I'm having a high idle issue with my '96 F150 with a 5 speed. I've been struggling with this problem for almost a year now. Cold it idles at 1000 rpms. Warm it idles at 1100 or so. (My truck has factory tach and these are approximate RPM values). I'm not sure what to make of this but while moving it wants to idle at 1300-1500; that is if I were to put the truck in neutral and coast while moving at any speed it will idle around 1500 and seconds after the truck stops physically rolling it will idle down to 1100.... Idle fluctuates between 1100 and 1300 while stopped and in neutral.

I haven't checked TPS for resistance or functionality - I'm not sure how to. I did remove and block off IAC and it dropped idle speed to maybe 900. While IAC was off I plugged it back in but it didn't do anything w/ engine running - not sure if it was supposed to. Putting it back on brought idle speed back up to 1100. I have not touched factory throttle blade adjustment - it has blue paint on it and looks like it hasn't ever been adjusted.

I was told this could be a vacuum leak issue but I've checked all the hoses so many times I never want to see a vacuum line again - and even so would it cause this weird of an idle fluctuation? Any ideas or other things to check? If anyone can explain to a college shadetree mechanic how to test TPS for functionality I'd appreciate it, I'm trying to diagnose and find the issue rather than throw parts.

Worse comes to worse I'll just keep driving it like it is - it seems to be more of an annoyance than a serious pressing issue.

 

" diagnose"

Pull the codes. If you don't have a code reader, some of the parts houses do and will pull them for free.

 

Yes pull codes is the first thing to do.

Next is check for vacuum leaks.
Carb cleaner: motor running and up to temp spray between intake manifold & head. Intake manifold from head to the air filter hose before the throttle body. Any place hoses connect to the intake and the other end of that hose for leak.

Smoke test: Look up on Youtube checking for vacuum leaks using smoke. It is pretty easy to do and the motor is off and cold might be better. This might show a leak that the carb cleaner may not pick up.
Dave ----

 

Forgot to mention that. Codes were pulled about three months ago. The only pertinent code was Bank 1 and 2 lean code. Found some rotten/disconnected vacuum lines behind the motor and replaced them, re-plugged in DPFE sensor (someone left it unplugged), replaced O2 sensors. The only other two codes were for the rear tank fuel pump (didn't have one until recently) and the Thermactor air system incorrect flow. I haven't checked the codes recently, and unfortunately I'm up at school so probably won't be able to for a week or so.

Thank you for the help.

Dave, I've used up a couple cans of carb cleaner and starting fluid over the last few months to try and track down vacuum leaks. Found a couple and fixed them but it didn't seem to solve this issue. I've sprayed multiple times around the TB, intake manifold, charcoal canister, vacuum tree, and whatever exposed vacuum lines I could get to, including the ones under the dash and the vacuum resevoir. The only thing I haven't sprayed or checked out is the EGR system. Might this cause an issue? Thanks for the help!

 

If you have not replaced ALL the vacuum lines I suggest you do so. The original equipment ones get old, break and crack and sometimes those cracks are just plain hard to find.

 

Best way I've found to track down vacuum leaks is to wait for a nice, calm sunny day then hook a hose to the vacuum tree, light up a cigar and blow smoke through the hose and watch for any creeping out around the engine.

It could be the EGR valve, I found a leaky one on my old '96 using the cigar trick.

 

Yes the EGR could be a cause be it a vacuum leak on the dia or the EGR could be stuck open a little and be the cause.
Dave ----

 

 

 

Baron - thanks for the advice. I'll get my hipster friend to go through an entire bottle of vape juice to help me out. Should this be done with the truck running or off?

 

Engine off.