Intermittent NO START, and fail running
 

SHORT STORY

an intermittent stall.

Thursday, Friday and Saturday morning, it would not start. I jiggled the fuse and wiring in and around the fuel pump and then it started.

Monday morning, it stalled for about 2 minutes while driving, then windmilled.
Then this morning, it stalled for about 3 seconds.

My tach goes dead, even though the engine is in gear and spinning in third gear.

Then it started running like normal again.

END SHORT STORY



BACKGROUND
In the last two months I have had four instances where I was driving along (usually accelerating) when suddenly the engine would die for about ½ second and then continue on as though nothing has happened.


This weekend, the car would not start and run in the mornings Thursday, Friday, and Saturday. I played with the fuel pump wiring in the fuse panel. But it is more of an ignition/computer issue. Primarily, because when I pull the fuel-pump fuse with it running, it dies slowly. This problem is immediately a dead stop.

Further information
When it stalled while driving, I was in second gear, clutch out, I noticed the tach was not “ON” or at different RPMs while I am in gear, and when I tried to pop-the-clutch to get it to run again. All of my other electrical was working (radio, heater, blower motors etc.).

It would windmill and not restart. I tried it four more times, and it then began to run without my fingers going in the trunk at all.

I have replaced the DME relay in the last two years, I got the 2–wire Head Temp Sensor update five years ago.

What I have and will do.
I have a spare DME relay, I will change this tonight. I will also clean and redo my #1 intake runner grounds.

Unfortunately, I fear this may be a CD unit, or coil failing.

Any other suggestions? Or things to test?

I know it's long, but what else can I do?

JWW? I may be sending you a PM.

Nothing else?

Intermittent electrical problems are a nightmare to hunt down when you're standing in front of the car. Sitting behind a keyboard thousands of miles away it's even harder. :>)

One interesting thing you mention is the tach not registering even when the engine was obviously turning. Makes me suspect the speed sensor on the flywheel.

These things are tough. I had a problem like this years ago and it took me a probably two years to figure it out. Try starting the engine and then wiggling every wire you can get your hands on to try and induce the problem.

Good luck,

Mike

No Start
 

I had similar problems with my 87 Carrera. I replaced a bunch of stuff: coil, CHT sensor, relay, flywheel sensor. Still had the problem. Then I did what I should have done from the start. Using Bentleys, checked all the inputs to the DME control unit when the car would not start at the plug to the box. Of course, everything was in spec, so it was the DME all along. They typically go bad from dry solder joints, so some people have had success with touching up the boards. I'd already done this a few years before, so it "lasted" a couple of years.

Good luck.

Thanks for the replies!

I am afraid it maybe the DME solder joints too.

Where there any particular areas on the DME board that you resoldered? Or areas that you thought where the cracks may be?

How are the flywheel sensor(s) tested? Is this in the Bentley?

Thanks again!

My car die when I make a hard turn , some time she can be restart and orther time she would not, and later I found out the ECU box is bad ( based on diag from the shop) , they quoted the $1200 for the new ECU, so I tried to touch up most solder joins on the ECU box and problem solve, so far have not seen the problem reoccure for the last 5 months

No Start
 

All of the resistances for the various sensors (CHT, flywheel, and the digital inputs) are given in Bentleys. I probed the DME plug to measure these resistances as seen by the DME. If they are all good, then your DME may be bad. It was in my case.

As far as resoldering the DME, there are two boards, analog and digital. If you resolder both 100% it will take 1-2 hrs. I masked off portions so I could keep track of what I'd done.

Good luck

Re: No Start
 

It's going to be a long night. :(

Thanks for the "masking tip". :)

It was shuddering most of the way home, while I was "cruising". If I got on the go-pedal, it would shutter, then suddenly it would run good and I would accelerate. It was almost like clutch chatter at 2500 rpms cruising going 60 + MPH. It was also like every other ignition pulse was working.

It did this in any gear at almost all rpms below 4k.

Then every once-in a while, all would be good. Then back to the shuddering.

This all happens during "warm-up". I noticed this time, it would run fine until the temp got out of the white mark (above 90 C). Then for the next 6 or 8 miles going home it shuddered. Then it ran very well like usual the rest of the way home (about another 8 miles).

Thanks for everyone's help.

After reading the bentley using their graph, it looks like it can only be a couple of things.

I replaced the German DME with a Hungarian unit. Seems cheesy, it rattles. :(

I soldered all (I hope) the outside circuit board solder joints. Nothing looked like it had a crack or anything though. 1/2 the bottom board looked like a lot of cold solders though. *shrug*
After I put the two boards back together and reseated my chip, I noticed there are more traces on the inside (top) of the boards. Ugg.

Well, if the issues continue tomorrow, then I will order the Speed Sensor, and TDC sensor, as these require a SCOPE to measure.

Everything else seems to check out.

I will double check my grounds, as per the Bentley, there is another ground inside next to the coil that I haven't done.

Thanks everyone for their replies!

I don't know much about the DME's. Cab 3.2 had a wild sensor prob that would come on after the engine warmed up. Routine testing was ok.. Then he used a heat gun on the flywheel sensor and Bingo when testing.

Hi Nick, don't replace the TDC sensor, that is only a diagnostic sensor, it has nothing to do with the engine running, it is test device only. You can try to test the, Ref/Speed sensors, but usually they will test ok with a meter, but the only real way to test is with a scope and check the sine. When it does die, have you checked to see if you have spark or not, if you do and it does not want to start, you might have a fuel problem, maybe a stopped up filter or strainer, or even a pump starting to seize up. Could also be the CHT sensor, even though you have a fairly new one, test it, it does not affect starting persay, it controls fuel, not spark, but it will make it run like crap. Another thing to check is the large vacuum line connection behind the heater blower, the one that tee's off from the metal pipe, it also has the little vacuum line for your cruise, on the y fitting, and the big vacuum hose for your brake booster, a lot of times these hoses at the fittings will crack, and cause a large enough vacuum leak and really screw up the way the engine runs.
As Ron said, I had a ref/speed sensor problem, if the outside temp was below about 70 degrees outside, the car would not start, no spark condition, checked everything, popped in a new DME relay, nothing... this went on for a few days, but after the car sat in the closed up garage all day and it got warm in there, it would fire right up, wouldn't turn over twice and it would start. That went for a couple of days, everytime I would try to start it after it sat getting warm in the garage all day, it would start right up, so then I decided to warm up the components, using a blow dryer, to see if it made a difference, first I warmed the DME unit, then the relay, no spark still, then the ref sensors wire connections, no spark still, rolled under the car and warmed the ref/speed sensors for about 10 seconds, rolled out from under, hit the key, it fired right up, ran perfect. Could turn it off and try it again, and it would start right up again and again..... So I left it till the next morning, was about 50 degrees outside and in the garage, tried starting it, would not start, NO SPARK, rolled under it with the blow dryer, heated the ref/speed sensors a little, rolled out, tried to start it, fired right up!!!! Went in and ordered 2 ref/speed sensors!! Replaced them, have not had a problem with the DME system since. I tried to find out what it could be that the temp would affect, before I discovered what was wrong with mine, but no one had an answer, kept being told nothing could be temp related to this problem. A Porsche tech helped me a lot, but he is over a hundred miles from me, but I would go there and he would help me trace the DME circuits and everything else that would control the running of these engines. He even had a colored DME circuit wiring diagram that was huge, like 2'x4' you could trace everything. I learned a lot from him about the DME system. Hope I was able to give you some ideas. Good luck.

"If you resolder both 100% it will take 1-2 hrs. I masked off portions so I could keep track of what I'd done."

Big mistake! Especially when you don't know what's REALLY the problem.
You may have now caused a real DME problem in the long run. DME units
rarely if ever fail.

Shotgun troubleshooting only causes more potential problems and wastes time & money!

Yes, I agree with Loren, it usually some other component of the system causing the problem. If you swap the DME unit and it still runs poorly, it must be one of the perifrial components. If it is not a fuel, or vacuum problem, check all the DME components, before doing a whole lot of soldering on the board.

OOps too late.

No change during the morning drive. It started up, then about 2 miles into it, it didn't want to idle. It would come down from ~ 2K and bounce to 0 bounce to 1500 rpms then die.

Pop the clutch and it would run.

About 1/4 mile in front of work, it sputters, hesitates and did not want to accelerate. VERY VERY erratic idle, no acceleration, coughing hacking and a little popping.

The drivers side of the DME was WARM (the amplifiers?) while the passenger side of the DME (other amplifier?) was cold!

I will call around today and ask the local shops to borrow/BUY TDC sensor and Speed Sensor my friend has an '86 DME that I may try before I buy a reconditioned one.

Does anyone know what the difference is between these two sensors?

http://www.pelicanparts.com/catalog/shopcart/911M/POR_911M_fuelMO_pg5.htm#item18

Meaning I am presuming the $52 one is the TDC center, while the $152. one (two required!!!) is the speed sensors???

Are there two of them?

Like I said in an earlier post, do not buy a TDC sensor, you don't need it. You can remove it and it will not have any effect on anything. It is ONLY used for testing purposes. The REF/SPEED sensors are the same part, your car has 2 of them side by side, in a holder mounted on the at the flywheel housing. 0.08mm air gap setting.

If the shop can loan you they DME box and swap with your box, may be you could pin point the problem.

If you have handsolder skill, would not take that long. I took my DME to work place and did it on spare time, would said 1 hrs and cost less than a new box

Thanks for the replies again guys.

Yah, I am still not sure what it could be. The local shop (only .5 mile away) said they would let me borrow some sensors. Now I called them and they said no. :( It will take a week for anyone (shop) to look at it to diagnose it.

Getting it towed home now... the shop said they can't garage it and it's too dangerous to leave outside at their shop.

Getting dark now, and it still won't idle, and runs bad.

3.2CAB
It runs fine for about 1 minute, do you still think it could be a vacuum hose???

I will double check the CHT sensor... actually, it seems like this is the culprit, as this was my problem when the car got TOO hot.

2nd -
I will try my friends DME, when I, and he get some time.

3rd -
Then I will order a speed sensor... I guess. Not sure what else to do.

Check out this web site (www.systemsc.com) on the Diagnostics page for
troubleshooting info and test values for the various sensors (ref/speed).

Note: Just bridge the CHT sensor with a paper clip to eliminate it.
Once the engine is hot, the CHT is ignored below 200 - 250 ohms.

Well... I have checked all but the computer and these sensors, as I don't have a scope.

Full throttle switch - Continuity when open all the way, else open in any other position --GOOD!


Idle switch - continuity when closed, else open --- GOOD!


CHTS 2400 ohms at 60 degree F --- GOOD!


Volume Air Flow Sensor (VAFS) - various ohms --- GOOD!


Intake air temperature inside VAFS --- 2500 ohms at 60 F ---- GOOD!


ISV - hums with ignition in the ON position --- GOOD!

So I have a computer, ignition coil, ignition pickup, and these two sensors left... right?

I am going to check the ignition coil now.

Just before I got it home on the a little rental trailer (yes they do actually fit, but it scraped up my front lip), I found a GAS LEAK! AHHHHHhhhh

And when it barely ran it RAN SOOOOOOO RICH! Black smoke, and I can smell RAW fuel out the exhaust.
So now I seem to have both a fuel and ignition problem else it's a bad computer.

Still, it my little mind, it sounds like a very intermittent speed/ref sensor that is not igniting the coil or fuel injectors synchronously...

I am also checking out http://www.systemsc.com/ now.

Thanks for all the replies!