Fast Idle Issue

An interesting new problem has developed in my 1987 328GTS. For the past several weeks(after warmup) the idle speed will suddenly jump to about 1400 rpms and remain there until I shut the engine off. When I restart it, the idle speed goes back to the 900 +_ which is the normal speed. After about another very few miles the idle jumps back to 1400 and remains there until I shut it off again. The idle adjustment screw can be screwed all the way in but has no effect on the idle speed when fast idle is happening. However, if I back it out, the idle speed increases. The sequence of fast idle, restarting, normal idle, then fast idle again is repeatable.

This morning while driving I was slowly accelerating and the engine suddenly completely died for a split second. Then it came back on and car ran normally again. I immediately checked the idle speed by depressing the clutch and it was in the 900 range as it should be. But it soon returned to the 1400 range about 1/2 mile later. Upon getting home I shut it off and restarted it. Idle went to 900 but after a short time it jumped back to 1400.

Seems to me in view of the fact it is so sudden, I have an electrical issue. Anyone have suggestions where I should begin my search.

I have a master on/off switch on the ground side of the battery that is over 10 years old. The fast idle problem began before the sudden loss of power incidents which have now happened twice.

To date I have cleaned the plug in connection on the warm up regulator. That had no effect.

 

Yes, that was the first thing I did - obvious things first. Throttle is closing against its stop.

 

I suspected that too so I pinched the line (which is original and quite hard) with a pair of rounded edge hose crimping locking pliers. I failed to mention this in my original posting. It had no effect but I'm not sure I'm pinching it hard enough. I'll remove it and plug it with a cork or something to ensure no air is passing through it.

 

OK, I'll loosen the locking screws and rotate it once it begins to idle too fast. I'll get on it tomorrow. Thanks for suggestion. Thanks everyone who has made a suggestion to date.

 

Here is what I've checked this morning. I cleaned the following connectors: warm up regulator, frequency valve, fuel distributor, throttle body switch. I have not gotten to the one on the AAV. However, I removed the inlet air hose that goes from the fuel distributor (uncontrolled air source) to the AAV and plugged it. I also plugged the nipple on the fuel distributor where it attaches - engine won't run without it plugged. Problem still existed. Engine started right up, ran for a minute or so at 900 rpm then suddenly jumped to 1500 rpm. The AAV rarely shut off the air flow because when I stuck my finger in it with the engine running I felt the vacuum coming and going. However, it had no effect on the high idle speed. This convinces me the AAV is not the issue because I competely stopped its air entry with no effect on idle speed.

The next thing I'll do is pull off the air line that goes from the idle adjustment bolt to the tee that is also fed by the AAV and plug it and see what happens. I squeezed the line with a hose friendly pair of vice grips but I cannot be sure I totally stopped the air. Removing it and plugging it will convince me.

I'll report findings once I get this done.

 

The idle adjustment flange/metering screw was removed. The exit holes at the fuel distributor elbow section for both metered and unmetered air were plugged. The line to the AAV was plugged and the idle adjustment bolt/flange was left loose. Engine was started and ran at 900 rpms. The idle adjustment screw would change the rpm (mostly only to increase rpms) but even with my finger over the flanged end where the air enters, the engine still ran about 900 rpms. After a few seconds the idle suddenly (and I mean suddenly just as before) jumped to around 2000 rpms. Completely plugging the idle screw hole at the flange had no effect.

The only line not checked is the one downstream of the tee and upstream of the aluminum pipe at the intake plenum.

 

TPS makes a click when throttle returns to idle. The click changes the resistance on the pins so TPS is working when it goes to idle.

 

At the risk of sounding silly here, I do not see any electrical connection on the AAV. There is what looks to be a heat sensor that screws into the reservoir. Could it be that the AAV relies on heat conduction rather than electrical heating? Pictures posted.
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Metering plate is not sticking. It moves up and down when engine if off and has been sitting for quite some time. If I push it down a little when engine is idling properly, engine wants to stall. It appears to be in same place at high idle.

Are you saying some AAV's do not have electrical connections?

 

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I'll check for it when the thing cools down. I cleaned the contacts at the temperature sensor in the second picture above. After doing that I started it up for the first time today. The first start cold idle was higher than before - about 1500 rpms - like it used to be quite a few months ago. (Prior to cleaning the prongs and slip over wire parts the idle would be only about 1100 when cold.) After a minute or so the idle fell off to 900 -1000 rpms as it should. I let it sit there idling until the electric fans came on. The idle never suddenly jumped to the 1400 range as before. It behaved. Then I drove the car for about 10 miles with the problem not returning. Perhaps it was a bad connection at that brass plug or an intermittant connection that confused other things. My plug is a switch that is closed when the water temperature is cold and opens when hot. It seems to me, however, that once the switch opens, it shouldn't matter if the connection is good or not. So I wonder if the problem has been fixed.

I think there is a vacuum leak or the AAV is not fully closing because I cannot get the idle below 1000 rpms when the engine is fully warm by turning the brass plug on the intake elbow. When it cools I'll unbolt the reservoir and remove the AAV and find the electrical connection that I cannot seem to find now. Hopefully the AAV internals can be cleaned as has been suggested in posts I found on a search on FChat.

Another test I think I'll do is open the idle air line right at the top of the right side of the intake plenum and plug the rubber elbow and the entrance to the plenum that the elbow slips over. That takes the AAV, and crankcase ventilation system out of the picture. That along with pinching off two small vacuum lines and brake system vacuum line should prohibit the engine from idling if there are no leaks.

 

Reservoir cooled down and I removed the AAV. As you said it has electrical connections. When I removed it from a tepid reservoir, the opening looked like 1/3 moon. I attached it to a car battery and after a couple of minutes it fully closed. It's in the freezer at the moment to see how far it opens.

 

Well I'm back to square one. I went for a ride this morning and everything was fine. After reading your post I checked the screw on the throttle and it is closing all the way. I can back off the screw to where there is space between it and the throttle linkage. That had no effect on idle speed of 1000.

After doing this I let it keep idling. Suddenly it jumped back to 1400 rpms. I shut it off and restarted but the idle went back to 1400. I think this thing is possessed. Perhaps I need an exorcist.