1991 5.8L

For the last month or so I have been troubleshooting a revving engine problem. When I'm letting my truck warm up in the mornings, after about 10 minutes of idling, the engine will rev up- at a guess around 2000 RPM and even higher than that sometimes (tach inoperable so I can't be certain)- and it will remain that high for either a few minutes or until I shut the truck off and restart it.

When I'm driving, it will rev when slowing down for stop lights or turns (not every time but often enough). With the snow on the ground, I'm forced to shift to neutral to keep from sliding through any intersections if it's slick out, and without the rest of the drive train restraining the engine it will rev up to that same range, and of course when I shift back into gear to keep going it's "slamming" into gear. But even if I don't have to shift to neutral it's still putting a bad pull on my brakes and I'm sure there's more stress on the transmission and related drive train parts.

I cannot nail down a pattern for when it does or does not rev up when I'm driving. The only constant is when it's warming up in the morning- that's the only time it's consistently revved.

It's getting to the point where it's dangerous to be driving it if the roads are slick, but as my DD I don't have much of a choice.

I've been checking and clearing codes throughout the process. Last week code 452 led me to replace the VSS. I also cleaned the IAC but that had no effect on the problem. Furthermore, the next day, I tried unplugging the IAC during idle and nothing changed, so I replaced that as well.

I pulled codes again this morning and the 2 codes that have consistently been showing up are 411 & 412. I didn't pay them much attention before on account of the VSS code and the bad IAC, but they're still there after that stuff has been remedied.

Also, just for kicks, I checked my FPR this morning as well and I can smell gas when I remove the vacuum line, so that means it's bad as well, correct?

What can I be checking to troubleshoot the codes 411 and 412? I don't know what all the computer relies on to regulate idle speed.

Thank you.

 

I assume there are no KOEO/CM codes?

Your KOER codes of 411 & 412 indicate the computer cannot control the idle RPM by modulating the IAC signal. You already disconnected the IAC and stated the idle RPM remained high. Typically that indicates you have unmetered air (aka vacuum leak) entering the intake

 

I agree with what you're saying, but what doesn't make sense to me is that if there is unmetered air entering the intake, why isn't the engine revving all the time instead of only now and then while driving or after so many minutes at idle?

 

Unplugging the IAC takes the computer out of the equation so only a few things are left. One previous statement you made "and it will remain that high for either a few minutes or until I shut the truck off and restart it." could be a TPS problem but once again the IAC takes the computer control out of the equation. It may be worth a shot to unplug the TPS and/or test the output when the truck is cold versus warmed up.

Intermittent high idle conditions can be tough to track down. You could have a gasket that is marginal which could allow in air when the parts it is sandwiched between warm up or perhaps a small crack someplace.

 

Shouldn't a 91 be generating 2 digit codes?

But anyways, if you replaced the IAC, and the idle still goes high with the IAC unplugged, I'd suspect a vacuum leak that opens up as the engine warms up.

 

Ford started using three digit codes in 1991.

 

I guess I should have clarified that- the engine was not revving when I unhooked the IAC. The engine was warmed up and idling normally when I unplugged it. Maybe next time it revs up I will try unhooking it then and see what happens.

I'm kind of stuck for now b/c the snow is falling right now and it's cold and windy, and I don't have a garage, so until I get an opportunity to check some of these things out, I'm gonna have to live with it.

Thanks for the advice so far guys.

Also, rla2005, you are right about the KOEO/CM codes- they both came up 111 system pass.

 

Unplugging the IAC at "normal" idle should have resulted in the idle speed dropping from some 700/800rpms down to 500. A noticeable change tach working or not.

If the idle didn't change unplugging the control had no effect? I'd look to that as still at least in part the problem.

And as suggested test your TPS, and see what its voltage reading is normal idle and then again when its idling way to high.

Wiggle the harness around, see if doing so has any effect whatsoever on idle speed.

Oh and not sure if you or anyone else mentioned it, I apologize if a repeat, make sure your throttle returns to full closed position every time and without fail.

 

Alright, so I got the chance to unplug the IAC when the engine started revving- and the idle dropped. When I hooked it back up, it revved up again.

So then I proceeded to check the TPS signal per FFI's instruction: Fuel Injection Technical Library » Throttle Position Sensor (TPS) As I was handling the plug/harness and trying to get it into a position to back probe, I noticed the engine idle changing. I took a moment and just fiddled with the connector, twisting it, pulling one way or the other, etc. Not with every movement, but it did affect idle speed.

I then noticed that a part of the weatherproofing "plug" in the back of the connector on the engine side was missing, like it had been torn off. So I think at least part of the problem is a dirty/corroded connector. I've been searching online for a replacement but with no luck- every kit I've come across is for the TPS that has blade pins whereas mine has the round pins.

I did test TPS output at normal idle- holding steady at .69 volts. I couldn't get the engine to rev, but my thoughts are that a corroded connection is making and breaking contact, and the lack of TPS signal during broken contact is causing the computer to go crazy.

Thanks again for the guidance- I'll keep you updated.

 

So I haven't been able to find a replacement connector kit for the style of TPS that I have. But, I can get new terminal connectors through NAPA, so I'll just replace those and do something to re-seal the portion of the weather plug that's missing.

Coincidentally, I haven't had any trouble with the engine revving since the day I screwed with the TPS connector, so I'd say that was definitely the source of the problem. I'll know for sure once I get the new terminals in place and see how it performs over time, I suppose.

Thanks again for the guidance.

 

So I ended up going to the local pick and pull and grabbed a connector for my TPS. I searched all over online and could not find a match for my style.

I got it installed yesterday, and not that 1 day is an accurate time frame for saying it's fixed, but so far so good. I cleared the codes and will drive it through the weekend- regardless of whether or not I have the revving issue again, I'm going to check them on Sunday or Monday to see what's there.

On a related note, since I clipped both TPS and engine harness side of the connector at the junkyard, I now have a TPS test connector!



Thanks again to those who helped out. I'll keep you updated if there's any more issues.

 

It looks like you have this one whipped! Great TPS test connector.

My old '92 F350 used to have an intermittent high idle that I finally traced down to corroded pins on the TPS connector. Amazing what a few ohms of resistance will do with the "ratch" algorithm on an EEC-IV.

 

That's what I've been learning more and more lately. Not just w/ my truck but with cars that belong to friends and family. Wiring seems to be getting smaller and smaller on auto's these days, which makes it that much more prone to even minor corrosion.

 

Well, it seemed to be straightened out for a while, but the last few days I've been having issues again.

Thankfully, it hasn't happened while I'm driving, just while idling. So I had time today to pull codes again:

(Definitions from Fuel Injection Technical Library)

KOEO/CM: 111 system pass

KOER:
411/412: Cannot control RPM during self test
116: ECT out of self test range 0.3 to 3.7 volts
311: Thermactor system inoperative

As for the 311, I know I have an exhaust leak where the manifold meets the pipe on the right side of the engine. I'm nearing the point where I'll probably need to replace the Y pipe soon- I'll start saving.

I'll get some jumper wires installed to test the ECT.

So now we're back to the original codes for the post. Given that I did replace the bad TPS connector, what would be my next step? I'm all for checking for vac leaks but I won't do that until there's a decent day or I can do it at my shop at work.

So, vac leaks aside, is it possible that maybe the IAC valve connector is loose or dirty or something that may be causing intermittent connection? Or maybe there's a problem in the circuit. Can somebody tell me how much voltage I should expect to see at the IAC w/ the engine running?

 

Reference this post from another forum:FSB Forums - View Single Post - IAC valve testing

With the key On and the engine Off you should get ~10.5VDC. What you will measure with the truck running is anyone's guess using a DVM. The PCM modulates the IAC on/off at a very rapid rate to control how much air is allowed into the intake. A DVM will not accurately display that voltage under those conditions.