2003 Ford Thunderbird, 3.9L V8

2003 Ford Thunderbird
Ongoing issue with stumbling/misfiring at lower RPMs or when you step on/goose the gas pedal (for rapid acceleration) while driving – i.e. like when you need to pass on the highway. Idle is smooth overall, but you can hear the occasional misfire.

Most recent repairs were to correct multiple lean codes, first on Bank 1, then a generic lean code. Most severe condition in freeze frame data showed STFT of 33% (LTFT approx. 3%) when ECT 186 degrees, ambient temp 86 degrees, Fuel pressure 55 PSI. My diagnosis was lean misfires due to a vacuum leak. Attempted fixes included: new lower intake manifold gaskets; new injector O-rings, new EGR valve gasket; new Throttle Body gasket; new PCV hose O-rings; inspect, check, re-tighten all vacuum hoses and fittings. Result was total fuel trims dropped significantly (from 33% DTC trip) until engine reached normal operating temp of 220-225 degrees. The trims went from very low both STFT and LTFT and the began to fluctuate sporadically as the engine went from through warm (approx. 175 degrees) to full running temp, then stabilized at about 22-25% (STFT+LTFT). Currently there are no DTCs tripped or pending. Misfires reappeared as the engine got close to and remained at full operating temp. Bank 1 O2S1 waveform is a little squirrely at idle – I assume that is related to the high total fuel trim of around 25%. Bank 2 O2S1 waveform is pretty smooth, total fuel trim is a little lower than Bank 1. There can be sporadic quirkiness to the waveforms on both banks as the engine heats up.

Past repairs to solve various DTCs included: new valve cover gaskets, all new plugs, all new COPs to solve cylinder specific misfire DTC; replacing all four O2 sensors (was getting a Catalyst Efficiency Low on Bank 1 DTC code). The entire Catalyst assembly has been replaced as well because the Bank 1 Cat had been removed (cut out) and reinstalled and was “suspect” in my opinion.

The engine runs fine at cruising speed, and runs fine under hard acceleration – AFTER an initial stumble/hesitation/misfires. Again, it runs fine when cold, cool, and slightly warm. The misfires/stumbling appear at/close to fully warmed up/normal operating temp (lower RPM and when you “stab” the accelerator). I have not yet rechecked for oil in the plug/COP holes since replacing the gaskets, plugs and COPs. Before I replaced everything, it was apparent that individual plugs and/or COPs had been replaced. I have been dealing with these issues over the past two years – although the total mileage is only about 8K in that time span. The car has 105K miles on it. I seem to be chasing DTCs over the past 2 years. I am out of ideas at this point and feel like I am just waiting for the next DTC to chase.

Would be great if you could post the actual code you are having trouble with. A misfire on a Ford has to happen often to set a code and pinpoint the problem cylinder/s. Since it seems to be fine until it warms up, maybe a faulty temperature sending unit.