Intermittent Ignition Problems - 911 (1988)
 

Hi,

Ok, I have been searching the forums for help.

Symptoms:
Start the car
Drive 20 minutes - Perfect
Stop the car
Car will not start again
Wait 30 minutes - No really every time its 30 mins.. and it starts.

Spark and ignition is perfect when working (Confirmed with Strobe)
No Ignition when the car will not start (Confirmed with Strobe)

Car Specs:
1988 911 Targa Super Sport
Sigma Sector S Alarm

Work so far:
New DME Relay
Checked the DME Harness pins 25/26 - 1155 Ohms drops to 1127 when not working
Checked the DME Harness pins 8/27 - 1170 Ohms increases to 1245 when not working

Questions:
Some forums say when I turn the key I should get a voltage on 25/26 pins and 8/27.. I do not.. is this normal?
What on earth could be stopping the ignition.

Please help me...

wild guess
 

aloha, i'm not sure if 88 3,2 has a crank position sensor but on a 89 3.6 motronic it was the solution to an oddball intermittent issue I worked on. easy to check with a noid light for a fuel injector (cheap at any parts store) as the sensor generates a volt and a half or so. if it doesn't, no fuel pump, spark etc as it tells the dme when the engine is turning, cheers
geoff

Just finished resolving the exact problem on my 87
Reference sensors were too far away from flywheel - problem happened after replacement of both reference sensors and 3 months of driving - my symptom was same as yours but I also did not get full range in revs - after 5500 rpm engine simply starved for fuel. Easy fix after working out problem

Remove left rear wheel whilst jacked up. Loosen 2x 10mm bolts and move reference sensor till it touches flywheel then move back a very small amount and slightly tighten all bolts. Once you think in right position use fan pulley bolt to rotate engine to confirm reference sensor does not hit flywheel. Tighten all bolts to required torque.

Happy driving

I agree with above - I changed out both sensors, speed and reference as they sit right next to each other. No more problems.

I recently had an intermittent problem too on my 85. It was the electronic portion of the ignition switch. The contacts get worn and sometimes everything is fine and sometimes not ... worth consideration.

Guys. Thanks so much. I will try those things..

A slight update. I have a multimeter on pins 25/26 and there is no voltage on cranking the engine.

The voltage from 8/27 is 0.9v.

Oh well.. I thought it was the sensor.. but the results from the multimeter are inconclusive:

TDC - Pins 25/26
0hms / Volts
1155 / 0 - Non Start
1125 / 0.125 - Starts
1058 / 0.143 - Starts
1233 / 0.124 - Non Start

RPM - Pins 8/27
0hms / Volts
1147 / 1.7 - Starts
1086 / 1.9 - Starts
1247 / 1.8 - Non Start
1226 / 1.8 - Non Start

I had an issue like this on my '88. It turned out to be not the reference sensors, but a bad connection on the CHT sensor. Make sure the plugs on all three sensors are locked in and secure. I had to take mine out of the bracket, and ran it that way until I could replace the broken connector.

If you can verify the injectors are pulsing you could rule out the variable reluctance sensors. If you short the Cylinder Head Temperature sensor mating connector you can rule that out.

If the injectors are working you will have to look into the DME itself.

Had a very similar problem on a 74. A several pin connector carrying electricity fore and aft thru the front "firewall" had a bit of corrosion at one or two of the pins. Unplugging and cleaning fixed the issue.

crank position sensor or cylinder head temp sensor.

Cylinder Head Sensor (CHT).. Pins 13/5 on DME Harness.

Cold Start (Working) - 4.66 KOhms on DME

Took for a drive and took over 5000 RPM to make sure there was no loss of power.. There wasn't and it was as always a lot of fun.. Until I hit a puddle!

Returned > Turned engine off > Start up.. No Problem
Waited 5 minutes > Attempt Start up.. No Start! (179.6 Ohms)

I am now just using this as a Diary.. Are we still thinking Sensors, they seem to be working correctly???

Should those noted pins for your resistance checks of the ref/speed sensors correlate to terminals 1 and 2 at the connector up top (engine bay) looks like the range should be 864 ohms - 1056 ohms (at ~77 degrees F) . Is she going high when warm?..?..///

Terms 1-3 and 2-3 should be >100K

Per Bentley.

Best!

Doyle

Ooops brain fart.

The CHT sensor has nothing to do with spark delivery.

You should however see if you have injector pulses to see the DME is getting reference signals. There have been cases of the ignition transistor in the DME having bad solder joints.


EDIT the cht will not inhibit spark.

CHT numbers look to be ok.....knowing:

(from the Porsche manual)

CHT and sensor temp is ~ 265 degrees F with an oil temp of 194 degrees F. This would give you 90 ohms.

212 degrees F gives you 160-210 ohms.

On the other side,....32 degrees F will give you 4.4k-ohms to 6.8 k-ohms.

Have you checked the DME for cracked solder joints? On mine that caused similar problems - runs GREAT then bam, no spark. Cool down a little and runs again as if nothing happened. The coil is driven by transistors that stand up off the board, so they are subject to vibration and easily crack the solder over time.

Chuck.H
'89 TurboLookTarga, 346k miles

Hi, I resolved it. It was simply the connection to the DME Relay. The connections looked good but they weren't making 100% contact.

Thanks everyone for your help!

So what'd you do to make good contact? Just remove and replace a bunch of times?