Intermittent surging/hesitation, '99Legacy

[zyewdall]

So.... my girlfriend's '99 Legacy wagon, EJ22, 5MT is having an intermittent problem.... most of the time it runs fine, but occasionally, it starts acting like it has a dead spot on the throttle... not having any power, till you push it a ways farther down, then engine kicks back in and it surges ahead. She said it was really bad tonight... so I took it for a drive, and didn't feel anything out of normal at all. But, it has done it to me before, a few months ago, and it felt either like a dead spot on the throttle, or a really bad fuel filter -- to the point where I wasn't sure if it was going to make it home. It almost felt like the engine was cutting out until you floored it.

But... then, the next morning, it was fine, and it didn't do it for a few weeks again. It's been about 6 months since I first noticed it.

 

Happens when the engine is warm, doesn't seem to be associated with wet weather or dry weather.

 

I haven't really replaced anything yet. Fuel filter seems obvious, but why does it run fine sometimes, and lousy other times? Air filter is fairly new. Not sure on the plugs, but I think they only have about 20k on them.

 

Ideas? Exorcist?

 

Thanks

 

Z

[Robert Harik]

Unplug you TPS ( throttle position sensor) and see if the problem goes away, if it does check the TPS.

[zyewdall]

Update... still doing it.... replaced the TPS, and no effect (used TPS). I cleaned the MAF and it got better for about a week, then back to normal.... hmmmm.....

 

Just drove it for a 3000 mile road trip, and it did okay, but it's still doing it. Mileage averaged around 25mpg -- low was 20mpg tank at 80mph, air conditioner blasting, going from Yakima to Spokane. High was 33mpg at 45mph touring through Yellowstone with the windows open.

[fishy]

Those symptoms sound VERY much like the symptoms that made me change the knock sensor out in my 98 Impreza. The result was excellent. Check out my Redneck Knock Sensor DIY thread to see the how-to video I did if you need some more info.

[1 Lucky Texan]

possible Engine Temp sensor (might be part of the same unit for the gauge in this year model) since it seems temp related.

[davebugs]

Knock sensor was a good guess.

 

Does it happen more when damp?

 

Some folks spray it with a spray bottle to see if they can replicate the problem.

 

Look at it and see if it's cracked.

 

Almsot always it won't throw a code ( or even a pending code).

[Fairtax4me]

Thinking Knock Sensor as well.

Front Oxygen Sensor can also cause intermittent trouble and not set codes.

[ferret]

I have seen this a few times where it was the Front Oxygen sensor. Happens after warmup. Sometimes so bad the engine cuts out completely, only to restart without a hitch.

 

When it's acting up, stop the car and UNPLUG the front oxygen sensor. This WILL set a code and the Check engine light. The engine is now running in OPEN loop on a preset of fuel mixture. Now drive the car. If the cutout is gone, the front sensor is a good candidate.

 

If not, my next candidate is the MAF. A good cleaning here is inexpensive using an aerosol MAF cleaner from an auto parts store. On some vehicles, you can unplug the MAF, cycle the key a few times and the engine will start and run again in open loop mode, although not very well.

 

Just a few thoughts after reading this thread.