Helping a friend diagnose his 2003 Chevy truck 5.3L

PO171, 174 Too lean.
PO300 missfire.

The post cat O2 sensors read very different from each other. One moves up and down in voltage like the pre-cat sensor the other puts out a more steady .7 or .6 volts.

The long term fuel trim on one bank is over 23%, this tripps the CEL.

I didn't think think the post cat O2 sensors affected mixure??

Or is it a clogged converter and the sensor is working correctly, telling me I need a new converter??

The truck stumbles and wanders at idle.

A good puzzle I think.

 

It's a 4.8 liter.
Well ,I tried to solve this for 3 days in our spare time and we decided he should bring it to the dealer.

I found a leak right after the MAF sensor, fixed that and it ran better but still missfired at idle. The truck drives fine otherwise just misses and throws a lean code at idle. We threw new factory plugs and wires in it for good measure, I knew that wouldn't solve anything but we where just trying to eliminate that.

After that I sprayed starting fluid all around seams and the intake gasket area and heard no change in RPM.
PVC system is similar to a crossfire, just an oriface no rattle ball, that looked good.
Took off the throttle body and the inside of the manifold are seriously caked with oil and crud but that might be normal for a 100000 mile chevy.

Internet research pointed towards a stuck EGR valve open at idle (that would make sense), but I couldn't find one or any EGR plumbing. Maybe Silverado's don't have one.

ST Fuel trims at Idle are over 30%, as soon as you give it a little gas or drive the truck they go to normal (+/- 5%).

I'm stumped!

 

Dealer says it's the intake manifold gaskets. Chevy has a problem with these shrinking with age. Fel-Pro makes an updated gasket to fix this. Guy said he could induce missfires by spraynig cleaner around the base of the I.M.. I'm going to install these for half the dealer cost Saturday (MOD MONEY, YAHOO). Will post results.

 

I did some looking in google, and that is one of the common reasons.

One source suggested starting fluid to test the gasket on the intake.

Either way the gasket replace is fairly easy and not expensive to do on your own.

 

Right. I did spray starting fluid around the manifold but didn't hear a change in RPM. The dealer tech said he could induce missfires by spraying under the V in the manifold, so I'll take his word for it.
You're right also it is a common problem so I'll replace them Saturday and post results.

edit: cold enough for ya?

 

Nine degrees in full mid-day sun... Oh yea this sucketh.

 

I replaced the gaskets and the truck runs great. They where leaking for sure. Not too hard a job just alot of steps, about 4 hours.

 

Time well spent. Good job!

Feels good to get rid of those nagging drivability issues.

 

Thanks man, It was a good friend's truck. I charged him half of what the two shop quotes where. Saved him money and feeding my SRT mod addiction. Win,win.

 

I have a 3.8 v6 ford that was giving the same codes, loss of power and missing.

$127, and a few hours of work and all is well.

New isolator bolts, inner intake gaskets, and top intake gasket.

Feel like I may get another 140 thousand on this pig.

 

Must be the MN winters killing gaskets.Good deal GDC.