6ocyclehum

This one has got me stumped-
Car won't hold an idle, unless you feather the gas pedal, and even then runs terrible. If I unplug the AFM, car idles perfectly! Can't drive it of course. Just swapped the AFM out, same results with both AFM's.
I've found plenty of info on testing the AFM- but what about testing the signal it gets?
Could the DME be bad?

6ocyclehum

Just checked the AFM plug with the key turned on- 5 volts is being sent by the DME.

944obscene

I was gonna say bad connection on the harness side of the plug. Pull the battery cable and then reset everything. Try it again. Also, check your AFM plug connection. Mine gets loose sometimes and needs a jiggle.

Jeff N.

Check for broken wires under the rubber cap near the connector.

6ocyclehum

Wires look fine. Is there any way to test the return signal from the AFM to DME? The TPS is fairly new, and makes no difference when disconnected. The ISCV is brand new, and when disconnected makes no difference. Also- I'm wondering if it could be a vac leak. Has almost all new vac hoses, but I have been getting around 13-15 on my vac gauge at warm idle. With the AFM disconnected, the vac shows about 12, but it's not fully warmed up. Also, when poking around the hood while the car was barely idling (AFM hooked up), it died and I saw a puff of black smoke come from under the intake- small, but it was there. The thought of yet another vac test, or pulling the intake makes me shudder........

BGA9512

Not to state the obvious, but if you could see the puff of smoke, one could deduce that a combustible liquid or gas was able to vent into the atmosphere. i.e. the system is not sealed as it should be. - There must be a pipe disconnected or cracked.

6ocyclehum

Yeah- that's what I was getting at. I know I have an exhaust leak, close to the engine, but this looked like under the back/rear of the manifold. I know the AOS has a leak. Could I be pulling 12 on my vac gauge and have a leak bad enough the car won't idle? Also, with the AFM disconnected, it will idle- can a vac leak cause this? Don't want to tear into the intake unless I'm pretty darn sure.

6ocyclehum

Bump-
Anybody know why it would idle with the AFM disconnected?

DrZ1

Do you have a known good AFM you can test w/ to rule it out. I would maybe remove air box and all boost tubes. Then re-install making sure everthing is tight an no cracked couplers. Esp the one on the cold side of the turbo compressor, I had a 2 inch split in that one. Also you can check all the vaccum lines. I would try this prior to pulling the intake.

Steve

hp18racer

I think that fuel delivery is controlled differently when the idle contact is made then when it is opened. I believe that engine RPM and the O2 sensor are used to calculate fuel delivery. Off idle the AFM and O2 sensor are used.

I would cut into the wire harness and make a test lead close to the computer where I could see the AFM signal with a multimeter.

I'd guess you get a crazy reading at the computer from the AFM. The problem could be in the wire harness, afm or computer's ability to read the AFM. Clark's garage has a procedure that tells what the AFM readings should be. (www.clarks-garage.com)

6ocyclehum

I've tried two AFM's both do the same- one is rebuilt. I guess I should check the signal going to the DME from the AFM and do another pressure test of the vac system. Car boosts OK, did anyway. Figuring this one really sucks! I wonder if a vac leak could be just the right amount of air, that it would idle with the AFM disconnected.Thanks guys-

hp18racer

With some creative use of PVC caps, air compressor, regulator, and air fittings I was able to deliver 15psi regulated air and pressurize the intake on my car. I've tried pressurizing it starting at the intercooler inlet and starting at the throttle body. I'd much rather look for leaks this way then by taking the intake apart and inspecting hoses.

6ocyclehum

Fixed- one of the hoses going to the ISCV had a pretty bad tear. Actually fixed it without removing the intake!

Zuba951

When the primary Load Sensor (AFM) fails the Throttle-valve angle sensor is used as the primary load sensor, the level of measurement accuracy required is greater. This greater degree of accuracy is achieved by the use of two potentiometers and higher quality valve bearings. The air mass of the intake air is calculated from the throttle valve angle and the engine rotation speed. Temperature-related changes in air density are taken into account by measuring the intake-air temperature. Taken from BOSCH Gasoline-Engine Management: Motronic Systems


Brian
86 951

Rogue_Ant

Just for accuracy, the DME does not use TPS angle, and will not run properly (if at all) without an airflow signal.

Zuba951's quote might be correct for some Bosch Motronic systems, but not the unit used in the 944 & 951.


-Rogue