RPM drops too low then returns to normal when comeing to a stop. Vacume leak??
 

I have a few small things I need to check into on my 86.

1. About 50% of the time when I put in the clutch (like when comeing to a stop) the RPM's will drop very low before retuning to it's 800rpm idle. Most of the time it returns to it's normal idle just fine, but sometimes it stays low enough that if I didn't blip the throtle I think it would die. Sometimes when it does this the oil pressure drops along with it to the point the warning light with flash on for a moment.
I would guess that this is being caused by a vacume leak, but I didn't find any leaks after a visual inspection and at idle the stock boost gauge is all the way to the left.(I think anything below 1 on the stock guage is vacume.
As I said this dosn't happen all of the time so it's not that big of a deal, but any help on this would be great!

I don't know if this makes any difference, but the car does have an Autothority MAF kit on it. I think it was doing this before and after the install, but I can't realy remember since I only drove the car once for less than an hour befor I put the MAF on it.

Could be a vac leak... Have you replaced the vac lines and the hoses under the intake?

Also try to raise your idle with the idle speed screw. If the idle is set too low, the ISV will pull it up to compensate. When you have a drastic change in engine rpm like when you let off the gas and let it idle, the ISV is not fast enough to compensate, thus the fall.

Does it hunt at all at idle, or just dip and then it stay where it is?

Check very closely the MAF to turbo inlet boot. Make sure you don't have a leak there, say under the bottom which is hard to see. ASk me how I know. Also if you are running a bit rich at idle this can cause the idle to drop. Do you have an adjustable fuel pressure regulator with the fuel pressure turned up to compensate for higher boost? A vacumn leak somewhere is the most likely cause.

It does hunt a bit at idle most of the time, but not all of the time. I have not replaced any vacume lines yet since I have only had this one for a couple weeks. I'll try the idle adjustment screw. Thanks, any other ideas?

well it seems that adjusting the idle up a bit helped. It still hunts around a bit, but it no longer drops too low. thanks.

Does that happen when the AC is turned on?

Definitely do vac lines... Also, check the TPS. May have to replace it.

"About 50% of the time when I put in the clutch (like when comeing to a stop) the RPM's will drop very low before retuning to it's 800rpm idle. "

Is there a difference in this behavior between when the car's cold vs. warmed up? The APE-MAF doesn't have an air-temp sensor and the DME needs that for cold-start enrichment.

"well it seems that adjusting the idle up a bit helped. It still hunts around a bit, but it no longer drops too low. "

That is the correct procedure to fix it; opening up the idle-adjuster screw with the engine running. But that shouldn't affect idle-speed because the idle-stabilizer will close up a little to compensate. Actually, you want to open up the idle-screw up to the point where the idle-stabilizer is fully closed when the car's warmed up and 100% of the idle-air comes from the idle-screw bypass.

The symptoms you had earlier was because the idle-screw as giving say... 75% of the idle air and 25% came from the idle-stablizer. What happens when you let off the throttle is that air is suddenly cut off and you only have 75% of what is necessary for normal idle. So idle-speed drops below 800rpm. Then it takes a fraction of a second for the stablizer to open up and restore idle-speed to normal.

So, by opening up the idle-screw (and closing down the ISV), you've got 100% of idle-air needed right as the the throttle is closed. Ideally, you only want the ISV in action for cold-starts and when the AC is running.

What of the a/c is running, and when letting off the throttle, and coming to an idle, it drops to under 800rpm and jumps back to 840+-40 rpm? is that a bad ISV? i have that same thing going on with almost ALL 944's in my area, so i guess that is normal right?

Check that the tps makes a clicking sound right off of the seated position. That clicking sound is a contact witch sends power to the isv to compensate for air after the tb closes. The tps has a adjustment screw to set the correct position so the sound can be heard on the return swing of the tb.

For that, you just loosen the mounting screws and rotate the tps and keep checking once and while until you get the click sound?

Yes the clicking sound should be heard on the return swing of the tb. Right before the tb closes fully. If you don't hear it after you try to adjust it than the tps will need replacement.

well, it worked before I swapped turbos so i assumed it will still work... if not, what do they go for new?