Hey all,

I just got done replacing the ISV and all the broken/busted vaccum hoses under my intake this week (nice little holiday project). It definatly made my idle smoother (I also replaced the DME temp sensor, resealed my injectors and replaced some other seals on the oil seperator, etc.).

So the car idle's well when it's sitting no A/C, etc. at around 850'ish. When I kick in the A/C is drops 75-100 RPM and struggles. I've tried the defeat ISV procedures over at Clark's, but that doesn't seem to make much difference frankly.

Tonight I took the plug off the ISV and started the car. From the middle pin to the right pin I get -13V, and from the middle pin to the left pin I get open (i.e. nothing). I can't find instructions on what this should be (even in the Porsche manual), and how to test the ISV directly (though being new it SHOULD be good).

I'm wondering if the driver in my DME has fried? I've seen posts to that effect before, but can't find them now!

Thanks!

 

The ISV is easy to check, at least electrically... disconnect it and use a 9v transistor battery and touch the center and first one and then the other of the outer electrical termimals. You should hear a nice little click as the internal cylinder rotates against it's stop. Don't use 12 from your car battery. I just did this last week. ...Bruce

 

My car has the same problem.... One question though.

I have a spare ISV ..... should I be able to blow through it when the ignition is off (or, off the car, as mine is)? Or, should it be in the closed position, so I can't blow through it?

 

Well, I'm not sure if the DME sends some sort of close signal when you shut-off the ignition but it seems to me the ISV will be sitting the last position it was commanded, which may be full open full closed or somewhere in between. I don't see how blowing through it will tell you very much, the pressure test is really more about knowing if the valve is leaking (especially under boost). You can look into one of the hose connections and see the window open and close (using the battery to command it).

Fireboy, I wonder if you have an open wire...Bruce

 

Possible. I haven't had a chance to check it this morning yet.

I do know when I started the car this monring from cold it idled like crap until it warmed up. Runs fine after warm though.

Might have been the idle screw being out too far though since I was messing with it yesterday. Unplugging the O2 sensor smoothed the warm idle too (I'm pretty sure the O2 sensor is toast, so probably best to leave it unplugged anyhow for the moment).

Waiting for the car to cool down again and will try it again. Hope to check the continuity later today. I found the pinout trace for the ISV to the DME in reading the schmatics in the shop manual. Should be pin 33 and 34 on the DME plug if anybody else is looking.

 

Well, doesn't seem to be the wiring harness, so that's a good first start. I took out the DME and checked each connector for all the main sensors I touched or replaced (i.e. AFM, throttle switch, idle valve, fuel injectors, etc.). I didn't do the power test to the AFM yet, just the temp sensor one. I also opened the DME to look for any obvious damage, of which I saw none other than a good amount of flux in some places.

It seems as if the enginer is 'starved' for air when it first idles. I say this because backing off the throttle screw brings the idle up, but it also makes it idle at 1100 or so when hot.

So I'll try the ISV with a 9v battery tomorrow and see what happens. Still not sure how to check the output from the DME to see what it should be or how to deterrmine if the driver part for the ISV in the DME is busted.

 

Is the ISV a new part? You should not hear any air coming out of it when capping one end with your thumb and blowing into the other.

 

yes, it's a 'new' rebuilt part. It passed that check before I put it in anyhow. Now it's under the manifold of course with no easy access other than to the plug on the top.

 

My car starting having the same issue today. Horrible stumble on start-up, but it is intermittent from one start to another. Ordered a new ISV, but also wonder if it could be DME related.

Now reading some of these threads I'm wondering if it is FPR related or AFM. Stumbles like a bad air leak, but I can't find a lead anywhere.

 

It's hard to say. I finally seem to have mine sorted now, after a day and a half of surging idle between 1200 and 1800 RPM. We'll see.

Here is what I learned from all this, for posterity (and since some of it's not written down) with thanks seperately to FRWilk and George Beuselinck of 944ecology.com.

1. Like many things on the 944, the ISV is PULSE driven, so other than the basic 9v battery test Bruce noted above, you really need a good oscilliscope and some kind of break out cable to watch it. Thanks here to FRWilk.

2. Be methodical about what you check and how you check it. First rule out vacuum leaks, since they are the easiet to find usually. Suspect this before ALL else.

My car has 20 year old rubber in many places, and until recently leaked and sucked air like water through a sieve. When I took off the intake to replace hoses, almost every hose under it had some amount of wear, if not a open wound resulting in a leak. There are no less than 11 vacuum lines connected to various parts of the intake or immediately downrange of a line that is directly connected.

The fuel ratio is surprisingly sensitive to air, so even the small temp. valve on the water neck going to the heater core at the back of the motor (it actuates the emissions system when the motor is warm and pulls from the throttle body directly) which has 2 small vaccum lines can result in an equally surprising amount of vaccum leak and air.

3. Then check the DME related components using the Porsche shop manual procedures in the back of Volume 1 in the L-Jetronic section. Some of these you won't be able to do again with out an oscilliscope, but the basic ones are pretty easy on the TPS, AFM, temp sensors, and fuel injectors. Measure both at the device itself AND and again the DME plug. Don't assume your wiring harness is good nor your ground without checking. Likewise, measure cold and at temp, especially for things in the engine bay since many of them are temperature related.

The manual can be found online if you don't want to drop the $300+ bucks for it, though it's NOT in a searchable PDF so you'll have to study it a bit.

4. Unplug your O2 sensor and run the car for a bit and see if that clears it up somewhat. Without the O2 sensor, the car will default to the stored map of 'known good quantities' in the DME and while it won't be optimum, your car SHOULD run without it. If it doesn't, then it's probably NOT your O2 sensor.

I think of the O2 sensor as a 'state of tune' sensor. It lets the fuel injection adjust constantly over a much wider range than the stored values depending on the tune of your motor. If your running rich (i.e. not enough air), it will cut back on the fuel to keep the fuel/air ratio maximized. Same is true for the running lean (typical of air leaks, too much air, not enough fuel), so the O2 sensor helps the DME figure out it needs to enrich the mixture, or add more fuel.

Many states, like mine in South Carolina, don't have mandatory emissions checks, so I could run forever without a cat and O2 sensor. That doesn't mean you SHOULD however, in my opinion, since your wasting fuel and gunking up your cat and possible your plugs.

5. As has been stated before, the ISV should 'hum' with the ingition key turned to the on position (i.e. lights on at the dashboard, but the car not started).

 

mtccl: have you checked ALL the lines, even the ones under the intake? I found I had several holes in mine and the nipple the ISV goes into UNDER the manifold apparently comes loose a lot as well from what I understand. Mine practically feel out when I pulled the intake off. JBWeld fixed it right up!

From what I have been able to find out, the ISV is almost impossible to test for correct operation other than the simple 'mobility' test Bruce mentioned above. That test will show it's moving right, but you still need to do a pressure check across it too (it has two large seals that apparently are known for drying out and leaking). If you have it out of the car, I think you can do this by just plugging the out port with a finger and blowing in the other end. It SHOULD be air tight.

I've seen some people mention that the ISV should flutter and hum with the ignition key on. The new one I put in my car does NEITHER, and yet it still seems to work just fine. Especially noticable when I turn on the A/C, since you can feel it kick in and 'idle up' the car.

As for the DME signal to the ISV, this can't be measured without an oscilliscope as far as I can tell, since it's a differential signal like the speed and reference sensors.

The AFM can be rulled out with the test in the shop manual as far as I can tell. One is a static test with the plug off for the temp sensor, and the other is an active voltage check with the ingnition turned on and the plug connected to check the potentiometer. The shop manual claims that the door will only effect lean or rich, but I'd think if it was busted it would be across all temps, not just start up.

I'd also check the DME temp sensor, both at the block and then later at the DME if you want to be certain. It's relatively cheap and easy to replace. Especially easy if you have the intake off to replace the ISV.

 

I was going to suggest vacuum leaks as you later seem to have found, but the idle being too low kept throwing me off.

In your last report you mention the idle surging between 1.200 and 1.800 rpm, which is what I've experienced when both the ISV itself sprung a leak, and when a vacuum check valve under the intake manifold broke as well.

Glad you sorted it out!

 

The ISV should vibrate with the key on. Although you can't HEAR the hum, you should be able to feel it if you put a finger on it, key on, engine off.

 

Switched my DME relay (with my spare in the glovebox) and problem went away...for now.

I have a new (used) ISV on the way and will check all the hoses next time the intake is off.

Mine acted like it had forgotten how to run.

 

Cool. I'll have to check it.

The car is generally running, though still not perfect. It's done the idle thing too me a couple of more times, but the last time I turned the car off and back on and it was fine.

The idle still is a bit erratic in returning to a consistent 900 or so. Today it ran fine but wouldn't idle below 1000 RPM regardless.

Two things I'm working on right now that might be related, but hard to say.

The first is the spark plug wires, which I found out day before yesterday are shot. They are arching across themselves and occasionly to the block, but the car isn't missing per say. Ordered new Magnecor wires from Pelican that should be here end of the week.

The other problem I noticed is when I was testing all the DME connections I'm only getting 5V to the AFM vice the quoted 8V in the service manual. That COULD mean that I have a weakening DME, which would explain alot.

Guy around the corner that runs a Porsche shop doesn't have the exact one, but has a 924 DME that I could pay them to test out. Would certainly be cheaper than buying a DME outright, even from a salvager (though I don't tend to trust electronics from parts places, since they don't tend to check them out, even if they claim they do).

Can anybody tell me where the ground connection is on the firewall that's listed in the manual and guides? I know where both of the ones are on the block, can find the two forward ones behind the headlights, but can't seem to locate the one on the firewall, unless I'm looking at the wrong thing.