I've got a bit of an issue with my 1988 F150 4x.4 It has a 351, c6 transmission. I don't know if this is important, but it has true dual exhaust, with the cat removed (not my idea...previous owner). It's two straight pipes, into 2 mufflers (not cherry bombs, they're mufflers), than into a tailpipe.

Anway, the idle is kind of erratic. It seems to take a long time for the truck to come off of fast idle. It will eventually come down to about 750 or so, but it's a bit shakey. Not super shakey, but shakey. I changed the IAC because I was able to get a good used one very cheap. It made a marginal difference, but the idle is still quite poor. The truck hesitates a bit when accellerating slowly (gradual accleration from 0 to 30) and from time to time, it wants to stall, but it doesn't actually stall.

The truck has new Niehoff wires, (midgrade quality), new autolite plugs and a new cap and rotor (Wells).

I've put the truck on my scanner and it returns two codes: EGR not working (that's verbatim from the scanner), and Thermactor not working (or something to that extent). I forgot to write down the actual codes. The thermactor is easy... the zipperhead that owned the truck took it off when the cats were removed. The EGR I've not checked into too far. But if it's seized shut, it shouldn't cause a problem should it?

I do want to fix the EGR, but I'm thinking that the problem may be vacuum related. So, I guess what I'm looking for from you folks is some ideas. Are there common areas or parts on these trucks that develop vacuum leaks?

Am I heading in the right direction? Any other ideas? Also, what's the best way to check for vacuum leaks. I've seen guys spray the whole engine with carb out. Does that work? If you don't think it's vacuum, what do you think I should check first. Are there other sensors that effect idle other than the IAC? Also, I should mention that it does that "almost stalling" thing whenever the AC compressor kicks in too.

 

Just bumping the post

 

Do you have a smoke machine? Exhaust gas analyzer? Breakout box? The first thing I woud do is hook these things up and go from there. There are many things that can cause this type of symptom and trying to troubleshoot with verbal descriptions is kinda dificult, thats why the tools mentioned above are a must...or I thinks so anyways.

The smoke machine would be the BEST way to trace vacuum leaks. The carb cleaner/propane/starter fluid methods don't work that well, they are a bit dangerous and they could cause other problems. Vacuum leaks can come up anywhere and trying to go for "common" areas is not all that great of a plan. Just hook up the smoke machine and look around...any and all leaks will surface.

Those codes are continuous memory so nothing to worry about. The computor knows that the EGR is not flowing and that no secondary air is flowing...that does not effect drivability. Those are soft codes (as opposed to hard codes) so, nothing to worry about.