No power at wide open throttle

[Zx2ner88]

Hello everyone, looking for some help here. I have an 02 trailblazer with an issue that has been ocuring over the last few weeks. At WOT, the vehicle has a significant loss of power at 40mph. It will accelerate rather slowly up to 40, then the speed will just hang there until i let off the gas, at which point it will shift hard into the next gear. No DTC set in the PCM, new plugs, new fuel filter, and throttle body cleaned with no change in the issue. fuel pressure is ~45 KOEO, ~52 KOER. I also ran it with the front O2 sensor unscrewed, rpms still hang. I do have access to a Tech2, if any other data is needed

 

[HARDTRAILZ]

Sounds like you need to be looking at the transmission. How is the fluid in there?

 

[Ziggy]

I had a car years ago that did kinda the same thing. Felt really lazy up to 25 MPH, then just wouldn't accelerate past that, no matter how heavy my foot was. Turned out my issue was a clogged catalytic converter. My engine couldn't exhale. If your cat is clogged to the point that it just can't breath, unscrewing the upstream o2 sensor is about the equivalent to blowing through a coffee stir stick.

 

[Zx2ner88]

@HARDTRAILZ: Fluid looks great, almost like new

@Ziggy: Probably missed it due to my run on sentences, but i did try this with no change

 

[Ziggy]

@HARDTRAILZ: Fluid looks great, almost like new

@Ziggy: Probably missed it due to my run on sentences, but i did try this with no change

That's what I was getting at with my coffee stir stick analogy. Have you tried blowing as hard as you can through a coffee stir stick? The bung for your o2 sensor is 3/4" wide vs. the exhaust piping itself being what, 2 5/8"? Is there a difference in the way the trans feels when accelerating lightly to 45 vs. WOT? It could be a trans issue... Weird that it isn't throwing any codes either way, though.

 

[Mooseman]

I know there were issues with the original programming involving the fan and transmission. Maybe try unplugging the fan and see if that helps. You will get a CEL but you'll know why.

 

[Zx2ner88]

@Ziggy: Okay i misunderstood. Everything feels normal under light acceleration, its just WOT when i have the problem. It does feel similar to when there is a plugged cat, except the rpms just stay steady, no misfiring or backfiring.

@Mooseman: ill give it a try, but the pcm has the latest software as of about 3 months ago

 

[Chickenhawk]

Exhaust backpressure test is quick and cheap. Unscrewing the O2 sensor is not always definitive. If exhaust backpressure is near 0 PSI at 2500 RPM, then suspect the transmission. Diagnose scientifically and try the more common and cheaper diagnoses first.

 

[Zx2ner88]

Got around to doing the back pressure test, read 6psi at 2500rpm. think i found the problem

 

[Zx2ner88]

Just a last update, took the cat off today, looked pretty bad on the front side as seen in the picture. I knocked the guts out, bolted it back up(temporary of course...), and she runs like new.

 

[Chickenhawk]

Can you post a photo of your coolant temperature gauge?

Have you ever had misfires/flashing CEL? What spark plugs were you running, and what is in there now?

Cats get plugged for a reason and we know most of the common problems. It would be good for you to know why it plugged, so that it will help prevent the new one getting plugged too.

 

[Zx2ner88]

Never had any misfires or even a cel with the vehicle. Cooant gauge stays steady right in the middle. Plugs are AC Delco iridiums. My best guess is that for the first 2 years I owned it, 90% of my driving was 1.5 miles to and from work, likely it almost never got up to operating temperature

 

[Chickenhawk]

Cool. Glad it's fixed!

 

[Daniel417]

Zx2ner88 did knocking out the cat throw any codes. How's your fuel trim levels? And did you get a noticeable difference in fuel economy? My envoy seems to be suffering the same restrictions @ wot.

 

[Mooseman]

It should throw a code to say that the efficiency of the cat is poor. That's what the downstream O2 sensor does. There is a way to defeat it. Google for spark plug defouler.

 

[Zx2ner88]

Zx2ner88 did knocking out the cat throw any codes. How's your fuel trim levels? And did you get a noticeable difference in fuel economy? My envoy seems to be suffering the same restrictions @ wot.

No codes yet after about 75 miles. Still to early to tell on fuel econony, but i was getting about 8-10 highway mpg just before gutting it. Ill update once ive driven a bit farther.

On a side note the spark plug non-fouler mod works great. I had a ford escort that i put a full length header on with no cat, never once did it throw a cat code

 

[Zx2ner88]

Thought I would update this since I haven't in months. The CEL did eventually come on. I installed the anti foulers but the still have a check engine light. I've read that adding steel wool between them can help so I'll try that and update later

 

[Mooseman]

I have read that using two defoulers with the O2 sensor to pull it futher out of the exhaust stream helps with particularly stubborn ones.

 

[Zx2ner88]

I do have two installed, one is drilled out to just slightly larger than the sensor head so it fits inside, the other in unmodified with the small hole. Im hoping the steel wool will "stabilize" the amount of exhaust going in

 

[Mooseman]

If that doesn't work, maybe try with just one. Maybe it needs to see "more" exhaust.

 

[Zx2ner88]

Thought I'd post one last update. I installed a piece of steel wool between the two non-foulers, which took care of the CEL. I even was able to pass my OBD emissions test with all monitors set.