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I have '99 Outback, 2.5L, manual transmission with 174,000 miles on it. I've had the car for about a year and a half. I've been living with and trying to solve this problem for a couple months: The engine stumbles on acceleration unless I'm very gentle on the throttle. It's worse when the engine is warm. Here's what I've done so far:

Read codes for EGR malfunction, misfire in cylinders 2 & 3, and knock sensor circuit malfunction. I cleaned the EGR tube and valve and verified that the valve moves and the diaphram holds vacuum. That took care of the EGR code. The car ran much better, but still not great. The misfire and knock sensor codes still came up after about 20 miles of driving.

For the second round, I replaced the fuel filter and spark plugs. No noticable improvement. I later found a maintenance record that showed the plugs had only 30,000 miles on them (NGK Iridium). I replaced them with a similar plug made by Denso.

The last thing I did was replace the knock sensor. This seems to have made an improvement, but I think it's still not as good as it should be. The CEL stayed off for a couple hundred miles but came on again when I didn't downshift soon enough and lugged the engine a little. Codes were again missfire in cylinders 2&3 and knock sensor fault.

Most of my driving is on the highway, and there it runs fine. The problems occur at under about 2200 rpm. Any suggestions?

Thanks!
Dave
 

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Coil or plug wires. Try the "mist" test. Get a spray bottle and mist the coli and wires while running at night and look for sparks or arcing. I had similar problems until I replaced my coil.
 

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I finally took the time to do this last night. I misted around, and when the coil is damp, I could see some arcing at both ends of the boots - where the boot meets the coil and where it meets the wire. Could not see any arcing when the coil and boots are dry. Does this really indicate bad wires? This area shouldn't get wet in normal operation.
Wet or dry, when I open the throttle quickly by hand, the engine stumbles. When I ease it open at a slower rate, the engine accelerates fine.
 

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Unless I'm mistaken,you should see any arc with mist test . There should be a tight seal between the coils and wires. I suggest you change the wires before moving to the coil. The Ngks from Rock Auto are a great price and a nice upgrade for my 99.

Not to scare you, but my 99 was stumblin a little bit at low Rpms and nothing else. Turned out my head gaskets we going. Just a thought.

I finally took the time to do this last night. I misted around, and when the coil is damp, I could see some arcing at both ends of the boots - where the boot meets the coil and where it meets the wire. Could not see any arcing when the coil and boots are dry. Does this really indicate bad wires? This area shouldn't get wet in normal operation.
Wet or dry, when I open the throttle quickly by hand, the engine stumbles. When I ease it open at a slower rate, the engine accelerates fine.
 

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Well, It took a while, but I finally got around to installing the NKG wires from Rock Auto. Car seems to be running much better, so hopefully that was it - though I don't like to assume something's fixed until after a few hundred miles...

Thanks for all the help on this. It had me stumped. FWIW, the old wires were Belden. They were installed by the previous owner, but I don't know how long ago.
 

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If it keeps it up you may have a catalytic converter going bad. The 98 OB I'm borrowing stumbles when you let the clutch out, but it seems like its starting to go away (oddly enough after I changed the oil)
 
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