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Posted by: 914ltd Aug 1 2011, 02:09 PM

Hi guys, I have an 84 Carerra in my shop (OK, it's not a 914 but there's more talent here than some other forums) with an intermittent no start. The car has no spark then will start and run fine. I have checked the DME and coil connections but nothing else. It is getting fuel smell at the exhaust during no start. Anyone know of a condition that might cause this? Thanks, Brad Mayeur

Posted by: Ferg Aug 1 2011, 02:19 PM

DME Relay?

http://forums.pelicanparts.com/porsche-911-technical-forum/230137-87-carrera-has-intermittent-no-start-problem.html

Posted by: pcar916 Aug 1 2011, 02:30 PM

That's always my first thought. I have both an '84 Carrera and a'95 DME engine in my 914 and I carry a spare DME relay in both cars.

Other than that, the DME won't send spark out to the coil until specific sensors report in successfully... I looked at the factory electrical diagrams to see which devices are energized at each ignition switch position.

Posted by: ConeDodger Aug 1 2011, 02:51 PM

The DME relay would be my first guess. They usually just go, not intermittent but as they are only $40 or so and you should always carry a spare, I would start there.

Posted by: phatnine11 Aug 1 2011, 02:57 PM

The CHT sensor! This sensor gives off very strange symptoms. Verify that the CHT is the updated 2 wire unit, this one adds a better ground.
Good luck,
Phatnine11

Posted by: scotty b Aug 1 2011, 03:14 PM

Order of most likely :

DME relay

CDI If it has been converted over ( Had a similar issue on a Maserati Bora that turned out to be the CDI. Interestingly enough, the Bora uses the exact same CDI the 911 does )

Coil

Posted by: Cap'n Krusty Aug 1 2011, 03:25 PM

Carreras with Motronic don't have CDI boxes.

The Cap'n

Posted by: r_towle Aug 1 2011, 03:33 PM

dme relay, cht, grounds grounds grounds.

Cap and rotor also...but I assume you checked that.
Coil also.

The DME relay was made when they were doing a crappy type of soldering....so was the DME itself.
There are pics on Pelican of all the bad solder joints in both the relay and the DME that cause all sorts of weird conditions.
If you look at the backside of many of the boards from that era of german cars you will see a discoloration and ring around the post of the resistor, capacitor etc.
That ring is a break in the solder and when it gets hot, its stops contact...when cold its fine.
Lovely thing to track down, but if you look at enough you can learn to see if with a magnifying glass pretty quickly.
It takes ten seconds to fix them....just reheat.
I did all the ones on the BMW Lights computer...took about 5 minutes...all problems gone.
I just re-solders all of them (acutally had a friend do it who is really good at soldering)
Its a common issue.

The ignition is controlled by the DME so start there.
There are a number of tests that you can do with an Ohm meter at the DME plug to help you diagnose the wiring, I suggest you may want to do that first.
There are breakout boxes to test the car live while running to diagnose all the other bits, but the CHT is a simple one to do at the plug in the engine bay (provided the wiring tests out good)
Those tests are in the factory manuals and in the Bentley manual also.
There is a good flow chart in the bentley for "no ignition"

Grounds seem to solve alot just by ensuring that certain circuits do not heat up.

Steve Wong has alot of articles on the DME and Carrera FI system.
I think its www.911chips.com.


Rich

Posted by: 914ltd Aug 1 2011, 08:32 PM

Thanks guys, I thought the DME relay controlled the fuel pump??? since the problem seems to be spark, I passed it by. I ordered one with the hopes that cures the problem. I'll post yea's or nay's.
BTW, I cleaned all the grounds and replaced the CHT 6 months ago chasing an idle problem. Brad