Backfiring Problems!

[Capri306]

Okay, first thing's first. I don't know what "vacuum pressure" is (sorry, but that statement makes NO sense). If the shop is referring to MAP being high or low, that's one thing. Having the aftermarket parts on an SD vehicle makes it difficult to pinpoint. You DO have the stock cam, right? Are those rockers 1.6:1 or 1.7:1?

If the EGOs (oxygen sensors) are bad or not swinging as they should, or even if they're fixed, there WILL be a trouble code for it. I'd pull the codes and post them here, if I were you.

On the other hand, I think you should just convert to mass air and be done with it. I did so on my '87 notch, and have never even once looked back. BTW, my car has an automatic, and before the conversion would knock horribly when above 3500rpm or so (sounds *similar* to your case), regardless of timing (within reason) or octane. It was mostly due to the inaccurate calculation of load caused by using 3.73 gears. Everything is right as rain now, and the mileage has gone up! FWIW, this car has almost 150,000 miles, still has great compression, and uses maybe a quart between changes (less since synthetic).

You might want to look into that fuel pump and/or fuel pressure regulator, though. I believe you have a lean condition brought on by:
1. Improper load calculation,
2. A vacuum leak (pretty common),
3. Lack of fuel delivery (pump/regulator),
4. Excessive EGR flow,
5. Timing is too retarded (a few shops STILL don't know how to do it right on EEC-IV systems!), or
6. The ubiquitous air pump check valve.
If I missed anything, it's because I've been up for quite some time, and I missed my nap.

Hope this helps. Good luck.