ScottLindblad

Hi-

I'm hoping I can get some advice or suggestions. I have researched posts but cant find anything similar to my issue.

I have a 1983 911 SC cabriolet. The car has run very well for last 12 years. The only problem I have had was that the CDI igniton unit went out 10 years ago after a loud backfire, replaced with used one and car has run great until today.

About a week ago (7-14-2019), I was driving the car and the engine would rev sporadically between shifts going to 7000 even with my foot off gas peddle. It only happened once and then the car ran perfect.

I was driving the car yesterday (7-21-2019) and the car ran perfect until I parked for the night.

I tried to start it today and I heard a backfire, the car didn't start and the the starter motor would not turn off by turning key to off position. The starter motor would just continue to spin motor until I pulled off neg battery terminal.

After a little trouble shooting, I found that if I removed the fuel pump relay, I can control the starter motor using the key to turn on and off. However, If I put the fuel pump relay back in, I can start the starter motor but I can't turn if off using the key.

I also replaced the fuel pump relay and disconnected the CDI ignition unit, the starter motor would start but I could not turn off starter motor using key and had to pull neg battery terminal


Summary: Starter motor issue & car won't start:

History-
1: Car was running perfect
2. Engine revved to about 7000 sporadically a couple of times between shifts during a recent drive, and then returned to normal operation and ran great for a few days.
3. 3 days later: from #2: Tried to start car, I heard small backfire and the starter motor would not disengage until I pulled neg battery terminal- then starter motor stopped.
4. Trouble shooting: Found that if I removed fuel pump relay I could control the starter motor with key- on/off worked.
5. Trouble shooting: Replaced fuel pump relay and disconnected Ignition CDI unit. Starter motor would start but could not stop starter motor using key.

Looking for any advice / suggestion / guidance on what may be the cause of the starter motor control issue, it appears to be effected by fuel pump relay circuitry. As the car also doesn't run now I assume the starter motor control is linked to non-start issue..


Thanks in advance for any help!

ScottLindblad

Doing a bit more trouble shooting....pulled air intake to check pop off valve....pop off vlave was blown off and the sealing epoxy was broken / cracked etc.

I assume this happened when I tried to start car and had the backfire....not sure if a leaking pop off valve would have cause the high rev issue I had while driving....I will replace and reseal pop off valve and provide update

car now running perfect....not sure what was causing the statter issue...but it appears as long as the care starts...i can then shut off the engine and the starter motor also stops....I will provide update in a week regarding the starter motor issue....but at the moment all appear ok...

dr914

usually a sticking solenoid causes the starter to stay running, however next time it happens, pull off the clipped on yellow wire to the solenoid and see if the starter stops. If it DOES, then the ignition switch should be suspect. (or maybe a bad wire in the harness unfortunately)