Engine has a miss and I can't find out why

[bcpianoman]

My ‘92 SVX has had an intermittent miss for the past several weeks. I’ve had it to three shops (one of them a Subaru dealer) but the problem persists. The problem doesn’t occur until the engine warms up to operating temperature. It seems to be less prevalent (or non-existent) in cooler weather (below 55 degrees). It is most noticeable when in 1st gear and pulling away from a stop (until I hit about 2500 RPM). When this happens the engine runs very rough. But it also happens in 4th gear when I attempt a slow acceleration at about 2000 RPM. Shifting down to 3rd (and increasing RPM accordingly) restores smooth operation. When it misses in 4th gear it’s almost like the transmission is slipping for a very brief instant, then re-engaging. The miss in 4th gear isn’t nearly as rapid as the miss in 1st gear described above. When the engine is at operating temperature it idles rough in neutral as well in warm weather (but not as bad as when in 1st gear).

So far the first mechanic checked all the spark plugs, pronounced them in good condition and gave up. The second mechanic (at the local Subaru dealership) plugged it in and the O2 sensor came up. That mechanic said that that is a common occurrence and he didn’t think that the O2 sensors were causing the problem. He found one ignition coil to be weak and replaced it. He also found a number of vacuum leaks and repaired them (although he thought there might be more). The third mechanic found that the fuel pressure regulator wasn’t allowing sufficient fuel pressure (only 28 lbs. instead of 34 to 36 as required). He replaced the fuel pressure regulator but that didn't help either.

Any ideas?