'99 Impreza Missing and Backfiring

Sounds like an air fuel ratio problem. Or probably timing. Is this happening during cold? Or it does that all the time?

When you changed the plugs and wires were they oily?Did you see any sign of corrosion on any of the coil pack towers. Is the check engine light on and if so have you have the codes read?Typical would be plug well seals that have failed alowing oil to soak the plug wire causing a misfire.

I am having the same exact problem. My 2000 Subaru Legacy GT has about 100,000 miles on it and it is now missing and sometimes backfires through the intake. It seems to be getting worse. Sometimes it may take several seconds to get up to speed from a dead stop, other times its perfectly fine. If you have fixed the problem, it would be great if you shared your info here. Any input would be great. Thanks in advance :-)

Any DTC. Or try to pull any DTC from ur PCM?

Yes, I had the car scanned about 7 months ago and it said the Knock sensor was faulty, but said it would not affect drivability so I did not replace it. I scanned it yesterday and I had the knock sensor and oxygen sensor code come up. I have replaced the oxygen sensor and will replace the knock sensor on Monday. I don't think these sensors would be causing a misfire this bad.

The misfire seems to get worse when the car has been driven a lot in one day. It also has a low rpm studder when you rev the engine from idle speed.

I am also replacing the spark plugs and wires to see if this helps.

Look tech guys.I think we need to verify the misfire with a scope or 5 gas here before we go any further. If there is a misfire. We wanna know if it is a lean/rich/density/ignition, mechanical misfire?. If it aint then we might look into reciprocating cpts that trigger knock sensor causing timing to retard or possible a fuel cut off ECM strategy. Agree?

IF the plugs and wires were done and done correctly the next thing I would look at is the mass air flow sensor as Subaru actually had a recall on the Mass Air Flow Sensor they used in a small window of production and I dont know for sure if your vehicle falls into that category or not. But the reallity is that some one who knows Subaru should look at the car and dont throw any more parts at it.I have worked on more Subarus than I care to even contimplate and I can tell you almost every typical problem for a given aera Subaru. But that doesnt mean yours suffers from the typical stuff. Needs to be Diagnosed Not just have the codes scanned.