My Suburban is having a bad temper, I have just replaced the two upstream O2 sensors just before towing over Vail pass in Co. The way there I had no problems, ran great ! But on the way back the Sub fell on it's face! It started running poorly, almost couldn't idle and was backfiring and lost power. I pulled over and let it sit for a bit, started it back up and it was idling rough, so I reved it up and blew it out. It then ran ok for a few more miles towing up this massive pass before it did this again!! It did throw some codes for the O2 sensor and one about it not being in the right gear? Also running lean code. I did limp back home and replaced the drivers side O2 again. But I have had a problem towing like this for years and have replaced so many things and no change. I really thought I had it this time when it went over fine, but coming back.. no luck. I will listen to anyones ideas on how to fix this, or if you have had any problems towing? I do have the 2500 with 8.1L and 4L80E tranny with a Edge programmer,Throttle body spacer, Airaid intake ac delco platnum gapped plugs to .045 and 110K miles. This all started around 36K miles FYI. Runs fine around town, just when towing my 7500lb trailer this happens

 

Thanks for the help guys..... some forum this is....

 

Might be no one else knows either. What has changed? You replaced O2 sensors as you suspect you might have a problem? Take the MAF sensor out, inspect, and clean it with approved cleaner. MAF and O2 sensor are in the fuel schedule loop. Just because you replaced the O2 sensor dosen't mean you got a good part.

 

I did have to replace the driver side O2 sensor again shortly after, Now it's runner a little better, but there seams to be a lag in the throttle response. I will try to clean the MAP sensor next. Thanks!

 

I am also suspecting bad catylitic convertors? How could I rule those out....?

 

Take it to an inspection station ( i'm pretty sure Colorado has emission laws) they can tell you. This issue with possibblty CC is back pressure, that I don't know how to measure on a car.

 

I remember on the early carb/cat equipped cars, you could detect excessive back pressure by poor manifold vacuum response. Not sure if that holds true w/ today's computer injected machines.

Also the exhaust had a whooshing sound on full throttle starts due to the plugged cat.

 

what year? flex fuel capable?

post all the codes and trouble shoot from there.

some things to consider:

the lean code is most likely the concern, the 02 is setting because its lean all the time. If your programmer can post live data; what is the fuel trim at idle, under load at 2500rpm? report results for both banks, long term and short term fuel trim.

If its fuel starved underload it could be a bad fuel pump. Towing increases the load so the problem is worse. You may need to check fuel pressure underload.

cai, and cai filters cause alot of problems especially for the newer trucks. the filters don't filter properly and the maf gets contaminated, skewing fuel metering.

plugged cats are easy to check; with either a tap test(rattle=bad) or bypassing them by removing the oxygen sensor from the exhaust.

 

The year is a 2004. Not flex fuel.
It has been driving around town without any troubles, except the a/c pump just went out The problem with the trailer normally happens at the eisenhour tunnel here in colorado, which is a very high altitude @ 11,013 asl. always while climbing the hill.
Can the transmission cause the engine to go into limp mode? If so, can I do anything to improve the tranny. It is a 4L80E...
When I tow to Nebraska or something , I never have that problem. Just on passes.
I'm going to do my exaust next, I want to replace the cats with high flow Magnaflows and do Flowmaster mufflers. Does anyone have any recomendations on exaust for the 8.1L ??
My Edge CTS programmer does not do fuel pressure.. the closest thing would be the air/fuel mix.

 

what unit is the air/fuel mix in? stoiciometric or as a percent like fuel trim?

If the truck is lean at both banks during accelleration the most likely items are low fuel pressure or a bad maf sensor. So report the fuel trim % and if its adding fuel you will need to check fuel pressure.

cai filters (junk) are the worst things for contaminating maf sesnors. They don't meet filtering requirements of a oem part.