Weird Misfire 1996 Mark

[txbear]

1st gen with only 75K miles started to misfire at low rpm (1200-1400) under load. No code, no engine light. Only happens when car is warm so smooth as silk for 1st 5 miles or so. Tried to simulate condition in driveway by putting in reverse applying brake and running the revs up. Car said "Nope, not gonna misfire cause ya want me to." So knowing it never had a tune up this is what I have done:

Replaced plugs and wires (Motorcraft)- No Change
Replaced Coils (MSD)- No Change
Replaced fuel filter (Motorcraft)- No Change
Replaced Fuel pump (Walbro)- No Change... well she does start quicker
Cleaned MAF sensor- No Change
Replaced air filter (Motorcraft)- No Change
Poured a butt load of Sea Foam in tank- No Change... bet my engine is super clean now.
Only use premium gas

If ya punch it she runs like a raped ape, highway speed is smooth and steady, this car only hacks up fur balls when you drive slow ~30-45 mph. I'm at my wits end, hopefully someone has a magic fairy wand and can wave it over my car. Suggestions would be marvelous, solution would be awesome! Thanks in advance for any help.

Bear

 

[ONLYTONY]

Could be a vacuum line. Gen 1 has a line that sits on the intake manifold about 1/2 way back. Check under dash. A line could have a slow leak, and when you rev it up, it gets enough vacuum.

 

[tixer]

I don't see a Throttle Position Sensor on your list of replaced parts. It's cheap, and easy to replace. (on the 2nd gen, anyway.) It could be a factor here.

The part is basically a variable resistor - think the volume knob from that old stereo that "crackles" when you turn it within the area you want to set it at, but works fine at high volume where you don't use it as much.

 

[txbear]

Thanks OnlyTony, will search anything vacuum related.

- - - Updated - - -

Thanks Tixer, will purchase TPS after vacuum line check.

 

[txbear]

UPDATE: Still having the problem. Looked at the vacuum more closely and still don't see anything out of the norm. Buying a TPS tomorrow and hope to install it this weekend barring family interference. I will update as I move through the issue.

 

[chris2523]

so are we sure its not converter shudder?

 

[94badbird]

Sound like front 02 sensors..my 98 did the same exact thing . No cel . But it did come on once for a day ..changed front 02 sensors with 10$ no name eBay ones and it's ran great ever since..lol I too did coils,plugs,mafs ect...

 

[ONLYTONY]

so are we sure its not converter shudder?

When I first got my 95 (18 years ago), I had the same problem when at low speed shifting into od, it would shutter. Trans filter, and Mercon 5 fixed it. Chances are you have Mercon 3 in it. Good call Chris.
Try it with the OD off.