2003 yukon 5.3 misfires

[silveradosid]

working on a 2003 yukon with the 5.3. it misfires on cylinders 5 and 8 and has low effiency code for bank 1. the misfires you can feel at idle but doesnt trip the ses light. as soon as you get to 2500 rpm ses starts flashing and scanner shows misfires on those two cylinders. if you take the engine up to 3000 rpm and higher the misfires stop. i have changed all the plugs and wires and the coils on 5 and 8 but that has not changed the misfires. i also changed the downstream O2 sensor on bank1 looking for some help as everything i have done has not changed a thing. the engine has 120k miles on it

[Jsdirt]

I'd put money on an intake gasket leak.

 

Does the condition seem to improve with heat? In other words, does it run pretty bad cold, then get better as it warms up? Fuel trims improving with temperature? If so, it's a vacuum leak, either from the intake gaskets, or any of the vac lines.

 

Flashing light means a cat-melting misfire. If you have emission testing where you live, I wouldn't drive it too far like that. Cats aren't cheap.

 

A bad MAF can do this too, if it's making the engine run too lean. Normally you'll see a code for that, or a Random Misfire (P0300) code, lean codes, or any combination of these.

[silveradosid]

thanks jsdirt when i changed the plugs they all showed that it is running lean, all of them looked white. temperature doesnt seem to effect how the engine runs. fuel trims are constantly fluctuating. i am trying to get this pos ready for an emission test. the only codes are misfire and the cat code

[silveradosid]

today i cleaned the maf and the throttle body ran the engine no change while it was running i sprayed starting fluid all around the intake and vacuum hoses made no change in engine rpm so i think the intake gaskets are good. i also hooked up a fuel pressure gauge at koeo has 55 psi running at idle it is 45 psi i am beginning to believe the cats are plugged

[Jsdirt]

Best way to check for plugged cats is by removing one or both upstream o2 sensors - if it runs better, then that's your problem. Also using an old school vacuum gauge can tell you alot, too. Low vacuum can be caused by plugged cats (as can valve train issues).

 

To absolutely rule out intake gaskets, I use just plain water in a plastic soda bottle with a 1/8" or larger hole drilled in the cap - squirt all along the gasket surfaces when the engine is cold, just after firing it up. Usually if the leak is big enough, you'll hear the water getting sucked in. You'll also get a big change in RPM, even if you can't hear any slurping water.

[silveradosid]

i am going to hook up a vacuum gauge just to confirm the scan tool is correct because it only shows 12 hg at idle which i know is to low unfortunately every time i get to work on the vehicle it is already hot many thx for your help jsdirt

[Jsdirt]

No problem.

 

You have to add the MAP or BARO reading to than number - sounds like it's pretty close to normal.

[silveradosid]

crap ok i will double check that