phucheneh
Diamond Member
- Jun 30, 2012
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That's a great point ISAslot
And he may be right, but can you disable OBD2 monitors on a stock PCM? I wouldn't think so; at least, not with just a software flash.
And when it runs the catalyst monitor, it should set P0420. Or maybe it has a rear O2 simulator- which is basically just a resistor to mimic the heater and a constant voltage supply for the O2 signal (PCM wants to see switching at front but a more constant voltage at rear). I dunno how well they actually work.
Can you confirm that all of the monitors have run? What do the front and rear O2 signals look like? Is it in closed loop (should be a simple PID for closed loop yes/no)?
Is the ECT reading normally? Around ambient temp after sitting overnight? Then rising steadily towards operating temp (prob 190-200 or so) during a reasonable warm-up period?
IAT reading ambient-ish? It should also rise as the engine warms up, but not a whole lot. Especially if it has a plastic manifold and the intake is ducted to fresh(er) air in the fender.
Assuming there's an IAT...I think there is. I want to say GM just stopped using MAP, but continued with IAT, after modern MAF's were introduced.
Speaking of MAF...don't bother watching that one. A good MAF and a not-quite-right one are pretty damned hard to tell apart. But in the absence of a DTC, 'new MAF' should equal 'good MAF.' A defective new part would likely be pretty FUBAR, rather than just being inaccurate-but-within-spec.
This is a bit of a tough one. In my earlier post, I was assuming everything was stock except for the air intake. Now I'm wondering if it's even POSSIBLE that all the parts are stock. As I think I already stated- if the truck is getting into closed loop, any change to the engine should be corrected for. Inaccurate mass meter or ECT, out of spec fuel pressure, different throttle body or injectors...anything. If O2 feedback indicates it needs to adjust the long term fuel trim away from baseline, it will do so. Right up until it hits the limit for the amount of deviation it will allow. Then it will set a fuel trim code.
edit: P.S. check the fuel trim numbers. There is at least one short term and one long term PID for each bank. They will be in a 'x.xx' number format. Some software may show it as a normal percent ('110%' instead of '1.10'). The max deviation should be 25%...so LTFT should be 0.76-1.24 and roughly the same on both banks.
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