AFM or Temp Sensor??

Blondie · 03-29-2007, 09:49 AM

After the car sat for a night, I try to start her the next morning and it fires right up to 1000rpm and sounds great but wants to die unless I keep the throttle open.

If I let it die/turn it off, everytime I go to start the car after that I'll have to pump the throttle like mad to get it to fire and then hold it at WOT just have it barely stay alive at a chugging 500rpm.

Anyone ever experience something like this?

dan476 · 03-29-2007, 11:49 AM

how cold does it get at night?

ae1969 · 03-29-2007, 11:53 AM

check the dme temp sensor resistance at the dme harness connector.

Temp sensor could be shorted going full rich.

Blondie · 03-29-2007, 02:04 PM

I got 4.42 k-ohms at the DME harness.

according to clarks, resistance should be:

At 15 °C approximately 3.3 k-ohms
At 30 °C approximately 1.46 k-ohms

It's 9 °C outside today. I can watch the multimeter fluctuate between 4.42 and 4.41. So is the higher resistance a result of the colder temperature or a shotty sensor?

ae1969 · 03-29-2007, 07:27 PM

Quote:

Originally posted by Blondie
I got 4.42 k-ohms at the DME harness.

according to clarks, resistance should be:

At 15 °C approximately 3.3 k-ohms
At 30 °C approximately 1.46 k-ohms

It's 9 °C outside today. I can watch the multimeter fluctuate between 4.42 and 4.41. So is the higher resistance a result of the colder temperature or a shotty sensor?

The sensors are known to fail. As you can see the resistance is so high that it thinks it is way below 0. Too rich.

onZedge · 03-30-2007, 05:50 AM

4.4K ohms @ 9degC is about right. The resistance increases with decreasing temperature. Check the TPS.

CPorsche · 03-30-2007, 07:19 AM

Sounds like a vacuum leak. Mine was doing the same thing and one of the hoses under the intake manifold had an air leak.

ae1969 · 03-30-2007, 08:56 AM

Quote:

Originally posted by onZedge
4.4K ohms @ 9degC is about right. The resistance increases with decreasing temperature. .....

Correct ... MY bad!

I misread the original post that it was always at 4.4K.

TPS and Vacuum are definitely possibilities at this point.

Blondie · 03-30-2007, 10:51 AM

Just tested the TPS
Numbers checked out beautifully at both closed throttle and WOT.

I want to agree with vaccuum leaks but its the fact that it'll start no problem (still without idling on its own mind you,) after its sat for a night.

CPorsche · 03-30-2007, 12:06 PM

The reason a vacuum leak can cause the problem is when trying to idle there is not enough air flow through the AFM to keep the AFM door open. The air is being provided by the leak and does not go through the AFM. If you will pat the accelerator the sudden draw of air will cause the door to open but as soon as you let it go to idle the door will close and the car goes dead. When my car had the air leak it would crank but would not idle. I could keep patting the pedal and keep it running, the best I recall it ran rather rough because of an inappropriate air/fuel mixture. I'm sure there are other things that can cause similar problems but and air leak could be the culprit.

Blondie · 04-01-2007, 05:35 PM

Ok so I pulled the plugs and their all black as night. Two of them (cylinders 2 and 3, looking from the front,) were wet.

I laid the plugs across the intake manifold and the spark is always there, in sequence on all cylinders, whitish/blue and stupid hot.

I can crank all day and get firing in a cylinder or two at random intervals. The air/fuel mixture seems incredibly rich...I'm thinking of taking pictures to show you just how black these plugs are. An uber rich mixture would explain why I have to hold the throttle wide open just to give the engine enough air to combust the fuel.

Fuel pressure checks out ok and none of the injectors seem to be leaking at all, which would lead me to believe its the computer adjusting the mixture.

So what would cause an extremely rich mixture? Temp sensor and TPS are good. Only sensor I havent checked is O2. AFM maybe? I tested it as best I could per 'Clarks Garage' aside from hooking up a 9-volt battery.

Extremely rich mix. What could cause it?

Techno Duck · Techno Duck's Garage

Fuel pressure regulator could be bad, thats about all i can think of to have as rich of a mixture as your describing. However if the fuel pressure checks out to within the norm's, that is basically ruled out.

ae1969 · 04-01-2007, 06:31 PM

Can you bring the car to operating temp.......

and check the resistance on that sensor again.

Blondie · 04-01-2007, 07:54 PM

I cant bring the car up to temp. because I cant get it to run long enough. Whatever was failing before must've failed completely.

blown 944 · 04-01-2007, 08:52 PM

My thoughts are to check the AFM if it is stuck open. My car will run even if the AFM is un attached unless you try to throttle it. So I don't buy any vacuum leak stuff. I don't think that is it however b/c it would want to go straight to redline.

If the AFM is OK I would think it is the DME.

You can also idle them w/o the O2 hooked up so I would rule that out.

Sounds to me like the DME is WAY over richening the injectors. Did it get wet?

This is all based on a accurate testing of the f/p. I however doubt that would even do it based on my running up to 50 psi with big injectors NA and still getting an OK idle.

Maybe you could try adjusting the FQS as a little tinkering may get it out of that mode. I think something has shorted resulting in a much longer PW. I am not as familiar with the inside ot the DME as I would like to be but I would continue to narrow it down as you have been.

If you want to try to at least get it to lean out to keep it running longer there is a flathead adjustable bleed screw on the side of the AFM that could be opened to get it to possible warm up a little. Maybe that will rule out the AIT.

I would look closely at the wiring to the AFM and other items.

Makis · 04-01-2007, 10:55 PM

I had a coolant sensor fail on me and had problems starting the engine. The coolant sensor when it failed was shorting the sensor to the ground and as a result the ECU was seeing low resitance causing weak mixture at start. Check again the sensor terminals resitance and ground. It should be open circuit. If this is OK then I would check ignition circuit for problems.

CPorsche · 04-02-2007, 04:30 AM

If it is running rich then I don't believe it is a vacuum leak. About 2 months ago a friend's 944 started running rich, it was so rich that there was a lot of black smoke. We put in another DME and it ran fine. Turned out to be a bad solder joint on the DME board.

Razorback1980 · Razorback1980's Garage

To get the temperature sensor to a heated state, put it in some water on the kitchen stove (women of the house LOVE it when we bring car parts inside) and heat it up to temperature. Then you can check the resistance.

Blondie · 04-02-2007, 10:08 AM

lol parts in the house..I'm no stranger to that!

The DME is a possibility....I fried my last DME and replaced it with a used one. Maybe this is where the Porsche gods have chosen to smite me? lol

When I told my parts supplier/944 guru and friend abt holding it at WOT just to keep it idling, he immediately said 'AFM'. He offered to have me bring mine in to match up the part number and see if a different AFM will put an end to all this.

Rest assured, I'll let you guys know

onZedge · 04-02-2007, 01:13 PM

Take the air cleaner cover off and look in at the AFM to see if it is stuck open. This happened to me. Black smoke. Would only run at WOT. Probably cooked the cat and maybe the O2 sensor too.