3.2 w/factory Motronic Running Poorly and Stalling
 

This is a stock 3.2l, good compression, typically solid running engine. Recently it has been somewhat hard to start when cold. Got this from the owner today.

"It was a little hard to start again today, several tries required and then rpm must be held at about 2k to run smooth (and not die) while warming up. After that it ran perfectly up to my job about 3 miles. A couple hours later I took it on a longer route home and though it started easy it ran poorly. On anything beyond the most mild acceleration the power would cut out and then come back repeatedly; kind of bucking with a cycle time of around a quarter second. Rpms were somewhere around 2-3.5k. As I babied the car home it died. The tach went to zero and I lost all power but the engine was still turning until I stepped on the clutch. After sitting on the side of the road for about one minute I restarted the car and drove it another mile until the same thing happened. I let the car sit a minute and tried to restart - tach reflected engine speed. It started and I held the rpm at about 2k for about 5 seconds before it died - just like someone had turned the key off. I waited another 5 minutes - exact same thing. I walked home and returned about an hour later - exact same thing."

I am guessing crank sensor but really don't know. Can anyone confirm or point me in the right direction on this?

Thanks - Chris

I would suspect one of the sensors. Also, take a look at your dizzy cap, rotor, plugs and wires.

something pretty serious is going on like DME relay cutting in and out, that's just a guess, could also be the rpm or crank position sensor I guess I don' t know.
I wonder if there is a big vacuum leak somewhere.

Dumb question here: did you replace the DME relay yet?

Hello. That's exactly what happened on my 84 when the speed and reference sensors died. I hope this helps.


Chris

Possible cap and rotor but because engine runs great when not exhibiting these problems I doubt it.
I wondered originally about the temperature sensor going bad and messing with the signal to the DME./?
The DME has not been replaced. Maybe this is a good place to start. (Hopefully it is not the DME control unit.)

Any other thoughts?

Additional email from the owner who managed to get it home.
 

"Stopped by the Porsche (parked in my neighborhood where it died) on my way home from work a few minutes ago. Started easy ran normal for a couple seconds and died - again like someone turned the key. Restarted and ran rough for a bit like it has been doing on a cold start - I held the pedal down trying to get the rpm above 1.5 or 2k where it smooths out - finaly succeeded it reved to about 4-5k and died again. Third try it started easy and ran normal. Drove it home gently, though it felt normal. Sitting in the driveway again..."

It does sound more like your sensors. Like what wolf76 said. Speed and reference sensors. I went through the same thing last year.

Check the DME Relay first, sound like fuel starvation. I doubt it is the DME itself, or the Speed and Ref sensors for now. You, if it were those sensors car will not start just crank.

Do the easy stuff first.

The owner should always have a spare in the glove anyways.

Here is my speed and reference sensor adventure from last year.

http://forums.pelicanparts.com/porsche-911-technical-forum/545644-need-help-power-loss.html

Just saw the wolf76 post. Are these type of sensor known to be erratic like this?
"Third try it started easy and ran normal. Drove it home gently, though it felt normal."

agree with Draco ...... fuel issue.... easy stuff first... when was fuel filter last changed? check connections on fuel pump.

Kipp, Sounds similar with the exception of the cold start issues he is having unless of course his cold start issue, which showed first alone, is unrelated to the stalling. I find it odd that the RPM's drop to 0 but the motor is still running. I think this may be a strong hint.
I could start by telling him to replace the DME followed by the reference sensors but rather just not make him start changing parts...

Don't think fuel because of the RPM zero out but engine still running.

The DME and the DME relay are two different things. Both are under the seat. But like what DRACO said about the DME relay, "The owner should always have a spare in the glove anyways."

Whoa Kipp, She is a Black Beauty :D

I read your thread, I am a bit concerned, and wish not to rain on your parade, Spark Plug Resistor? I checked Bentley and could not find a reference to a Spark Plug Resistor. I did not even know we had that on our "84-89? I could be wrong here but very strange.

Also, before the Speed and Reference Sensor was changed did your wrench verify the OHM's to verify that it was indeed bad?

I went through a nightmare where I moronically twisted the sensors into the housing and consequently snap the internal filaments to the brand new sensors. Car would not start for the life of me. 11 hours later and a 50 mile humiliating tow trip to Tony Callas, he checked the OHM's showed me my new sensors were faulty and then he swapped them. :eek: Expensive Lesson Learned. At least he did not yell at me for hacking up the housing when I removed the old sensors :D

The way it was explained as long as the OHMs are 5X5 there is no reason to swap them, frayed or not. But honestly, if I saw the fray I would have replaced them anyways for extra measure.

But bottomline, I am just glad you got your Beauty on the road again.

Again please it is only my 2 cents,

Jim

"Drop to Zero" and car still running? Hmmm, that would be the ref sensor but the car would not start in that case. Wierd???

Brain Fart: does your owner use the factory alarm? When I first got her I started using the alarm and immediately after that my relay went out. just a thought?

No alarm in this car. Transplant into a 914. Done right about 13 years ago so not a new install without a track record.

Well, the spark plug resistor. Maybe that's an incorrect term. He replaced the part of the plug wires that get plugged into the plugs. He did state that they needed to be replaced. As for the sensors, he showed me the Ohm reading and explained all that too me.

As for the sensors. The insulation was crumbling off along with a intermittent short. He replaced both sensors and the car has been running great ever since.

I have to tell you about my wrench though. He charged me a flat fee of $150 to diagnose the problem. He is not one to just throw parts at it. He got in there and figured it out. It did take a while for him to do that. But he got it done without over charging me. He has my business.

Sorry Chris for hijacking your thread.

Sorry Chris!

Then I conceed sir Kipp, :D, Chris check the OHMs on the sensors to eliminate that as the culprit.

No issues guys. I appreciate the help. I forwarded the info. thus far and suggested starting with the DME relay since yes it is good to have a spare.

Anymore advice will be appreciated. If there is any success once this has been solved I will be sure to post.

- Chris

I have these exact sypmtoms on my 86. replaced ref sensors, cht, fuel filter, dme relay, checked dme itself, idle control valve and it still persists. I am ging to my mechanic next week and i have full faith he will crack it, this guy knows these cars inside and out. I will let you know what we find

Hey guys - I am the current owner of the discussed vehicle. I had some time and tools available this morning so did some work for Chris. First I put an ohmmeter on the CHT - 500 ohms at about 50 degrees is too low. Then I unplugged the connectors and tested the speed and reference sensors. Came back with same numbers on both. Looking at the connectors I noticed one of the three clips (elect contacts) had been pushed back in the housing. I removed the rubber backing and pushed it back into place and heard it click. I tested it's security from the front and then buttoned everything up. I left the CHT disconnected to richen the mixture for starting. Started like it should no gas, first try, idled fine. I plugged the CHT back in after about 30 seconds of warming. It stumbled for a sec but then returned to smooth idle. I took the car for a drive everything was perfect.

So in summary there were 2 issues. CHT going bad causing hard cold start. Intermittent connector failure on speed sensor. Car starts using the other sensor (reference) and then switches to the speed sensor - this is why it would start and then die. The connector issue was likely caused when the speed sensor was reconnected after the PPI compression test.

Chris needs a new CHT sensor. $50 and 10 minutes to replace.

I'm going to miss this car. Don't get married! ;o)

Never a bad idea to replace the flywheel sensors and CHT. If they are original, they will at some point fail. Another OFTEN missed preventative maintenance issue, is the 25 year old ORIGINAL fuel injectors. Take them out and have them over hauled. You will be amazed at the difference in performance and economy as well!

OUTSTANDING. Glad to hear the problem was resolved so quickly.

gotta' watch that fuel pump (pressure readings), too!

best,

doyle

A successful ending and hopefully a helpful post for someone in the future.
 

Thanks to all!

-Chris

PS. Hey Jon, Be careful what you post or my wife will think I bought a sixth Porsche!:eek:

First thing I would do is pour in some Chevron Techron and let it run as much as you can. This has helped me several times in the past and will only cost $14.00 for the good stuff. I'm just saying that before you jump for the parts, clean the fuel system. I keep a bit in my tank every few months or so. Nick

Try the BG 44K, much better than Techron.

Congrats on the Fix!