1990 cougar has 3 mechanics stumped

i have a 1990 mercury cougar its having a weird issue its like it surges when driving at higher speeds you feel a small push pull feeling, also going up hill its like i loose throttle at about 20mph then i feather the gas for a second and it comes back and this repeats at about 40mph or when i i need to pass or speed up in general i took it into my ford dealership they plugged it in told me it was the egr valve the dpfe senor and the tpi sensor wich all have been replaced still having the same issue took it back in he told me it was my altenator and fuel relay switch both replaced brand new as well still having the same issue most of my family is all mechanics and there completely stumped and this didnt come on slowly i noticed it last week on the way back from work o ive also replaced spark plugs properly gapped fuel filters hell replaced the suspension air filter oil change the works and well were all at a loss of what it is

Have you had the fuel pump pressure tested? That’s the first thing that I would do. Check the fuel filter as well. I had a '92 Ranger that acted the same way. It turned out to be a bad fuel pump and a partially clogged fuel filter.

If the fuel filter hasn’t been replaced then do that and if that doesn’t work I suggest you replace the fuel pump.

Got me thinking it could be a computer related problem, but then everything could be a computer related problem. Maybe the dealer people can try to re-input the computer program, reflash the PCM in other words. Sometimes that will erase mysterious problems.

In the old days of 1987 model cars, Ford Tempo/Mercury Topaz, a bad oxygen sensor could cause the same problem. I don’t know if a slightly wayward one can cause it any more. The old system was so simple a cave man could figure them out.

Now, I can’t fix anything by smashing it with a half inch ratchet. I have to skillfully select the proper hammer.

ok so just incase it was the fuel pump i already had one i bought through napa, installed the new one according the chiltons manual just got done taking it for a spin started up the hil and again at about 20mph it did it again but i took it to the hwy anyway and yea still the same issue o and we changed the fuel filter before hand "missleman"
pleasedodgevan2 when we had it at the ford auto shop he said the computer wasnt throwing out any more codes and everything looked great we even swapped the unit just in case same issue so im guessing its not the puter any other thoughts

Have you checked the fuel pressure regulator?
And have you testing the engine for vacuum leaks?
Have you bench tested the throttle body injector(s) (I assume a 1990 is TBI?)?

And don;t forget…a wandering ignition timing will present as a wandering engine. I assume this is a distributor based system, so have you checked the distributor shaft for play laterally and axially? Is the spark advance vacuum operated?..and have you checked that for function?

Well I’m sorry replacing the fuel pump didn’t fix this issue. It sure sounded like a fuel issue to me. There may be a timing issue like TSM stated. I also am wondering if perhaps the exhaust system may be blocked and there is a back pressure problem.

i havent checked out the regulator can seem to locate it and vaccum leaks was one of the first things we checked and its not tbi its direct port injected distributor looks great ill get back to ya after i try the regulator thanks

cougar yes one mechanic stated it could be something in that area but so far i havent found it one guy said its the Catalytic Converter so instead of checking it to see if it was the issue i said hell replace it any ways brand new from rock auto had that put in 2 days ago im about to check the fuel regulator now

also i know it has nothing to do with this wel it does at the same time i didnt have this issues until i hear a buzzing coming from my driverside rear fender but that turned out to be a illuminated sensor for the lights and my rear tail light was out after i put a new light in that sensor hasnt buzzed but thats exactly when i started having this issue so i dont know how its realated but i swear its gotta be

I had a Chevy 305 1997 that did this. What I found was a bad distributor gear. It was wore out to the point of being paper thin. I replaced it and it ran fine after that. I was told that the computer could not keep the timing right with the slop in the gear.

It might be a good idea to pull codes and see if anything is present. A problem like this could point to an EGR problem but it’s unknown to me how the shop arrived at the conclusion they did.
All of the tests related to the engine control codes should be run. You can find out how to do this on-line as it’s easier than my posting a number of paragraphs on it.

Another possibility could be a TFI module fault; especially if this problem is more prevalent when the engine is warmed up. Maybe the ignition timing is going in and out of being retarded. Those modules are a known fault and proper inspection of the timing is a good idea if the distributor or module has ever been dinked with at all.

There’s another possibility related to an intake bypass if the car is the supercharged version but I’m assuming that it’s not.

“It might be a good idea to pull codes…”

1990 is pre-OBDII, yes?
Did they store codes back then?

Yup. It’s pre-obdII.
They did, but not as standardized and not with the range of information that OBDII systems do.

Oh yeah! Now I remember the blinking lights on my 85 & 88 Accords.
But I think all the stored codes would also turn on the check engine light: no pending codes.

I replaced a knock sensor on my 1995 OBD1 Corolla. The MIL never came on, but there was a DTC.
I forgot to clear the DTC, and it was still there years later. On that car, codes do not “clear themselves”

As a matter of fact, even on OBD2 cars, not all PCM DTCs turn on the MIL.

While not having nearly as many codes as OBDII, the EEC system on that Cougar still has about 70 or 80 of them (ballparking) including memory.

Regarding problems without indications, about a week ago while in my Lincoln I happened to notice the cruise quit working and the speedometer died. Otherwise, there were no symptoms and the car was driven the last 80 miles home like that with the assumption that the speed sensor had gone belly-up.
That did turn out to be the cause and there was never a CEL illuminated or any codes even being set.

Maybe if the vacuum air was monitored it would show something up. If there is a vacuum advance maybe the problem is there.