fast idle when engine is hot

[wagonboy]

2.7L MFI.
I'm experiencing a problem with the idle speed when the engine is warm. It's idling at 1900-2000 rpm but only when hot. Has anyone experienced anything similar and what did you try in order to fix the issue?

My engine idles fine when cold but as the temp rises the idle will not drop back to 1000 when after throttle is applied and released. At roughly 150 degrees the idle speed is about 1500-1600rpm and when fully up to temp it's idling between 1900 and 2000rpm. At 150 degrees I can pull back on the throttle linkage by hand and make the idle speed drop and it will stay there after I let go of the linkage but it rises again when I blip the throttle. As the temp rises to 180 and beyond I can no longer pull back on the throttle linkage to make the idle drop. It almost seems as if when the engine is cold, there is enough slack on the throttle linkages allowing for the throttle to remain fully closed and as the heat rises, the engine is expanding until it's fully up to temp and taking up all the slack plus a bit more thus preventing the throttle from fully closing. Something seems to be sticking.

I've checked the hand throttle and the adjustment looks fine. There is clearance on the adjustment when the handle is all the way down. I've even made sure that nothing like the mats are resting against the gas peddle. One odd thing I have noticed though is that the hand throttle will put the rpm up to roughly 5000rpm when fully up. That's at least 1000rpm too high from what I've read. That said, the hand throttle when fully down doesn't seem to be hindering the closure of the throttle.

Sincerely,
Marc

[wagonboy]

Once the engine is up to operating temp nothing I try will bring the rpm down from 2000 except shutting the engine off and restarting it. As soon as I give it some gas it settles back at 1900-2000 rpm. This only occurs once the engine is up to temp. Has anyone experienced this?
Any help appreciated. Thanks in advance.

[Mr9146]

Sound like it's hanging on the distributor advance. Check the timing, check the vacuum pot on the distributor (if you have one), check the advance plate to make sure it's free. If you have a problem with the distributor, get it reconditioned. You can't tune MFI with a bad distributor.

Talk to Kurt - www.partsklassik.com

[beh911]

What is the CO at idle?

Is there any adjustment left in the bleed screws on the throttle bodies?

What is the health of the throttle bodies in general? Heat can affect tolerances which can lead to leaning out mixture, affecting idle

[Chuck Miller]

wagonboy,

The two above post are right on the money…

We do not know if this is a symptom that you’ve been living with, or just came on, or was there when you newly purchased the car.

I’d take the air cleaner off, get some kind of a unisync and see if you have the throttle air screws anywhere close to reading even…. if not, adjust until they do and see if your idle comes down.

Hot idle on your stock engine should be about 900 to 1000 RPM

My .02

[Frank Beck]

Wishful thinking that it's related to accel rod adjustment...
MFI isn't black magic but it does take proper baseline adjustments and as mentioned above, T-bodies and distributor that aren't worn out. These two issues are evil bedfellows and will definitely cause runaway idle.
Have a pro look at it.

[wagonboy]

I found a few forums over on Pelican with exactly the same problem as I'm experiencing - right down to it beginning after a recent rebuild. Unfortunately none of the posts seem to come to any definitive conclusion about what they did to ultimately fix this kind of problem.

When warm the car will idle fine at 900-1000 rpm after startup as long as I don't give it some throttle. Once I hit the throttle a little the idle moves up to 1900-2000rpm. It will slow down with just a little clutch action so it's really not difficult to drive with this problem.

Today we'll have a real close look at the distributor and vacuum lines. My guess at this point is I have a vacuum leak somewhere.

Warn TB's: This I'll definitely get into should the distributor and vacuum lines all prove to be operating as they should but it's hard to imagine that worn TB's would cause the engine to idle at 2000rpm when warm but allow to engine to idle normally when cold. Also, if TB's were worn I might expect revs to rise perhaps 200 or 400 rpm higher than normal at idle - not the extra 1000rpm I'm getting and the waring of TB's should be more progressive.

One other thing I just thought of... Before I had the engine work the throttle used to stick up around 2000rpm when i came to a stop - not always but often enough. It made it hard to get the car into first gear but a quick blip of the gas peddle usually was enough to get rpm down to a normal idle before trying to engage 1st. Precursor to the current problem possibly?

[Mr9146]

When warm the car will idle fine at 900-1000 rpm after startup as long as I don't give it some throttle. Once I hit the throttle a little the idle moves up to 1900-2000rpm. It will slow down with just a little clutch action so it's really not difficult to drive with this problem.

This is most likely a mechanical distributor problem.

Most likely the advance mechanism is worn out or sticky (plates, springs, weights) and the distributor is hanging on the advance ... dragging the idle down with the car and in gear and your foot on the brake lets the advance weights close and the distributor drops the advance thus slowing down the engine speed.

I'd start there.

[vicrola]

Similar issue on my '72 E w/ MFI. Replaced all the vacum lines w/ fresh ones. Problem solved...

[wagonboy]

We played around with the vacuum and timing today. Here's what was discovered:
Vacuum lines seem to be working as designed. Although I don't own a vacuum gauge, removing the vacuum from the housing on the distributor results in rpm rising as it should from idle and drops again when re-installed. The lines appear new and have a good seal around the base of #1 & 4 cylinder, at the "T" split and at the housing on the distributor.
A check of the timing with vacuum lines installed and at 1000rpm idle showed roughly 2.5 ATDC (halfway between the two marks on the pulley. So we retarded the timing to 6 degrees. The result didn't fix the fast idle completely but instead of 1900-2000 rpm when hot I was getting this down to 1600. It was also sticking less often now and although there was a significant delay after blipping the throttle past 2500 rpm, it was often dropping down to 1000rpm after hanging up around 1600 for about 6-8 seconds.
Another observation was after a short drive with the engine now at 190 degrees and idling at 1600rpm, releasing the clutch in gear slightly and then pushing the peddle back in resulted in getting the idle to drop to the normal 1000rpm. This was not occurring two days ago so yes we have progress.

For now, unless I get myself a CO meter and something to measure vacuum to play with, I might be finished toying around. The car is very drivable with lots of power and the added rpm at idle when hot isn't affecting drivability. I'll continue to keep an close eye on it. BTW, idle at 1000 when turned on hot is smooth and even. Even when it's sticking at 1600 or even 1900-2000 yesterday, the engine sounded fine. No popping, back firing or other.

With each "issue" I'm getting increasingly bold about what I think I can accomplish in the garage. Although I have much to learn about these old cars, I'm enjoying the experience. Thanks for everyone's responses and suggestions.
Regards,
Marc