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!