Idle surges 800-1100 only when hot

[TCMdocs944]

I got my engine back together and on the second road test I found the exact same aggravating problem as before!

Engine starts right up an idles at 1100(high but perfect)
Warms up and around 1/4 mark runs great
1/4 - 1/2 seems to have only slight stumble on accel. if you baby it

Ok, heres the problem...at this point with the engine around 1/2 heat on the guage,
the idle surges from 1100 down to 800 then back up to 1100.
This happens persistantly at one second intervals.
It does not effect drivability or happen when laying on the gas. Only at idle. Sometimes it will not happen with the ac on but if the heat goes up even more it starts. I have searched many times both before and after the rebuild and am still stumped .

Ps. When I remove the vacuum hose from the fuel pressure reg the surge is quicker, maybe twice per second.
Help.

[Porsche944]

Here is a test: Unplug the Throttle Position Switch and see if the idle still hunts.

If the hunting stops then I would suspect either the TPS is out of adjustment, the throttle plate mechanical stop is out of adjustment, or both.

Typically the throttle stop is the culprit, particularly if the engine had a low idle and a non-Porsche literate "mechanic" got his hands on it.

To check this out you need to pull the throttle body and loosen the TPS hold down screws. With the TPS loose, use an 8mm wrench to loosen the throttle stop jam nut, and then turn the stop screw out until it does not touch the throttle stop arm.

Clean the throttle bore, throttle plate, pivot points, throttle plate seat, idle by-pass port, and both vacuum ports with some carb cleaner, and then verify that the throttle plate will close completely. If you hold the throttle body up to a strong light you should be able to see a very small amount of light between the plate and the seat.

Once the throttle is clean and seating properly, turn the mechanical stop screw in until it just touches the stop arm, turn the screw an additional 1/2 turn, and then tighten the jam nut. This sets the throttle plate to its proper "fully closed" position.

Now you can adjust the TPS. To do this, hold the throttle plate closed and turn the TPS until you hear it click. Tighten ONE of the hold down screws. Test the adjustment by slowly opening the throttle and listen for the switch to click. You should hear the click just as the throttle plate comes off the seat. Verify that the mechanical stop screw is touching the stop arm. Now you can tighten the other hold down screw and the throttle body is ready to install.

The final step is to hook up a dwell tach, start the engine, and use the idle by-pass to set the idle at 950 +- 50 RPM.

What is causing the idle to hunt is the fact that when the throttle plate is closed, the engine is SUPPOSED to be at idle. If the DME sees a throttle closed signal from the TPS, but the RPM is above a certain value, the DME assumes the engine is in an over-run, or "trailing-throttle" condition, like when you are using the engine as a brake.

Since a trailing-throttle condition wastes fuel, the DME turns the injectors off until the RPM drops to a more "normal" value. If the mechanical stop or TPS are holding the throttle open too far, the idle will hunt.

[TCMdocs944]

Cliff - Thank you soooo much for your time in that explaination. I will do the test and adjustment per your instructions.

Just to make sure...
Do you think this happens only when hot due to expansion of the throttle body and not the plate, causing a "leak" of sorts?



Another note- when I blow into the vacu fitting on the throttle body closest to the firewall I get only light resistance.

[Porsche944]

Hey no problem!

I have my doubts about a heat related vacuum leak, but it is still a possibility.

A high idle when cold tells me that someone "tweaked" something to get the engine to idle smoothly, which usually indicates that something was interfering with cold idle engichment. Under those conditions I would expect the hot idle and cold idle to be different, and in this case the difference may be just enough to put the engine in an over run condition, causing the idle to hunt.

While you have the throttle body out make sure you clean both vacuum ports. The front port controls the evaporative emissions purge valves, and the rear port supplies vacuum to the FPR, fuel damper and one of the emissions purge valves.

When the engine is at idle the front port will have 0" of vacuum, and the rear port should have between 14" and 18" of vacuum. The only time the front port has any vacuum (about 5" on both of my cars) is when the throttle plate is just starting to open. This momentary vacuum is enough to operate the purge valves and purge the evaporative system.

Since the rear port supplies vacuum to the FPR, a low vacuum will cause a higher than normal fuel pressure which can contirbute to idle and fuel mileage problems.

[ae1969]

That is a great write up Cliff.

I am having an idling issue as well.....and this has been a very informative post on the TPS functions.

[TCMdocs944]

After studying P944's posts, and studying them again..I also referenced

calibrating body throttle and air flow ?

I did this:
1. In my rebuild I had already thoroughly cleaned the throttle body.
2. First I carefully turned the throttle cam by hand, heard a click in the first ten degree's.
- Didn't completely satisfy me but good enough for initial test.
3. Ran the temp up to 1/3
4. Adjusted the idle screw down to 900 approx 4 turns clockwise



5. Road tested extensively and BADA-BING ..

BELTS BROKE!







Just Kidding THE PROBLEM IS SOLVED

I have a bit more fine tuning to do but it now warm idles at around 900 and cold about 1000, no surge! Goodnuff.

Much thanks.

[TCMdocs944]

BTW - I think the "mechanic" was probably covering up either a vacuum leak (i found many) or the Engine temp Sensor that literally crumbled in my hands during the rebuild.