1988 F250 351w efi
Hello all,
been dicking around with this old truck for a while now. Got it started up the other day, now the issue I am experiencing is a high idle (sounds like my 7.3) and I can’t get the RPM’s over 2000. Climbed the hill on my driveway at about 3 mph. Can’t seem to find any vacuum leaks. Any help would be appreciated.

 

Have you checked timing and manifold vacuum? Is the distributor vacuum advance working? There isn’t much to go off of here for details yet...

High idle sounds like either a lot of advance or a vacuum leak. Are you getting a lot of backfiring? I’m a bit concerned that your “sounds like a diesel” may be pinging...

 

What is the idle speed? Have you pulled codes? The problem here could be mechanical or electrical.
High idle could be a sticking throttle cable or a warn throttlebody, sometimes if the shaft in the TB is badly worn air will leak in around it, or the blades themselves may get stuck partially open. Note there is stop screw on the TB but that isn't an idle screw, it's just there to prevent the blades from closing too much and getting stuck in the bores so it should not be messed with. But have a look to see if the previous owner messed with it, if it's cranked right in it could be holding the blades open more than it should. And disconnect the throttle cable to see if it is restricting movement at the TB.

With everything in good mechanical condition the computer controls idle speed via the IAC(Idle Air Control) solenoid on the side of the throttlebody based on feedback from the TPS(throttle position sensor). When the TPS is in the closed position the computer modulates airflow through the IAC to achieve a warm idle speed of about 650rpm. So the IAC valve has to move freely and the passages must be clear, and the TPS must produce the correctly voltages. That is where reading the codes come in, if the TPS is out of range or is showing a higher(or lower) closed voltage than it should a code will be set.

The motor not revving over 2000rpm suggest low fuel pressure or a very restrictive exhaust, clogged cats are possible but not that common while weak fuel pumps are very common in vehicles that have been sitting, so get a pressure gauge and attach it to the fuel rail.. you should be getting close to 40psi before the engine is started.

 

Ha. I missed the “efi” detail!!

 

Okay, got the idle to between 600-700 rpm by adjusting the distributor. It is backfiring after about 1800 rpm, and when it gets to 2000 it starts bogging down and backfiring a lot. The cat was just put on less than 1000 miles ago, I know that doesn’t mean anything, just hoping to not be a exhaust blockage. I will grab a fuel pressure tester tonight or tomorrow and check what I’m getting. Thanks guys. I’ll update with my findings.

 

Yeah that gives the result you were looking for but timing is probably way retarded now, leave it for now and reset to spec with a timing light after you sort out the bigger problems.

 

Pulled exhaust. Cat was broken up and melted. Put a straight pipe to the cherry bomb now. I can get it revved past 2000 now, but still can’t get much power. Gonna plug brake booster line tomorrow and see what happens. Any other guesses if that doesn’t help out?
Thanks guys