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Diagnosing a difficult electrical problem
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bsmyth292
New User
Sep 5, 2012, 11:46 AM
Post #1 of 4
(1850 views)
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Diagnosing a difficult electrical problem
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1995 Subaru Legacy Brighton with 207,000 miles.
When it runs it runs great, still. Problem is, over the past couple years a problem has developed:
49 times out of 50 it will start with no problem. That last 1 time out of 50, the car will turn over but engine won't catch.
Removing the spark plug reveals no spark on crank. So, I've got an electrical problem.
Now, eventually the car will start again. It will take anywhere from a couple minutes to several weeks, but eventually the engine will catch. I can't tell you how many times I left the car overnight in a parking garage somewhere, to come back the next day thinking I needed to tow it, when I tried "one last time" and the engine started right up.
I replaced the spark plug wires, the cam position sensor, the ignition coil, and the ignitor (which on this model is separate from the coil). The problem still exists. Of course, given the infrequent nature of the problem, I've gone long periods of time thinking that I've found the problem and solved it when, what do you know, a month later the problem returns, and I'm stranded somewhere 20 miles from home.
A few times I've had it towed to the mechanic, and when it gets there it starts up fine, so something has jarred something else temporarily back into place.
I've come to the conclusion that it may be bad wiring, maybe a bad connection to the ground somewhere. My mechanic told me that often wires start deteriorating under the chassis and an intermittent short may develop.
I'm at a loss as to where to begin troubleshooting bad wiring. I look under the hood and start from the battery and all the various wires etc. just disappear into the innards of the auto and I'm not sure where to start.
Any advice where to start and how to proceed?
Why don't I take it to my mechanic? --He and I have talked about it, but the problem is is that every time I bring it in, the car ends up working. Like I said, it only fails maybe once out of every 50 times. It's hard to reproduce the problem on demand. So, I can't reproduce the problem easily or frequently enough to show him what the problem is.
Advice is greatly appreciated!
(This post was edited by bsmyth292 on Sep 5, 2012, 11:48 AM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 5, 2012, 12:16 PM
Post #2 of 4
(1827 views)
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Re: Diagnosing a difficult electrical problem
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This being that intermittent it would be nice if would stay broken long enough for some testing. Generally lack of fuel delivery or lack of spark but if it lacked spark I would think IF fuel was being delivered with attempts it would struggle a bit once it makes up its mind to start.
If no other code info (maybe not) and that erratic wild guess is maybe fuel pump relay just doesn't make
good contact inside. Testing would do nothing while it is all well.
You could try priming it with starting fluid if a few "no starts" are happening and if that suddenly works know it's fuel related OR whack/tap on the pump relay with the plastic end of a screwdriver and some will snap to. If convinced a trick made the car start toss it. Under dash inside - I think driver's side with others and owner's manual should specify exact one + location. Looks like this.......
On the wild maybe list and catch it fast when happening,
T
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nickwarner
Veteran
/ Moderator
Sep 6, 2012, 2:27 PM
Post #3 of 4
(1767 views)
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Re: Diagnosing a difficult electrical problem
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When you have started it 49 times, skip to time 51 and it will run fine.
Worth a shot , right? You could have a bad crank sensor. You would want to carry a spark tester and a noid light in the car so you can see what is missing when you can't start it. Since this is so intermittent you have to have things with you to capture the failure when it happens. The noid light plugs into the injector wires and emits a light flash when it gets a signal from the PCM to fire. See if you are missing spark and injector pulse or just spark. Spark and injector pulse missing together indicates the PCM isn't seeing a signal from the crank sensor. If you have someone with you, have them crank it while you wiggle the wiring harness around. If you suddenly fire up you know the wiring needs to be looked at and repaired.
Now the bad news. If you cannot get this to act up for you while you have the ability to test it, you may have to actually let it get worse so that you can pinpoint the cause.
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Hammer Time
Ultimate Carjunky
/ Moderator
Sep 6, 2012, 3:13 PM
Post #4 of 4
(1762 views)
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Re: Diagnosing a difficult electrical problem
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It's not a difficult electrical problem. It's an intermittent electrical problem. It's all in the timing. You can't fix what isn't broken and it isn't broken for 49 starts. The problem would be easy to diagnose if it was present when being tested.
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