1991 3FE runs worse the hotter it gets (1 Viewer)

[JGrizzle]

hey all, trying to fix an issue I've had since I got this truck in January.

1991 3FE, desmogged. 200k miles. Currently getting 10 ish mpg with top freeway speed at 80 on dead flat with no wind.

Recently checked/replaced vacuum lines and rubber throughout, rewired EFI relay to successfully fix no start issue, checked belt pulleys and components for drag/wear, adjusted throttle cable with great success, corrected timing to a little advanced of FSM spec, checked all fluid levels, replaced all spark plugs with verified gap (old ones were white on the spark end). Replaced the intake plenum gasket

Here's the deal:

Truck starts right up every time. She runs great when cold, getting worse as the engine heats up. If we do any heavy driving - mountains, wheeling, interstate - and the engine gets really hot, it runs like crap. By crap I mean:

-Rough idle below 2k RPM
-No low end power from a dead stop and no acceleration at any speed unless pushing 3k rpms
-Engine shakes under load at low RPM
-When stopped in the driveway with trans in D, the engine sounds like it's burdened and RPMs drop to 400-500

From what I've read, I suspect either a bad Engine Coolant Temp Sensor, a bad fuel pressure regulator or??????

I feel at a loss for where to go next. Don't want to replace the ECT sensor without cause because they're not cheap....

Can anyone help?? I'm relatively new here, so let me know if I've omitted something needed to help diagnose.

Thanks in advance you kind and sage-like wisdom-givers!

 

[jdayment]

Alot of different variables as you already covered I would recheck for vacuum leaks especially with a desmog its easy to miss something next step would be to download the FSM and actually test your ECT and CST sensors in lieu of replacing them. My truck is smogged but a few years ago I experienced something similar to find out it a vacuum line came off put in a new one and all good to go.

 

[82much]

I have a 3FE also. It sounds like the desmog left a vacuum line open some where

 

[FJ80Steve]

Test the AFM. Do the ohm tests from the FSM cuz its quick and can eliminate or pinpoint the prob. Mine had similar symptoms. Rough idle, ran like crap below 2k rpms but cleared up around 3k. The wiper arm inside will wear a groove in the lower portion of the potentiometer, where it gets used most.

 

[JGrizzle]

Test the AFM. Do the ohm tests from the FSM cuz its quick and can eliminate or pinpoint the prob. Mine had similar symptoms. Rough idle, ran like crap below 2k rpms but cleared up around 3k. The wiper arm inside will wear a groove in the lower portion of the potentiometer, where it gets used most.

Thanks Steve,

Would you mind telling me what AFM means?

 

[FJ80Steve]

Air Flow Meter. It gets confused for a mass air flow meter a lot, its the doo dad that mounts to the air filter housing. It measures the amount of air coming into the engine so it knows how much fuel to add with said air.

DO NOT REMOVE THE TWO SCREWS BESIDE THE WIRING CONNECTOR. It will be toast if you do. I cant for the life of me figure out why they are even there in the first place..

 

[mct75]

I cant for the life of me figure out why they are even there in the first place

It gives the parts guy something to do!

 

[JGrizzle]

Awesome, I'll check it out and report back.

If anyone has the FSM spec for ohm test on that it would save me some digging...

Thanks all for your help!