Help! 87 nt 5sp street car. Over 4000 RPM engine bucks, pops, very erratic when warm, but OK when cold. Can't be driven over that RPM when warm. We (son who is a BMW tech, and I) recently rebuilt the engine w/all new OEM parts. Ran great for 10,000 miles until this. Mass air flow/ignition wires/ignitor & coils/injectors/ecm/fuel filter & pump/KN filter/O2 sensor/RB header/low restriction cat & cat back exhaust/wiring harness repaired/etc. etc.-almost everything replaced. Yes, we already added the additional ground to the pressure sensor harness. No evidence of any mechanical damage or seal problem w/engine.
Our latest theory: Could the resistor (that runs the injectors, on the right wheel well) be blanking out when warm? Some other solenoid or sensor?

 

Does it buck and pop if you gingerly take it over four grand? Or just when you are on it hard?

Probably not this, but do you have orifice in the boost line? Sometimes called a pill?

Ever go to your afm and take the plug off. Put a ohm meter on the brown/black wires and check for resistance? Should be two brown/black wires. About the third one from each end. Should be under one ohm. Same with the boost/pressure sensors ground wire.'

Ever swap the injectors. The primarys to the secondarys?

 

Thanx for your reply. Answers:
Pops regardless if on it or only lightly.
No boost line, this is a non-turbo.
Will check the resistance on the mass air flow connections (assume afm = mass air flow sensor?)
Replaced injectors, do you mean we should try switching them, or did we incorrectly install them? No, didn't do either.
Mazda tech thinks it is a fuel problem, but filter, pump are new & fuel flow OK. Will check the check valve in the fuel line.
We also think it may be vapor lock in a line near the new exhaust (we installed header, pipes, etc.) but it ran OK for some time w/this installation before trouble turned up.
BTW, we didn't do all of the upgrades to cure this problem, only mentioned what we did so far. Trouble happened lately, but not immediately after any installation or replacement. Also, compression OK, checked w/Mazda rotary compression meter.

 

have you tried to swap out the boost sensor (called that on n/a) it's on the strut tower it's what you said you grounded. the pill is in the line that goes to the block that's what hailers is talking about. also you changed all igniters and coils, have you tested them??? the reason i ask is i had this same problem for a long time and i always thought it was the tps(which could be your problem) but it turned out that one of the trailing coils was bad i replaced the coil and viola it ran perfect again! so to sum it up, test the boost sensor, tps, and the coils. also have you checked the plugs? fouled plugs could do this also but only if they were severly fouled. oh and remember boost sensor for n/a on passenger strut tower. pressure sensor for turbo.


james