Just bought an '05 Outback 2.5XT. Engine has just been rebuilt. When I start it up there is a knock which quickly goes away. The temperature gauge has started climbing erratically, doesn't always do this. I noticed last night that the engine knock was occurring while accelerating from a stop. This doesn't seem to happen when the engine is cold. The engine also idles rough, to the point of shaking the car a little. It's worse when engine is hot and stopped with the car in drive or reverse. The rpm's seem too low, maybe around 500. As a diesel mechanic myself with no experience on boxer engines my first thought was timing; fuel knock, temp increase, rough idle, etc. Any suggestions??

Check coolant level. Low coolant will cause temp variations.

These engines are picky about thermostats so of the coolant level is correct you will probably want to pop the thermostat out and put a genuine Subaru stat in it.

 

The knock could be several things. Very often the timing tensioners fail on those engines and will start bouncing up and down.

 

A cold knock for just a few seconds could be due to a poor quality or incorrect oil filter.

These engines are known to have piston slap which will make a deep knock sound until the engine is warm.

 

ECU will set engine idle speed and that's not adjustable. As strange as it may sound, vibration when in gear at idle can be due to poor quality front axles.

Possible the timing is off, but that would generally result in a steady misfire at low RPM, and would set a code and turn on the CEL. The engine would be very rough and very low on power until 2,500-3,000 rpm.

Thanks for the info. Coolant level is good. Have checked multiple times & tried to burp cooling system. Engine was just rebuilt so; new thermostat, w/p, radiator, timing belt, tensioners. No check engine light yet. It also vibrates when in park, just more noticeable when in drive & rpm's are lower. Oil light also flashes on when rpm's are low (hot, in drive/reverse and stopped). 

Thanks for the info. Coolant level is good. Have checked multiple times & tried to burp cooling system. Engine was just rebuilt so; new thermostat, w/p, radiator, timing belt, tensioners. No check engine light yet. It also vibrates when in park, just more noticeable when in drive & rpm's are lower. Oil light also flashes on when rpm's are low (hot, in drive/reverse and stopped). 

Do you have receipts that the timing belt and tensioners wrere replaced, or just previous owner's, or dealers comments that this work was done?? If engine was just rebuilt, do you have any guarantee on the motor?? Why was the motor rebuilt?? A knocking noise and flashing oil light are not good signs, as a mechanic you certainly know that. I would change the oil and filter to see if that helps.

 

New thermostat could be one of those short spring cheapies that don't work well. Those never work well.

Thanks for the info. Coolant level is good. Have checked multiple times & tried to burp cooling system. Engine was just rebuilt so; new thermostat, w/p, radiator, timing belt, tensioners. No check engine light yet. It also vibrates when in park, just more noticeable when in drive & rpm's are lower. Oil light also flashes on when rpm's are low (hot, in drive/reverse and stopped).

Need to put a pressure guage on it ASAP. Flickering oil light means the oil pressure could be as low as 2-3 psi at idle.

I have the receipts that the work was done & i already changed the oil & filter. I do have a warranty on the engine & the place that did the work is supposed to take care of it, just trying to do some trouble shooting on my own. The flickering oil light definitely bothers me, the mechanic also replaced the oil pump. 2-3 psi is definitely scary! I have heard of the braze cracking on the suction tube, which is probably worth checking. Thermostat is a good idea, not sure if they used genuine or not. From what i know the radiator went south & the engine overheated. I found the car for sale with a newly rebuilt engine. Hopefully I didn't buy a can of worms!