mark kibort

I ran the car over to CT automotive late in the day to get it on the rack to do a "nut and bolt" before next weekend's race, and reved the engine as i pulled in to the shop. Suddenly, when the RPMs cam down, they only came down to 2000rpm. I blip the throttle, and its still hanging up at 2000rpm. then, it starts hunting between 500 and 2000rpms until I pull the idle switch connection. then it stablizes at 2000rpm. Im puzzled. what could cause this?

after about 30min on the rack, i start it up and its fine. however, i rev the idle (blip the throttle) and suddely its back at 1800rpm again. 5 min later, i start it , its reving 1800rpm and i drive away as soon as i got to .5mile down the short road as im driving kind of hard to the next light, the idle comes down and its fine . I blip for down shifts, I go near WOT a couple of times, and alll is perfectly fine, as it has been all week. Does anyone know what could cause this? increased fuel pressure in the lines, sticking injectors, ecu???
Im at a loss.

mk

AO

Possible causes:
MAF is not fully seated
Tear in the rubber elbow
Massive vacuum leak (Pretty much same as above)
Sticky throttle linkage
Sticky ISV

mark kibort

I checked the MAF, it seemed to be seated
it sure sounds like it could be a false air leak. I opened up a vaccum line to see if that increased the engine speed and it didnt, so the leak has got to be pretty big.
throttle linkage is fine, as even the microswitch is engaging meaning that the throttle is closing
The sticky ISV valve sounds intereting, but why would it get engaged or effected by reving the engine out of gear.
Why would the problem almost entirely go away after driving normally under load a bit?

??????????

kind of a strange


mk

aggravation

It's that prototype driver side head expanding and pulling on the throttle linkage..

mark kibort

That would be head "contracting"

mk

hacker-pschorr

Random idle sounds like an ISV since usually a leak is consistant (I said usually).

I've been down the sticky / dirty / broken ISV on three of my cars. My Audi would go from 2,000 rpm idle to not running, then perfect for a day or two.

mark kibort

of course it now has to screw up now that its buried in the intake! AHHHHHH!

Is there anything i can spray on it , in it, or shake it with that could help?

what does it do exactly? meter air for controlling the idle oscillations?

if it stuck, then why was the engine hunting over the high rpm range of 2000rpm to near 500rpm until i pulled the plug on the idle switch. doesnt the ISV get disabled with the idle switch (ie past part throttle)?

If its leaking, like a false air leak, thats bad and that intake might have to get pulled to fix. If its leaking, like an internal issue, could cycling it some how fix it, or at least get it to stick closed? this might be a good option if the isv is disabled when the car is anythig but idle.

thanks for the ideas

mk

Mako 928

What about intake manifold gaskets. Did'nt you use the old ones? Could be possible.

mark kibort

old ones and they were torqued to spec. they looked brand new so we didnt change them. why would the problem go away with drving on the road with some load. why would the problem show itself, only when i over rev the car.
HHMMMMMMMM. Im thinking out loud, maybe the super high vacuum of a rapid decel of the engine rpms, works a "gate" open on the ISV or something else that could have the power to remain closed in other circumstances.

it only happened when i reved the engine pretty high in idle. on the freeway, this doesnt seem to be an issue as maybe the much lower vacuum of part or near full throttle resets the seal.

thoughts?
mk

Mako 928

Well you could try the WD-40 trick for the ISV. I believe there is a hose that routes to the ISV towards the front of the manifold. ( I am more familiar W/ 85'86') Spray into the hose and see if it makes a difference. Seems to be a temp fix but at least it would identify the problem.

aggravation

The part that actually opens in the ISV doesn't look like it would be moved by pressure of any kind.

mark kibort

Thats a good idea.

Im thinking it might be sometype of spring loaded valve or door that could be sucked open at extreme manifold vacuum. (ie rapid decel or high rpm decel)

Ill do some tests on the freeway to see if can do a decel and then put the clutch in and see where the idle ends up.

this could be one of those chasing your tail type of hunts!

I hope it is not a air leak, like those injectors that dont seem to fit that tight. I did push on all of them while it was idling high for a few minutes and it didnt seem to do anything.

mk

AO

Like you said... it has to be a pretty big air leak. I'm guessing the ISV is sticking open. Try the WD-40 trick. Might work... might not.

mark kibort

Thanks. Now, if i can only find that hose that feeds it.

mk

Bill Ball

Pull the rear hose off the large Y that comes off the driver side of the manfold and passes by the throttle linkage. That rear hose snakes down under the fuel rail to the ISV. Squirt some WD-40 or PB-Blaster into that hose for about 10 seconds. Then start the car with the hose still detached, plugging the Y with something. That should suck the lube down into the ISV.