I have a JDM 2.0 turbo in my sand rail that had a similar problem and I'll find out if I have fixed the problem this upcoming weekend. Anyway here is what I observed:
When the engine was cold the pressure was around 75psi once it warmed up the pressure would practically be nonexistent at idle. The engine would produce pressure under cruising speeds and acceleration; however, would hover around an unreadable pressure at the gauge on the dash when idling. A friend that owned a mechanic shop said that it was probably due to the bearings being worn and were therefore unable to keep the pressure up. I talked to a few other friends that worked at Crawford Performance and their prognosis was the same. Spoke with the owner at a shop who produces engine packages for buggies from JDM imports (Johnson in spring valley, ca), he said that it is not uncommon that the pressure relief valve will stick giving the same behavior. My decision was to either buy a new block $1500 or try a new pump ($107), I opted for the new pump. When I removed the old pump I found the plate that is responsible for holding the pump gears together was loose. I found this is really common problem on these engines on NASIOC. Com's forums. So I replaced the pump anyway and used loctite on the new pump's retaining plate as I do not ever intend on doing any maintenance on the pump itself; just assume replace it if a problem arises. The engine now idles around 30-40psi at idle once warmed up. Will need to test it under heavy load condition though.
The mechanic shop owner I referenced above gave me a helpful tip, if you suspect that you may have damaged the engine by driving it with low pressure and need to make a determination if you want to invest the resources to fix it, you can take the oil to a tractor maintenance shop and they can analyze the oil to determine if you have worn through the outer coating of the bearing and into the copper cores by the contents of material in the oil.
If you do the work yourself be sure to follow Subaru's procedure for compressing the idler pulley. Also be absolutely sure to put the timing belt on correctly as damage to the valves will occur.
Sunday, October 14th, 2007 AT 9:12 PM