Hello all, having an issue with my 1988 F-150 4.9 six and T-18 gearbox. When I start the truck, the tach goes to about 2 k and settles down to 1500 until it warms up and settles down to 800 or so (normal for early EFI). Well it started a new thing... about half the time I start it now it DOES NOT go up to 2 k and come down. It starts, and only goes to about 800 and stumbles (like it has a fuel pressure issue) and then sometimes it surges and the tach goes back and forth between 1k and 800. The exhaust smells really rich sometimes also....When it does this, I shut it down and then restart the truck, about half the time the re-start will solve the problem and the tach will jump up to 2k-1800 and then settle down to 1500 until it warms up (like it is supposed to). All the tank pumps and fuel pump are new, as are all the relays. The ECM was refurbished by a company at great expense about 2 years ago... all the sensors, coolant temp, water temp have been replaced. EGR and MAP were replaced a couple of years ago too. Any Ideas? really stumped.....

Scott in Dallas

 

Pull codes, do the KOEO and KOER tests and see what the computer spits out If there are actuator problems the KOER test will identify them.

Off the top of my head it sounds like there may be a vacuum leak somewhere in the intake ... that could explain the truck running rich and the occasional surging. Put a vacuum gauge on the manifold tree or off of a vacuum line and see where you're at. Idle readings are 18-21 in Hg for an engine with no leaks and good compression.