Idle Issues

[Black91SHO]

I have a '91 and it has intermittent idle problems. When I start it up in the morning, it might idle high like it's supposed to, but sometimes it won't. Instead, it'll bob at around 700 rpms, then eventually kind of smooth out to around 900. When I'm around town, it sometimes does the same thing, so even when it's warmed up, it does it.

No codes. I've changed the idle air control valve, but it didn't fix anything.

Any ideas?

 

[itwonder]

Search forum for idle reset procedure. If someone has messed with the throttle plate stop screw on the TB, you may need to do the base idle adjustment too.

 

[sperold]

It still sounds like the Idle Control Valve to me. Take your new unit and put some lubricant / cleaner on the working part and look at the new gasket. Look at the mounting surface for the gasket as well.

 

[Black91SHO]

Thanks. I'll see if either of those work.

 

[rubydist]

I would also look hard for a vacuum leak. It doesn't take much for your idle to pop up a couple hundred rpm, and if it leaks at a hose that has a varying leak rate, that could be your issue too. Some of the hoses flex as they warm up so the amount of air leaking in can change, and most of these cars have 20 year old vacuum hoses everywhere - some of them are rotted out by now, especially the ones that handle a lot of fuel vapor.

 

[Black91SHO]

So far so good after the idle reset, but I haven't driven the car much to test it. I'll let ya know if it happens again. Thanks!

 

[Black91SHO]

I spoke too soon. The idle problem is back again.

I haven't tried doing anything to the IAC valve because it's brand new. I don't see how a brand new unit could screw up exactly like the old unit I just replaced, so I'm thinking that can't be it.

A vacuum leak doesn't seem likely, because the loping idle would be persistent. My idle is fine usually, but sometimes, upon starting the car, it lopes/bobs around 600-700 rpms. I rev it once or twice and it normalizes.

My guess is something electronic, like the IRCM or DIS, but I'm not sure how those effect idle. I just know they can have an effect. I'll probably swap one of those out next and see if the problem persists.

Any other ideas?

 

[DeepPower]

I would suspect the Throttle Position Sensor (TPS). This is a potentiometer-type device, the resistance tells the electronics the position of the throttle. If it goes bad, or has dead spots, your idle will vary wildly.

The TPS is basically a resistance material that is contacted by a wiper. As you press your foot on the throttle the wiper moves along the resistance material. The wiper or the resistance material can get dirty.

Do this - with the engine off and the key out, RAPIDLY press your foot on the throttle, up and down, up and down, as fast as you can for about 15 seconds. This quickly rubs the wiper against the resistance material and can (temporarily) clean it off. Then start your car as normal. If your idle gets better then you need to replace the TPS.

BTW, I replace the TPS every 20K miles as a wear item. While replacing the TPS I suggest you also replace the two Philips head screws retaining it with hex bolts. Ford put in screws made of soft material that strip easy. If you replace them, please do us a favor and let us know the type of bolt and thread you use.

 

[Black91SHO]

Thanks for the info.

The idle doesn't vary wildly. It's normal most of the time. But sometimes when I start the car, it drops to 600-700 rpms, approximately, and bobs. If I rev it a couple of times, it goes back to normal. When the engine's cold, it should idle at 1,500 rpms, but when this little glitch happens, it sits at 600-700 rpms, bobbing. In other words, this is always the same, as opposed to varying wildly.

Doesn't sound like the TPS. But then again, in my few hundred thousand miles and 13 years of owning 5 SHOs, I've never once had a TPS problem, not even on my 308k-mile SHO, so I don't know what it looks like.

Now that I've described it in more detail, does it still sound like the TPS to you?

 

[DeepPower]

Now that I've described it in more detail, does it still sound like the TPS to you?

Could be. Try my method of cleaning the TPS anyway. It won't cost you anything except 15 seconds.

 

[Black91SHO]

I'll try it, but since it doesn't happen but about every 4th or 5th start, I'm not certain I'll be able to tell if it worked or not.

 

[DeepPower]

I'll try it, but since it doesn't happen but about every 4th or 5th start, I'm not certain I'll be able to tell if it worked or not.

I understand. Intermittent problems can really try your patience. Don't give up.

 

[Black91SHO]

Oh, I don't mind troubleshooting it. I'm in no hurry. I do need to fix it before I sell it, though.

 

[rubydist]

I have had a brand new iac be sticky right out of the box, so the fact that your iac is newer is not relevant.

 

[Black91SHO]

I may pull it off and clean it. But the fact that the car's doing exactly the same thing instead of changing just a tad would imply the IAC wasn't the issue.

 

[Black91SHO]

Tested the TPS. It seems to be fine. Now the idle issue has gotten worse. Not only does it Bob at start hp sometimes, it now randomly kicks the idle up higher, and it varies up there a little, and will sometimes idle out. I think the fuel pump is bad. I heard a crazy noise from it the other day every time I'd turn the car off. It only happened a few times, but it was wierd. I'd never heard that before. Took me a few tries to figure out it was the fuel pump because it sounded so odd and wasn't easily localized in the back.

Any other ideas?

 

[rubydist]

lots of times there are multiple issues at once on these cars, since they are now getting old like me...

if the idle surging is not the iac, then there must be an intermittent vacuum leak. one good candidate for that is the line that is on the charcoal canister purge - below that solenoid the vac hose is subject to lots of fuel/vapors so it is rotten in nearly every SHO I see - so when that solenoid opens to purge the canister it actually lets in way more air that it should. I would be looking for a bad vac hose somewhere like that.

 

[Black91SHO]

Thanks. I'll check that line, but typically, but a vacuum leak causes the idle problem to be there all the time. When I used to own RX7s, that happened a lot. It sucked (no pun intended), because they have so many vacuum hoses. This problem is intermittent.

I'm going to swap the MAF from another car. That'll sometimes cause weird issues like this due to faulty air intake readings causing fluxuation. I'm putting off changing the fuel pump till I try a few more part swaps off parts cars. Fuel pump's kind of a pain to change.

 

[rubydist]

that is why I mentioned the canister purge solenoid - it is not open all the time, so a vac leak on the other side of it could appear intermittent. there are a few other things that vacuum operates that are not always on, that is where I would be looking for the vac leak.

 

[Black91SHO]

Cool. I'll check it out. Thanks.