1991 Aftermarket Ford Mustang Has Engine Miss

Reader Question Hi, I’d just like to thank in advance for any help you can offer. I own a 91 Mustang GT, with a very modified 5.0 motor. It has aftermarket heads, cam, intake, injectors, mass air meter, etc…I could list parts for hours.

The car has a persistent misfire at part-throttle that seems to be getting worse. The car will start breaking up, audible through the exhaust, and buck and jerk violently. Heavier throttle inputs (not WOT) will usually be met with a bogging-down and heavy loss of power, occasionally an exhaust backfire, and then a normal, hard pull after anywhere from 2-5 seconds. The camshaft I run has a lot of overlap, but the driveability problems posed by the camshaft clear up at approximately 1600 rpms, beyond this range, the misfire becomes very noticeable.

It clears up completely at wide open throttle, and runs excellently, but anything less than wide-open presents me with problems. The plugs are not fouling but look to be lean (very white), but when I add fuel, the problems do not disappear and in fact get worse, as the backfires get more prevalent.

Plugwires are brand new and have shown no effect replacing them, as with the cap and rotor, TFI ignition module, and ignition coil. I have taken the aftermarket CD ignition box out of the loop as well to no effect. Base timing is 5 degrees above stock, in an appropriate range, with no detonation present, and I have adjusted all of the rockers arms to spec (aftermarket adjustables, not factory bolt-downs)

I have gone over the motor and have no vacuum leaks that I can find, the motor holds a steady 9 inches of vacuum. TPS voltage is within spec at idle, and produces a smooth sweep up to peak voltage, which is also within spec. Mass air meter and injectors are aftermarket, and the meter has been calibrated to the injector size. EGR system is totally disabled and block off, as the heads have no EGR passageways. There is also no smog pump, EVAP return (purge solenoid disconnected, purge line left to vent to atmosphere), and no catalytic converters.

I get no check-engine light but have scanned for stored codes and find O2 lean codes for both banks, but the light is never tripped. Air:fuel ratio has been confirmed as 13.0:1 at wide open throttle, but has not been checked at part throttle.

I’m sure this is a lot more complicated than the questions you usually get so I understand if I don’t get a reply back, but I figured I’d ask. Thanks again, I’ve just been trying to track this problem down for 6 months and am honestly stumped and can’t afford to just ‘throw parts at it’. Every mechanic that’s looked at it has been stumped as well.

-Brooks

It clears up completely at wide open throttle, and runs excellently, but anything less than wide-open presents me with problems. The plugs are not fouling but look to be lean (very white), but when I add fuel, the problems do not disappear and in fact get worse, as the backfires get more prevalent.

Plugwires are brand new and have shown no effect replacing them, as with the cap and rotor, TFI ignition module, and ignition coil. I have taken the aftermarket CD ignition box out of the loop as well to no effect. Base timing is 5 degrees above stock, in an appropriate range, with no detonation present, and I have adjusted all of the rockers arms to spec (aftermarket adjustables, not factory bolt-downs)

I have gone over the motor and have no vacuum leaks that I can find, the motor holds a steady 9 inches of vacuum. TPS voltage is within spec at idle, and produces a smooth sweep up to peak voltage, which is also within spec. Mass air meter and injectors are aftermarket, and the meter has been calibrated to the injector size. EGR system is totally disabled and block off, as the heads have no EGR passageways. There is also no smog pump, EVAP return (purge solenoid disconnected, purge line left to vent to atmosphere), and no catalytic converters.

I get no check-engine light but have scanned for stored codes and find O2 lean codes for both banks, but the light is never tripped. Air:fuel ratio has been confirmed as 13.0:1 at wide open throttle, but has not been checked at part throttle.

I’m sure this is a lot more complicated than the questions you usually get so I understand if I don’t get a reply back, but I figured I’d ask. Thanks again, I’ve just been trying to track this problem down for 6 months and am honestly stumped and can’t afford to just ‘throw parts at it’. Every mechanic that’s looked at it has been stumped as well.

Thanks

-Brooks

Hey there Brooks,

Without being there to test anything, I would be looking for an engine misfire. Distributor cap and rotor that are touching, loose, cracked, worn out distributor shaft etc.. See if you can duplicate the problem while by power braking the vehicle. You might even want to spray some water from a plant sprayer bottle on the spark plug wires while you are power braking…if you have a bad or cracked wire it will ground out when the water hits it.

Blessings,

Austin Davis