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1989 930 Engine quits, backfires, restarts

Thought I would ask if this is familiar to any of you. Low milage 930 driven sporadically
but always starts easily, boost functioning and basically the car has been terrific. Full engine rebuild about 2,000mi ago, sent to a very qualified shop to check everything after my purchase, and came back even stronger with even idle.

After a short run (into boost occ.) of about 35 minutes, in traffic the engine stumbled and as I thought is was a fuel issue, tried to feed it gas...a few "pops" then a few loud backfires and I coasted to a stop and cut ignition. Would not restart or fire at all. While waiting for AAA, 15 minutes later it restarted and I nursed it home without issue. Optimistically I thought it would not happen again, but today the same happened again after about the same driving time. This time cruising at about 40mph.

A friend behind me said they saw a few puffs of black smoke then bigger ones (backfires) and being a general mechanic, he thought I should replace the CDI box because it appeared to be ignition. I should mention that I couldn't hear the fuel pump due to a fan running on the rear fender oil cooler.

I know it could be a failing coil (works after cooling down?) but where would you start if you can't get it to happen in a garage? I plan to replace relays and check connections. Any input appreciated.

Last edited by Pfan; 12-29-2017 at 03:41 PM..
Old 12-29-2017, 03:38 PM
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I had an issue where my over boost sensor didnít have a good ground and caused sporadic issues with fuel and ignition.


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1989 Slate Grey 930...still tinkering
Old 12-29-2017, 05:03 PM
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Thanks for the hint. Just looked into that switch and its function online. Certainly might be the culprit if the connection is bad or loose. Didn't happen under boost fortunately. I'll check that first.
Old 12-29-2017, 06:47 PM
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Def possibility. Idle the engine and wiggle the terminal. They do go bad. Plenty of threads. Or the dreaded yellow relay??
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 12-29-2017, 09:53 PM
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Some progress and questions. Grounded the wire end at the overboost switch, giggled the end to see if a bad connection or broken wire was the culprit. All ok. While running (cold) I also giggled the yellow relay to see if that would break the gound ...all the relay pins appear tight. All ok. I made a jumper for the yellow relay to ground 87A if if happens again (breaking the jumper kills the pumps). So still could be bad overboost switch or relay or ??

Is the popping and loud backfires when it shuts down indicative of fuel starvation or ignition failure? Trying to find the right path to find the cause. Thanks again for any input
Old 01-01-2018, 03:45 PM
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You need to wiggle the o/boost terminal while connected to the wire. That is the earth circuit for the pumps. If it breaks the circuit the pumps stop. Earthing the wire elsewhere has disabled the switch, so no fault will show.
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 01-01-2018, 05:00 PM
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Thanks Alan. Good point. I know I just ruled out the wire spade terminal and not the switch. Any idea re the backfires?
Old 01-01-2018, 05:17 PM
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Move the 14pin connector around as well, a bad pin connection (i forget which ones) can also shut the car down. fuel pump shutdown will get you a backfire
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Old 01-01-2018, 05:21 PM
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Quote:
Originally Posted by Pfan View Post
Thanks Alan. Good point. I know I just ruled out the wire spade terminal and not the switch. Any idea re the backfires?
The switch is spring loaded (1.1 bar) to make a circuit for the earth for the pump relays.
The internals of the switch are of dubious quality and do fail - and usually erratically/intermittently.
Yes a fuel shut down will strangle the engine for fuel = backfire.
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 01-01-2018, 06:04 PM
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I pulled the 14 pin connector on the relay block, inspected (all seemed ok), then reinstalled. I will try moving that block and the OB switch terminal next. I'm going to get a new OB switch and pull the old one and test. By comparison, what would a intermittent coil/CDI box do re backfires??
Old 01-01-2018, 07:04 PM
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You can test the OB sw just by wiggling the terminal while engine is running. Or you can pull and test with multi meter - ohms, between terminal and body of sw while wiggling terminal.
Ign/CDI cutout would be a miss rather than backfire - spark interruption. Backfire generally lean mixture.
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 01-01-2018, 07:57 PM
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Tough to diagnose until total failure. Bad coil, bad green wire, bad CDI, etc will produce similar symptoms. You need to go through the entire ignition circuit and check all the wiring and components.
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Old 01-02-2018, 03:48 AM
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Also check the rotor on the distributor and spark plugs. I've had both go bad and giving similar trouble. Both also after about 30 min of driving to get heated enough.
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Old 01-03-2018, 08:51 PM
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could be:
cd unit
coil
speed relay under seat. - go thru and resolder all the joints.

these would top my list. $$ is on the coil
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 01-04-2018, 08:47 AM
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Quote:
Originally Posted by RarlyL8 View Post
Tough to diagnose until total failure. Bad coil, bad green wire, bad CDI, etc will produce similar symptoms. You need to go through the entire ignition circuit and check all the wiring and components.
hey brian, I paid for an adjustable WUR,. my boost CP and warm CP are not separately adjustable.
when are you going to make this right?
not answering my PM's is not a solution for me
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 01-04-2018, 08:49 AM
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Thanks for all of your tips. I'll start the process this weekend when I visit the car in my warehouse. I also found the "sticky" that highlights resolving this sort of problem. Will update as I rule out causes. GC
Old 01-04-2018, 05:10 PM
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Update. Idled the car, shook, shaked, rattled, jiggled, pulled-inspected- reinstalled, etc the OB switch connector, the yellow relay, the ign delay relay, the fuel pump relays, the OB wire, the green dist wire, the 14 pin rear relay chassis plug, the speed relay(s) under seats and .....none of it shut off or affected running. So its erratic and/or heat related.

Thought I would provoke a shutdown by getting it real warm, driving on boost then cruising, idling for 20 min then drive some more. Results....ran perfect after over an hour (no surprise since I now have my OB jumpers).

The one thing I did see though was the green wire exiting the distributor was bent at a fairly hard 90 degrees and it was rubbing on the fan strap. There wasn't room to see if it was worn at the contact point (and the heat shrink started past there) so I bent it slightly so it wasn't pressing quite as much.

Cant be that easy.
Old 01-06-2018, 12:15 PM
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Well you could try pushing the green wire back hard on the fan housing and see...
You may have simply reinstated a bad contact pin somewhere? The 14 pin plug for instance. Time will tell........
We wait for the next chapter :-)
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 01-06-2018, 01:08 PM
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Quote:
Originally Posted by T77911S View Post
could be:
cd unit
coil
speed relay under seat. - go thru and resolder all the joints.

these would top my list. $$ is on the coil
if you got hot and it ran fine then I would start with the speed relay under the seat.
if you have a spare coil I would carry it with you.
next time it dies, if it will not restart check for spark.
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 01-08-2018, 03:18 AM
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I had something very similar and it was charging/battery issue. Good luck. As stated earlier it ia hard to diagnose until it fails.
Old 01-08-2018, 02:12 PM
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