I was about 5 miles from home this morning when my engine shut down and I coasted to a stop. It’s a 1977 911s that had a 3.2 liter (DME Motronic injection) installed about 7 years ago.

I replaced the DME relay with the spare in the glove box while I was still on the side of the road, but that didn’t help.

After about 5 minutes, I was able restart the engine and go about half a mile until the same thing happened again. I was able to repeat the pattern (drive short distance; stall, wait; repeat) until I called me wife to tow me back home.

I’m assuming that it might be fuel related. I didn’t hear the fuel pump run while restarting. Back in my garage, I can run the fuel pump by jumping terminal 30 and 87b on the DME relay. The fuel pump is only 2 years old (could be premature death) and a new fuel filter is on order (it’s been too long since I changed it last).

I haven’t started to systematically troubleshoot the DME system yet (if anyone wants a copy of the factory test plan they can download it from my site http://www.coreone.net/documents/dmetestplan.pdf ) Does anyone strongly disagree with my assumption that’s it fuel (as opposed to spark).

Does anyone have a simple test that can be done roadside to check for fuel – I miss being able to lift the throttle plate on a cold CIS engine to listen for the cold-start injector.

Thanks

Lou Parrotta
77 911
99 Boxster

 

I'd not simply focus on fuel. There are other items that could fail intermittently with heat. Look at icemans current thread on "Carrera performance", his buddy is having similar engine issues when it is warmed up.

 

Good gravy!! Ask and ye shall receive.......must....buy....'scope!!!


Edit: you trained the wife to operate a tow vehicle? And nice 3rd post btw.

 

yup my buddy is experiencing the EXACT symtom as we speak .. when I got stuck at the side of the road with him I fortunately had a spark plug in my pocket .. I hooked it up and although it cranked like mad I got a ton of spark but no fire. you might want to try that simple test first . I can also give you some additional info on my scenario that may or may not help you.

Before dying totally it bucked a couple of times like it was not getting fuel or somethng or the ignition was cutting out . WHen I had him just dip the clutch it was oscilating like crazy . like it was hunting and the condition was way rich, then it died.
If you have read the thread you know a good way to eliminate the CHT sensor by now .. when it dies after warm up ground the single wire pin or jumper the double wire harness ( the harness is the side NOT on the bracket ) . This is assuming you have checked for spark however.
From what i have read this is highly indicitive of a craped out DME but i don't have one to swap so I am trying other things.

 

I was able to reproduce the problem tonight in my garage (engine started and idled for about 30 seconds).

I pulled a plug wire and I had spark. Fred you may want to check the Harbor Freight spark tester before you need it. I don’t think it will work with the deep reach to the 911 spark plugs and the short wires. It looks like it will work fine with other engines.

I also had the generator light so I don’t think it’s supply power related.

I put my voltmeter on the 87b pin of the DME relay and I got 12 volts while cranking. BUT I DON”T THINK THE FUEL PUMP RAN AT ALL. I need to double check my wiring diagrams, but I’m pretty sure that 87b is the feed to the fuel pump. I also need to put my volt-meter leads directly on the terminals on the fuel pump.

I’m getting closer. A little more due-diligence; but I think that I eliminated some major components and failure modes tonight.

 

Lou,
According to the Bentley manual the DME pinout for an '86-'87 (It should be similar for other years) is:

85B (Blk/Wht) - Pin 20 of DME
87 (Red) - Pin 35 of DME (Through a star connection feeding injectors and idle speed positioner)
30 (Red) - Battery
87B ( Yel + Red/Grn) - Yellow goes to O2 sensor and Red/Grn goes to Luggage Compartment Fuse Box 2, Fuse 3..... Then travels out of Fuse 3 to the Fuel pump as a Red wire
86 (Blk) - Goes to Alarm System
85 (Brn) - Goes to ground point 407

It seems you will need to check Fuse 3 and its connections as well as the fuel pump.

Good luck

 

I had a similar problem a few weeks ago and it turned out to be a loose wire on the fuel pump.

 

Battery over-charge, over-voltage, will cause the CDI to misfire (tach "jumping") engine over-rev detector trips due to misfire, kills fuel pump circuit, over-rev circuit latches up due to over-voltage. Turn key completely off, wait..wait,....over-rev detector unlatches, restart.

Or, set by the road a few minutes with heavy battery load, lights, etc, key off, restart, drive x miles, battery again reaches over-charge....

Replace the alternator/regulator and CDI...

 

I got myself confused last night, but I think I got back on track this morning. The voltage at terminal 87b looked OK, but the voltage at the fuel pump was all over the place. The pump would make some sounds, but it did sound quieter than normal. Finally, I disconnected the fuel pump completely and checked the control voltage to the pump – everything looked right.

When I checked the resistance on the fuel pump it measured .5 ohms. That would have resulted in about 24 amps at 12 volts. I’m surprised I didn’t blow the fuse. Bentley manual calls for 2-3 ohms (actually 4-6 amps – Ohms = Volts / Amps). I ordered a new pump tonight.

Hopefully, I’ll have some good news next week.

Yes, James those are the Harbor Freight line clamps. They usually come in 2 styles; I think Walt has the ones that are more pliers shaped. The Harbor Freight ones are similar to the types that are used by Bentley in their manual photos.

 

I'm just loving the DME Test Plan. Makes nice reading! Thank you, Lparrotta for posting it.

 

Just an update, after spending a couple of weeks troubleshooting it myself (and replacing the fuel filter and pump), I finally turned it over to the professionals.

It seemed the fuel regulator was the problem (the pressure was too high). I guess I would have evidentially figured it out given infinite time and tools. BTW the Harbor Freight vacuum/fuel pressure won’t work with fuel injection cars (max pressure – 10psig).

Now if the heat would only break, I could spend a little more time driving my non-A/C car.

 

The fuel pressure regulator is controlled by vacuum. Was the diaphram broken? I'd like to know what failed in the FPR.

Harbor Freight has a Master Kit that works for both CIS and Motronic fuel injection.

 

Yes it was a broken Diaphram (insert joke here).

 

I thought it might be. Rarely happens, to others that is.

ahem.

 

Want to bet?