Hi. After just having fixed a rich running engine( (MAF sensor & relay) 350tpi ) it's now very lean, to the point where it stumbles as you drive. We first thought it was an EGR problem, because it does run better with it disconnected. So there's a new EGR installed and no difference. There's good fuel pressure, the o2 reads fine and the vaccum is good.
We are completely stumped. Any ideas would be helpfull.
Thanx.

 

Do the plugs read alike a lean condition..i/e white.ashy? If not, then maybe your stumble is not A/F related?

 

16th,

Remember to check the TPS voltage. Obviously, you should inspect the vacuum lines, including the PCV and brake booster.

Another possibility is a leaky connection between the MAF and ducting. There is also a possibility on your '87 that the plastic air resonator box (in front of the throttle body) has splits or cracks. GM had a TSB on this problem, where the seams would develop undetectable cracks and cause a lean condition.

 

Hi again.
The TPS is set and the vacuum guage reads OK.
I have a 3" airfoil, which seems to be intact.
The car is in the shop right now. The mechanic has a high rep to uphold and is suppose to be the best at this kinda stuff, but he is completely baffled at this point.
The engine runs better at cold engine temps, getting worse as it warms up. It also runs better with the EGR dissconnected. We installed a new valve thinking that was the problem ( opening much to quickly ) that wasn't the case.
Could it be the EGR silenoid? It also runs better at higher rpm.
What's puzzling is that the o2 reading is fine. You would think it should show a lean code, if it was running lean. But everything indicates, that what it's doing.
Thanks for the imput, gives me ideas to throw at him.

 

If the O² sensor shows a correct mixture, does it agree with the gas analyzer? There should be one in a well-equipped shop.

If the analyzer and O² sensor show a normal mixture, what's the problem? The "stumbling" could be something other than mixture control. Injectors that don't produce a good, conical spray pattern will create the correct mixture reading from teh sniffer and O² but can result in very poor driveablity and throttle response.

 

The gas analyzer is reading around 160, which is to high. Like it's trying to compensate for a lean condition.
At this point, I'm only going on what I'm told. This stuff is over my head.
The engine was rebuilt 500 miles ago and the injectors were cleaned and metered at that time. There was nothing install that should affect the operation.
I'm told that a shoot of propane smoothes it right out, even though the vacuum gauge reads OK

You seem to know your stuff. Any other ideas?
By the way, super looking engine compartment.

 

Upon thinking about it, I obviously misunderstood what I was told.
The 160 reading could not be from the gas analyzer. I'm not exactly sure what that reading came from, but he did say it was trying to compensate.
Guess I'll have to leave it in his hands. No point in me passing wrong info.
I'll post the cure.......hopefully