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2000 OBW 2.5l
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Discussion Starter #1
My wife just came home and I noticed the engine was making peculiar noises before she shut it off. When she turned the car "off" and removed the key from the ignition, it remained running (diseling) at a low RPM... like maybe 1 cylinder was dieseling.

I pulled the spark plug cables and that didn't stop it, so next step was to remove the serpentine belt. With the help of a breaker bar, that did the trick. (I probably could've shifted it into gear and killed it, but oh well).

Anyway, what might cause these symptoms?
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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Was the car overheating?

When you say you pulled the spark plugs, you mean the cables? All of them or one?

When a car "diesels" it is detonating the fuel in the combustion chamber without the availability of a spark. The extreme heat in the chamber is igniting the fuel. This usually happens in fuel injected vehicles when it overheats.

How was it getting fuel? A partially stuck open injector.

Can you check the ECM for DTCs? Will it start up normally now?
 

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2000 OBW 2.5l
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Discussion Starter #3
Was the car overheating?

When you say you pulled the spark plugs, you mean the cables? All of them or one?

When a car "diesels" it is detonating the fuel in the combustion chamber without the availability of a spark. The extreme heat in the chamber is igniting the fuel. This usually happens in fuel injected vehicles when it overheats.

How was it getting fuel? A partially stuck open injector.

Can you check the ECM for DTCs? Will it start up normally now?
Sorry, fixed above post. I disconnected all of the plug wires at the coil.

Car was at normal temp at the time, no obvious codes thrown (no CEL), will scan to see if there are others.
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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All it takes is fuel, oxygen and a chamber temperature over 495F to obtain ignition. You most likely have a leaking injector or leaking fuel pressure regulator. Now to find out which one.

Pin point testing won't necessarily tell you. You can pull off the vacuum hose from the regulator and look for evidence of fuel. If its wet, its leaking. The best time to check is right after shutting the engine off while there is still pressure on the fuel rail. The AF sensor and LTFT data would tell you if its rich, but not which bank its on. If you can look at live data, and the LTFT is negative at idle and high rpm, then its rich.

Two ways to go about checking the injectors if its not the regulator. Unplug all the injectors so you won't have to fight with them later and plug them back in. When/if the car does it again, start unplugging injectors until it stops. The one that stops it is the failed injector.

The other is remove and pressure test. You need the tools for that and you may as well buy a set of injectors over the tools.
 

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2005 OBW 2.5L, 1989 Subaru Justy, RIP Blu
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7,355 Posts
How many miles are on this car. Next time this happens DO NOT do what you did.


NEVER EVER EVER that is extreemly dangerous. In fact that shouldnt have even worked.


Starve it for air by clogging the intake.

Like was said you can have a leaking injector. What does your motor oil smell like and what level is it at. How does it run otherwise. Also you may have some carbon buildup in the engine, that would give the heat source for a leaky injector to ignite an air fuel mix.
Luckily checking the fuel pressure is fairly easy with a cheap pressure gauge.
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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When it diesels, after ignition kill, the coil is dead, the ECM is down and the alternator can't supply enough amperage to charge the coils. If the key was on, yeah, shock of your life, but in this case, it would have been a large fluke that the coil would've been energized.
 

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2000 OBW 2.5l
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Discussion Starter #7
Thanks for the feedback guys.

Regarding the injectors vs. the fuel pressure regulator, riddle me this...

If the fuel pressure regulator is leaking, and no injectors are leaking, how would this scenario be possible? Would the injector(s) not stop the fuel?
 

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'14 Subi OBW, '18 Subi Forester
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One would think that without active participation from the ECM (key off), no injectors should have received an active pulse to energize their coils and fire. That favors a leaking injector and sufficient residual fuel line pressure to keep it spraying for a while.

I'm still not sure how you managed to derail the accessory belt, or exactly what roll that had in stopping this....
 

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Discussion Starter #9
I'm still not sure how you managed to derail the accessory belt, or exactly what roll that had in stopping this....
In hindsight, I'm almost positive that removing the belt played no role in the engine stopping. In the haste of the moment, all I did was effectively disconnect the A/C. Derp.
 

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06 OBW 2.5, 05 Forester, had 03 H6 OBW
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Thanks for the feedback guys.

Regarding the injectors vs. the fuel pressure regulator, riddle me this...

If the fuel pressure regulator is leaking, and no injectors are leaking, how would this scenario be possible? Would the injector(s) not stop the fuel?
The regulator senses fuel demand by measuring the vacuum in the intake manifold.

When the regulator leaks it leaks fuel directly into the intake until the fuel pressure drops.
 

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Excellent! I didn't think about a regulator diaphragm failure. We've just gone from port fuel injection back to a throttle body / carburetor.
 

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01 VDC, 05 R Sedan, 06 BAJA EJ257
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Learn something new everyday, eh?

But, it could be either. If an injector is staying open, the pressure in the rail is feeding one cylinder. If its the regulator, its feeding a lean mix to the intake. That's why you check for fuel in the regulator vacuum hose right after shut down while pressure is still on the rail.

Either way, the engine will run odd throughout because the AFR will change and the LTFT will show it.
 

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Some thoughts/questions:

The idea of a leaky injector and/or fuel pressure regulator seems quite possible. But I was wondering how long the engine could continue to run after the ignition and, presumably, the fuel pump, are turned off. As far as I know, there's no "fuel system reserve pressure tank", most of the fuel system is metal pipe which can't expand/collapse, and the liquid fuel itself can't be compressed. So the pressure after the pump is turned off must be due to the short lengths of rubber hose (in the fuel lines) that can stretch a bit, thereby providing some residual pressure. But it seems to me that this pressure will dissipate very quickly when fuel is used. So, how long did the engine continue to run?

Another source of fuel in the intake after the throttle body is the evaporative emissions purge control solenoid valve. This is normally closed, but opens to draw fuel fumes from the charcoal canister and fuel tank when driving. If the valve remains open, the fuel fumes being drawn into the intake can cause poor idling, and perhaps even some dieseling.

It's been a few days since the original report. Has the car been used again, and has there been any sign of the same problem?
 

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2000 OBW 2.5l
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Discussion Starter #14
The engine ran for about 5 minutes after the ignition key was removed. It sounded like only 1 cylinder was firing at that time and the RPMs got progressively slower and slower until it finally stopped.

I'm hoping to get under the hood this afternoon to check it out. Somehow in the midst of all of this, the battery became drained (only 18 months on it).
 

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Last time I changed the fuel filter on my '02, I got a pretty good pressurized shot of fuel out of the upper hose. Probably on the order of 2 oz or so. At low RPM, that could power the engine for at least a minute.

Remember that while water is incredibly stable in terms of volumetic expansion from a few degrees above freezing to the boiling point, carbon based liquids like gasoline do have a somewhat linear expansion with temp. So if the lines around the engine heat soak when the ignition is shut off, expansion could fuel this leak longer than you might think.
 

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Discussion Starter #16
Quick video I took the night of. It's dark, but you get the gist.

 

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Plain OM, the pressure on the fuel system doesn't drop off immediately after the key is turned off. The fuel is slow to drain back in to the tank, so at shut off, if there is a fuel system leak, the pressure in the lines and rails will feed the low vacuum being created and detonate.
 

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I appreciate that the fuel supply lines normally retain pressure after the car is turned off -- as Fibber2 noted, there can be quite a splash when disconnecting a hose if the line pressure isn't relieved first by removing the pump fuse and turning over the engine for a bit. (This causes the ECM to pulse the injectors, thereby dropping the pressure down so that disconnecting the fuel line doesn't lead to the "pressurized shot of fuel" Fibber2 experienced.)

What I was trying to examine is how long the pressure in the lines and rails can sustain the dieseling engine, because it seems to me that while there is residual pressure, it is sustained by a very small volumetric area (the expanding circumference of the rubber hoses) and therefore will be run down quickly. Fibber2 suggested at least a minute, which is still a far cry from the 5 minutes reported. But it's really an academic question and not one to spend time on.

Still like to know if the car is exhibiting the same or similar symptoms since that first episode.
 

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2005 OBW 2.5L, 1989 Subaru Justy, RIP Blu
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Well that is assuming your pressure regulator is working properly. If it is keeping the pressure high it may be taking longer to bleed down. All you need is one cylinder with a leaky injector and a hot spot in the cylinder to deisel. Keep in mind that the ideal combustion AF ratio is 14.7 air to one part fuel. That is a very small amount of fuel. The cylinder itself can be pulling the fuel in from the bad injector.
 

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Back of the envelope calculation: My OB turns around 2400 RPM at 60 MPH, and delivers around 24-25 MPG steady state. That's approx 5 oz/minute consumption at cruising.

Drop that to less than 1/4 the RPM, and derate it further for near zero load and torque production - just enough power to keep the engine rotating, and you get maybe a half ounce of fuel per minute? Academic exercise, but interesting!

To the OP - next time just drop the tranny into gear and it should stall out. I used to do that with an old Ford until I got crafty and installed an electric throttle stop solenoid from a much newer car on my old beast.
 
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