2002 Avalanche 5.3 O! M! G!

Lolz!

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Ain't that the truth... LMAO!!! It took me long to reply with info cause I had to wait to get paid and my back went out on me. The work I'm used to is following after someone else's work, fixing their mistakes, blah blah blah. What I'm not used to is the factory just all of a sudden not doing something that they are used to doing for decades. I guess it's better engineering, I guess... lol. It took me, a couple of weeks ago, a few minutes to try and remove the pcv valve which is now part of the valve cover. I think since I can't check the pcv valve I can at least clamp that hose off that goes from the intake manifold to the pcv valve off to see if fuel trims get better or not.

Lol, I feel like I'm at step #1 again in figuring this stupid truck out. I actually am after getting that O2 problem fingered out

what you're testing when you pinch off the PCV hose is for an internal intake air leak. Check for vacuum in the dipstick tube when doing the test.

chevy/gmc 5.3L are known for an oil consumption issue due to the PCV valve and the baffle in the valve cover.

you still have a vacuum leak... your LTFT's get better with higher RPM.

air entering through the exhaust manifold can cause the 02's to read lean. when doing the exhaust check (putting a rag in the tail pipe) check the fuel trims. if they go to 0 or start correcting, that is a leak that you have to repair. any air leak in the exhaust prior to the 02 will effect the 02 reading.

with those positive LTFT's of 21 and 24 at idle, I think its more likely you have an internal intake leak. that's if you're sure you have no external vacuum leaks.

Thanks for keeping me on the straight and narrow

I was going to ask if your fuel trim numbers were negative or those were just dashes in front of the numbers? I'm with scannerjohn on this with a vacuum leak if those are not negative numbers.

Nissan Technician

I also assumed that those dashes were negative signs. If they are not... then forget almost everything that I said.

Really sorry about that guys... the - is meant to be = not negative numbers. Again sorry didn't mean for that to throw everyone off.

I edited one of my posts above because I messed up. Sorry about that.

Well, Went and plugged the PCV vacuum line... nothing. Pinched off the Vacuum assist line to the brakes... nothing. Brought my scanner out by the engine and started spraying brake cleaner looking for leaks... nothing. This truck is really kicking me in the butt. I'm going to pull out my little smoke machine since it's a very nice day out with hardly any wind. Maybe I'll see some smoke somewhere today. I also checked the exhaust for leaks like scanner john said... nothing as far as I could tell. Nothing was obvious I should say.

SJ where else would an internal leak be? I'm really at a lost now. I checked for exhaust leaks incompletely... I was by myself and was basically listening for leak. I'll have my wife help me later with the scanner.

the internal leak would be at the intake manifold gasket bottom side of the manifold. you will not be able to use water, propane or brake cleaner to detect the leak as its internal. the presence of vacuum in the dipstick tube is caused by that leak. if you changed the intake gasket, it may have not sealed correctly.

Thanks for the heads up on the leak. I haven't changed that gasket yet because I saw no smoke. and I forgot to check the dip stick for vacuum. Just a thought... would I have to block off the Pcv valve? That would create vacuum at the dip stick too wouldn't it?

Another thought.... what if I pulled the dip stick out and run another smoke test? It should come out the dipstick tube wouldn't it?

Thanks again

you clamp off the PCV hose and the fresh air inlet PCV tube(passanger side front of the engine). when you run the engine this would produce pressure at the dipstick, but if you have an internal intake gasket leak you will have vacuum at the dipstick instead. smoke will not work because your checking for vacuum not pressure. the leak would be pulling extra air in from the crankcase into the intake runner causing a lean condition.

Hey ScannerJohn… I owe you a Thanks and a coffee. Went out today and hooked the smoke machine up and waited for a few minutes and looked the engine over good and saw nothing. Pulled the oil fill cap off and we got smoke. Pulled the dipstick and got nothing at first. Then it start coming out of the dip stick tube too.

Appreciate everyone's help on this!!!

I'll write back when I get those gaskets in and check my fuel trims again. My guess is that the worse part of the leak must be at cylinder #6 because of the misfires.

Thanks again scannerjohn

Awesome! Glad that you found a problem!

When you change those gaskets, make sure that your manifolds flat and not warped.

If they are warped you have a few options.
1. You can build a lapping rig and lap them until they are flat
2. You can send them to a machine shop and they will flatten them
3. You can replace them
4. You can attempt to keep adding more sealant where it matters to get it to seal.

I've had really good luck with #1. I've done it many times.

I have some heads off an Intrepid with a 2.7 that I lapped by hand. They were cast so it wasn't so easy but not too bad... just time consuming. Kept checking it with a straight edge with .002-.003" slop. I ordered some graphite head gaskets just to make sure it seals good. I couldn't finish it because I got involved with this dag-gone truck. First thing this spring... but not until I finish this truck. So, so glad to have found that leak finally. It was making me nuts!!! Just hope that it's done after I fix the intake..

Well... put new intake manifold gaskets in and it seemed slightly better using one scanner then in hooking up the GM scanner it basically showed the same readings as before even after a trim reset. Both long terms are at 25 and the short terms are fluttering around in the single digits sometimes hitting 10. What an unexpected disappointment

I was in a hurry and forgot to check to see if the intake manifold surface was flat before putting all back together

with new gaskets, unless you pinched them or pushed them out of position when putting the intake back on, I would say they sealed Ok... your just getting too much air. unplug the MAF when engine at hot idle and read the trims. if they get better then your MAF is not reporting properly. I know you said you replaced it, but its worth checking.

One last thought. Put you foot on the brake when you start the engine. Does the peddle drop/bump when you start? Hold the brake pedal down and check trims. Do they get worse?