no changes and then sprayed that at the air cleaner and it did nothing. They both acutally caused the idle to stumble.

Could it be something in the valve train that is causing the fluctuating low vacuum?

 

Might be valve lash. Try running the valves again. Also might be intake gasket leak.

 

The are hydraulic lifters and the lash was set correctly.

 

see if u can find a vacuum gauge key chart that should tell you what the low vac means