3.2 DME Issue?

[Gordo2]

Folks,

I'm brand new to the Motronic world with the recent purchase of an '85 Carrera. Having problems getting the 3.2 to run consistently. Here's the symptoms:

-Starts OK, and idles fine once warmed up.
-Intermittent but very frequent sputter when rev'ing the motor (in neutral).
-Intermittently sputters and acts like it's going to stall when accelerating.
-Allows intermittent acceleration up to red line, but then the RPMs fall and the engine nearly dies.
-I'm fairly confident I'm getting adequate fuel flow: 1 mile trip around the block (RPMs dropping and then accellerating again) turned the catalytic cherry red (I assume from unburned fuel making its way into the cat).

I've read some threads that seem to point to replacing the DME relay as a starting point to fix a bunch of fuel delivery problmes. Do you think its possible my DME relay is bad, and if so, how would it produce the symptoms I'm experiencing?

Thanks, Gordo

[ischmitz]

The DME relay usually fails in a way that the engine dies completely. Sometimes you'll notice cutouts before it finally gives up. It is a very good idea to carry a known good spare relay in the glove box. So go ahead and get one and try it.

What you describe sounds more like a mixture-related issue. And that is most likely not caused by the DME relay. You need to check everything affecting mixture. Here are some pointers where to start:

- how is your O2-sensor?
- make sure there are no intake leaks anywhere
- check the CHT (head temperature sensor. If it is a 1-wire Porsche suggests upgrading to the later 2-wire
- check the fuel delivery. There is a procedure where you jumper the fuel pump and measure the delivery rate in ml/min. This is the only proper way to determine if you get enough fuel.
- is the fuel pressure regulator working?

Regards,
Ingo

[vreference]

If you think you are getting constant fuel then I think I would inspect the ignition; From coil to plugs, everything in between.

[kidrock]

Originally Posted by vreference

if you think you are getting constant fuel then i think i would inspect the ignition; from coil to plugs, everything in between.

+1.

[dfink]

I will vote for leaking, malfunctioning injectors and or faulty reference sensors, and or, vacuum leaks in any of the multiple rubber hoses, and or the vacuum lines came off of the regulators on the fuel rails, and or a faulty air flow sensor.
And or all the above.

If you decide you need parts let me know as I have a complete intake system from an 84 3.2 that I converted to PMOs. You have however entered an area that is not trivial in expense. Personally I would expect the DME to be at the end of the list.

First you can visually inspect all the rubber hoses. Unless replaced already I will bet the setdown connection for both the brake vacuum and PCV connection are cracked. I would also expect the large rubber hoses going to the oil tank to be cracked. It is very easy for the small vacuum lines to come off that run to the fuel rail regulators. They are located on the front of the fuel rails may have to remove heater blower and air filter box to check hoses.
The referrence sensors are a bit more difficult and they are expensive. Again if not replaced I can almost guarantee that they need to be. Carefull messing with them. You can visually inspect from below the car from the drivers side. Don't mess with them much if the look intack as the rubber will fall off pretty easily.
Thats enough to start with the rest gets a bit more complicated and you start getting to some real money when talking injectors and or air flow boxes. If the injectors are actually leaking you should have a gas smell at idle and the car should run a bit better at speed because it need more fuel there anyhow. This leads my final best guess at the air flow meter.....

[Gordo2]

I was hoping for an easy fix, but it sound like its going to have to be a systematic approach.

dfink, thanks for the info an offer on the parts. Oddly enough, I'm just trying to get the motor up and running right to assess what I want to tear into this winter as I rebuild it (just bought the car). I'm planning on going to PMO's as well (intended to do so before I bought the car - appears Motronic is quickly reducing any doubts I had about the conversion - I love simplicity...).

Thanks again for the feedback guys.

Gordo

[dshepp806]

Cherry red? better check that mixture for sure.

Best,

Doyle

[gregwils]

I would agree with Ingo that your problems sound beyond those an old DME can create. My '86 was running poorly although didn't exhibit problems nearly as severe as what you describe and a new DME resolved all of the issues. I would suggest you buy a new one and install it, and remove your existing DME and stick it in the glove box as an emergency spare. If it doesn't improve performance, it's still a good idea to carry a spare and you can rule out the DME relay from you diagnostic effort. If it does resolve your some or most of your problems, I would order another DME relay so you have a decent spare for the glove box.

[aston@ultrasw.c]

If you do end up needing to have the DME checked out, I can thoroughly recommend these guys.

I think the fellows name is Jose, anyway he is very knowledgeable and willing to help you troubleshoot your exact problem. He helped me fix a prob that had stumped all the experts.

http://www.porschedme.com/

[wnsgc]

Just so I'm clear, are you saying the DME is not just working or not working (in other words, all or nothing). I carry a new/spare DME at all times, and I believe my DME is original (thus 24 years old). Should I change it and keep the old one as the spare or should I just leave it alone? I have no bad symptoms at the present time, and as a matter of fact, just installed a Steve Wong chip, I just wasn't sure if it could perform even better with a new DME.

Thanks.

[kidrock]

Scott,

I believe in Murphy's Law. If it ain't broke, don't fix it.

That being said, the best insurance you can possibly have for your 3.2 is a known good spare DME relay in your glovebox.