Hanging Idle
 

Tried searching but couldn't find what I needed.

Pickup is the 91 in my sig.

What could cause a hanging idle? It will drive great but when I put the clutch in to shift it doesn't immediately idle down, takes a second or 2. When stopped I can blip the throttle and it acts normal.
Ill have to check but Im thinking I can blip the throttle with the clutch in while moving and its normal also.

Quite a lot of changing from a stock pickup on this but I maintained most of the sensors and such. EGR is blocked and turned off in ECU. Running an A9L ECU.

Ideas? Where to start?

Could just be a dirty throttle body, gummed up enough to slow the closing of the plates. If you're going to clean it do not scrub or scrape on the inside of the barrels- there's a thin coating that shouldn't be removed. Just spray off what will spray off and leave the rest.

It could also be a dirty/sticking IAC valve. Carb cleaner works well to clean them.

Ive got less than a 1,000 miles on all nice and cleaned up parts since I put the engine together, so dirtiness shouldn't be a factor.

I had these symptoms when my tps was bad. There are simple ohm tests that can be performed with a voltmeter. Get a haines manual. . .it has simple tests for all sensors as long as you have a voltmeter. You can test them in the same time it takes to sit down n post a thread

TPS tests fine

Iac test out good?

I tested the voltage across the connector to the IAC, got almost 12v. Book says to check out the ECU. Mine is completely tuned up, possibly just some settings inside need to be changed?

Bear in mind that bad parts can test good. The most direct way to test the IAC is to start the engine, then unplug the IAC valve. If the engine then starts bucking and stumbling or dies, it's functional- if there's no change in idle, the IAC is bad.

What about the throttle cable itself? What's its condition, because even if the throttle body is good and clean, a sticking cable can cause the same problem. You could have someone work the gas pedal while you watch to see if it is sticking.

Also, how sure are you that there aren't any vacuum leaks?

The IAC is modulated on/off by the ECM, a DC meter is not an effective tool to test it. Ryan has a great write-up of how it works: Ford Fuel Injection » Idle Air Bypass (IAB)

Many times a "hanging idle" is caused by a sticking throttlebody or cable or a TPS that has a varying resistance every time it closes. A bad IAC usually manifests itself with a "rolling idle" or intermittent dying at low engine speeds.

My '92 F350 had a varying idle speed issue that was driving me crazy. The TPS tested good with both resistance and voltage checks. I eventually found the connector was corroded with an ugly green color goo. Cleaned it, lubed it with dielectric grease now it works like a charm.

If you are slowing down the computer is supposed to raise the idle when you depress the clutch. I don't remember why? But I vote for dirty linkage since it is the easiest fix and you say you can hit the throttle and make it go away. Either way it would be good to clean it up.

This may not have anything to do with your truck, but........... "hanging idle" is a common problem with modular motors in the Mustang crowd. In their case, the ECU is programmed to inject extra air via the IAC every time you let off the gas to shift gears. It's designed to reduce the unburnt fuel emissions created by suddenly letting off the gas. Sometimes it can get pretty bad. To fix it, many guys install a restrictor plate in the IAC port or have the condition fixed via a custom flash tune.

Ok, so I messed with it today. Checked the cable going to the TB, with it off I moved the TB by hand everything moved smooth no hang-ups, snapped back. Took it off and checked the TB itself and it moved freely also. Started it up, kept the cable off and revved it up by hand, still hangs when I let off.

Also while it was idling I sprayed some TB cleaner on all the vacuum connectors and there doesn't seem to be any leaks on that front.

The connector to the TPS looks clean, no corrosion.

Thinking I might get a new TPS and throw it in there, ust to make sure its not the problem.

Any other ideas?

If the TB & cable are OK and are not hanging up, then the extra air that is causing the hanging idle has to be coming from the IAC. You can try a new TPS to eliminate it as a source, but to me, the engine speed is dictated by air flow...which leads back to the IAC. If the IAC is clean & not sticking, then I would look at the ECU tune. Not sure how the IAC is made on these, but you may be able to experiment with different size holes in the IAC gasket (to regulate air flow)until you find the perfect size that will let the RPM drop faster but still idle. This is a common problem with the 4.6L Mustang (my '02 Taurus did it as well) and "Mr. Gasket" sells a special IAC gasket with a smaller hole just for this purpose. Most Mustang guys just have it corrected when they get custom flash tunes. I'm not 100% certain this is your problem, but it sounds eerily familiar. Good luck.....hope you get it figured out.

Well finally got around to messing with it again, between finals, work, family stuff, and just plain laziness, Ive been busy.

I got another TPS to throw in(I work at an autoparts store, so buying and returning is no big deal) but ended up not doing it yet. Just don't think thats it, since its basically new.

I warmed up the engine a bit then unplugged the IAC and it cuts off, so I'm thinking that isn't the problem.

Is there a better way to check for vacuum leaks than the spray method? I have a handheld pump. I hooked it up to the tree on the manifold, while running its getting about 14" and when I blip the throttle and it hangs, the vacuum goes up to about 25-26". So I'm thinking the idle vacuum is a bit low compared to what Ive seen on here.

Also when I unplug a line from the tree while running, nothing happens, so that makes me think there is already a vacuum leak that I cant find.

Any ideas how I could find this thing?