I have a 90 f250 5.8 auto that will try to run but dies. It won't idle at all and to keep it running I have to pat the pedal constantly. When I try to rev the engine it dies. I've got spark, air, and fuel. Good fuel pressure. Can't find any vacuum leaks. Check engine light doesn't come on at all. I'm pullin my hair out. Thanks.

Even though the light isn't on, check for codes. Also, be sure the light comes on before you start the engine. If it isn't then the light may just be burned out or non-functional for some reason.

I suspect throttle position sensor, but that is just an outright guess without better info to be sure.

I attempted to retrieve the codes using a test light. Turned the key to the on position and the test light lit and stayed lit. Never flashed a single time. Would a tps really cause that big of an issue?

If the ecm doesn't know what the throttle is doing, it can't meter the fuel properly or respond to changes in position properly. But as I said, it is an outright guess.

As far as the test goes, you have to at least get an all clear code. I haven't heard of using the test light to retrieve codes. Most times, the flashes are the light in the dash. GM uses a grounding like that to trip the diagnostic function of the computer. Not sure on the Fords as I have generally used a code reader to get the codes. I know there is another way to do it, but not remembering it at the moment.

Would a tps go bad just because the truck sat parked for a few months? I've drained the fuel tank and cleaned everything so I know it's getting good clean gas.

They generally don't, but they also don't tend to give warning either. They are essentially a wear part, as they are a contact variable resistor. It could be coincidental, however there is still something to be found yet to be sure that the computer is working correctly. There has to be an ok code if things are working correctly.

Here is a link which tells you how to manually pull codes plus the site will also tell you what the codes mean http://oldfuelinjection.com/?p=13g

I brought a code reader for my truck (87 f150 5.0) from amazon just like this one and it works well (saves having to count the light flashes ) and comes with a detailed instruction book and code definitions plus a diagnosis as well - here is the link to it http://www.amazon.com/INNOVA-3145-Ford-Digital-Reader/dp/B000EW0KHW

Checking the codes with test light just as that link describes, the light stays on. It doesn't flash a single time. Something is definitely not right. Even with the key off the light will still stay lit. I can unhook the ground from the small plug and the light still stays lit. Sounds like something shorted or cross wired or something. I don't know I'm not really experience in electronically controlled engines.

Sounding like the ecm may be done in.

That's what I'm afraid of. They are scarce around here in junk yards. Hopefully it's just the ecm and no underlying issues.

I keep coming back to the icm. I know they do weird things when they fail and I know they can pass the autozone tests and still be faulty. My question is, how hot should an icm get under normal operation. The reason I ask is the module passed several tests at autozons, but it was so hot it couldn't be touched.

kiwi f150 said:

Here is a link which tells you how to manually pull codes plus the site will also tell you what the codes mean http://oldfuelinjection.com/?p=13g

I brought a code reader for my truck (87 f150 5.0) from amazon just like this one and it works well (saves having to count the light flashes ) and comes with a detailed instruction book and code definitions plus a diagnosis as well - here is the link to it http://www.amazon.com/INNOVA-3145-Ford-Digital-Reader/dp/B000EW0KHW

Click to expand...

Good info, Mark! Thank you.

Sounds to me like a idle air control...

You can unplug the IAC And the TPS and the engine will run and drive fine. You can unplug them all and it will still run fine. All but for the Thick film and the Dizz it self.

If its not the Thick film or the Dizz then its the computer.

Depends how you define "fine." Will it run? Yes. Will it run and drive right? Not so much. With the tps unplugged, the ecm has no way to determine load or even that you are trying to make it move. It then uses the vacuum signal as a reference, and throws a code. To run right, the tps must remain plugged in. It is more critical than the temp sensor and some of the others to help the ecm determine what load is being requested and how hard you are opening the throttle plate. If you unplug all of the sensors at once, it likely will run poorly if at all. You could unplug it for testing, but I would not recommend it for any longer than that. The throttle response would be pretty bad. It isn't as cut and dried as pointing right at the electronics, you still have motor under there that also uses vacuum and fuel systems to run right. All have to work together to make the motor run right. I have seen more than once a vacuum leak or even bad air intake hose make motor run bad.

Had one customer come in with a Ranger that just ran terrible most of the time if you step into it much. He had been to shops and the MAF and other things had been replaced. Nothing was helping the truck and he was just frustrated about it, I went out and helped him look at it, as the description of the problems did not really match a failed MAF. What I found was the air intake hose had disintegrated to the point that when it flexed, the air was drawn right through the hose, bypassing the MAF completely. I patched it with duct tape and told him to get a replacement hose for it. It ran great with that simple little fix that numerous others had totally overlooked. So it doesn't always point at electronics when there are issues. The hard part here is none of us can walk out and check it out. We are relying on the poster to be our eyes and ears, and that is not always an effective way to diagnose, as they do not know necessarily even what they are looking at like those of us that have been doing it for a long time do.

A guy is talking about a truck that really does not stay running at all with out any codes. Any sensor bad on a good running truck will go to limp mode and run and drive.
If it had a big vacuum leak it would idle high and not come back down and still run fine and not die. 1990 is not a MAF truck and not a ranger and would not run lean because of a vacuum leak. So thats out and gone.

Now I can put my hands on about any good non MAF truck from 1988 to 1995 started unplug sensor by sensor and it will stumble on the MAP sensor for about 30sen and regain control and you can go back to unpluging them and it most likely not going to die out untill you get to the IAC and i bet if you go and try to started it. It will start and run. and drive. It will not like you and the MPG is going to suck. But it will do it. Now if it cold as hell out and you unplug them all and try to start it its not going to start. but if you let it warm up some first it will do it in the cold.

Go do it You might learn a lot about the way a OBD1 ford computer thinks. And why it worked so much better then the GM junk of the same years.
I've done it many a time.

Now if you told me the truck runs and drives and stalls or tries to stall at stop lights or with your foot off the gas. I say no. it the TPS or the IAC is bad and check you volts at the TPS and make sure it under .99 at idle and it near a full 5 volts at WOT.

Sad part is you cant watch the computer info when its running on a scanner like you can on a OBD 2 computer But a ford computer will do a KOER test. But i dont know if it will do him any good.
if it will not stay running and theres now codes. and it got the right fuel. and its got good compression and it looks like its got spark.{that doesn't mean its getting the right spark] I would look long and hard at the Thick film and the Dizz before i lay down the money for a computer. Computer are not really know for going bad unless they have been wet. And a 20+ year old spinning part is in the heat is more likely going to go bad before one that cool and dry all the time with no moving parts at all.

He can get a Dizz with the thick film on it for 50 bucks from rockauto Or the Computer for 200+ plus a two or 3 week turn around time for the send out and return .
I've Been down this road more then once.
So we can leave that up to Murph2006.

HAy big one here, that i over looked that has happen before.

Is the EGR valve stuck open. it will not run or try to rev up if it open at any time. Only time that thing open is at a cruse like going down the highway for a long time and any time you try to get in it. It should close.

some time the spring in it will brake and will not hold it close. and it puts a hole in the diaphragm. or the vacuum valve that open it goes bad and the vacuum opens the EGR as soon as the truck is started.

Look at that .
If you want to make a block off plate for it and see if it doesn't fix it. If it does well there you go. replace the EGR and dive on.

Sorry should of said that first. Ive had that one drive me up a wall more then once.

Please do not assume I know nothing about how these trucks run, I have spent plenty of time diagnosing. Maybe I read into your post more than what you intended, but the way you wrote it sounded like you were saying to have them all unplugged. You now state one by one. I do know they will go into limp mode and turn the light on. However, they can make an engine hard to start in limp mode.

An EGR open is just another vacuum leak, so now you agree with my statement of maybe a vacuum leak causing issues. When the TPS failed on my 88, it would waver the idle speed without touching the pedal. It put out no codes, as the ecm couldn't tell the difference between me pushing the pedal and it have the resistance vary through heating of bad connection. I unplugged it, and yes, it would run, but not good at all. It also had gotten hard to start.

Fuel pressure could also be at play. Just having fuel is not good enough, it needs to have enough pressure. A bad regulator could flood it out. The injectors could possibly not be pulsing. There is a lot at play here.

He also has stated there is no standard check code. He is reading absolutely no codes, so the ecm is already suspect. Did you read that before posting? Yes, most times sensors will not shut an engine down, but they can be let go long enough to foul plugs and make hard starts. I did not advocate nor will I ever advocate throwing parts at a problem without testing.

Im glad you know all. Have nice day.
Hope the man finds out what wrong with his truck