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I verified you guys are right. Gotta bring car, new ECU, and key fobs to Subaru dealer to get it all programmed. Can't plug and play with ECUs, at least not on the '06 Tribeca. Can't just grab another ECU and throw it in and expect the car to run.

 

I have checked wiring extensively but didn't check for voltage on the ground pin for injector 1 at the ECU. Just go from the ground pin to ground, correct? Won't I have some voltage drop across the injector?

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Hello. You have completed some good troubleshooting.  I am not a believer in coincidences. If you did not have this issue before the head gasket work then the odds are less then 1% that you have a ECU problem post swap.  You mentioned that you have 130 psi in that cylinder.   That is low.  I searched around and you should be around 185. Google "low compression and misfire".  Check something like this out

 

http://www.subaruoutback.org/forums/66-problems-maintenance/157514-p0304-help-tried-everything.html

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I have checked wiring extensively but didn't check for voltage on the ground pin for injector 1 at the ECU. Just go from the ground pin to ground, correct? Won't I have some voltage drop across the injector?

In a properly operating circuit you'll see the voltage drop Only if the ECU is grounding the circuit. With key On,You should see 12v at both pins on the injector (plugged in) and you should have 12v at that injector pin on the ECU.

Voltage only drops across the load when a ground is present. With no ground, voltage will remain the same all the way to the end of the circuit, or the point at which the circuit is open.

So if/when the ECU grounds that injector circuit, the voltage will drop to near 0.

 

If you have 12v reaching the ECU with the key On, start the engine and see if that voltage fluctuates with the engine running. (Presumably the ECU will be pulsing that injector ground to spray the injector) Steady voltage at this point will confirm the ECU is bad.

 

If you have no or low voltage reaching the ECU, that will mean a break or poor connection in the wiring.

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I have 12 V at the ECU both with key on and with it running. No fluctuations - no grounding of the circuit to cause the injector to pulse.

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Ok, now as a final check to rule out a short to voltage, while checking voltage at the ECU (key on) Unplug the number 1 injector. Voltage should drop to 0.

If voltage stays up there is a short to voltage in the harness.

 

It's not easy for an injector driver circuit to burn out unless there is a short directly to supply voltage (no load, thus high current passing through the circuit) or an internal fault in one of the components in the circuit. What you don't want is to install a new ECU and have the same problem because of an issue in the harness that caused the old one to be damaged.

 

So with the injector unplugged if 0v showing on the meter, wiggle the major parts of the wire harness all around and make sure the voltage doesn't jump up instantaneously, due to a temporary short.

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Update: Found a used ECM on ebay. Took car, key fobs and ECM to a dealer. Dealer reprogrammed new ECM. VOILA!! Problem fixed!

 

So apparently this was that 1% probability that the ECM failed shortly after the head gasket work and the two problems were unrelated.

 

Definitely goes against reason, but hey, it's fixed now...

 

Thanks to you all for all the help!

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Update: Found a used ECM on ebay. Took car, key fobs and ECM to a dealer. Dealer reprogrammed new ECM. VOILA!! Problem fixed!

 

So apparently this was that 1% probability that the ECM failed shortly after the head gasket work and the two problems were unrelated.

 

Definitely goes against reason, but hey, it's fixed now...

 

Thanks to you all for all the help!

 

good job tracking this down and thanks for the feedback. 

 

i don't think it's likely - but curiously wonder what's the pro's would say to whether or not a long term misfire (caused by the ECU) could instigate, precipitate, or exacerbate headgasket failure?

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In my case, the misfire started about 40 miles after the head gasket replacement.

 

I'll let you know what fails next! :)

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I have over 200 miles post ECU swap, and no problems. Hope it stays that way :). I got a used ECU off ebay for $145 and it cost $115 to have the dealer program it.

 

Not as expensive as it could have been. I was glad I didn't diagnose it with a credit card, as some shops would have done...

 

Following some info on these forums, I boiled my diagnosing for a misfire down to the following steps:

1. Verify spark

2. Verify fuel and air

3. Verify compression

 

Once I saw the injector wasn't working (but all the others were), I then:

1. Checked for power at the injector

2. Checked for a pulse with a noid light

3. Gave the injector 12 V while engine was running and the misfire went away.

4. Traced the wiring harness and connections back to the ECU.

5. Tested the pulse coming out of the ECU to verify no injector pulse.

 

Hope this can be helpful to someone else...

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