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1989 Ford F150 Rough Idle

Hi all. I recently picked up a 1989 Ford F-150. It has the 300cc 4.9 I6 Engine and a T18 4 speed Tranny. It didn’t run at all when we first got it and the guy claimed the fuel pump was bad. It ran fine on starting fluid but no dice otherwise.

We got it towed home and threw some gas in the tank (maybe half gallon at most) and wanted to try spraying the intake to get her running enough to move it. Eventually it seemed to stay running on it’s own no problem. It stayed running for several minutes, although we did shut it down due toi the heat gauge starting to go a bit high (didn’t have much coolant in it).

We fired it up the next day without fluid and ran great. We’ve been fixing minor things here and there mostly on the interior. Something we noticed within the past coupe of days is it being a bit wonky with idle. It will generally start pretty much fine and sit about 1k RPM. It then generally proceeds to rise on it’s own to about 1.5K RPM then will come down rather quickly and die, or if it catches it self, will run at about 500RPM barely alive. Sometimes giving it some gas will let you rev it up to 2-3k RPM and maintain, sometimes it does nothing and will cause it to stall quicker.

After it’s warmed up, it is a bit steadier. I can generally give it some gas and it will consistantly accelerate immediately without as much hesitation if any. The idle can still bog down to about 750 or so but it generally doesn’y really threaten to stall. At first I’m pretty sure we just burnt through what little gas we put in it so today we put in about 5 gallons of gas. This did not help unfortunately.

Before we put in the 5 gallons, the gas gauge showed somewhere between 1/8 and 1/4 tank of gas. After the 5 gallons it’s showing being completely full past F. It actually pegs out. Sometimes when it’s running the needle will pivot a bit.

I’m not sure whether to lean more towards fuel delivery problem from a failing sending unit / pump or more towards a vacuum leak problem or a bit of both. I’ve uploaded the Vacuum diagram I have from AllDataDIY as reference. In the meantime I’m going to look for a good way to test fuel pressure but if anyone’s heard of this as a common problem and things to look for. I definitely would appreciate the assistance. Thanks!


Idle Air Control valve is a common problem with idle problems on Fords.


Get the engine up to operating temperature.

And while the engine idles, take the handle of a screwdriver and tap on the IAC valve.

If the idle speed changes or the engine stalls, that’s the problem.


I’ll give that a try. Easy test to do. I will report back. Thanks!

Unfortunately that does not seem to be it. It definitely idles better once it warms up though. Keeping it running when it’s cold is nearly impossible. I had to start it at least 12 times from it just out of nowhere dieing. Once the needle got above C, it could run.

Then look for a possible problem with the coolant temp sensor for the computer.


I am seeing how that’s another common problem and causes issues similar to mine. What i’m not sure is why if that is failin, why I’m not getting a code for it. I know my diagnostic system works because we pulled the MAP sensor to check for a change and it instantly threw the money light on.

Because your truck has the OBDI engine management system.

OBDI isn’t as sophisticated as OBDII.


Makes sense. I found the expected resistance values for the sensor. Thankfully the sensor is at the very front of the engine. My 91 Camaro with the 3.1 isn’t as easy to access. I will report back probably tomorrow with the results. Thanks again Tester!

Well we got somewhere, the external fuel pump is big dead. Relay is good, getting 12 volts to the pump but the pump does not kick on when the key is turned on (ground is good). Getting new pump later today to see if that was the entirety of the problem.

I suggest that you check the wiring diagram and voltage to the pump. The Ford crash switch was wired in somewhere in the truck and it may be between the FP relay and the pump.

I am getting 12 V to the fuel pump connector with key on. Relay clicks on. Unless I’m missing something, it’s just the pump.

That does seem to confirm that the pump has failed @birdbrainxzx.

Update. We got the new fuel pump in today and she’s running like butter. No idle issues, revs great.


Congratulations @birdbrainxzx. Maybe you rate Owlbrainz now.

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