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Old 11-09-2019, 10:29 AM   #1
davefedex5358
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Member#: 309719
Join Date: Feb 2012
Location: Dayton OH
Vehicle:
2012 Forester XT

Default 2012 Forester XT hard starting when hot, rough idle, engine misfire at idle when hot

Vehicle: 2012 Forester XT Touring, 110,000 miles

Engine: 2.5 L 4-cylinder - EJ255BPRJB

Transmission: TZ1B56WWAA - aka 4EAT

--- The problem ---

When the vehicle is started from cold, it starts, idles and run normally - no complaints. But when the engine is hot, the idle deteriorates, misfiring occurs (P0301, sometimes P0301 and P0302 or even all 4 cylinders misfiring, triggering CEL).

If the vehicle is stopped when hot and restarted within a few minutes, it's hard to restart and misfiring occurs triggering CEL. If the vehicle is allowed to cool (say, 2+ hours), then there is no problem.

Whether cold or hot start, the vehicle runs fine on the highway, good (great!)performance. This problem may have been present for many months but simply not noticed because of the way the vehicle is used.

The problem can be duplicated.


--- More about the vehicle ---

The vehicle normally gets 23-24mpg on the highway, less in city. This has not decreased. I consistently use 93 octane gas, usually from Mobil-serviced pumps.

The vehicle does not use oil (currently running Rotella T6 5w40, previously Mobil-1 5w30). Oil pressure is 100+ psi on startup, 80-85psi on highway (engine turning 2600-2800rpm.

The BOV was replaced a year ago with a GFB recirculating BOV.

The vehicle had a COBB (AT SF 93) off-the-shelf tune. Removing the tune did not impact the problem (but it has always run better with the tune in place).

Timing belt and all rotating components were changed at 106k. The problem existed before and after the timing belt service. There were no oil seal leaks detected during the timing belt service.

Based on Defi electronic boost gauge, the TD04 turbo will build and hold 12-13 psi boost. The inside of the intercooler is clean, slightly tacky to touch. The turbo is not pumping oil.


--- Steps taken and tests done so far ---

- Spark Plugs replaced: both old and new plugs were NGK LFR6AIX-11 gapped to .029". The old NGK plugs only had 20k miles and were not remarkable in appearance.

- the MAF was cleaned

- Cylinder pressures taken: 120-124 psi all cylinders. A leak-down test was done on Cylinder 1 with normal results.

- A smoke test was done but no vacuum leaks were found.

- The front O2 sensor was replaced with genuine Denso part 2349108

- All four injectors were replaced, including new lower seals, with genuine Denso part 2970013

- The throttle body was cleaned (it wasn't really that dirty, but...)

- No timing issues detected, based on visual belt inspection and electronic monitoring.

- The battery was load-tested at a battery shop. It tested okay (remarkably good for a 4-year old battery).

- The on-plug coils were switched about among the spark plugs - no change or improvement.

- the fuel trims were checked - all normal

- I have been systematically cleaning all grounding points and engine bay electrical connections. There are a few still to do, but most of the accessible grounding points are done.

The problem is still present.

----

Has anybody encountered a problem like this?

Any suggestions on what to do or check next?

I'm not a stranger to Subarus (this is my 3rd, and second turbo Forester) but I'm out of ideas. Several professional mechanics have been over this vehicle with the "fine tooth comb" and can't find anything. It hasn't been to a dealer (yet) but I'm not sure what else they can do.

One suggestion was to remove the exhaust manifold and inspect the exhaust valves and guides, to see if a valve guide was loose or had slipped.
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Old 11-10-2019, 02:30 AM   #2
blue wrench
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Member#: 349542
Join Date: Mar 2013
Default

There is an old bulletin Subaru issued for a few model years, including yours, where they noted that a bad wiring joint at the engine harness could cause random intermittent codes. Might be worth looking into. Subaru TSB 07-96-15
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Old 11-10-2019, 09:34 AM   #3
davefedex5358
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Member#: 309719
Join Date: Feb 2012
Location: Dayton OH
Vehicle:
2012 Forester XT

Default

Quote:
Originally Posted by blue wrench View Post
There is an old bulletin Subaru issued for a few model years, including yours, where they noted that a bad wiring joint at the engine harness could cause random intermittent codes. Might be worth looking into. Subaru TSB 07-96-15
Strange you mentioned this. I have had some random codes. I opened the bulk-head connector and cleaned all the contacts, reconnected, repeated several times. Ditto for the large connector adjacent to the ABS system. I have also been doing these same steps to all the electrical connections in the engine compartment as I come across them. All the 'random codes' have vanished.

But, the problem above is not a 'random CEL' issue. The hot engine is really idling rough, misfiring, almost to the point of stalling. You can feel it or hear it - you don't need a code reader.
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Old 11-10-2019, 01:56 PM   #4
some dude
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Join Date: Nov 2005
Chapter/Region: MWSOC
Location: MIC|-|IG/\N
Vehicle:
06 WRX
CGM

Default

Could a bad fuel injector(s), maybe one side fuel rail? be leaking fuel in to cylinder?
Maybe cylinders with fuel puddled inside makes for hard cranking? Trying to compress liquid? And misfires from too much fuel?

I'm no pro, just the only thing I thought reading this.
Good luck either way
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Old 11-10-2019, 03:07 PM   #5
davefedex5358
Scooby Newbie
 
Member#: 309719
Join Date: Feb 2012
Location: Dayton OH
Vehicle:
2012 Forester XT

Default

Quote:
Originally Posted by some dude View Post
Could a bad fuel injector(s), maybe one side fuel rail? be leaking fuel in to cylinder?
Maybe cylinders with fuel puddled inside makes for hard cranking? Trying to compress liquid? And misfires from too much fuel?
I was convinced that was the problem... leaking too much fuel into the manifold so the ECU was closing the throttle and starving the engine. But, as noted above, I replaced all the injectors with new Denso parts - truly a PITA. But it did not solve the problem.
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