Dougs951S

So guys the 944 runs beautifully! I really enjoyed driving it around town today with the radio playing and the roof off. The only problem is that the car idles at ~1275 rpm when fully warmed up. No vacuum leaks that I can detect; I've deleted everything non essential so my vacuum system is extremely simple. I've deleted the emissions equipment, the AC, and all the HVAC vacuum controls so all I have running on vacuum is the fuel pressure regulator and the brake booster, plus the AOS and ISV. Everything else is capped off and no amount of me spraying starter fluid all over the engine bay can detect any vacuum leaks. The car pulls 17" HG at 1250 rpm idle when fully warmed up, which is a lot lower than I'd expect a new engine with good compression numbers and a 4* advance cam key to pull (the compression and leakdown numbers are great by the way!).

I sealed up the ISV with RTV; it wasn't leaking to my knowledge; it was more of a precaution. The AOS has new O rings and the intake gaskets are new. The idle screw is currently turned all the in (closed) on the throttle body, which also has all new seals. The throttle stop is set correctly; the TPS idle contact clicks when the throttle is closed; the cable is adjusted correctly (and the cruise control is deleted) and I've held the throttle closed with my hand so ensure it isn't getting hung up on the new seals.

What I noticed though, is that the ISV is not working. The car runs exactly the same with the ISV unplugged, zero change in idle and it is a known good unit that was working on the car previous to the engine swap. In fact, the only electrical component that is new to the car is the TPS since I used a different throttle body than was previously on the car.

Now, I know that the car will only engage the ISV when the DME detects that the idle contact on the TPS is closed. Additionally, the car has a somewhat choppy idle when it is first fired up cold. All of the above has lead me to deduce that despite audibly clicking, the idle contact is not working on my TPS.

Correct me if I'm wrong, but I believe the idle map is somewhat more rich than the the "cruise" map would be at say 1% throttle, which is where the car seems to think I'm at when the throttle blade is closed. This leanness I believe is the cause of the car's rough cold idle. Furthermore, the timing and fueling is off because the car still thinks it's cruising, and I believe this is causing my high idle. Dd you folks agree? The only other possibility is a vacuum leak I somehow can't find.

Sorry for the book guys, just thought this might be a useful post for the archives.

odonnell

Doug was that the TPS that was on the rebuilt throttle body I gave you? If so that worked when it came off my car. You can test it by looking for continuity between the ground pin, and the idle pin, when the TB is closed and it clicks. Same with WOT switch. There are only 3 pins so you should be able to work out which ones are which. IIRC it lists the corresponding DME pins on the male connector, so you should be able to look at a DME pin diagram to see which pin goes to what.

Dougs951S

yeah it was Michael. Thing is, my isv isnt working either and that is controlled by the idle contact. I csnt think of what else would be causing me to idle at 300 rpm faster than I want.

thomasmryan

Is your FQS at 0 fuel and 0 timing?

DME temp sensor functional?

V2Rocket

If there were any leak it'd have to be very small.
Leaving one of the 10mm under-manifold hoses wouldn't even let the car start cold, in my experience.

Filler cap O-ring good? Sounds like you're getting a little extra air from somewhere...

Dougs951S

So you are saying a bad idle microswitch will not cause the symptoms I am describing? Yes the filler cap and dipstick o rings are new. The car got a top to bottom rebuild, no O ring was left untouched. I'm baffled where it could be leaking vacuum from.

EDIT: Checked the oil filler cap and sure enough, the new seal was sitting cockeyed, no way it was properly sealing. I fixed it and the cold idle is definitely smoother. Perfect actually. I dont know if that fixed the issue, I didn't let it warm up. Something else odd I noticed: The car started up and idled at 950 rpm, which is exactly where I want it. However, it did this with the ISV unplugged. The ISV is a PWM type motor and its position will remain fixed at the location it was in when power was removed. I'm wondering if the ISV is "stuck" open? Why then, isnt it operating when it used to work fine? The weird thing was, when I hit the brakes the idle rose ~75-100 rpm and stayed there steady till I shut the car off after maybe 90 seconds of idling. Does this indicate a failing booster?

V2Rocket

What about the o-ring inside the venturi "Y" pipe to the booster?

Maybe the wiring to the ISV is messed up?

Tom M'Guinn

You can take the ISV off the car and bypass or plug the hoses. Then, with the ISV in your hand, plug it in and start the car. You can then look inside and see if it is doing anything or stuck in one position. Try reving the motor, turning the ac on and off and turning the idle screw way in and way out -- you should see the valve inside trying to compensate. If you have a known good spare, you can start by plugging your car's connector into it and looking inside to see if the DME is controlling it.

The middle pin of the ISV should have 12 volts when the ignition is on, so that's easy to test.

I'd plug the brake boost vacuum lines for testing (in the garage, not driving) to see if maybe it is leaking inside. 17" is quite low for 1250 rpms. I wouldn't suspect it at a stock 840 idle, but at 1250 with a closed throttle that's pretty low. So I'd keep hunting for leaks...

Dougs951S

Can someone help me crystalize my understanding of our ISV's? I'm still chasing my high idle, and determined that my TPS is 100% fully functional, including the idle microswitch. I plugged in a spare ISV into the harness while the key was on and noticed that the valve was not operational. This lead to me testing for power at the plug, which is where I need help. I know the valve is a PWM valve; it always has a pin which is hot at +12v with key on, and the ground is pulsed to it by the DME. I'm not sure what the third pin is for. I tested for 12v between all three pins, and not only is there no voltage but it appears I have no continuity between any 2 pins with the key on. I was at one point familiar with the related circuitry and posted about it some time back here >>> https://rennlist.com/forums/944-turb...n-the-dme.html

It's been some time now though, and I could use some help from the electrical guys. My test seems to indicate to me damaged ISV drivers, but I'm not positive and want some feedback before I tear into a DME that in all other respects, seems to work fine.

Edit: sorry for the above post, I meant to post this as a separate thread but I was not paying attention.

Tom M'Guinn

The ISV has three pins in the connector. The middle pin (pin 2) should have 12 volts. The DME pulses pin 1 to ground to move the valve in one direction, and pulses pin 3 to ground to move it in the opposite direction (i.e., one pin to open the valve, one pin to close it.) You can test an ISV with two alligator clips and the car's battery. Connect the middle pin to the positive terminal of the battery and then tap one of the outer pins to ground and the valve to snap to the open or closed position. Then tap the other outer pin to ground and the valve should snap the other way. (Don't let the ground hit the hot #2 pin of course!)

You should be able to confirm or rule out the ISV as the source of your high idle easily though -- just remove the hose going to the i/c pipe from the ISV and block off both both the hose and the pipe (or put duct tape on the i/C port to block it, and put the hose back on. If you physically block off the air path from the ISV and still have the high idle issue, then air is getting in somewhere else. If blocking it off solves the high idle, then it's either the ISV itself (which you can test per above) or the driver in the DME...

Dougs951S

Tom, you're on the right path. Isolating the air path is my final test to confirm the high idle is being caused by excessive air bypassing the throttle, but I have also determined that I'm not getting 12 volts at the ISV plug. Also, FWIW this is concerning my NA. See my other thread for more info.

Tom M'Guinn

Ah, N/A, ok. All the above still applies, other than references to I/C pipes. Just run those easy low-tech tests and you can confirm it's the ISV or move on to other potential causes.

Dougs951S

Its definitely the ISV but the valve is confirmed good on the bench. I'm not getting power at the plug. I know the drivers in the DME pulse the ground; so if no power is present does that pretty much mean its a harness issue? Where does the power at the center pin on the ISV plug come from?

Tom M'Guinn

Power on the center pin starts from pin 87 in the DME relay, runs through pin 2 in the 14-pin junction box on the firewall, then goes into the harness where it is crimped with 5 other wires, 4 of which supply power to the injectors and 1 or which supplies power to the center pin of the ISV connector. Since your car is running, it's safe to say your injectors are getting power, but there is no continuity from the power pins on the injectors and the power pin to the ISV. If so, it's highly likely that the power wire to the ISV is frayed under the rubber boot right were the wire goes into the hard plastic of the connector. Peel the rubber boot back and check.

Dougs951S

Wait....14 pin junction box...you mean the one the PO hacked out?! Stand by for pictures. I forsee wire tracing in my future to rebuild snipped circuits.. I had to rebuild the fuel injector harness to get the car running when I first got it.