Has been slowly loosing power.

[awg1011]

As I mentioned in my intro, here's my post on my power issue. This is on an 07 LS with the 4200 and is just shy of 140k. It has been slowly loosing power over the past several months. most of the time I'm light on the accelerator so it wasn't been very obvious early on however lately even trying to get up speed on the freeway it has been noticeable. Right now it's driving like I have my camper hitched and will barely scuff the tires on loose gravel. The MIL is not lit and no codes are stored or pending.

An average week of driving for it is 4 miles a day 1 mile at a time. I work 1 mile away and go home for lunch. Then on the weekend we usually take it shopping to a near by city that is 20 miles away.

After some searching and reading (mostly here) I starting leaning towards the CAT slowly clogging. I decided to check the back pressure and change the spark plugs and clean the throttle body at the same time. The throttle body was very nasty BTW. The back pressure was .5 psi at idle and 1.5 at 2500 rpm. Any thoughts on those results?

When I removed the O2 sensor, it came out very hard and took most of the threads out of the manifold. The engine was hot when I removed it, thinking it would help it come out easier. I cleaned the threads on the sensor and it did tighten back up.

 

[gmcman]

Those back pressure readings are very high for the stock exhaust, I don't think I managed .3 psi at 3000 RPM.

I would look for some hairlike fibers protruding from the tailpipe which could be the resonator coming apart, or the cat could be plugged up.

 

[Mooseman]

You can also do a backpressure test at the downstream O2 sensor and see if the backpressure is present there too. This would tell you if the cat or something past that is causing the pressure.

 

[Chickenhawk]

Post a photo of your coolant gauge when it is fully warmed up. It is almost certainly your cat, but replacing the cat without fixing the root cause will just cause the new one to slowly clog as well. The most common causes of cats clogging include driving for more than a few minutes with a flashing check engine light (indicating a serious misfire) or long-term driving with a bad coolant sensor or thermostat.

 

[awg1011]

Thanks for the responses. One thing I've had trouble finding is specs on back pressure and come across a lot of conflicting info, some say 2psi at 2500 is OK and others say way too high. Very confusing to say the least. However, I'm inclined to believe all the experience here on this particular engine.

I hadn't thought of back pressure being caused beyond the CAT so next chance I get I'll retest on the downstream O2 port. If it tests lower, I may remove the CAT and physically look at it. If there's any question, I may just gut it for now and see what happens. I don't see any fibers of any kind coming out the tail pipe, it's fairly clean looking.

At this point I don't have a good means of getting a dash photo, my phone is a flip phone with no camera and my tablet no longer takes clear photos, but I can tell you that at OT the coolant gauge sits steady on the little line just before the bigger 210 line, always has since I've had it. The temp sensor on the scanner, it fluctuates between 183 and 185. I doubt the Tstat and coolant have ever been changed and is probably due.

I only really get time to work on this on weekends, hence the slow responses from me.

Oops! I almost forgot, The MIL did come on yesterday -
P0411 "Secondary Air Injection System Incorrect Flow Detected"

Could too much back pressure cause this? Related to power issue? Or totally unrelated?

 

[Blckshdw]

the coolant gauge sits steady on the little line just before the bigger 210 line, always has since I've had it. The temp sensor on the scanner, it fluctuates between 183 and 185.

You should be over 195 (the temp the t-stat is supposed to open). For me, my truck rarely goes over 200, I know for others they run a little warmer.

 

[gmcman]

awg1011, at about 3 psi your engine will likely barely run. At 2 psi it's going to be severely choked off.

I have tested the stock exhaust as well as a Walker aftermarket. I don't even get a reading at idle and 2500 rpm is barely 1/4 psi.

Your back pressure readings are very high...I'll post my vid in a little while.

 

[gmcman]

I lost about 3mpg using the walker exhaust....that particular system almost doubled my back pressure.

 

[coolasice]

Thanks for the responses. One thing I've had trouble finding is specs on back pressure and come across a lot of conflicting info, some say 2psi at 2500 is OK and others say way too high. Very confusing to say the least. However, I'm inclined to believe all the experience here on this particular engine.

From service manual 1.25psi max at idle, 3psi max at 2000rpm

 

[Chickenhawk]

I would suggest ignoring the service manual. Those readings are only indicating the maximum acceptable values before a warranty replacement. Performance would be down long before you reached 3 PSI at 2000 RPMs.

awg, your coolant temp readings are too low and indicate either a bad thermostat or a bad coolant temp sensor. Either way, the result is the same - too rich a mixture being dumped into your cat because the engine perpetually thinks it is not fully warmed up yet. It has created a long-term rich mixture which has slowly clogged your cat. Such slow progression rarely lights the Check Engine light because the cat is still working to clean the exhaust - but the flow is vastly restricted.

Our advice is simple. Don't worry about trying to measure any more readings. Change the thermostat, coolant temp sensor and the cat, and your performance will return.

 

[gmcman]

Also, so not to possibly add to your issues, I'm assuming you used AC 41-103 plugs when you changed them. These motors are kinda picky.

 

[awg1011]

Also, so not to possibly add to your issues, I'm assuming you used AC 41-103 plugs when you changed them. These motors are kinda picky.

Yes, that is the plug I used, I learned my lesson years ago on the Jetta with using different plugs and now I only use the recommended plugs in all my engines, big and small.

For kicks and grins, I tested back pressure after the CAT and the needle never moved. I unbolted the CAT, but didn't remove it, and stuck a scope down the inlet and it looked close to 50% plugged. I decided not to gut it, it's worth much more for scrap intact. I have a new CAT coming, it should be here Wednesday. Hopefully I can get it in some evening as we might head out camping this weekend.

The scanner I've been using is an inexpensive (OK, cheap) bluetooth scanner. I've seen warnings about them being inaccurate. I was able to get a hold of a newer Snap-On scanner and got a consistent 15 degrees higher temp. I do plan a coolant and Tstat change in the near future.

Most days my TB never reaches OT, could this be contributing to the clogging of the CAT? I know short trips aren't always good for engines.

Sorry the pics from the scope didn't turn out or I would have posted them.

 

[Mooseman]

Most days my TB never reaches OT

If you mean that it doesn't get to 210, then yes, this will kill you new cat if you don't take care of this FIRST.

 

[DocBrown]

Sounds like you are on the right track, but to Mooseman's point, change the thermostat first. It's pretty easy but you'll have to remove the alternator to get at it easily.

 

[awg1011]

Thanks everyone for the help. Just a quick update. The new cat made a world of difference, even towing the camper it had more power than it had with the old cat without the camper. In the very near future, I will change the t-stat and coolant, however, there's not much I can do about the daily short trips at this time, but I know to expect the cat to slowly clog.

Thanks again!

 

[Trailblazzin]

What cat did you purchase and from where?