Hello, I am a mechanic and I am pretty much stomped on this. My son has a 99 forester 2.5 automatic with about 275,000 miles on it. It was always peppy and ran great. The timing belt idler pulley bearings failed and the pulley burned it's way out through the timing cover and flew out. Of course , it jumped time and shut off . Long story short , he pulled the engine because it bent ALL the valves. So, after getting the heads checked and resurfaced, replacing all the valves, lapping them in , replacing the head gaskets with updated Subaru multi-layer gaskets, and putting it all back together, it has a SIGNIFICANT power loss up to 4100 rpm, then it seems to come alive. It does not miss. it starts and idles fine. It has NO check engine light or codes. Of course , we adjusted the valves on assembly, checked timing belt timing ( twice ), replaced crank sensor, mass air flow sensor, coolant temp sensor, checked intake boot for cracks, double checked that all wires and hoses are hooked up, cleaned all ground connections, tested TPS signal by graphing with scan tool with no evidence of any drop outs or glitches, cleaned throttle body, checked for exhaust restrictions, going as far as cutting the exhaust system apart to make sure that no critters got in there and clogged something while it was apart. Even steam jennied the cats out to make sure they were open. Welded it back together and installed it- same thing. Unplugged the knock sensor and drove it- of course the check engine light came on then , but no difference in performance. New fuel filter - no difference. checked fuel pressure- within specs. Tried another ECM- same thing. Even when you shift it manually, it just lays down when you pull out or climb a hill. New battery. The ONLY time he got it to act normal was to disconnect the battery overnight . Then hooked it up the next morning and drove it . He said it acted fine for about 5 minutes, then started losing power again. Well, he's going off to college in a few weeks, and here we are, STILL trying to figure this out . Any ideas please? Thanks in advance for any responses.
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Oh yeah, I forgot . We also did a compression test and cylinder leak down test. It checked out O.K.

Edited December 9, 20214 yr by moqezoro

This sounds like a good one... a few things:

- timing, yes I know you said you double triple checked it. Which markers did you use to line up the cams (don’t use the arrows, always the tabs on the rim of the cam wheel)? And this is a DOHC engine?

- after reconnecting the battery did you let the engine idle for 5-10mins before touching the throttle?

- after a drive when it was “normal” then not, are there any stored codes?

- the spark plugs and leads new, if so, what was used?

- fuel pump healthy or quite old? And on that one, fuel lines connected correctly? It’s easy to get a return line mixed up with the IN line on the fuel rails 

I hope some of these suggestions help or give you something to look at further.

Cheers 

Bennie

yeah, really doesn't seem like a sensor/ECU issue, seems like something more 'physical'.

 

as said, triple check cam/crank timing. Locate the 'tooth count' for that engine. Bring the crank/cam marks to their 'belt install' positions and count the teeth between everything. Something may be a tooth off. Wonder if a 'dropped' valve guide could come-and-go???

maybe some odd vacuum leak comes-and-goes??? - maybe try some vacuum gauge readings - may have to rig a way to do that while revving/driving?

1+ for timing!