Totally perplexed!!!!!

[skyhigh483]

Hello, I have a '03 impreza 2.5 rs. It has 283,000 miles. Auto trans. Car runs great 99% of the time. lately on an occasion it seems like it won't take fuel, when you step on the gas it just dies, spits sputters and wont go over 20 or so, the engine won't take the load. a lot of bucking but will idle fine then all of a sudden it's fine no prob. it threw one code: bank one sensor one, (O2 sensor) I had it replaced. then it was fine 2 weeks later it did it again the fuel filter, coil pack, and the sensor that controls the air flow into the gas tank(the dealer told me that this was the problem), it worked for about 2 weeks. over the weekend it happened again CEL came on and again the car drove fine the next morning the CEL shut off on it's own. It does seem that this only happens when it is driving and on hot days I am totally lost and I have a decent amount of experiance working on vehicles but please help?

 

Thanks,

Brett

Edited May 4, 2010 by skyhigh483
typo

[unibrook]

Do I understand right that you recently replaced the front 02 sensor?

 

If not, then do it.

[skyhigh483]

yes I did thanks

[Fairtax4me]

Might be a knock sensor problem, throttle position sensor, or my favorite... a vacuum leak. Or any number of other things, but knowing the actual code that is being set when the CEL comes on will hep. Even if the light turns off it still stores the code and can usually be read even after the light is off.

 

It might help to get the freeze frame data that the ECU records when the code is set. You'll need access to a fairly high end scanner to get this info.

[skyhigh483]

do you know where the knock sensor is located? or the throttle position sensor, are they things I can get to? thanks for the reply

[skyhigh483]

What about the sensor behind the cat? maybe? what do you think?

[gbrand]

o2 sensor behind the cat will not affect runability.....its main function is to make sure cat is operating. double check and make sure something didnt get knocked loose during the front o2 sensor replacement.

[Fairtax4me]

Knock sensor is easy to replace but usually sets a code. The TPS is a bit more involved but only because you need to adjust it so the ECU gets the proper reading from the sensor at 0 throttle.

 

You could keep throwing parts at it, or do some real diagnostic work and fix the problem without spending a jathousand dollars. I don't know of any sensor that controls air flow to the fuel tank. Find your service receipt and post exactly what part or parts they replaced along with any other info they might have put in the description.

[skyhigh483]

The o2 sensor ws replaced after the coil pack and fuel filter the vlave for the gas tank controls the air going into the gas tank as the gas is being used they said it closes and stays shut and creates a vaccuum. they also said it is a typical problem in fact they keep the part in stock because it is so common. it worked for a while as well as the o2 sensor it only seems to last a couple weeks before it messes up again, I guess the only way is to get a high end diagnostic I am assuming the dealer is the way to go right?

[skyhigh483]

Fairtax4me? where is the knock sensor at? what exactly does it do?

[JT95]

Fairtax4me? where is the knock sensor at? what exactly does it do?

 

 

Knock sensor is a little doodad guy that is bolted onto the backend of the top of the engine with a wire plugged into it. Remove your airbox off the throttle body and you should be able to reach it if you are creative. I replaced the one on my 95 Legacy and it was a pretty straightforward procedure. That was a few years ago, though. I had a code that specified the knock sensor needed replaced however, so you need to check your code at Autozone or wherever.

 

 

 

I do remember an issue I had with my 95 several years ago. I had bought the car used (150k) and had been driving it about a year. Like you said, pressing the gas did nothing for the engine when it would take a weird spell to pretty much go powerless if taken over a certain low RPM. I don't recall any check engine light though. It would just do it at odd times. I replaced the fuel filter and thought that fixed it, but it did the same thing a couple weeks later. I took the fuel lines off the filter and turned the car on to see if I had pressure. Let it shoot some juice outta the line for a bit, put it back together, and it never acted the way again. Never figured out what the issue was or why it had acted that way, but since it stopped I didn't worry. That does nothing for your situation, so I have no idea why the hell I am rambling on about it. Sold the car a few years later and that was the only engine mystery/issue it had experienced in the years and miles I drove it. Thing was humming at 230k when I last drove it. Best car I ever owned.

Edited May 6, 2010 by JT95

[JT95]

http://www.customersrevenge.com/node/611

[Fairtax4me]

Fairtax4me? where is the knock sensor at? what exactly does it do?

 

As already described the knock sensor is on top of the engine block tucked away under the air box. It's easy to see and change, but the question is, is yours bad? It's hard to tell without being able to see what is going on with the engines ignition system while it is running. This means having the car hooked up to a computer and streaming info in realtime from the ECU to look for abnormalities.

 

Generally if the knock sensor goes bad, the ECU sets a code.

 

What does it do? The knock sensor senses knocks. It picks up the ringing sound created in the engine when detonation (spark knock) occurs. Repeated detonation can be detrimental to the health of the engine, and also increases harmful emissions into the atmosphere (mostly NO2 gas). When it hears spark knock occuring, it tells the ECU to retard (dial back) the ignition timing advance in an attempt to correct the situation that may be causing the knock. This creates a drop in power, but generally doesn't cause the car to "fall flat on it's face" as if it has lost all power. It will usually only last for a few seconds then return to normal, if you notice any power loss at all.

 

A bad knock sensor can cause the type of problem you are having, but so can a ton of other things.

 

I think the dealer fed you a load of bull with the fuel tank air valve thing. It may be common for them to go bad but a slight amount of vacuum in the fuel tank will not cause any noticeable drop in fuel pressure going to the engine. The fuel pump is capable of upwards of 50psi. You would need to have about 30 inches of vacuum in the fuel tank to cause any drive ability problems, and the fuel tank just isn't sealed well enough to hold THAT much vacuum.

The valve may very well have been faulty and needed replacement, but I just don't see any way that it would be causing the problem you are experiencing.