I have a ongoing issue with my idle. The truck is a 90 originally with a 300 5 speed. I transplanted a 94 351 roller block. Truck runs great except for the high idle. Replaced Tps sensor 2 times & IAC 2 times. Start truck up, and it goes high idle for 10-15 secs and drops to 750 like it is supposed to. tap the gas once & it goes to 1500 & stays there unless i tap the gas again. took throttle cable off to rule out a binding issue & still it does it. throttle plates also do not bind that i can see. tps closed voltage is 0.87 v & wide open is 4.56 volts.Unplug IAC & idle stumbles for a about 30 secs & then idles perfectly at 750 rpms. truck runs great with IAC valve unplugged, but I want to fix the problem. replaced all vacuum lines, checked egr & evr. found no leakage. If it was a vacuum leak, wouldnt it idle high all the time? idle goes back to normal when i tap gas once.Also goes to 1500 between shifts.throttle body has been cleaned several times. would coolant temp sensor or air charge sensor cause this? any help is appreciated

 

well i got a 97 f350 with the 351 in it. I had a idle problem with mine but it would idle very low when I started it. So i replaced my IAC with a new one from advance auto parts, it works good but I do notice it jump around from time to time. Have you checked your EGR, maybe theres a hole in the diaphram on the EGR valve. I had to replace mine because it wasn't working. Have you tried to pull codes on it to see if theres something not working.

 

Sounds like you've ruled out the TPS. It could be another sensor like the ECT but I'd take another look at the TPS.

I had a similar issue on my 88 250 351. This symptom drove me nuts for 3+ months. I changed IAC and ACT sensors, lubed the throttle body shafts and the throttle cable and seriously considered replacing the throttle body shaft bearings before again revisiting the TPS which as relatively new. The TPS was the problem - it increased resistance as the truck reached 195 degrees. It was very tricky to diagnose. After deciding to replace the TPS, I bench tested the old one (a Ford product) with a heat gun and a meat thermometer to verify the output resistance changed with the temperature. I picked up the new TPS from Advance Auto (has a 1 year replacement warranty) so if the idle get wacky again before a year is up, I'll just replace it again at no charge.

Are you getting any codes? I tried lowering the output voltage on my old one to .86v attempting to stop the high idle. It didn't work to fix the idle and it caused a "TPS out of range" code to appear in the key on engine off test. I had to have the TPS set at .9v to eliminate any codes.

 

Considering what you have done already I suggest you try another TB and IAC valve on it, sounds like the throttle plates are not fully closing at times but of course it works perfectly when you're watching it, changing the whole TB will also eliminate the linkage if it happens to be the issue. Temp sensors will not cause this issue.

 

What about the throttle body gasket? If you can't see the plates sticking, perhaps there's a break in the TB gasket. Before you replace the TB as Conanski suggests, check the gasket- and give gooping both the TB and IAC gaskets a try. Run as thin a bead as you can, and I caution you to give the goop 24 hours to fully cure before you try starting it up.

Also, when you cleaned the TB, what did you use to do it, and did you do any scrubbing? If you used TB cleaner, you're good- even carb cleaner is supposedly okay- but scrubbing or sanding on it is a no-no. There is a special coating of some kind on the inside of the throttle body that helps seal the throttle plates when they are fully closed. Scrubbing of any kind will remove this coating. It has kind of a yellowish tint to it- are you sure you didn't scrub this off and now unmetered air is slipping thru the TB?

I tried running a search but couldn't find anything, but almost anybody else on this board can confirm what I'm talking about.

 

Thanks for all the replys. This is the second throttle body I have tried. I didnt use anything abrasive on the throttle body, just TB cleaner. new gaskets on upper phlem,throttle body & IAC valve. I even siliconed them on after they didnt fix the idle. I guess ill try & find another throttle body and/or another TP sensor. But if it was a vacuum leak, it would idle high all the time right? EGR is new also. thanks again for the replies

 

Not necessarily- it depends on how bad the leak is. If it's a big leak, then yes, it would likely idle high all the time. But a smaller leak is likely to cause stumbling issues, or possibly what you're describing.

I recently had a broken vacuum line on my truck that, every time I hit the accelerator at cruising or highway speeds, would cause the engine to buck. Once I fixed the leak the problem went away.

I don't know how you're checking for vac leaks, but give this page a look. The method it suggests works really well.

Ford Fuel Injection Vacuum Leaks

Also, perhaps you're looking in the wrong place. Have you pulled and checked the vacuum reservoirs? There should be one on each side of the engine bay- it looks similar to a small coffee can. These are prone to rusting out on the bottom and can also cause leaks that are difficult to detect. I'm having a problem with mine right now.

Just another suggestion. Good luck and keep us updated.

 

I also want to note that the more I think it over, the more it makes sense that there's a vac leak somewhere. If you unplug the IAC and it smoothes out, that means there's unmetered air entering the engine from somewhere.

Other than checking the vac reservoirs, one other thing I can suggest- and make sure the engine is up to operating temperature when you do this- is to unplug all your vacuum lines from the various sensors and canisters and what not. Leave them hooked up to the manifold only. Using vac line plugs or bolts of random sizes, plug all the loose ends of the lines.

Start the truck.

One by one, unplug each vacuum line and reattach it to its proper sensor/canister/valve, etc. If you hook up a certain line and the stumbling/high idle issue starts, you've likely found the problem.

 

after 2 years of headaches, the problem is now fixed. I hooked the meter up to the TP sensor with engine running. closed throttle was showing 0.83 volts. I opened throttle body just a hair & volts went to 0.97 volts & it stayed there. rpm also stayed high, could also feel butterflys kind of catch or bind a little. I left the meter hooked up & adjusted throttle stop screw until I had 0.9 volts on tp sensor.Throttle plates are now not sticking & rpm drops when I shift gears, instead of going up. Took it for a test drive & its working great. thanks to everybody for all the help