Help please - 95 Thunderbird runs rough @low revs

Hi all, i have a 1995 Ford Thunderbird with the 4.6 V8. I have owned it since it was a year old, it now has 70,000 miles on it. A couple years ago it developed a problem while driving. Hard to explain, but seems like it is misfiring or there a fuel restriction somewhere. This shows up when i floor it, the transmission kicks down and the engine struggles to accelerate, you can feel when it clears up, the car shoots forward. It seems like if i smoothly step on the pedal it is better, but not always.

This problem is also noticeable when the revs are low, like just before the transmission shifts down, like when climbing a hill, you can feel a shudder, if i accelerate and the transmission shifts so the revs are higher the problem goes away. Also, shutting off the overdrive so the revs increase helps too. It really feels like the engine to me, although it does not go as far as to backfire.

2 years ago i replaced the auto transmission and torque converter with a factory one direct from Ford.
To diagnose what i think is an engine problem i have:
replaced the fuel filter (wasn’t dirty)
removed the injectors and had them cleaned and tested (they were fine)
checked the fuel pressure (fine)
checked the vacuum (fine)
removed the exhaust to look for cat blockages (nice and clean)
replaced the plugs, wires, and coils
checked that the timing was in spec, although i don’t think there is a way to adjust it

The car has the early version of OBDII - it does not have any active errors.

Please help with some ideas, all i have left is to take it to the shop. Or maybe replace the computer.
The car starts and idles fine, cold or hot, and when it is not acting up, still has excellent power. It does sit inside all winter though.
Thanks for any ideas.

If that car has a mass airflow sensor (which I think it does) clean it if you haven’t. I’d also clean the throttle body. A really really close inspection of the intake snorkel would also be good - for those tiny, razor-like breaks that are almost impossible to see.

I’d also maybe do a basic check of the throttle position sensor - its quite easy if you have access to a scantool or you can do a basic check with a voltmeter. (If you back probe it, key on it should probably read in the vicinity of .5V or so at closed throttle and increase to 4.5V or so at WOT.

I’d also check out the EGR system. The most straightforward way to eliminate it as a source of the trouble is to temporarily disable it.

Thanks cigroller. I looked at the airflow sensor, very clean. The intake looks pretty good too, and it has a sticker that says Don’t Clean. Fine. Didn’t see any cracks in the intake either. I put an ohmmeter on the pins for the TPS, when i moved the throttle it was very smooth, no big jumps at all. I disabled the EGR, the vacuum side of the valve, this almost seems better, like there was an improvement, but the problem is still there. I will disconnect the hose from the EGR vlv to the intake and seal that next to see if it gets a little better.


Doesn’t seem to be the EGR, but i did get a no flow fault during that testing. I have since done a compression check in case there was a cracked head, nothing wrong found.
I did find that when i use high octane fuel, the hesitation and surging are less frequent. I am thinking it’s something to do with the timing. Anyone have any experience with this?