I'm working on a 95 F250 with a 5.8 E4OD. It's had a high idle ever since it was bought a few months ago. It runs great just idles high. The previous owner already changed the goodies such as TPS, IAC, EGR and it's other components, O2 sensor, and coil. I put on a new tune up including cap, rotor, plugs, and wires. Now, I was starting to think that the TPS was bad because sometimes when it is in idle position, you can hear it fluctuate. I was also open to the idea the IAC could be stuck. Just because the parts are new, doesn't mean they're good. I believe he got them from Auto Zone when he changed them

I check codes and got these

KOEO:
111: We all know that's good

Continuous codes:
172: HEGO Shows always lean
452: Insufficient input from vehicle speed sensor (I will add that the fuse for the speedometer blows once in awhile. Could a bad VSS cause that?)

KOER:

172: HEGO Shows always lean
411: Cannot control RPM during KOER low RPM check


So, I had it idling and I unplugged the IAC. Nothing changed and it even drove fine. I inspected the IAC, it's fine as can be. THEN I noticed something. The throttle screw (looks like an idle adjustment screw) looks to be turned very far in. My guess, the PO had a low idle issue, decided to turn it in. I compared it to my 92 F250 with a 5.0 and couldn't believe how far in the screw is turned. It's nearly all the way in

I wouldn't doubt that's my issue, or at least a start. So my question is, How far is the screw supposed to be in? Would it be the same as my 5.0? And when I do adjust it, I'm gonna have to loosen the TPS so it adjusts correctly, aren't I? I'll post pics if I can! Thanks Guys!

 

I'm guessing that the depth is according to the desired idle RPM.

 

I read somewhere that it's supposed to be closed. I guess turn the screw out all the way, let it close, turn it in about 3/4 turn. But everyone on those threads denies it and has other opinions haha. I'm confused!

 

With the IAC disconnected or removed, the engine should just BARELY be able to idle and may even die.

Disconnect your IAC and adjust your screw so that is the case.

Then, disconnect the battery for ~10-15 minutes so the computer forgets the idle settings, then hook everything back up, connect the battery, start it up and let the computer re-learn the idle settings.

 

Thank you! Do you think I should loosen the TPS sensor? Something tells me it was replaced AFTER the throttle screw was turned in.

 

The screw on the TB should not normally be touched, but in your case the genie is already out of the bottle so you're gonna have to try and get it back within spec. To do this you'll need a multimeter and a couple small pins, use these to probe the Dk Green/Lt Grn strip and Black/White stripe wires on the TPS and then adjust the screw until you read between 0.9v -0.99v on the meter.

 

Um, hmmm......

I dunno......

The TPS doesn't have a motor and can't actually *do* anything on its own, BUT it sends a signal to the computer and the computer reacts accordingly.

I probably wouldn't fool with the TPS until after I disconnected the IAC and reset the screw, and then only if I was still having problems trying to get it to idle correctly.

No matter what route you take setting the screw, the computer is going to have issues about it but removing power will let it forget them.

 

Actually the computer takes a reading from the TPS every time at startup before the engine fires and uses this as the "baseline" for adjustments during that drive cycle, so you don't need to disconnect the battery to clear it's memory. The TPS itself has very little adjustment range(if any) on its mounting screws so there is really no need to touch it as it won't accomplish much.

 

You're probably right, I'm just speaking from my own experience of when I've done things like this.

Personally, I guess I just *feel better* with the computer having forgotten about all stuff in the past when I'm trying to re-establish a baseline like this.

I hadn't learned of the 0.9 - 0.99 thing until afterwards, did the "barely running" thing instead and left the TPS alone, didn't touch it, didn't put a meter on it to adjust the engine speed or anything.

It would make sense that the computer uses whatever position the TPS is in as its baseline and adjusts from there (like you say) but I work in hi-tech for a living and am generally not real trusting of electronic things sometimes.

 

I already adjusted it and it seems fine, but I'll try the voltage test next. I asked about the TPS because I thought maybe it would be turned all the way back (idle position) and by turning the screw, it would prevent the throttle plates from moving. But now I realize that can't exactly happen.

But anyway, away from my confusing brain fart, I unhooked the IAC, the engine was at operating temp, I turned that screw maybe 5-7 turns before it even wanted to surge haha. Once it nearly stalled, I turned it in a little bit and hooked the IAC back up. It's purring like a kitten now. But I'll try the TPS voltage that Conanski just mentioned next.

Like I said, I was concerned the TPS was in "Idle" position and couldn't turn back anymore. It's hard to explain. I wish the guy knew to not adjust it lol.

Thanks guys!

 

When i first got my truck, it idle real high. For the first year of owning it I just lived with it, then my valve covers started leaking.

While replacing the valve cover gaskets, I found that I had a vacuum leak where the upper and lower intake meet (gasket completely missing one 3 of the runners) . After replacing all the gaskets in the intake, my truck no longer had a high idle, nor did it flash lean O2.

 

Welcome to FTE.

How did you discover/find that leak?

 

I never would of though of that. With the miles he has on this truck, He should keep a gasket set around haha

 

Thanks, I've been lurking for some time now.

While i was taking the intake off, so that I could get to the passenger side valve cover, I noticed that there was no gasket on three center intake runners,

 

No it can not. The VSS is the source of the voltage (it generates it) and not the load.
But the 452 is from the fuse blowing.