87 with an unusual Idle

[shrtshck]

This past Friday was out for a ride with my son and noticed that on the way back the engine was idling at 2100 RPM.
Today it started OK, idle was a touch high (haven't adjusted anything) but after warming up it went back to the 2100 rpm idle. The unusual part is that if I hold the car in gear and brake the engine down about 1000 rpm it will hold there at idle. As soon as the engine is revved up the high idle returns.
Removing the oil filler cap made no difference to the idle.
Have checked all electrical connectors on the ngine bay, none were loose.
No obvious hose disconnects.

Anyone seen this before???

[Tippy]

There's quite a few turnbuckles with ball joints on the throttle linkage. Maybe there are some needing lube.

Check the easy stuff first.

[Justin@Athens]

Do a search, classic dirty idle control valve symptom (ICV)... Might also climb under the car and lube up the throttle linkage. Check the return spring on the linkage in the engine compartment too.

[shrtshck]

Had the engine out for top end rebuild two years ago, cleaned and lubed all the linkage points during reassembly so I'm confident there is no binding.
Will start with a search on the idle control valve, recall seeing some posts concerning cleaning the ICV.
Thanks for the lead

Blair

[shrtshck]

Removed and cleaned the ICV with carb cleaner, it looked pretty clean beforehand but ran cleaner through it anyway. Put it back together and went for a drive and still have the same odd symptoms.
Once warmed up it will idle at 2000 rpm, but if I pull the engine down to 1000 rpm it will stay at about 1200.
Tried removing the oil filler cap again, can just detect a very slight decease in rpm.
Prior to last Friday the car has always had a rock solid idle, no hunting when warm and nothing like this behavior.

Any other thoughts?

[kidrock]

I don't mean to beat a dead horse, but I would suggest (as others already have) to check the portion of your throttle linkage which is attached to your tranny housing. This is a known issue that seems to always occur as the weather warms up. Take the 3 minutes to check the linkage where it is attached to the housing and liberally squirt all the bushings with a lube of your choice.

[Tippy]

He did that already. Now it's the typical vacuum leaks and ICV checking/cleaning.

[Quicksilver]

The fact that it varies would tell me that it is the ICV. Other vacuum leaks can make the problem worse.

I just finished an odyssey that could be described as a "rubber explosion" on my car. My car was basically running good but was surging up and down heavily on cold idle and had a few cylinders running cold at idle. I found rubber messed up all over. Even the plastic manifold spacers were toast. All the vacuum lines are on the back side of the engine where you can't see them. Unless they have been changed they probably need to be changed. You even need to check the brake vacuum line under the front hood. I'm just happy I got to it before the fuel lines could cause an engine fire.

[bigel]

Originally Posted by Quicksilver

...All the vacuum lines are on the back side of the engine where you can't see them. Unless they have been changed they probably need to be changed.

Hey 'Quick' did you change the vacuum lines yourself? If so, how did you access them?

Not looking for a 'how to' explanation, just a one or two sentence answer that will give me an idea of how many hours I'll need to schedule.

Thanks.

[SJBuff]

I'm experiencing the exact same thing on my 75. I'm looking into vacuum leaks now.

[dtxscott]

A way to rule out the ICV is to jumper it on the panel to ensure it is centered.

If idle comes down to 800-ish, then the ICV needs attention. If not, then keep looking.

[Quicksilver]

Originally Posted by bigel

Hey 'Quick' did you change the vacuum lines yourself? If so, how did you access them?

Not looking for a 'how to' explanation, just a one or two sentence answer that will give me an idea of how many hours I'll need to schedule.

Thanks.

I was going after an intake gasket leak so I had the whole intake off the car. For the next 3 weekends the project stalled as I found more "rubber explosion®" that involved parts that are unobtainable over the weekend.

Unless you do a lot of work on these motors I don't see how you could inspect all the lines without pulling the intake. To do the fuel line you absolutely need to pull the intake and that one REALLY needs to be done on the older cars.


I'm going to be posting a writeup when I have got time because I found a bunch of stuff for a car that was basically ok.

[Tippy]

I forgot about the easiest check. There is the idle microswitch behind the throttle body.

With engine off, you should hear a tiny click when rotating the throttle shaft ever so slightly from idle to just off idle.

If it clicks, it should be adjusted ok. Need to do continuity test to see if it works.

[shrtshck]

You could be on to something Tippy, the mirco switch makes the "tick" as the throttle is opened, but checking electrically the idle switch is not switching between continuity and open.
Spent some time with a multimeter and the service manual;
ISC valve checks OK at 23, 21.4, and 42.9 ohms
Full throttle switch is OK

Since the issue came up suddenly I am leaning towards an electrical issue, prior to Friday the idle was always stable and at the proper rpm.

According to the manual a failed idle switch can cause a 1200 RPM idle, this is what I see if I drag the engine down in gear to about 1000 rpm.

Only thing that doesn't fit is the 2000 rpm idle if the engine is revved.

Now to find an idle switch

[Tippy]

Simple test is to disconnect it while idling. It should idle high when disconnected but throttle remaining closed.

[shrtshck]

Have ordered a new idle switch from our host.
Disconnecting the plug while idling made no difference to engine speed.
Used a jumper on the wire to mimic closing the switch and idle immediately dropped to the correct level.

[Tippy]

Excellent. Good to hear.

[McconnellSilva]

There's quite a few turnbuckles with ball joints on the throttle linkage.

[bigel]

Originally Posted by McconnellSilva

There's quite a few turnbuckles with ball joints on the throttle linkage.

PICs???

[Lorenfb]

"A way to rule out the ICV is to jumper it on the panel to ensure it is centered."

Not really, especially if the ICV or the DME ECM is bad.