I'd love some help diagnosing this mystery problem **Now with Video!** (see page 3)
06-21-2011, 03:34 AM
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I'd love some help diagnosing this mystery problem **Now with Video!** (see page 3)
I'm new at diagnosing electrical issues with cars and would love some help on this one:
After some spirited driving on Sunday evening my car sputtered a couple times, recovered for a bit, and then started surging between 500-1000 rpm at idle. It then died. I tried starting it again - it cranked, got to 1000 rpm, and then died again. I left it there for 1.5 hours and went for dinner, when I got back I was able to drive home - I made sure the engine never made it up to normal operating temperature by shutting it down whenever I could (most of the drive was downhill, and thank goodness for manual steering).
I've read up on clark's garage about the speed and reference sensors and why they might go wrong, but I just had them replaced at a local shop - I had a similar problem a few weeks ago and after they were replaced the car ran fine until now. I'll call them tomorrow to see if they checked the sensor gap (I removed the bellhousing not too many miles ago and may have accidentally moved the bracket while removing the sensors). If this is the culprit, how can I check the spacing on the bracket without damaging the sensors?
If they checked the sensor spacing, where else should I start looking? Injectors? I'm kind of overwhelmed at the moment, and would appreciate any insight! Thanks
After some spirited driving on Sunday evening my car sputtered a couple times, recovered for a bit, and then started surging between 500-1000 rpm at idle. It then died. I tried starting it again - it cranked, got to 1000 rpm, and then died again. I left it there for 1.5 hours and went for dinner, when I got back I was able to drive home - I made sure the engine never made it up to normal operating temperature by shutting it down whenever I could (most of the drive was downhill, and thank goodness for manual steering).
I've read up on clark's garage about the speed and reference sensors and why they might go wrong, but I just had them replaced at a local shop - I had a similar problem a few weeks ago and after they were replaced the car ran fine until now. I'll call them tomorrow to see if they checked the sensor gap (I removed the bellhousing not too many miles ago and may have accidentally moved the bracket while removing the sensors). If this is the culprit, how can I check the spacing on the bracket without damaging the sensors?
If they checked the sensor spacing, where else should I start looking? Injectors? I'm kind of overwhelmed at the moment, and would appreciate any insight! Thanks
Last edited by YoungerThan944; 08-16-2011 at 03:25 AM.
06-21-2011, 11:18 AM
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Verify your fuel pressure, and a new fuel filter might not hurt..
also check the fuel pressure regulator and fuel damper (pull the vacuum lines and check for fuel in them)
also check the fuel pressure regulator and fuel damper (pull the vacuum lines and check for fuel in them)
06-21-2011, 12:19 PM
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New fuel filter within the last 1000 miles. I'll take a look at the connectors and fuel damper/pressure regulator when I get home from work today. Anything else I should take a look at?
07-05-2011, 02:59 AM
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So after waiting on and installing a few unrelated parts I got back to diagnosing the issue. I took a drive up the hill, the car died, and I bump-started and engine braked home. 
Checked the connections to the S&R sensors, they appear to be fine. It starts and runs fine when it's cold and dies when it's hot. Would the car start in the first place if the S&R sensors or connections were bad?
I removed my water temp sensor connection before the car had warmed up, and the way the car died at the top of the hill was very similar to the reaction of the motor when the sensor was pulled - however, the motor didn't die when I pulled the water temp. It would drop to ~500 rpm, rev to 1000, and repeat every 1.5 seconds or so.
Checked the fuel pressure damper/regulator, there is no fuel coming out the other end.
Checked the coil, it seemed to be ok according to the clarks garage procedure, and the cap and rotor were replaced within the last 5000 miles.
I will check my fuel pressure tomorrow.
Would the car cut fuel if pressure was too high?
I just want to drive in this beautiful Vancouver weather!!
Checked the connections to the S&R sensors, they appear to be fine. It starts and runs fine when it's cold and dies when it's hot. Would the car start in the first place if the S&R sensors or connections were bad?
I removed my water temp sensor connection before the car had warmed up, and the way the car died at the top of the hill was very similar to the reaction of the motor when the sensor was pulled - however, the motor didn't die when I pulled the water temp. It would drop to ~500 rpm, rev to 1000, and repeat every 1.5 seconds or so.
Checked the fuel pressure damper/regulator, there is no fuel coming out the other end.
Checked the coil, it seemed to be ok according to the clarks garage procedure, and the cap and rotor were replaced within the last 5000 miles.
I will check my fuel pressure tomorrow.
Would the car cut fuel if pressure was too high?
I just want to drive in this beautiful Vancouver weather!!
07-05-2011, 11:00 AM
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I am no expert but I also say there is a problem with one the S&R sensor connections. When I was fidling with mine to get the tach working there were a couple of times they caused me similar to what you describe. There is a good diagnostic checklist for them on Clarks IIRC.
michael
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07-05-2011, 11:34 AM
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So after waiting on and installing a few unrelated parts I got back to diagnosing the issue. I took a drive up the hill, the car died, and I bump-started and engine braked home.
Checked the connections to the S&R sensors, they appear to be fine. It starts and runs fine when it's cold and dies when it's hot. Would the car start in the first place if the S&R sensors or connections were bad?
I removed my water temp sensor connection before the car had warmed up, and the way the car died at the top of the hill was very similar to the reaction of the motor when the sensor was pulled - however, the motor didn't die when I pulled the water temp. It would drop to ~500 rpm, rev to 1000, and repeat every 1.5 seconds or so.
Checked the fuel pressure damper/regulator, there is no fuel coming out the other end.
Checked the coil, it seemed to be ok according to the clarks garage procedure, and the cap and rotor were replaced within the last 5000 miles.
I will check my fuel pressure tomorrow.
Would the car cut fuel if pressure was too high?
I just want to drive in this beautiful Vancouver weather!!
Checked the connections to the S&R sensors, they appear to be fine. It starts and runs fine when it's cold and dies when it's hot. Would the car start in the first place if the S&R sensors or connections were bad?
I removed my water temp sensor connection before the car had warmed up, and the way the car died at the top of the hill was very similar to the reaction of the motor when the sensor was pulled - however, the motor didn't die when I pulled the water temp. It would drop to ~500 rpm, rev to 1000, and repeat every 1.5 seconds or so.
Checked the fuel pressure damper/regulator, there is no fuel coming out the other end.
Checked the coil, it seemed to be ok according to the clarks garage procedure, and the cap and rotor were replaced within the last 5000 miles.
I will check my fuel pressure tomorrow.
Would the car cut fuel if pressure was too high?
I just want to drive in this beautiful Vancouver weather!!
NO the DME will not cut fuel for excessive fuel pressure; it has no way of sensing fuel pressure.
Seems to me you are onto something with the head temp sensor / DME direction of your testing... IMHO you should keep testing in that direction...
ALSO check the O2 sensor output.
If I had to place a wager, I would be betting on either a bad O2 sensor, or a bad DME temperature sender in the head..
07-05-2011, 12:38 PM
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Thanks for the quick replies!
When I had the same thing happen to me a bit over a month ago the shop I took my car into diagnosed the problem as S&R sensor outputs not reaching the DME. They replaced the sensors and the car ran fine for two weeks before having the same problem.
One of the wires was exposed when I pulled back the connector boot.
Should I get a new connector or try to splice the wires correctly?
The water temp sensor was replaced a month ago, as was the O2 sensor - the water temp sensor could be faulty, I'll follow the clark's garage procedure to see if that turns anything up.
When I had the same thing happen to me a bit over a month ago the shop I took my car into diagnosed the problem as S&R sensor outputs not reaching the DME. They replaced the sensors and the car ran fine for two weeks before having the same problem.
One of the wires was exposed when I pulled back the connector boot.
Should I get a new connector or try to splice the wires correctly?
The water temp sensor was replaced a month ago, as was the O2 sensor - the water temp sensor could be faulty, I'll follow the clark's garage procedure to see if that turns anything up.
07-05-2011, 12:53 PM
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Thanks for the quick replies!
When I had the same thing happen to me a bit over a month ago the shop I took my car into diagnosed the problem as S&R sensor outputs not reaching the DME. They replaced the sensors and the car ran fine for two weeks before having the same problem.
One of the wires was exposed when I pulled back the connector boot.
Should I get a new connector or try to splice the wires correctly?
The water temp sensor was replaced a month ago, as was the O2 sensor - the water temp sensor could be faulty, I'll follow the clark's garage procedure to see if that turns anything up.
When I had the same thing happen to me a bit over a month ago the shop I took my car into diagnosed the problem as S&R sensor outputs not reaching the DME. They replaced the sensors and the car ran fine for two weeks before having the same problem.
One of the wires was exposed when I pulled back the connector boot.
Should I get a new connector or try to splice the wires correctly?
The water temp sensor was replaced a month ago, as was the O2 sensor - the water temp sensor could be faulty, I'll follow the clark's garage procedure to see if that turns anything up.
07-05-2011, 02:23 PM
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I'll take a look at the items you recommended tonight and see if I can figure anything out.
07-06-2011, 10:37 AM
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Sounds to me like a DME issue. I had a very similar problem on a Mustang I used to have. The idle would hunt, surge and carry on (but it would restart after stalling). It turned out to be a bad ECM (DME in Porsche speak). The problem was in the TPS circuit. I could set the TPS at its proper lead voltage witht he throttle closed (in a Mustang its set at .98V while car is off and key is on). As soon as the car would run for a few minutes the voltage would jump around causing the ECM to think that the throttle was opening and closing. It took me a little while to figure it out, and I finally just replace the ECM....
Now, im not saying this is what is going on in your car, but I would not be surprised if it were the case.
Now, im not saying this is what is going on in your car, but I would not be surprised if it were the case.
07-06-2011, 06:40 PM
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Here is a post with pictures I don't know to patch it in to another post but you plug LED's into the DME plug and spin the engine if the LED's flash the sensors are working. I have used this method and it works great. Maybe some can post it correctly.
Okay Roman. I hope I don't confuse you here. I had a no start problem a couple of months ago. I went through everything. The car was new to me and I had never seen it run, so there was a huge learning curve. Along the way I had to determine if, in fact, the reference and speed sensors were working since I could not "test" the DME. Incidentally I have a 1984 N/A, so this all applies to your car.
The following information refers to Clark's Garage document IGN-02 and the Porsche shop manual section on testing the DME and associated sensors. Porsche uses an oscilliscope to test everything. I don't have one, could not find one and I certainly wasn't buying one! So, lets use LEDS to test.
First, start by buying two LEDS. Radio Shack #'s 2760022 & 2760041
One lead on each LED is shorter that the other. That is the negative side, which is important. Mark that one black with magic marker on each. Now remove your fuel pump fuse and disconnect the 35-pin connector from the back of the DME. Using Clark's Garage document IGN-02, page 5 "DME plug connector" locate and mark pins 8,25,26,27.
8 & 27 are the Speed Sensor outputs; 8 is negative, 27 is positive. Insert your RED LED (2.6 volt) between the two. Remember your + & - !
25 & 26 are the Reference Sensor outputs; 26 is negative, 25 is positive. Insert your GREEN LED (2.1 volt) between the two. Remember your + & - !
It should look like this:
Make sure the leads don't touch. Now crank the engine over as you would normally. If everything is kosher your green LED should be blinking and the red LED should be almost steady as the engine spins.
This worked like a charm for me, helping me to determine that the DME was bad and ultimately solving my problems. If I didn't explain well, please ask for clarification. I'm no teacher! Good luck sir!!!
Kerry Chadderton
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12-16-2010, 02:50 PM #179
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Okay Roman. I hope I don't confuse you here. I had a no start problem a couple of months ago. I went through everything. The car was new to me and I had never seen it run, so there was a huge learning curve. Along the way I had to determine if, in fact, the reference and speed sensors were working since I could not "test" the DME. Incidentally I have a 1984 N/A, so this all applies to your car.
The following information refers to Clark's Garage document IGN-02 and the Porsche shop manual section on testing the DME and associated sensors. Porsche uses an oscilliscope to test everything. I don't have one, could not find one and I certainly wasn't buying one! So, lets use LEDS to test.
First, start by buying two LEDS. Radio Shack #'s 2760022 & 2760041
One lead on each LED is shorter that the other. That is the negative side, which is important. Mark that one black with magic marker on each. Now remove your fuel pump fuse and disconnect the 35-pin connector from the back of the DME. Using Clark's Garage document IGN-02, page 5 "DME plug connector" locate and mark pins 8,25,26,27.
8 & 27 are the Speed Sensor outputs; 8 is negative, 27 is positive. Insert your RED LED (2.6 volt) between the two. Remember your + & - !
25 & 26 are the Reference Sensor outputs; 26 is negative, 25 is positive. Insert your GREEN LED (2.1 volt) between the two. Remember your + & - !
It should look like this:
Make sure the leads don't touch. Now crank the engine over as you would normally. If everything is kosher your green LED should be blinking and the red LED should be almost steady as the engine spins.
This worked like a charm for me, helping me to determine that the DME was bad and ultimately solving my problems. If I didn't explain well, please ask for clarification. I'm no teacher! Good luck sir!!!
Kerry Chadderton
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12-16-2010, 02:50 PM #179
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07-19-2011, 01:46 AM
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Sorry for the (very) slow replies. Things got rather busy lately.
That wouldn't be the cheapest repair, but it would be nice if that was the sole cause of all my strife haha
Thanks for the procedure!
I ran both of these tests using only a 2.1V LED (radio shack/the source didn't have any 2.6V).
My reference sensor inputs look fine, it flickered at what seemed to be the right rate.
When I did the speed sensor test with the 2.1V LED, it flickered roughly at the same rate as the reference sensor. I would expect, as I used a lower-voltage LED, that the signal should be constant just like it should be with the 2.6V LED. Is this assumption wrong?
When I fired up the engine with the DME and DME relay installed, it started cleanly to 1300 rpm before falling immediately back to zero. Would not having the speed sensor read correctly cause this?
Sounds to me like a DME issue. I had a very similar problem on a Mustang I used to have. The idle would hunt, surge and carry on (but it would restart after stalling). It turned out to be a bad ECM (DME in Porsche speak). The problem was in the TPS circuit. I could set the TPS at its proper lead voltage witht he throttle closed (in a Mustang its set at .98V while car is off and key is on). As soon as the car would run for a few minutes the voltage would jump around causing the ECM to think that the throttle was opening and closing. It took me a little while to figure it out, and I finally just replace the ECM....
Now, im not saying this is what is going on in your car, but I would not be surprised if it were the case.
Now, im not saying this is what is going on in your car, but I would not be surprised if it were the case.
One lead on each LED is shorter that the other. That is the negative side, which is important. Mark that one black with magic marker on each. Now remove your fuel pump fuse and disconnect the 35-pin connector from the back of the DME. Using Clark's Garage document IGN-02, page 5 "DME plug connector" locate and mark pins 8,25,26,27.
8 & 27 are the Speed Sensor outputs; 8 is negative, 27 is positive. Insert your RED LED (2.6 volt) between the two. Remember your + & - !
25 & 26 are the Reference Sensor outputs; 26 is negative, 25 is positive. Insert your GREEN LED (2.1 volt) between the two. Remember your + & - !
It should look like this:
Make sure the leads don't touch. Now crank the engine over as you would normally. If everything is kosher your green LED should be blinking and the red LED should be almost steady as the engine spins.
8 & 27 are the Speed Sensor outputs; 8 is negative, 27 is positive. Insert your RED LED (2.6 volt) between the two. Remember your + & - !
25 & 26 are the Reference Sensor outputs; 26 is negative, 25 is positive. Insert your GREEN LED (2.1 volt) between the two. Remember your + & - !
It should look like this:
Make sure the leads don't touch. Now crank the engine over as you would normally. If everything is kosher your green LED should be blinking and the red LED should be almost steady as the engine spins.
I ran both of these tests using only a 2.1V LED (radio shack/the source didn't have any 2.6V).
My reference sensor inputs look fine, it flickered at what seemed to be the right rate.
When I did the speed sensor test with the 2.1V LED, it flickered roughly at the same rate as the reference sensor. I would expect, as I used a lower-voltage LED, that the signal should be constant just like it should be with the 2.6V LED. Is this assumption wrong?
When I fired up the engine with the DME and DME relay installed, it started cleanly to 1300 rpm before falling immediately back to zero. Would not having the speed sensor read correctly cause this?