First Post! I have a '92 F150 XLT Lariat 4x4 with a 5.0 engine and a manual tranny. For about a year now I would sometimes start up the truck and the RPMs would seem to hang around 2K and over the next few seconds slowly come back down to a normal 700. If I drove off in this state it would continue for the duration of the trip. If I shut the truck off and restarted it immediately the problem would go away and the truck would idle normally for the duration of the trip.

Starting a few days ago this has become a problem that no longer responds to my bandaid fix of simply restarting the truck. Now she idles at around 2k and will not drop, period. After a couple hours of reading forum posts about high idle problems and solutions I decided to remove the IAC and clean it to see if that resolved the issue. It did not. BUT if I leave the IAC installed and unplugged the truck idles at at 600-700 RPM. As soon as I plug the IAC back in the RPMs jump immediately to around 2K and hang there for about 30 seconds before slowly coming down to 1000-1200 RPM. My OBD1 port hasn't ever worked so no info there, and I'm not getting an engine light.

I've smoke tested for vacuum leaks and haven't located any, but I didn't expect to since this seems more like an electronic/mechanical problem to me. Any advice on where to go from here? Test the TPS? Check the ECM? Is there more information that I can provide?

 

I would say yes, test the TPS and check the ECM for leaky capacitors.

 

for vac leaks, it's better to just remove all the vac lines at the manifold and plug them. You can have big vac leaks where the smoke wont go to tell you that they exist. That, and it's so simple to remove all the vac hoses from the intake and plug them, that it is the lowest hanging fruit for troubleshooting (ruling out, vs finding an issue)

 

If the vac lines are original on a '92, you might want to plan on going thru them all. The TPS can cause some erratic behavior. If you change the TPS, it would be a great time to change out those smaller coolant lines that go to the throttle body. They can have slow leaks that you won't see but then they let loose and make you have bad words.

 

Damned if I can find any vacuum leak anywhere, but again, I didn't really expect to. The problem seems to be electronic. With the IAC plugged in the truck idles around 2K, but with the IAC unplugged she idles perfectly at about 700 RPM.

I'm wondering if the engine temperature sensor could be causing the issue. I know better than to just throw parts at the issue but honestly the sensor is so cheap and easy to replace I think I might as well give it a shot and see what happens.

 

Have you run the Key On Engine Off (KOEO) tests and Stored Code display? Was anything flagged?

If you have not, run the engine for a few minutes before running the KOEO tests and Stored Code display. Otherwise you will most likely trigger a few sensors out of range codes.

Given the symptom that shutting off, then restarting the engine fixed the issue previously it does point to a failing TPS.

 

So you heard me, huh?

 

STOP! Don't replace anything without testing it first. All you need is a multi meter. If you don't have a multimeter, buy one... They're cheaper than the sensor you're planning on replacing. Disconnect the sensor and use the ohm setting to compare the value of the sensor to this chart. Replacing parts without testing is a surefire way of replacing functional, high quality OEM parts with Chinese parts that may be DOA, out of tolerance, or work for 3 months and fail. Ask me how I know.

 

Sorry to go off-topic for a bit, but would you happen to have a resistance to PSI scale/chart for the well-known "oil pressure gauge fix"? I did this years ago in my '92 F150 (which is dying), and I'm planning on doing it in my '92 F250 soon (which is replacing it), but I don't recall what oil pressure sender I used back then, I can't find the chart/scale, and based on what I'm finding now, the sending unit looks different than what I used previously. I was thinking of re-creating it (the resistance chart) with a couple of 120(?) ohm pots wired in-series this weekend, but if you know where to find this...

I don't know if this info is in there, but I have ordered the '92 Ford Shop Manuals, but as paper copies (I like paper), and they haven't arrived yet.

(I suppose I could just swap gauge clusters and oil sending units....)

Either way, thanks!

Jim

 

Tried the KOEO test today (had to repair my OBD1 reader first) and got the code 23 twice, which suggests that the TPS is malfunctioning or out of range. Good call rla2005!

I'll pick up a TPS today. Thanks for the advice, always a learning experience here.

 

Since the idle comes back down when disconnecting the IAC, I would not have expected a vacuum leak. I have never had any luck trying to clean one of them. Since you have a TPS code I would be checking the voltage and resistance of the sensor. It might still be the IAC but at least you have a starting point.


Using the one here or your favorite search engine, look up oil pressure gauge modification. I used an ‘87 or older sending unit and bypassed the resistor on the gauge cluster circuit board.

 

also check the ecm as was suggested.

 

Have seen tremendous amount of threads about High idle issues with theese trucks. Especially high idle on cold start. Im old school 73 years old and here is my thoughts that might help someone. Im not a certified tech but have been playing with theese damn trucks a long time. First of all, 87 and up f150s are ALL way over engineered. Mine now is a 1991 f150 4.9 eng. When I first got it it idled high all the time. #1 problem like everyone says is fix vacum leaks. After replacing vac. hoses only High idled at cold start for about 4 sec then setlled down. Anything over 2k idle anytime is too high i dont care what anybody says. I replaced manifold air temp sensor, coolant temp. sensor, map sensor and stupid IAC. This brought cold start idle down from almost 3k to 2600. Still way too high. Didnt replace egr,egr solenoid or tps as they appeared to be working. Dont have scan tool or all that crap. The 2600 at cold idle only lasted for 2 seconds at this point. Much improve but still not good for engine. Here are the last 2 tips that helped A LOT ! Saw a video about starting the truck in nuetral instead of park to help with this. I was sceptical but it did help ! Started in neutral it goes to 2100 then settles down. The last thing I did to help is turn the key on, put in nuetral and then PUMP THE GAS ONE TIME before starting. Doing this the truck starts at 1800 then settles down. All the afore mentioned is in cold weather 40 to 60 F. Above that the truck starts at about 1600 then settles down. I cant for the life of me figure out why pumping the gas one time lowers start rpms by 300 but it does. I know there is a fuel press. regulator but dont know how Obd1 system can know this. The only reason I tried this was because I remebered that way you were to suppose to start carbureted vehicles. Maybe someone can shed light on it. Im convinced designers are told not to make anything that will last more than 5years. Thats why they quit making 7.3 turbo diesels, 300 CI inline 6s, Slant 6 Chrysler engines ! Poor folks like us have to reverse engineer stuff to mak it last.

 

Mystery solved. Replaced the TPS and she now idles at ~700 RPM and returns to ~700 RPM normally when shifting (manual).

Now back to figuring out what's squeaking when she's running...lol.

 

Thanks for the update.