Stumble and backfire between 2,000-3,000. Stumped.

[Mellowgorilla]

Hello. I have searched on the forums, as well as elsewhere, for a solution and have not come up with any sort of fix. I have a 1991 Legacy L N/A AWD that has been stumbling for a year. With the engine hot or cold, when the RPM's reach approximate 2K, it will shudder like it is starving for something. Once it hits about 3k, it will clear up as if it is an old Subaru. When I bought it, the timing belt was installed wrong with the two cams timed advanced and retarded. The timing has been corrected, though the computer has retarded my timing -22 degrees at idle and it gets far negative with higher RPM's.
I have replaced the timing belt, the heads since some were laying around, Oxygen sensor, plugs and wires, air filter and fuel filter. I have checked the knock sensor and noticed the wire was burnt and has been replaced. The MAF, TPS, ECM have all been swapped with another unit and gives the same results. My Alternator is pulling 14v+ on a constant. and my fuel mileage is still around 18 mpg...
I am completely out of ideas, I am more than willing to listen.
Thank you.

[mike-tracy]

Could be the coil pack, an intake manifold gasket and or an exhaust manifold leak (pre o2 sensor).

[Mellowgorilla]

I have swapped the coil pack with a 95 L, it has made no difference with either car. I will check the Intake manifold and throttle body for leaks. I have hosed down the engine to clean dirt off with no stumbling. I assume my vacuum lines and gaskets are solid. My exhaust is complete only one exit.
I have rather good compression and vacuum pressure. My idle is close to 48 cm/hg (18.89 in/hg). Once I start driving, it gets close to 0, somewhere around 4 cm/hg (1.57 in/hg). My cylinders read all four virtually 170 psi. The part that sucks is in an Automatic, every gear wants to remain at 2-3k. And 3.5k is close to 90 mph (on a closed track. Yeah, we'll go with that). If conditions were ideal, I am sure I travel over 110. Not that it is worth the gas or deer dents.

[Mellowgorilla]

One dumb question to clarify. If the timing mark is on the left side of the pulley from the front of the engine and the pulley spins clockwise, the ignition timing is advanced or retarded? According to the FSM, my timing should be -20 degrees +/- 8 if pointing at 11 o'clock. The idle timing (-22 as listed above) is within tolerance. Still confused, been racking the brain for too long.

[Legacy777]

The engine timing really isn't something you can adjust, and IMO isn't a good indicator of anything. As long as the cam timing is correct, the ECU will adjust and control engine timing.

You said you replaced the knock sensor. Have you used a multimeter to check the knock sensor wiring from the engine bay to the ECU to make sure it has 0 ohms (full continuity)?

Any stored codes? Have you done the d-check (active diag) with only the green connectors?

[Mellowgorilla]

I had a knock sensor code caused by the shorter stock wire. The new wire gives me 3.8vdc from the computer and the wire has virtually 0 ohms.
I had a chance to speak with the one guy from Advance that knows what he is ttalking about. As I have found so for, there is one at each store... He suggested I check the fuel pressure regulator. The vacuum line smells of gasoline, I ordered a new one, I expect it by Thursday.

[Legacy777]

Are you talking about the vacuum line going to the fpr?

[Mellowgorilla]

Yes, going to it.

[Legacy777]

Ok, yeah you shouldn't smell fuel in the vacuum line. Are you sure it's in the vacuum line and not coming from the fuel lines?

[Mellowgorilla]

The line appears to be the same diameter as the remaining vacuum lines to the engine, about three inches long and bent at a 180. The line came off with a small twist.

[Legacy777]

Yeah, that's the correct line. If you're planning to replace the fpr, let us know if it fixes things or not.

[Mellowgorilla]

I replaced the fuel pressure regulator and it ran better than before but still stumbled. I replaced the vacuum hoses since they were all factory, it ran worse. At this point I am rather upset but drove on it anyway. I was to the point of dumping in a 2.5 motor (or just the block and sensors) but I felt I should wait til the spring to take on a task like this.
I decided to clear the codes again by plugging in the white (I have seen also a black instead in a 93) and green connectors. After one mile the Check Engine light showed me a code 32. I changed my Oxygen sensor even though it was replaced almost a year ago. No more stumble or backfire! I am so relieved!

Thank you both for all your assistance!

[Legacy777]

Ok, well I'm glad to hear things are working well now. Hopefully that was the problem, and it'll be good to go.