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Old 12-21-2016, 05:13 PM   #1
v2rocket_aka944
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Default EJ25D ECU gurus...what will kill fuel pump signal?

I have been chasing a weird problem with my 1997 Legacy Outback 2.5 AT for a few months now.

On occasion, and only in very humid weather (80%+) the car will hiccup and stumble and stall while driving (after it has already warmed up and driven fine for 5-50 miles). In dryer (0-80% or so humidity) weather, regardless of temperature or altitude, the car runs fine 100%.

I went through and cleaned all the engine bay electrical harness connections and dieletric-greased the s*** out of them all for water ingress protection (thought it was a rain issue at first).

I replaced the spark plugs with new OE Denso Laser, new Subaru plug wires, and did the valve cover gaskets, thinking I was getting a short.

After these "fixes" the car still stumbles/bucks/stalls on occasion.
It will have its problem, I will pull over for 10-30 minutes and let it sit, and then it will start up and run fine for several more miles before suffering again.

I recall when the bucking is happening there is a distinct "whistle" noise that could be one of two things...either the muffler baffling reacting to a sudden pressure change? Or the fuel pump cycling on and off. I am more suspicious of the fuel pump, as the muffler only "whistles" at high throttle/RPM conditions, but the stumbling can happen at 2000rpm and 10 mph or cruising down the highway at 3000/70.

Looking over wiring diagrams for my year car the fuel pump is controlled by a simple relay whose coil is activated internally by the ECU. What I am wondering is...what sort of input/lack of input (faulty sensor?) would make the ECU cut power to the fuel pump relay intermittently, making the fuel pump go on/off as I believe it to be doing? Classically humidity issues would point to ignition, but why would a coil or igniter pack shorting out somehow cause the ECU to cut out the fuel relay?
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Old 12-21-2016, 05:27 PM   #2
jamal
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How new is the coil pack?
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Old 12-21-2016, 05:52 PM   #3
v2rocket_aka944
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As far as I know, original.
I have suspected it as well, but again, how does a shorted out coil affect the FUEL PUMP circuit?

Would think that the pump would continue happily running even if the spark isn't going anywhere due to a short.
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Old 12-21-2016, 06:21 PM   #4
Charlie-III
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Does it stall when raining?
Do you get CEL's? If so, what?

Try this, outside, dark area, spray bottle with water, engine running, mist the coil pack.
If it stalls the engine, you have a failing/cracked coil and it's shorting to ground. Over time and miles, the outer housing fails.
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Old 12-21-2016, 07:02 PM   #5
v2rocket_aka944
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Charlie -
I thought it was a rain thing at first but it has driven through torrential pouring rain recently without incident, but on very foggy mornings (but no rain for weeks) it will still have the problem. The rain idea is what led me to change plugs/wires in the first place without result.

I have done the "mist" test on MAF, TPS, coil pack, ignitor pack, individual spark plugs, cam sensor, crank sensor, and pretty much everything else under the hood electrical. No result.

I stepped it up and sprayed the running engine with my garden hose, at first from a distance to just get everything wet and then on each individual component to thoroughly douse them as my patience ran out. Completely soaking all the parts produced not even a single hiccup while idling in the driveway.
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The reason I am asking about the fuel pump control specifically is that I have noticed when the hiccup/stalling happens I am usually just cruising down the street/highway and get a bucking/jerking, sometimes once and then sometimes it will be rapid-fire every second or two. Giving it more throttle keeps me moving forward but does not stop the bucking. When the car is bucking I hear a distinct whistling type sound which to me sounds like the fuel pump when you first turn the key to ON and the pump primes the fuel lines. With each buck is a short whistle sound, like the pump switching on/off/on/off.

But seeing how the fuel pump is controlled (relay triggered by ECU) I am wondering what input (or lack thereof) to the ECU would cause the ECU to, even momentarily, open the contact on the fuel pump relay. A bad sensor, or maybe the ECU as a whole is losing power?
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No CEL whatsoever when this happens, or afterwards.

Last edited by v2rocket_aka944; 12-21-2016 at 07:09 PM.
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Old 12-21-2016, 08:01 PM   #6
Charlie-III
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OK, I'll think some more. No CEL and the sound is interesting.
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Old 12-21-2016, 08:45 PM   #7
v2rocket_aka944
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What do you think of the odds of humidity specifically affecting the crank angle/cam position sensors?

Being VR-types I can't really picture an internal failure since it's just a coil/core (magnet), other than an internal (inside the plastic connector portion) failure due to age/heat cycling. However, I would think aiming the jet-mode of my garden hose straight at each of these sensors and their respective plugs would make any such problem quite apparent?


My unfamiliarity with the Subaru ECU and systems (I have only owned this car since March '15) leads me to think of the fuel injection as it is set up on my other car, an old Porsche...obviously there will be many differences due to component manufacturers and simple age (1985 EFI vs 1997), but the ECU needs crank and cam position, MAF signal (with built-in IAT), O2/coolant sensor correction, and TPS...(this much TPS)+(that much MAF) = (engine load)*O2*ECT = spark firing time and injector open duration.

As far as I know all the other sensors and such are for the OBD-II "monitors" and not directly affecting engine running.

Last edited by v2rocket_aka944; 12-21-2016 at 08:51 PM.
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Old 01-03-2017, 05:35 PM   #8
al_s14
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Dont overthink it.If fuel pump does not prime during key turning without attempting start then your inputs are not the issue.

Make sure you got power coming in and out of ECM, same on relay as well as ground and key on signal.
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Old 01-08-2017, 09:47 AM   #9
Charlie-III
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1998 Legacy 2.5GT
Silver Sleeper Wagon

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Since you pretty well ruled out an engine bay issue, wonder if it's inside the cabin wiring/connection issue.
Do you have leaks inside the car?
Maybe check connections at the ECU?

Whistle may also be something in the "evap emissions" like a purge valve.
Hmmmmm........ where does the whistle seem to come from? Wonder if you have a gas tank venting issue? When it happens again, open the gas cap, listen for a rapid pressure change. Once open, reinstall the cap and try starting.
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Old 01-09-2017, 01:47 PM   #10
v2rocket_aka944
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update..
due to other suspicions i swapped the MAF (autozone reman) for an old Subaru MAF from the boneyard...

in recent weeks of humid/rainy weather the problem has not duplicated...i have put 500+ miles on it without issue so far.
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