Car won't accelerate past 40mph



93 Mercury Cougar 3.8L V6 with just over 83,000 miles

l’ve been trying to figure out this issue for ages, but can’t seem to find the root of the issue. When driving and accelerating to 40mph, the rpms drop and GET STUCK at exactly 1000rpm. Forcing the accelerator down makes the engine bog and sputter and not go any faster. No check engine lights whatsoever.

The idle is very rough and low in park (700rpm compared to the regular 900) and very low in drive (500 compared to 650-750). When stopped in traffic for a while, it begins to shudder and shake like it wants to stall right there. Now let me run down the extensive list of parts l’ve thrown at it, bringing only temporary results:

Replaced catalytic converter
Replaced IAC valve
Replaced O2 sensors
Rebuilt transmission (runs great, so l’ve ruled that out)
Replaced 1fuel injector
Replaced MAF sensor
Replaced motor mounts
Cleaned throttle body extensively

I asked a guy at a Firestone and he said that l needed a valve job. Asked another guy at another place, he said l needed a fuel system cleaning. Asked another guy under a tree, said l needed a new speed sensor. Can anyone else please give me ideas of what could be the root issue? l was thinking carbon build up over 22 years maybe? Almost all 83k of those miles have been city miles…

This car is my baby, all l want is for it to be able to handle highway speeds without a hiccup. Please don’t tell me to junk it, or that the repairs are not worth the value, l have no interest in ever selling it. l’m keeping it and plan to see 100k hit the odometer and many more miles! :slight_smile:


Have you tested your fuel pressure?
Have you done a compression test?
Vacuum gage?

I think in the end you’re going to discover that you have valve problems, perhaps valve timing off and floating caused by well-worn chain, sprockets, and/or guides/tensioner. Perhaps the valevtrain is shot (weak springs and worn parts) allowing the valves to “float” (a condition wherein at a specific speed they can no longer close at the speeds necessary, and they basically just never close completely at that speed, preventing the engine from going any faster). Perhaps both of these conditions exist.

One thing I’d suggest is not changing any more parts until you do some diagnosis.


This is an OBDI vehicle. And OBDI didn’t provide the diagnostics that OBDII does.

But what you want to do on an OBDI vehicle is check the four major inputs into the computer.

These are as follows:

The coolant temp sensor.

The crankshaft position sensor

The throttle position sensor.

The MAP/MAF sensor.

The only two on that list that can effect acceleration is the throttle position sensor and the MAP/MAF sensor.



This is an OBDI vehicle. And OBDI didn’t provide the diagnostics that OBDII does.

But what you want to do on an OBDI vehicle is check the four major inputs into the computer.

These are as follows:

The coolant temp sensor.

The crankshaft position sensor

The throttle position sensor.

The MAP/MAF sensor.

The only two on that list that can effect acceleration is the throttle position sensor and the MAP/MAF sensor.



Could have Bad Gas depending on age.(Check that you have good fuel pressure)Change filter and fuel pump as needed.You’ve replaced a good number of things,I would continue, replace coolant tempSENSOR,Crnk Pos sensor,MAP sensor,TPSensor,and EGR valve.
Let us know how it turns out.


Sounds like a transmission issue(likely electrical), possibly a bad reading from the speed sensor?

Or potentially the torque convertor/lock up not working correctly since it wants to stall in drive.




I ve had three cougar 3.8 s and never had this problem. sounds like timing may be off tho


also, there is a in line fuel filter under the car, can t remember if it is under passenger or driver side door. I did not see where that was changed in your post.

great cars tho


how long has this gone on? did the car ever accelerate well? thats a loaded question since its an underpowered V6 in a heavy body car. you have a new cat. fuel pressure is ok. fuel flow with bad filter is a next possibility. i would wonder about performance change before/after trans rebuild. did it stay the same or get better? a boggy motor with poor acceleration prior to trans work and than having the same bog after trans work would lead me to think trans was fine.


WOW Thank you SO much everyone, this is super helpful!!

I have not checked the fuel pressure or had a vacuum test done, but l was told by a couple mechanics that the compression in a couple cylinders was somewhat low (110psi in some while 125psi was in the rest).

I did a bit of research on the valve float issue, and saw that it occurs often at higher rpms/over-revved engines, but to my knowledge it’s never been driven too hard (grandmother has been known to speed though haha!). She has been the only owner and bought it brand new. I came across the manual and saw that she documented a “possible engine hesitation problem” in 2000, but never bothered to fix it. Fast forward to 2015 and now l’m here trying to decipher just what is wrong.

I’ll definitely have the sensors checked, l know for a fact the electrical system has been a bit shoddy, but l always thought it was just blown fuses from age. Timing could also be a thing, the belt is as old as the car. Changing filters will be the next move since its cheapest, l probably should’ve done all the basic replacements first, shame on me.

The car drove fabulously before the trans started slipping at age 20, and drove fabulously as soon as it was fixed…but the engine issue must have always been there since 2000. But it was always known to have such great pickup and a smooth ride all its life, even on its good days in between guess-repairs.

l’m 19 and as soon as l got my license, l was allowed to drive it, and now l’m playing catch up with poor maintenence. The car has sat for many years at a time, and gas has never been siphoned out after its “vacations”. It sat for 5 years before l got my license, which was the longest time. It’s sitting right now for almost a year, but this time l’ve left the tank near empty to prevent possible further bad gas contamination. Can bad gas really cause this much of an issue? l’m really hoping fuel flow will be the issue, does anyone recommend getting a fuel system cleaning done at the dealer/firestone?


Also l forgot to mention the exhaust fumes smell absolutely horrid, it’s not a very stinky/sulphury smell, just pure chemical smell that burns my eyes and makes my throat sore within 10 seconds of smelling it in the garage. Does that point to something specific? It’s not a color really…but l’d take the rich sulphur scent it used to have before l did all these repairs, over this terrible smell.

You may be wondering why was l so close to the exhaust to get this bothered. l heard that the exhaust sometimes has a bit of a “sucking” feeling that indicates sticking valves. I went to test it out one day and couldn’t even test it well enough because l was on the brink of death from the smell. :wtf:


I don’t necessarily think it is all of the problem but could be most of it. Based on the compression readings the engine is worn out.
A 110-125 sucks to be honest. An engine in good shape should be in the 180 range.

You need to step back and make an engine decision before spending any more money or thought processes on it.


Ohhh really? So you think a rebuilt/remanufactured engine would remedy all of these problems? I think starting over with a refreshed engine would be awesome, undo all those years of no maintenence, bad gas, etc. l’ve been having my eye on a remanufactured engine on ebay for a while now… :smiley:

Also, do you think the wrong type of oil and gas used can kill an engine? 10w30 has been used in the car for as long as l can remember, and premium shell gas. As soon as l took over, l started using 5w20 and regular grade shell gasoline.


Would buying this be any good? How would l even take this thing to a shop to be put in? I can imagine a scenario like “Here, put this in now”


fuel filter should definitely be changed if you had old gas. and fill it all the way up afterwards to dilute any bad gas remaining


I wouldn’t purchase a remanufactured engine from EBAY. You want to purchase the engine from a local supplier.

If there’s an issue with the engine you want to deal with someone locally. And if you have to return the engine you bought off EBAY, you pay the shipping. Which isn’t cheap.



Sitting around with an empty gas tank is bad for a car, especially with old gas. I would remove the fuel filter, catch the old gas in a drain pan, turn on the key but don’t start the car to get as much gas as you can out. Put in a new fuel filter and fresh gas with an appropriate amount of a fuel system cleaner like Berrymans Chemtrol of Chevron Techron .
10W30 is what this car was designed for. Premium gas is not needed for this engine.


Oh l see, where would l find a supplier in chicago? If l took the it to the dealership and requested an engine rebuild/engine replacement, will they take care of finding all the parts? l don’t trust independent shops in this area anymore, they’re pretty sketchy and l’ve had to get very acquainted with rockauto for all the parts l’ve replaced.


great idea oldtimer, will do! :smiley: