JA6X

Greetings all,

I have this posted in the 4.2 engine forum, but it seems not many read that, so I thought I'd post it here. It is a 4.2 ART motor.

First a quick update on the 6sp MT conversion. I've put approximately 14K miles on it and have nothing but good things to report, with the exception of the current problem. It works as expected and has been trouble free.

Now for the two issues I'm currently facing. I have a, let's call it a consistent intermittent issue with a slight stumble/misfire/hesitation, under light loads, here are the DTCs

16486 - Mass Air Flow Sensor (G70)
P0102 - 35-10 - Signal too low - Intermittent
17536 - Fuel Trim; Bank 1 (Multi)
P1128 - 35-10 - System too Lean
17549 - Load Calculation Cross Check
P1141 - 35-10 - Implausible Value - Intermittent

I also had a Bank 2 S1 O2 sensor issue reported.

Here's what I've done so far.

Replaced both S1 O2 sensors which cleared up the 02 DTC
Replaced all of the small vacuum lines, both combi valve lines were crumbling
Replaced the MAF with a reman Bosch unit - No Joy, same issue
Cleaned the MAF connectors

I'm still suspecting the MAF and will order a new unit if the consensus points us in that direction. Additionally, checking the short term fuel trims, Bank 2 is fine, Bank 1 is all over the place, and MPG has gone to **** ~13MPG currently. I still suspect the MAF because it seems the problem will manifest itself when the underhood ambient temperature rises and stabilizes, when I first fire the car up, and for about 10-15 miles, she runs perfectly. Then as the underhood termperatures stabilize at the operating temperatures, the problem of hesitation/stumbling/misfire will occur. One additional point, once the RPMs cross about 3,200, everything clears ups and she runs like a champ. Once they fall below, sometimes it's fine, sometimes the problem will manifest, more often than not, the problem returns. Any ideas?

Issue 2

Which may be related, or not. when I replaced the vacuum lines, and fired the car up, I noticed the upper pod for the variable intake system (IMT) was still inflated. It is my understanding, that both pods should be pulled back. I double checked the vacuum lines, all good, bypassed the pierpont valve and both pods retracted. I then switched the pierpont valves to check if the upper was defective, and the same condition applied, the upper pod stayed inflated. I did clean both of the "bearings" up and both flaps are easily put through their range of motion with no binding whatsoever.

I'm hoping you guys have some ideas as this is driving me crazy. As always, thanks in advance for any help.

Best regards,

Joe

JA6X

Found the culprit, bad Fuel Pressure Regulator. She runs like a champ and the fuel trims are much better over a 500 mile snippet.

SloopJohnB@mac.com

It is interesting there is no EZ way to check fuel pressure. SOP on GM engines...even a schrader-type valve and cap to make it EZ.