Troubleshooting a start-stumble-shutoff Issue - after rebuild
12-23-2017, 12:19 AM
#1
Racer
Thread Starter
Solved: Troubleshooting a start-stumble-shutoff Issue - after rebuild
Purchased a 951 that had been sitting for about 10 years. The plan was to reseal/refresh the engine. But before I got started I wanted to make sure it ran. After a new fuel pump and bad DME replaced, I was able to fire up the engine. It ran for about 30 seconds before oil and water started spewing out. Shut her off and started my journey. I've posted a separate thread with pictures.
Fast forward to now. After putting everything back together, I was finally ready to fire the engine. Took a few tries, a can of engine starter, and a battery booster, but the engine fired up. But it would only stay on about 10 seconds and shut off. Did some more troubleshooting and found that if I manually held the barn door open in the AFM, the car ran perfectly. If I tried to use the throttle to keep the revs high, the engine would eventually shut off. But if I held the barn door open with one hand and turned the throttle with the other, I could rev the engine up and down. I ran the engine for several minutes (holding the barn door open, maybe 15 min. The engine ran well and didn't notice anything out of the ordinary (while I held the bard door open). Once I released the barn door after the engine was warm, it ran but rough and seem to stumble at times. Good news was no oil leaks or coolant leaks.
I purchased a used AFM but no change. Also note, that if I started the car with the temp sensor on, the engine would not respond to my throttle input well. In other words, I couldn't use the peddle to keep the revs high and the engine for shutting off. The engine was erratic and eventually shut off. Also, I did clean the tracks of both AFMs and moved the arm down to contact the strip on a new track.
After some more troubleshooting and reading, I figured out that if I disconnect the engine temperature sensor (NTCII) the engine starts right up and stays on. The only issue was that the revs go up and down, not violently or drastically, but definitely noticeable. The other interesting thing is that the revs are very consistent, its not a stumble or erratic. After reading more threads, I did purchase a new sensor and no change. At this point I started to suspect wiring. I double checked all physical connections and also looked for possible vacuum leaks. Note, I did install a new wiring harness from LR. Per several threads, i started to test the harness connection with a multimeter.
One thing I found was that I'm not getting any continuity when I test pin #16 of the KLR plug with the DME connected (KLR not connected). I did also try jumping the KLR pins #9 and #16 to bypass the KLR, no change. From what I read and understand, this might mean I have a bad DME. I did open the DME and I do see signs of corrosion. This is the second one and it did start the car before I started the tear down. So perhaps it already had corrosion and it got worse over the past year or so. The DME has not gotten wet while I've owned it.
To recap, I can only get the car to run well by manually working the barn door. I can also get the car to start up and remain on if I disconnect the engine temp sensor (NTCII), but the revs consistently goes up and down.
I have a separate thread asking if anyone has a DME I can borrow in the DC DMV area to test. In meantime, does anyone have any further suggestions or tests I can try?
Thanks in advance for your help.
Juan
Fast forward to now. After putting everything back together, I was finally ready to fire the engine. Took a few tries, a can of engine starter, and a battery booster, but the engine fired up. But it would only stay on about 10 seconds and shut off. Did some more troubleshooting and found that if I manually held the barn door open in the AFM, the car ran perfectly. If I tried to use the throttle to keep the revs high, the engine would eventually shut off. But if I held the barn door open with one hand and turned the throttle with the other, I could rev the engine up and down. I ran the engine for several minutes (holding the barn door open, maybe 15 min. The engine ran well and didn't notice anything out of the ordinary (while I held the bard door open). Once I released the barn door after the engine was warm, it ran but rough and seem to stumble at times. Good news was no oil leaks or coolant leaks.
I purchased a used AFM but no change. Also note, that if I started the car with the temp sensor on, the engine would not respond to my throttle input well. In other words, I couldn't use the peddle to keep the revs high and the engine for shutting off. The engine was erratic and eventually shut off. Also, I did clean the tracks of both AFMs and moved the arm down to contact the strip on a new track.
After some more troubleshooting and reading, I figured out that if I disconnect the engine temperature sensor (NTCII) the engine starts right up and stays on. The only issue was that the revs go up and down, not violently or drastically, but definitely noticeable. The other interesting thing is that the revs are very consistent, its not a stumble or erratic. After reading more threads, I did purchase a new sensor and no change. At this point I started to suspect wiring. I double checked all physical connections and also looked for possible vacuum leaks. Note, I did install a new wiring harness from LR. Per several threads, i started to test the harness connection with a multimeter.
One thing I found was that I'm not getting any continuity when I test pin #16 of the KLR plug with the DME connected (KLR not connected). I did also try jumping the KLR pins #9 and #16 to bypass the KLR, no change. From what I read and understand, this might mean I have a bad DME. I did open the DME and I do see signs of corrosion. This is the second one and it did start the car before I started the tear down. So perhaps it already had corrosion and it got worse over the past year or so. The DME has not gotten wet while I've owned it.
To recap, I can only get the car to run well by manually working the barn door. I can also get the car to start up and remain on if I disconnect the engine temp sensor (NTCII), but the revs consistently goes up and down.
I have a separate thread asking if anyone has a DME I can borrow in the DC DMV area to test. In meantime, does anyone have any further suggestions or tests I can try?
Thanks in advance for your help.
Juan
Last edited by 944M3; 12-27-2017 at 05:51 PM.
12-23-2017, 01:14 PM
#2
Do make sure there are no air leaks. I missed a small air line after the AFM, recently, with similar outcome. There is a small, 1/4" line to the AFM boot, not visible from the top.
Last edited by Alan 91 C2; 12-23-2017 at 01:32 PM.
12-27-2017, 05:58 PM
#3
Racer
Thread Starter
Solved.
Big DUH!! Moment. As part of the rebuild, I installed an adjustable fuel pressure regulator from LR. I ASSumed that the default pressure was 3 bar (at its lowest setting). But after speaking with Dave at LR he walked me through adjusting it and bingo, the car started right up and idled perfectly. He also took some time to walk me through some other troubleshooting steps, which I really appreciated.
Basically, what we think was happening was that when I disconnected the engine temp sensor, the DME pumped more fuel, thus compensating for the very low fuel pressure that the regulator was set at, and thus keeping the engine at idle.
Hope someone else learns from this thread. Though it sucked, I did get to learn more about how the DME, AFR, temp sensor and afp regulator work.
Juan
Big DUH!! Moment. As part of the rebuild, I installed an adjustable fuel pressure regulator from LR. I ASSumed that the default pressure was 3 bar (at its lowest setting). But after speaking with Dave at LR he walked me through adjusting it and bingo, the car started right up and idled perfectly. He also took some time to walk me through some other troubleshooting steps, which I really appreciated.
Basically, what we think was happening was that when I disconnected the engine temp sensor, the DME pumped more fuel, thus compensating for the very low fuel pressure that the regulator was set at, and thus keeping the engine at idle.
Hope someone else learns from this thread. Though it sucked, I did get to learn more about how the DME, AFR, temp sensor and afp regulator work.
Juan