Rough engine, power is low/surges, misfires when cold

We’re talking about a 2002 Subaru Forester with only 80,000 miles on it.

This winter, the cold seems to have taken its toll. The fuel economy is bouncing around between 16 and 24 MPG and the car runs a bit roughly. Especially when it’s colder (<30 F) there is often a lack of power/RPM when depressing the gas pedal. The problem is worst in city driving, but exists on the highway also. It doesn’t go away during my 15-20 minute commute. For a period of a couple weeks, it was even easy to induce misfires (I think that’s what they were; quiet pffft pffft pffft sounds during severe lack of power) by fully depressing the gas. But even if you half-depress and hold the pedal steady, the car will accelerate weakly and then sometimes surge forward. Other times it seems to be idling strong and running rich. Now that it’s warmer (>40 F) the problem has lessened, but I’ve also started feeding it 93 octane gas and the car feels rougher than normal.

A year or two ago, I replaced the front oxygen sensor and spark plugs (prior to problem). This winter, I took it to a mechanic who couldn’t explain the problem and replaced the spark plug wires.

Based on my reading, it seems like the only other most likely culprit is the manifold air pressure (MAP) sensor that’s mounted on the intake manifold. I haven’t worked up the effort (and can’t find my multimeter) to do an electrical test of the MAP sensor’s output itself. I’m also not keen on just buying a $270 part without sound evidence, nor paying for ill-informed guessing by lesser mechanics (than yourselves, of course!). I also have a sneaking suspicion that the automatic transmission could be slipping – when I drive with 2 adults and two kids in the car up a hill, it seems more underpowered than days of old.

Any ideas?

Thanks for your help.

First, is the ‘Check Engine’ light on? Has anyone put a code scanner on it and pulled codes?

Even of the light is not on, you still should check for codes. You could have some stored.
Did you use OEM replacement spark plugs? I’ve seen Champions seriously deteriorate very quickly.
Did you “read” the old spark plugs when you removed them? Spark plugs can tell lots.

Bad ignition coil, injector or spark plug? If no have a compression test done.

Thanks for the reply, BustedKnuckles & the same mountainbike.

MIL is not illuminated. I didn’t know there could be codes without the light (I’m pretty sure you’re looking for PO301, PO302, PO303, or PO304 misfire codes). Mechanic didn’t mention any codes.

Yes, I’ve only used OEM parts. O2 was a 22791AA00A and I bought the plugs from the dealership.

Yes, I tried to ‘read’ the old plugs. I’m not experienced, but used a chart for reference… didn’t see anything noteworthy.

In 2011, I had the oil tested by Blackstone Labs (report attached) and the only problem was slightly high lead (but this was somewhat to be expected because the car was being abused driven 10 minutes at a time).

Resolution I gave up and brought the car into a Subaru dealer, who hooked it up to a diagnostic machine. I was told that all of the sensors were within tolerance, but was that the air-fuel/front O2 sensor seemed to have a slightly slow response. They replaced it and the car now runs much more smoothly.

Sincere thanks for the followup post. We often never hear the ending.
Happy motoring.

Thanks right back. I rely heavily on the Web for answers to life’s persistent problems, so it’s the least I can do to try to populate some answers for others to find. :slight_smile: