Yukon XL Denail 2009 Severe Hesitation and Poor Gas Milage, real head

pfluegel · April 15, 2017

Not to be to cliche...but Help please.

So here is the full run down. About a year ago my Yukon started what i can only call a severe hesitation, it got so bad that i took it to the dealership. They had it for a month and a half and claimed to have replaced everything but the computer with "known good" parts. I was unwilling to spend 2000 on a computer and reprogram that may not be the problem.

This year it got so bad i bought a computer from LKQ and had it programmed by the dealer. It seemed better but never went completely away. I have since replaced the throttle position sensor, Maf Sensor, Cleaned the K&N filter and completely cleaned the throttle body, all of which the dealership claimed to have already swapped, but I just couldnt believe that once all that was done it didnt fix it. I considered trading her in, but just cant part with her. She has no codes and idles fine, its just when I press the pedal it feels like its just bogging down.

It is odd to me that when I replaced the computer, and cleaned the maf it got so much better. I am desperate. My hope is one of you is that guy who knows everything and know exactly what it is. I have read every forum i could find and just cant find anything that matches my symptoms exactly.

Please Help,

Thank you in advance

Podor · April 15, 2017

It may be a fuel delivery issue. It could be fine until under load. I would recommend putting a fuel pressure gage on it and driving it to duplicate the issue. If the fuel pressure drops significantly, you could have a restriction or weak fuel pump.

pfluegel · April 15, 2017

I have a tourque pro, not sure if it will read fuel pressure, any idea what it should be?

pfluegel · April 15, 2017

Also, why would putting a new computer help initially? I guess I am just held up because i do things and it seems to help short term. I will get my hands on a fuel pressure gauge and let you know what I get.

Podor · April 15, 2017

Also, why would putting a new computer help initially? I guess I am just held up because i do things and it seems to help short term. I will get my hands on a fuel pressure gauge and let you know what I get.

I have no idea why it would have gotten better with the new PCM. If it went away completely and came back I would focus on that some more. It's possible it was learning the incorrect condition and adjusting. FP should be between 55-60 PSI. It's always good to check the basics when things get challenging. It could be a plugged converter also.

pfluegel · April 16, 2017

Ok, Fuel pressure according to my torque app is right between 42&44 PSI and doesn't seem to be affected by wide open throttle in any way. Do these things have some sort of Fuel filter somewhere? I didn't see one along the rails or where i thought it would be. If not is it time for a fuel pump?

SpartanAutoworx · April 16, 2017

Check the Fuel Pump Control Module under the rear spare tire.

Podor · April 16, 2017

Just to verify, there are no history or current codes, right?

As long as it maintains 42 to 44 while driving under a load, I don't think that is your issue. I would highly recommend using a mechanical gage to verify. The sensor could be reading wrong and the computer could be compensating. Garbage in to the computer, garbage out.

You can do a somewhat easy test for a restricted converter. Pulling the upstream 02 sensors temporarily and driving it will open the exhaust enough to make a difference in performance. You can drop the y-pipe. If it drives better, you probably have a restriction.

pfluegel · April 17, 2017

Check the Fuel Pump Control Module under the rear spare tire.

How would one go about checking that particular module?

pfluegel · April 17, 2017

Just to verify, there are no history or current codes, right?

As long as it maintains 42 to 44 while driving under a load, I don't think that is your issue. I would highly recommend using a mechanical gage to verify. The sensor could be reading wrong and the computer could be compensating. Garbage in to the computer, garbage out.

You can do a somewhat easy test for a restricted converter. Pulling the upstream 02 sensors temporarily and driving it will open the exhaust enough to make a difference in performance. You can drop the y-pipe. If it drives better, you probably have a restriction.

Ok, i can borrow my buddy's fuel pressure gauge, i had wondered if perhaps i was getting bad data. There are no codes pending or otherwise.

Regarding restriction, if i drop the ypipe and remove the first O2 sensor and our drives fine, i have a clogged cat? And do you mean remove the wires to it, or leave it wired and remove it physically? On that note, wouldn't clogged cat trigger a fault code?

Podor · April 17, 2017

You can either drop the y pipe or remove the 02s, you don't have to do both. The hole from the sensors will probably be enough to feel the difference. In some cases, dropping the y pipe is easier, but you may run into an issue with rusted nuts and studs and replacing the gaskets between the manifold and pipes. Unplugging the 02s will set a code, but it isn't a bid deal to drive it. You can just clear the codes when your done.

pfluegel · April 18, 2017

You can either drop the y pipe or remove the 02s, you don't have to do both. The hole from the sensors will probably be enough to feel the difference. In some cases, dropping the y pipe is easier, but you may run into an issue with rusted nuts and studs and replacing the gaskets between the manifold and pipes. Unplugging the 02s will set a code, but it isn't a bid deal to drive it. You can just clear the codes when your done.

Alright, I will remove the O2s, If it conutinues i can cross that off the list of could bes.

oh, remove all 4 o2s?

Podor · April 18, 2017

Just the upstream ones before the cats.

pfluegel · May 4, 2017

Ok, a few updates. I got so frustrated that i took it to the dealer and essentially told them to take it on trade or i would light it on fire in their parking lot. Cooler heads prevailed and they showed me a really nice truck that was more expensive than my current mortgage. I was able to get a hold of the guy that teaches the drivabllity techs through a local college and he said right off the bat, and I quote, "i bet you have a f@#$ing K&N air filter". I let him know i indeed did have one and he said pull it out and replace it with a paper one. Clean off the original MAF sensor, and reset the computer. It is indeed driving 95% better, fuel economy is up along with power. However, it still stumbles, though far less frequently, still hasn't returned to previous MPG, and is now throwing a code P0333. This is a knock sensor bank two. It has been replaced, still throwing the code.

I know that is a lot of data... I know how to test a knock sensor with a single connection, but this one has two, so I have no idea what to do now...

although it is significantly better.