3.2 Intermittent cutoff on track (video included)

GOOD HANDS · 08-12-2019, 03:15 AM

So my 88 3.2 has started missing and cutting off under load. Consensus from 3 separate mechanics is it's electrical based on symptoms. Other opinions here welcome. Car either misses badly or completely cuts off. Once it cuts off you can start it right back up with no issues. (see video) However some sessions it runs perfectly. So far we have bypassed kill switch, replaced cyl head temp sensor, reference sensor, all plugs and wires, coil, DME relay, put another DME in to see if old one was bad, coil, and supposedly gone over the car with a fine tooth comb for ground bad wires, fuses, fuse connections etc. The only weird symptom is the oil level indicator on the dash has starting going haywire since this started happening. Next step is to take it to the dyno and try to recreate it hopefully. Any other ideas? I'm about to pull my hair out.

In video car misses at 22, 50, 1:06, 2:28 then totally dies at 2:35.

https://www.youtube.com/watch?v=l20_KW-t5Xw

FrankM_ · FrankM_'s Garage

Did you try to disconnect the oil level float (on oil tank) completely so it can be ruled out (as it is you're only clue) ?
Gotta say, it doesn't look like that could cause a problem though (potentiometer between indicator and ground).

I would check, recheck and clean all battery connections and the ground point (MP I), alternator ground strap and transmission first ground strap. Then check the important ground points for the engine / DME / injection operations (MP X, MP XII come to mind) and the fuel pump connections (and MP II as its ground).

GH85Carrera · GH85Carrera's Garage

On my car it started misfiring and the ultimate culprit was a bad coil. If you go order a new coli for the car from Porsche it will be a silver Bosch coil, and total junk. About half don't even work right from the box, and often go bad in short order. I have yet to find a good aftermarket coil that is the proper voltage and works right.

I finally sourced an old OEM Bosch black coil from a scrap yead, and that coil work perfect and has for years.

GOOD HANDS · 08-12-2019, 05:59 AM

Quote:

Originally Posted by GH85Carrera

On my car it started misfiring and the ultimate culprit was a bad coil. If you go order a new coli for the car from Porsche it will be a silver Bosch coil, and total junk. About half don't even work right from the box, and often go bad in short order. I have yet to find a good aftermarket coil that is the proper voltage and works right.

I finally sourced an old OEM Bosch black coil from a scrap yead, and that coil work perfect and has for years.

Yup silver coil was installed to see if coil was a problem. It wasn't. Reinstalled the original black one.

proporsche · proporsche's Garage

your oil level is just a different issue-bad ground or the cork gasket is to thin and the unit is grounded......i would check the distributor rotor....just in case...
Also check you engine harness plug connector next to the left rear shock....
Ivan

after watching the video what chip do you have in?

GOOD HANDS · 08-12-2019, 08:19 AM

Quote:

Originally Posted by proporsche

..i would check the distributor rotor....just in case...
Also check you engine harness plug connector next to the left rear shock....
Ivan

after watching the video what chip do you have in?

All inspected and replaced.

proporsche · proporsche's Garage

and the chip is from SW or where? Also you did say the cut out was the same with the silver coil...but do you have an option to use black spare instead the silver-they have very very bad reputation..as mentioned above..
Also, do you think it is heat related?
Ivan

peppy · 08-12-2019, 09:07 AM

I can offer no help, but did enjoy the lap around Summit Point. It's been about 25 years since I've been around that track.

GOOD HANDS · 08-12-2019, 09:08 AM

Quote:

Originally Posted by proporsche

and the chip is from SW or where? Also you did say the cut out was the same with the silver coil...but do you have an option to use black spare instead the silver-they have very very bad reputation..as mentioned above..
Also, do you think it is heat related?
Ivan

Chip is from Behe. But we changed out the entire DME with another chip so that was not the problem. Yes cut out was the same with new silver coil so black was put back in. It's not heat related as it usually happens first lap out. It actually seems to get better the longer you drive it.

proporsche · proporsche's Garage

so this would lead me to the fuel pump ,it looks like on video it is absolute cut off...what do you think?
ivan

Fly911 · Fly911's Garage

I had the same thing with my "87 Carrera on the track. On my car the tach needle went to zero when the engine was cutting off, it looked to be the same with yours.
It turned out to be a faulty DME relay, a very common problem on the Motronic Carreras. I bypassed the DME with a direct +12V to the Motronic, and no more issues. But you can't have a permanent +12V to the Motronic, so now I have two DME relays side by side, and can switch over in 30 seconds if it fails.

GOOD HANDS · 08-12-2019, 10:48 AM

Quote:

Originally Posted by Fly911

I had the same thing with my "87 Carrera on the track. On my car the tach needle went to zero when the engine was cutting off, it looked to be the same with yours.
It turned out to be a faulty DME relay, a very common problem on the Motronic Carreras. I bypassed the DME with a direct +12V to the Motronic, and no more issues. But you can't have a permanent +12V to the Motronic, so now I have two DME relays side by side, and can switch over in 30 seconds if it fails.

We put in 2 new DME relays just to make sure and we switched out the entire DME box with a different one with a different chip. No change in the problem.

GOOD HANDS · 09-23-2019, 09:09 AM

Well I think we figured it out. After all the work done to the car we believe it's a broken gas tank or something is in there blocking the fuel flow. I did the test and tune on Thursday for the PCA club race at Summit point and the car ran flawlessly the first 2 sessions. I was so happy until the third session. Spit, sputter. cut off. The gas gauge was reading between a quarter and half a tank so I wouldn't think it was gas related however that was the only thing that changed over the day. I filled the tank up and it ran great again. Kept it above a half a tank all weekend and it ran perfect. In speaking to my mechanic he explained there must be something in it that must have broken off and when the tank gets low it was blocking the fuel intake. With more fuel I guess it's floating to the top or something like that. After 2 months of chasing this I really hope this is it.

gasgas · 09-23-2019, 05:54 PM

Quote:

Originally Posted by GOOD HANDS

Well I think we figured it out. After all the work done to the car we believe it's a broken gas tank or something is in there blocking the fuel flow. I did the test and tune on Thursday for the PCA club race at Summit point and the car ran flawlessly the first 2 sessions. I was so happy until the third session. Spit, sputter. cut off. The gas gauge was reading between a quarter and half a tank so I wouldn't think it was gas related however that was the only thing that changed over the day. I filled the tank up and it ran great again. Kept it above a half a tank all weekend and it ran perfect. In speaking to my mechanic he explained there must be something in it that must have broken off and when the tank gets low it was blocking the fuel intake. With more fuel I guess it's floating to the top or something like that. After 2 months of chasing this I really hope this is it.

Are you thinking it is this part?
Strange Repeated Fuel Pump Failure 3.2 Carrera

I'm trying to figure out a similar issue. Your video is similar to the issue I'm trying to figure out. Except, once it dies, it wont start for a while (15 mins). My issue seems to be triggered by quick driving through hills.