Fast Idle Problem

[Tonylito]

1991 GW

I did extensive search but have not seen the resolution to this.

Status - new engine, new carb, messy vacuum.

So just did all the adjusts - choke, timing, etc.

The jeep starts and runs better than it ever has.

The idle is set to 650rpm. Read then its idling with timing light.

When you 'drive' the throttle does not return all the way to base idle (650) but rather somewhere around 950-1000rpm.

When I am actually driving the car .... it does to about 20 mphs without me touching the gas. I can hit the accelerating and I cant get it back to 650 idle.

When under the engine .. you can 'force' the throttle back to its 650 position. So it appears the throttle plates are not closing and the throttle is not being pulled back.

When the car is off - it springs right back to the 650 position and isnt hung up.

I have looked for obstructions on the linkage et al and the fact that its only happening when running - think is might be vacuum related.

Cannot do anything to adjust the throttle screw - has no impact.

[will e]

You might have a blown power valve. Try adjusting the idle bleeds in all of the way and see if it still runs.

[Tonylito]

what are the idle bleeds? It is a brand new carb < 1000 miles. Its a motorcraft 2150.

[tgreese]

An inconsistent idle can come from a worn-out carburetor body. When you say new, do you mean new to you or newly manufactured? Is this the factory carburetor? The factory carburetors are being made new, though not with the altitude compensator that the Jeep carbs come with. If it's not the factory carbureotr, disregard the rest of my answer.

I think he means the idle jet adjustment screws. The idle air bleeds are not adjustable on this carburetor. There are two idle screws at the base of the carburetor, one for each venturi. Turn them in and keep track of how many turns, so you can put them back. The engine should stall with both screws turned all the way in (closed). NB the idle circuit only works at low speed, so you need the engine speed to be well within idle range, like 600 RPM.

Suggest you read the TSM about this carburetor. Each of the circuits is well described. Does not have to be the '91 book - any TSM covering the 2150 carburetor will do.

[letank]

if your charcoal canister check valve/ diaphragm -under the cap that is labelled do not open- and related circuit have a small leak it can induce a higher idle... Play with the different hoses and clamp each temporarily with a hemostat or a visegrip...

[will e]

Yes, I used the wrong terminology. Sorry!

[Tonylito]

1) it is a brand new 2150 from I-5 auto (doesnt mean its not bad) but less than 1000 miles. it does have the altitude compensator.
2) it is factory I guess - motorcraft 2150.
3) The idle adjustent screws were adjusted by a professional mechanic with 30+ years experience.
4) Can you elaboroate "NB the idle circuit only works at low speed, so you need the engine speed to be well within idle range, like 600 RPM."
5) "charcoal canister check valve/ diaphragm -under the cap ... small leak" ..... can you help me locate this? The actual diaphragm has a leak or the hoses connected? is this the culprit?

[tgreese]

Just that. The idle circuit has no role in fuel delivery above idle. Fuel control transfers to the high speed circuit at something like 700-800 RPM. The idle mixture screws are part of the idle circuit. There is more about this in the TSM.

You should still close off the idle jets as advised above. The engine should stall when you either close the idle mixture screws or the idle speed stop (throttle flap fully closed). If the idle mixture screws stall the engine, and the idle speed stop does not, you have a vacuum leak, or something else is holding the throttle open.

[will e]

When you close the idle screws just count how many turns each side took, that way it's easy to put them back to where your guy had set them.

It does sound like it could be vacuum related, as you mentioned in your earliest post but I don't understand why 'forcing' the idle back would work if it was a vacuum issue.

Does the truck have an anti diesel solenoid? You might take a look at its operation.

If you turn the truck off while it is idling high and then turn it right back on, does it still idle high or does it go back down to where you expect?

[marc]

You can also put a vacuum pump/gauge on different legs of the vacuum system to see if they hold vacuum. Mine were in the choke pull off and air cleaner vacuum motor as well as one cracked hose. Plus,can't too much pressure on the choke spring cause something like this t least in the BBD?
Marc

[marc]

You can also put a vacuum pump/gauge on different legs of the vacuum system to see if they hold vacuum. My leaks were in the choke pull off and air cleaner vacuum motor as well as one cracked hose. Plus,can't too much pressure on the choke spring cause something like this t least in the BBD?
Marc