Intermittent running issues - 88 Carrera
 

I've been trying to figure out this problem for about 2 years now. I've thrown parts at the car trying to chase down this intermittent running problem and it is still persistent. The car will at times run perfectly normal. Then suddenly it will start stumbling, appear to run very rich, and die. It will not re-start. Then after sitting for an hour or so it will start up again, but usually exhibit the issues again after a few miles. Yesterday we drove it several miles, got it up to full operating temperature and it ran great. Started it up and drove it up to operating temperature a few more times, and no issues. A few weeks ago, it stalled after about 2 miles and wouldn't run well the rest of the day. It's very unpredictable when the issue will occur.

Items that I have replaced:
Cylinder head temp sensor (new improved version)
AFM
Fuel filter
Air filter
Spark plugs
Spark plug wires
Distributor cap and rotor (sealed with silicone grease for humidity control)
DME relay
DME has a replacement chip in it (Steve Wong brand)

I'm at the point where I suspect the DME itself is faulty. I took a picture of the bottom of the board, as I hear this is the typical place they fail. Can anyone tell if there is something wrong with this board?

http://forums.pelicanparts.com/uploa...1412607375.jpg

Before searching for a replacement DME, are there any other suggestions to chase down this issue. Troubleshooting has been difficult due to the nature of the problem.

I'm in Indianapolis. Not sure why that isn't showing up anymore under my name...

I don't know of anyone in this area that can test. These boxes are similar to ones used in old BMW's from what I've heard so there has to be someone around.

...do you by any chance have one of the newer Bosch coils???THE ONE WITH THE YELLOW AND RED AND BLUE STICKERS ON THEM THAT COME FROM BRAZIL...I was once chasing down a similarly mysterious demon, turned out to be the pricey new coil...good luck...

I once accidentally loosened the CHT sensor plug when changing the fuel filter. It would start, run and die.

Disconnect the O2 sensor, if you haven't already tried. Cheap test

I played around with the CHT sensor plug and it seems ok. Same with O2 sensor. I may replace the O2 sensor with a generic Bosch one anyway just to rule that out though.

What about the flywheel sensors? I haven't touched those yet, mostly because their pretty salty $$$. Could they be a potential problem?

The problem with trial and error tests is that the car always dies after you're a couple of miles down the road. Then you start fiddling with plugs, dizzy cap, and other various things and try to start the car a few times. After fiddling with a few plugs, the battery begins to die from cranking. Then you're out of options. Either keep cranking and get it towed, or wait a few hours and try to start it again. Like I said, troubleshooting this has been very problematic. It has unfortunately meant that the car has been stored for the majority of the time over the last few years and we forget about it for long periods of time.

Thanks for the advise thus far. I'll reach out to ischmitz and see about having him test the DME.

Have you tried the connector box that houses / connects the leads to the dme relay. I had a similar problem and had extremely bad corrosion on the the barrel connector for the hot lead coming in. Replaced it with the help of Doyle Shepperd and runs like a champ.

Have you taken a magnifying glass and looked closely at some of those soldered connections? I had similar problems an old Honda (I know different in many ways but there are similar symptoms). Changing a bunch of parts didn't do anything. The car would just buck and die randomly every few weeks, but turn right over after about half an hour of just sitting. Turns out the control circuit board had some soldered connections that were not optimal, so over the years the solder, where weak, just didn't hold or it cracked and this resulted in instability depending on a bunch of factors like running temperature. Those soldered points should have a nice rounded mound shape to them, with plenty of solder for a good connection. I just resoldered the weak connections and that car never died on me again for the two years I kept it. I don't know if this may solve your problem, but just check yours to see if there are any points that don't look good.

What is his best place to buy a replacement DME, if that is the issue? Ballpark cost?

You can use the BMW ones, which are evidently the same sensor for way cheaper. Do a search on here and you'll find info about it.

If you can find someone local with an 80's Carrera the fastest test is to put your DME in their car. An 84 through 88 will work. If their car runs with your DME then the problem isn't the DME.

Do not put a good DME in a questionable car. There are a few failures like a short that can damage a DME and you don't want to damage someone else's DME for trying to help you.

Can someone tell me more about the flywheel sensors are? Are these called the crank position sensor and cam position sensor in other cars? What does the 3.2 engine have? Are there 2? I'd like to find where mine are, and look up the part cost, just for future reference. I wasn't able to find it in the PET catalog.

Speed reference sensor and crank position sensor. I just replaced mine with the BMW ones. Plenty of threads on this with the BMW part number, which our host sells. Pain to do but would be my next step if I was u. Getting old ones out the bracket was the hardest thing for me. I took the cradle out in the end and set gap with .8mm washer.

Well worth doing regardless.
Ed

Have you looked over the fuse panel closely, especially the fuel pump fuse?

The fuse holders, fuse end clamps will weaken over time, especially the ones running high surge currents as is the fuel pump.

Look at the condition of the yellow fuse/holder.


http://forums.pelicanparts.com/uploa...1412957944.jpg

sugarwood, you have a PM.

the CPS is a good possibility.

here are some places you might re-solder on the DME. of course based on a pic. the other option is to jsut hit them all with the iron.

http://forums.pelicanparts.com/uploa...1412959633.jpg

Could be Flywheel sensors or the DME. If you can take nice close up pictures of the back of the board near the 2 large heat sinks I can ussually find the issues. PM me if you need to email the pics. I fix like 3 of these every month!

I downloaded your pic but can't zoom in, it is not high enough resolution.

Your pic looks like someone may have done some solder work already on the board. I see what appears to be solder flux that has not been cleaned off. Factory solder job would never leave flux behind. I always clean boards after I work on them. Better high res pictures would really be helpful.

Here's thread showing my typical repair done properly:
http://forums.pelicanparts.com/porsche-911-technical-forum/652375-dme-bad-solder-joint-repair.html

If you have a steve wong chip you should be aware he also refurbishes, repairs, replaces, DME boxs, I sent him mine from my 88 just because it was twenty five years old. He reported it had a bunch of failing solder points that were waiting to make my life tough.