I'm experiencing a weird driveability issue and am wondering if anyone has any ideas as to the cause. The application here is an 89 Mustang LX, 5.0, AOD.

The problem: A few minutes after startup the engine begins to run rough and sometimes backfires through the intake at part throttle. At idle it runs rough with no backfire. At full throttle it seems to run fine. Eventually, the problem disappears and the car runs perfectly for the remainder of the drive. The problem disappears quickly when the car is stopped (such as at a long traffic light) but takes much longer to disappear if I keep driving (up to 10 minutes).

It does this almost every time the engine is started, regardless of ambient temperature or engine temperature. I can drive it 50 miles, turn it off then restart and it does the same thing over again. Occasionally, it does not do it at all or at least not bad enough to really notice. The engine always starts easily.

The codes I get all relate to the thermactor system, which is expected because I've removed it all. I also have no catalytic converters.

What I've replaced: cap, rotor, wires, distributor (including TFI), IAC, EGR, TPS, O2 sensors. I've monitored the voltage output of the MAF and I've seen no difference between "good running" and "bad running."


So.... WTF is going on with this thing?

 

run compression test possible sticky intake valve causing backfire through intake than frees up after it warms up just a guess

 

Connect a fuel pressure gauge when sitting and driving. What were results of cylinder balance test? What brand plugs..copper right?