Idle problem, help needed.

[72_911T]

I can't seem to find an answer to my problem in any previous threads. I took the my 72T MFI out Sunday for an hour, warmed it up on the freeway all seemed fine. At the end of the freeway off ramp I slowed down for the stop and the car just died. Started up fine but would not hold an idle. I had to really work the hand throttle to keep it running. I parked for 5 minutes and restarted. Ran and idled fine. Then later the same problem returned. I notice the RPMS drop way down even between shifts during normal driving. Any suggestions as to where to begin?

[jtratza]

I've got a 1987 911 with mass flow sensor ( MFS has about 70,000 miles on it) that is giving me similar fits. Every other day it won't hold a steady idle. A few months ago from time to time it would not take anything more than part throttle and wouldn't idle steady but rather searched from near dying to 1,500 rpm and back again. At that point we replaced the O2 sensor, cleaned out the fuel filter (maybe I should just replaced it but its only a year old) and adjusted the fuel mix. I also put the spare DME relay in but no change. Seemed to run fine for a couple of weeks until it began to give me idle fits every week or so searching from near dead again to 1,500 rpm. At that point I cleaned the fuel tank and replaced the idle control valve. The idle control valve was thought to the the culprit because if I disconnected it the car would go back to a steady idle. Unfortunately its still doing it on a daily basis. Runs fine but idles crappy every so often. I also replaced the throttle switch a couple of years ago.

Any ideas would be appreciated 'cause this is really getting annoying in daily traffic on the way to work.

[72_911T]

Have you done a search through the vast storehouse of previous knowledge? I think this is always the best place to start. I would guess your 87 is strapped with a lot more equipment that could cause your idle problem. My search revealed several possibles for your vintage vehicle, but very little for an older car like mine.

[NYSCAR]

Check your ignition, plugs, cap, rotor, points, coil, wires, and perhaps the engine ground to make sure everything is ok (easy to do). If everything checks out there, how long has it been since you've changed your fuel filter?

Brad

[Rot 911]

Sure sounds like the hose came off to your warm up regulator, the regulator itself is sticking or your cold start solenoid will not turn off.

[jtratza]

Thanks guys now I have all sorts of things to check this weekend! I'll check hoses, contacts, regulators and cold start solenoid. I'm checking the previous posts on the idle subject this morning for more ideas. Its been one year and about 8,000 miles since last major tuneup

[john walker's workshop]

try adjusting the screw that hits the micro-switch bolted to the #1 stack until it doesn't trip the switch, or pull the wire off the pump soleniod. (just for test purposes). that way the solenoid won't pull the pump rack to the off position at closed throttle. perhaps the speed relay won't let go of the soleniod so the pump can return to idle position below 1300 RPM.

[72_911T]

Kurt, I went through the WUR about 5 months ago and that was the first thing I checked last Sunday so I know the hoses are ok. I read the Haynes book and they talk about a cold start solenoid but for the life of me I cant find it in the car. Haynes also says something about a microswitch.

[72_911T]

J.Walker, you beat me to it! I know where the microswitch is and will do that test. Can you help me locate the cold start solenoid and the speed relay? Do you know of tests to check these components?

[FA-18C]

is the speed relay the same as the rpm sensor/relay? should be the left aft relay in the engine compartment...I've been chasing a nuisance backfire when I lift throttle, the mcroswitch appears to be working fine with a test light, plan on getting into the other animals during the holidays...

[john walker's workshop]

the solenoid i'm referring to is on the rear of the MFI pump. (rear, as to rear of car). not the cold start solenoid. when it gets power from the speed relay, (rpm transducer), it pulls the pump rack back beyond the idle position, basically shutting the fuel off, until the relay drops the power to it below about 1300 rpm, where the rack returns to the idle position. use a test light on the wire to the soleniod. raise the rpm to 3000 or more. quickly close the throttle to close the microswitch and you should see power for a moment until the rpm falls below 1300, and then no power at idle.

[72_911T]

It sound like you have got it. While I was struggling with the hand throttle to keep the motor running It was about 3000 rpm and below that it would drop off to nothing. What is more likely to fail the relay or solenoid.

[john walker's workshop]

the wiring is the most likely problem. the relay is next in line. warren posted a lot of good info lately on the relay and the wiring. do a search.

[72_911T]

FA-18C: I did some more searching and came up with a wealth of information on the speed switch. Check this thread from Warren Hall. This may help solve your problem.
Good Luck.
http://www.pelicanparts.com/cgi-bin/vb/showthread.php?s=&threadid=8074&highlight=rpm+transducer
John walker- Thanks I'll work on this this evening our maybe weekend.

[mstehno]

Check your fuel adjustment. You may be running rich and fouling plugs. I just experienced this on my 76'. I'd run it hard for a bit then would have a hard time at idle. Adjusted the fuel and seemed to be fixed.

[jkeyzer]

Find the big cylinder on the front of the MFI pump with the wire connected to it, and disconnect that wire. This will disable the overrun fuel shutoff and if the problem goes away, you'll know something is awry with that circuitry. I believe if you also pull out the middle fuse in the engine compartment fuse block, you will also deactivate the fuel shutoff.

Check the dwell of your points as part of your ignition tests.