Intermittent Idle Hunting

mickey356 · 12-03-2009, 01:54 PM

I ever since I had the co adjusted to pass inspection (5 months ago) I've had a tiny bit of idle hunting when I start the car. It starts and goes right to idle at 950 and after 15 secs it'll go up and down from 950 to 1150 for about 5 secs and then back to 950. I never thought it was a big deal.
The night before last I left work and all of a sudden a huge surge from 1800 down to the bottom almost stalling, gen light lighting up, etc... and the warmer the engine got it didn't help. The next morning same thing so I swing by my buddies garage and we hook up the smoker to check for any obvious leaks, none. I start it up and the surging/hunting had stopped. All day yesterday with no problems. Today it started hunting again but not as bad as the other day but enough (400-1200 and back) then it would level out at 950.
I did the forum search but don't know where to start. Since there wasn't any smoke can I rule out vacuum leaks? Is it something as simple as a mixture adjustment? If so, how can the mixture get so far out of whack without making adjustments? What things should I be asking for when I take this in to find the problem?
Thanks

(FYI - 83 SC w/3.0 with cat bypass and no o2 sensor hooked up. )

ths911 · ths911's Garage

search - ICV (idle control valve). you will find more than you need. just lived this myself.

good luck.

Zef · Zef's Garage

Dirty ICV...Remove, clean, install, enjoy.

kidrock · 12-03-2009, 02:33 PM

In a nutshell, yes. A rich mixture will definitely cause idle hunting. When you had the co adjusted to pass inspection, it is likely that you're Air to Fuel ratio is not correct. Also, a vacuum leak will cause the same dilemma. I'm not sure what you mean by the absence of "smoke", but in any case, the presence or absence of smoke is not an accurate gauge of whether you have a vacuum leak.

do a search on "idle bounce", it's easy to find. The thread is about 6 pages long, but it is very informative. Somewhere on the fourth page I believe, is where the air/fuel mixture is discussed at length, and is relatively simple to adjust.

I'm mostly referring to Carreras with the DME, though...CIS cars with vacuum leaks are a bit notorious in this department, IIRC.

Good luck, and keep us posted.

rusnak · 12-03-2009, 03:07 PM

Use a strong light and a large inspection mirror to check the vacuum hoses. All of them. Look especially at the small hose from the throttle body to the ambient air valve, the case breather hoses, cruise control hoses, fuel pressure regulator hoses, etc. A smoke machine may not necessarily replace a good visual check. Also try starting fluid on the intake gaskets.

It sounds like a vacuum leak to me.

mickey356 · 12-03-2009, 03:56 PM

I will keep you posted.
I just had to shoot home from work and go right back and had no problems so it's definitely intermittent.
I've always, well since the mixture adjustment, sensed it was too rich. When I parked at work I rev'd it a bit and it practically smelled like raw gas. I'd say it's set too rich if my nose has anything to say about it.
I'll be reading tonite and I'll have a few hours tomorrow to look around.
Thanks again for the help.
I guess me "search skills" need some work. I never seem to type the right search words.

rusnak · 12-03-2009, 04:02 PM

A vacuum leak to the fuel pressure regulator may make it run rich, but not if the O2 sensor is healthy. You could have a bad or old O2 sensor. Is it the factory one, or the unversal 3-wire? Vacuum leaks are usually numerous and small. When they cumulatively equal a large amount, that is when you get idle hunting.

mickey356 · 12-03-2009, 04:45 PM

OK, I'm a little confused (not hard to do). Is there an ICV on my 83 SC. I see from another thread that a guy with an 82 was looking for his and didn't have one.

rusnak,
The O2 sensor on my car is not connected. That said I have a new one and a relay and was planning on eventually hooking it up to my cat bypass since there is a bung for it.

Should I be searching for leaks before I put the O2 sensor in because that's the way the car has been running, or should I hook up the O2 and start looking for leaks with it hooked up?

rusnak · 12-03-2009, 04:57 PM

Ah...ok sorry.

Since you have an SC you can pretty much delete all the answers here so far.

Your CIS w lambda control is probably running in limp home mode. It is probably running very rich like you said. I'd hook up the O2 sensor, and get things back into spec.

ths911 · ths911's Garage

No - the SC doesn't have a ICV like the 3.2 Carrera however if you search ICV it will bring up many idle bounce links for your review.

Sorry, I should have spent a few seconds more explaining that in my original post.

GothingNC · 12-04-2009, 03:42 AM

I am having a similar situation with my 82 Targa

Most times it idles rock solid at 950 but when it is warm out (above 80 degrees) and while sitting at a light in traffic for a few minutes the idle will suddenly fluctuate from 850 to 1050 for a minute or two with a rich exhaust smell. If I give the gas pedal a quick tap it revs up and after releasing drops to about 600 RPM then bounces to about 1,100 before settling around 950 for about 10 seconds

After a few minutes (or if I turn off and restart the engine) the idle stablizes and it might not happen again for a few days or weeks.

I'm wondering if my 02 sensor relay under the seat is going bad or it is running slightly rich.

mickey356 · 12-04-2009, 09:19 AM

Stopped by my buddies garage today and we checked the co. It was way too rich. I don't know what caused it to start running so rich; maybe since I've been driving it at higher r's to make sure I get to operating temp I "knocked something loose"?

Anyway, after adjusting the mixture it seems much crisper. I'm still suspicious of vacuum leaks so I will reserve my excitment for a while until I know the hunting is gone.

kidrock · 12-04-2009, 10:43 AM

Great news, Mickey. When you get a spare 5-10 minutes, pick up a can of carb cleaner spray, and spray around all hose ends and intake areas briefly with the motor idling. If your motor tries to die when the carb cleaner hits a certain area, you've found your vacuum leak.

mickey356 · 12-05-2009, 03:26 PM

No Joy! My car still has the "urge to surge". All seemed fine yesterday after leaving my buddies garage but last night leaving work it start hunting again and then today pretty much all day, some times worse than others. Sooooo, I guess it wasn't just the mixture. The exhaust really really smells rich like gas. And it seems like I'm burning up a lot of gas, but I don't know if it's just me being hyper-aware now. I suppose I should be pulling a plug or two to see if they're gumming up, no?
Being that I'm an idiot when it comes to Lamda CIS is it safe to say that this is still likely due to vacuum leak or is there another likely culprit?
Thanks, Mickey

Didn't have any carb cleaner handy and didn't get a chance to check yet. Maybe I'll try tomorrow. Thanks Kid Rock.

Goth · 12-05-2009, 03:32 PM

Could it be a bad/loose connection to the temp sensor? Only guessing as I have a 3.2 (not 3.0).

Just trying to help...

rusnak · 12-05-2009, 03:35 PM

You need to re-connect the O2 sensor.

Goth · 12-05-2009, 04:12 PM

Quote:

Originally Posted by rusnak

you need to re-connect the o2 sensor.

+1

Bob Kontak · Bob Kontak's Garage

Have had similar problems on my 81. Though for sure it was my aux air valve. Read a lot of posts and one fellow mentioned to plug the decel valve hose. I plugged the hose with a fat nail and the rpm hunting went away. After I did this the RPMs actually would go up to 1500/1800 on cold start and die down to 950 after a minute or so and then stay there. It had not done this for years.

Additionally, to add context, in 1997, at a Euro car shop, a mechanic grabbed a big Channel Lock plier tool and squeezed the decel valve (to picture this - he made it skinnier - like crushing the can a few millimeters) - actually left marks in the space ship disc - and the rpms dropped instantly. I am thinking this was an indication that the decel valve was on it's way out back then and my recent issues may be a result of this.

I am in no way am certain this is your problem but it's a 30 second test with no risk of screwing things up - so worth a try.

Agree with above posters on the O2 sensor use vs. not.

T77911S · 12-07-2009, 04:31 AM

when the O2 is removed, does the frequency valve stay open or closed or does it go into a default cylce frequency? i would look into the FV and related circuit. perhaps the fuse for the relay is dirty and not making good contact or it is the relay itself. i think it is fuse 18.

high idle could be the decel valve. it is controlled by vacuum and i think it can be adjusted. i put a hand vacumm pump on mine to check where it opened.

if your car has vacuum retard, check that out, loss of vacuum would cause an rpm jump

Flat6pac · 12-07-2009, 05:22 AM

You cant do any adjustment to the injection with the lambda probe hooked up. Unplug the O2 sensor and adjust to what youre looking for then hook it back up. If you dont, the system under the seat compensates itself against all your adjustments.
Bruce