2003 AWD losing power/acceleration under load

[DigitalVortex]

First and foremost before I get into any details, altho I can't recall names, I've lurked this forum for years finding solutions to a multitude of issues over time and am greatly appreciative.

That said, the issue at hand which I've seen some stuff in the forum that may apply, but I'm not certain. The basic symptom is that the vehicle has declined in its power over the years. But most recently, there is a mountain between our home and place of business and the car will not pass 40MPH with the pedal to the floor when attempting to traverse said route. Further, the engine RPMs are high at that point and basically it sounds like it downshifted for initial power, but never allows an upshift as it gets speed to traverse the steep grade. The car has about 155K miles on it.

It also has an oil "loss" issue that I'm putting a quart in every 700 mi or so. I "think" that is valve stem seals that need replaced. Would that be related?

I know that compression is good (as of a couple months ago when I was testing to diagnose the oil loss). Fuel injectors are good. The best part of all is that there is NO CEL. On the off chance that something had been stored, I still plugged in my OBD II scanner. No DTC codes are found.

Past experience in a different car (94 Saturn) tells me it could be a cat issue. I'd like to be more certain before going down that road because exhaust work is just a nasty job without a lift. Further, it's expensive.

That's everything I can think of at the moment. I'd love some suggestions of further diagnostics I can run (by all means, point me to threads I failed to find). Any help, greatly appreciated.

[Rayven01]

I'd agree with the cat diagnosis, although with that significant of a power loss it's odd that it's not showing a code. My wife's minivan had the exact same symptoms you describe and it was the cat, which went bad due to a faulty EGR valve that had previously been replaced. Hers showed P0420 though.

[DigitalVortex]

@Rayven01 - I appreciate your feedback. And at least I'm not nuts in thinking there should be a code if it is the cat and it's as bad as it is performance wise.

Since the post I made, I have replaced the air filter. Although I haven't had the opportunity to road test it since, my wife tells me it's better BUT not what she recalls being normal. I need to road test it myself again. I think I also need to take the time to check out the throttle body and see if it's gummed up too.

I guess what bugs me the most about this (aside from the problem itself) is that with the degree of power loss, I'd really expect a code to hit. We're in a "hi tech" age of the auto mechanics diagnosis and it's not like the days I recall when I'd road test, listen, and diagnose that way. I can't help thinking that the lack of code means that there are multiple trouble spots and no single one of them is pronounced enough to flip a code.

[JohnO]

The trouble codes are mandated as part of the emissions controls on all cars these days. They are only intended to spot things that affect emissions and not more general engine problems. The normal way to diagnose a plugged catalytic converter it to remove the front O2 sensor and attach a pressure gauge. The back pressure is measured as the engine is revved with the car in neutral.

[DigitalVortex]

@JohnO - Thank you for the information. I was unaware of that. It brings me to a logical question however. If the vehicle is not throwing any catalytic converter related codes, can one be fairly certain it's going to pass emission inspection?

It's due for state inspec/emissions inspec next month. I guess I'm wondering if I were to just get the inspection done early, what my chances are that the trouble is found at that point?

Is there a known pressure reading I should get if I pull the O2 sensor and do that test? I understand the concept of doing that would be to determine flow through the device. That being said, if there's no "known" value, then I'd think we'd have to drill holes in the pipe fwd and aft of the converter in order to see a pressure differential which would suggest that the unit is plugged (or not).

How valid would using live readings from an OBD II scanner be in determining the health. We have two O2 sensors in there. When I run the car with live readings the, the #1 sensor varies considerably up and down, often between about 0.028V to 0.872V. The #2 sensor stays more consistent at 0.3V, sometimes jumping up to as high as 0.82V, but typically staying consistent between .3 and .5V.

Does anyone know of a good way / test location to check for the airflow into the throttle body? I'm thinking I could use a vacuum tester in there maybe. But again, I'm not sure where the best place to test that would be AND I'm not sure what kind of nominal values I should be seeking to insure the health of what's going on.

Thanks again for the assistance!

[JohnO]

The OBDII error codes *are* the emissions test these days. They don't use the sniffer in the exhaust any more, the cars test themselves and report with the CEL. So if the CEL is off you will pass the test. The car has to have been run long enough since the last error code reset for the tests to have been run, 10 or 20 miles in most cases. This is done through "rediness codes" which need to be satisfied before the emissions test. Some states will allow a car to pass with one code still present, others require no codes.

Search youtube for videos on back pressure testing for catalytic converters. I've seen a couple good videos before. I think there is a special pressure gauge that is needed so you may be better off having it done somewhere if it is only once.

What you describe for the O2 sensors is right. The front one is used by the ECU to set the fuel mixture and it constantly cycles from rich to lean as the control loop in the engine does its thing. The rear one is only used to check the function of the catalytic converter, so it should stay flat indicating that the converter has used up the rest of the Oxygen in the exhaust and therefore the converter is working properly.

[circuitsmith]

How valid would using live readings from an OBD II scanner be in determining the health. We have two O2 sensors in there. When I run the car with live readings the, the #1 sensor varies considerably up and down, often between about 0.028V to 0.872V. The #2 sensor stays more consistent at 0.3V, sometimes jumping up to as high as 0.82V, but typically staying consistent between .3 and .5V.

That's just how it should be: front sensor cycling, rear sensor pretty steady.
Stock intake system? There's a mini filter (with activated charcoal) between the regular air filter and the throttle body. That could be clogged.

[DigitalVortex]

That's just how it should be: front sensor cycling, rear sensor pretty steady.
Stock intake system? There's a mini filter (with activated charcoal) between the regular air filter and the throttle body. That could be clogged.

Yes, stock intake system. Ok, so there is the throttle body and big hose connected to it, running to filter housing. Just before the filter housing is the MAF sensor (I think). Where-abouts is this mysterious filter you speak of in relation to these things?

I haven't gotten back in here to look in a couple days with other stuff happening. One of the things that happened was the starter dying on the Vibe. I can accept it...anything over 150K mi on the original isn't shabby in my mind.

I'm getting it inspected Saturday morn and I wager they find nothing to fail inspection. I'm considering a pressure test on the system though to make sure I'm not missing something. Outside of some air restriction causing this somewhat extreme sluggishness, I can't really think of anything else it would be.

[DigitalVortex]

Inspection this morn. As already assumed, passed all around. As I had it done at a local tire place that does light mechanical work as well, I found out they don't have the tools for a back pressure test.

But the guy made an effort to be helpful and so I thought I might bounce this off the forum because, well, I don't know. He explained that the catalytic converter is operational at running temp. Therefore, if I don't notice any issues when the engine is cold, then I likely do have an issue with the cat because once it starts functioning, the problems would be encountered at that point if being caused by the cat. Further he said that, altho it would be noisey, I could remove the top side O2 sensor which would offer an air outlet and test -- if the problem disappears, I have a cat problem, but if it persists, the problem is somewhere else. Any thoughts on this?

The warm vs. cold item makes me ask another question. Will the O2 sensor readings be any different when my scanner is hooked up on a warm vs. cold engine? At this moment, I'm thinking I ran my previous test with a warm engine, but my memory is fuzzy.

I still haven't checked that other filter that was mentioned in a previous post. Partially because I haven't gotten to it and partially because I don't know where it is exactly (never knew it existed). I hope to pull the car into the garage tomorrow to look at some things again. Today with the snow in the NE, I'll be putting the tree up and then digging out -- assuming we get anywhere near the 4-8 inches predicted.

[JohnO]

The catalytic converter only works when it is hot because it needs to be that way for the chemical reaction to occur. Nothing about how it restricts the exhaust will change, so I don't see how the cat being hot or cold will affect your problem.

I wouldn't remove the O2 sensor and drive the car. The sensor is needed by the ECU to control the fuel mixture. I have heard of others with cats plugged so badly that the engine wouldn't rev to 4000 rpm with the transmission in neutral. In this case the O2 sensor could be removed and the engine run to check for improvement without driving the car. So you might see if your engine has any trouble revving to high rpms with no load.

I'm not sure what the small filter that was mentioned is, either. Maybe in the PCV hose that connects near the throttle body, but I doubt that would be your problem.

[LoneWolf]

I know this post is a few years old but I can relate to the P042 code popping up and nothing else...I have an 03 Vibe GT and was losing my acceleration (tort) and couldn't figure out what was going on with the car since it wasn't popping any codes other than the Catalyst code...For craps and giggles I went and bought 4 new coil packs and whola, i got my acceleration back....So question is, have you replaced the coil packs since you have had the car??