Mechanic thoughts needed. 2003 sierra

Okay, so not my tow vehicle. But my Dad is having problems with his 2003 Sierra with the 5.3. We put a reader on it. Says mass flow sensor fault, as well as multiple Mis fires. He did a new MAS, plugs, wires, and fuel filter. Truck is still acting the same. At idle it is fine. Drive it down the road and acts up. Scaner shows it faulted at 2460 rpm, 30% load, and 57 mph. Map sensor seems fine, and throttle position as well. The fuel rail read outs seemed strange. Bank 2 had a negative number, while bank 1 was positive. That normal?
Any one have experience in this area?

Mattman said:

Okay, so not my tow vehicle. But my Dad is having problems with his 2003 Sierra with the 5.3. We put a reader on it. Says mass flow sensor fault, as well as multiple Mis fires. He did a new MAS, plugs, wires, and fuel filter. Truck is still acting the same. At idle it is fine. Drive it down the road and acts up. Scaner shows it faulted at 2460 rpm, 30% load, and 57 mph. Map sensor seems fine, and throttle position as well. The fuel rail read outs seemed strange. Bank 2 had a negative number, while bank 1 was positive. That normal?
Any one have experience in this area?

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My much better half had a 03 Yukon with the 5.3 in it that was acting the same way turned out the plastic intake manifold had warped and was pulling air between it and the head. I found it by spraying starting fluid around where the manifold bolted to the heads. Replacing the manifold fixed the problem, but they are not cheap. About 600 bucks if I remember right. The 5.3's are also known to drop lifters as well that will cause misfires at different RPM's with misfire codes, as bad coil packs could as well, but bad lifters, or coil packs shouldn't throw MAS codes. Check the intake. Hope this helps.

Plugged cats if anyone is curious.

Mattman said:

Plugged cats if anyone is curious.

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My old 1996 Dodge Ram 1500 4x4 started acting strange up in the mountains coming home from a camping trip in Glenwood Springs, CO.

Cruising along I-70 at 70 mph and all of a sudden it just slowed down to around 15 mph and wouldn't go any faster.

Pulled off in Eagle and the only garage in town thought it was the tranny going out and said they couldn't fix it and that we would have to go back (about 30 miles) to Glenwood Springs to get it fixed.

We dropped the trailer (our old Coachman) at a rest area/museum (they gave us permission to leave it there) and drove back to Glenwood Springs . . . without the trailer we could get it up to 50 mph.

Turned out the catalytic converter died (clogged) . . . about a $1200 repair plus two nights in a local motel since we had to leave the trailer behind.

Took this shot just before we left the campground that day!

My old '98 Dodge did the same thing. About a month after I paid to have it fixed, I got a letter in the mail saying they had been forced to extend the emissions warranty, including the cat. Sent them my receipt and got my money back.