[scubasteve]

i have a 1990 mustang lx with world jr. heads forged pistons 512' 220/220 cam 1.6 roller rockers.24lb. injectors prom75m maf.calibrated 190 fuel pump. the car idles rough and gets poor fuel mileage 10-13mpg.if i start the car after sitting for a minute or two the car stumbles and you have to give it gas until it smooths out. if i unplug the iac nothing happens,it should stall. the iac is not working at all. i get code 86 and 95 get fuel pump interruption.got 12volts to fuel pump relay.checked for vacuum leaks with propane and carb cleaner. cant find any.need help.the colder it gets the better it runs.

[EagleAutosports]

First, I'd suggest replacing the IAC, if the engine doesn't change when you unplug it then it's faulty. That could be the idling issue you have, but as far as the bad fuel mileage goes, the problem could be your regulator. Is it a stock regulator? You might need to get an adjustable unit and then retune the engine with a different pressure setting because it sounds like it's not operating properly. Stock is 39 psi, you might need a little more with those heads and cam. Is the intake stock as well? Throttle body?

[scubasteve]

adjustable regulator set at 39 psi.checked with a vacuum pump holds good vacuum. cobra cast intake. i checked the iac with 12 volt suppy and works good.been stumped a long time with this problem. this problem existed with the old motor with 180k on it. i believe its electrical.

[youngfordguy]

i have the same issue with, i have a 68 ford pickup that i shoved a 5.0 in and im running a painless maf harness and ive been having the same almost exact issue... when i unplug my maf the damn truck ran better so i replaced my maf and drilled and adjusted my tps and also replaced my iac but it still has trouble at idle not nearly as bad though, any other ideas??

[Finnstang]

There can't be too many reasons left for your problem. Newly built engine, new MAF, checked IAC... what's left for these symptoms are:

[Finnstang]

There can't be too many reasons left for your problem. Newly built engine, new MAF, checked IAC... what's left for these symptoms are:

- engine coolant sensor
- throttle body sensor
- EGR-sensor
- wires and terminals
- ECU