p30 stumble problems/ fuel pressure regulators

P30 stumble fuel pressure update: Took a 1400 mile walkabout. Fernley Nv to Sacramento and back, over Donner pass. I got 8.25 mpg, fully loaded with toad. Fernley Nv to kingman az just below 11mpg, this is a pretty flat run. No problems at all. Lots of power, brakes worked perfectly, engine temp never going over 200 but outside ambient was in the 50's going over Donner pass.

Down town vegas was another matter through. Ambient temp went to 114, turned on dash AC, engine temp went to 220 mechanical. As the ambient temp dropped to 98 when we drove up the 95 and the engine temp dropped to 192. Did I mention it was stop and go 15 mph traffic?

I mentioned before, I had a still had a slight misfire on cylinders 1 and 2. I pulled the bosch plugs(11k miles) and replaced them with AC delco's iridium plugs. Only one missifire per 32 cycles on cylinder 8 when engine is cold and that goes away. Guess my old 454 vortec does not like german engineering!!!! Or bosch parts are crapola )-:

I replaced the up stream O2 sensors and the -fuel trim reduced from -7% to -4% (+-1%). I can't figure why I would be still getting a negative fuel trim unless it is due to the elevation that I'm at. 3500 feet. Replaced the vacuum sensor and the ECM temp sensor. I replaced these sensors because they were showing a weak voltage and a low engine temp on the tech2 code reader. Because of the new fuel injectors I now have a very smooth idle and improved gas mileage. After the problems with the bosch plug wires and plugs, I'm a little concerned that the bosch injectors will give me a problem down the road a bit.

The fuel trim still troubles me, I pulled the idle selinoid, cleaned it, tested. All is perfect so I figure out why the computer would reduce the fuel? Yes, I know what the books say: low fuel trim, poor air intake or 02 sensor problems, but my sensors are all new, exhaust system is sealed, and the 02 sensors are new and reading the correct voltage. The low -negative fuel trim is only at idle, not at 1500rpm's. I'm sure most of you remember that high fuel trim +percentages mean a vacuum leak or additional air is getting in past the MAS and MATS so you get fuel added to maintain the 14.7:1 air mix. Your ECM programming sets your fuel injector pulse width but then alters it slightly when one of the sensors tell it there is a problem with the mixture. Mine is negative, so that means it is reducing the fuel (pulse width) due to less air? Checked the air filter to see if it was clogged, it was clear.

So, I'm out of ideas for the - fuel trim, any suggestions?

Anyone have any suggestions as to why this is happening? Any practical experience with this elevation and -fuel trim?

Thanks

Update: Checked my engine readings. still getting negative fuel trim values, -5 to -7. Coach seems to be running perfect. Idle smooth, temp in a good range, gas mileage varies between 8mpg over passes and 11mpg the flats of the desert.

I'm tempted to replace the idle mix valve to see if that is what is causing the negative values. Any one have any experience with this?

fuel trim of -5 to-7 is not bad I would be thinking more of a vacume leak maybe from intake manifold gaskets if you could find somebody that had a smoke machine it would be easy to see

Update to my update!!!!!

got an engine light again. Hooked up my teck2 code reader and guess what? I have another brand new bosch O2 sensor failing, bank 2 sensor 2. So, I ordered 2 new AC Delco units and replaced both downstream sensors. All back to normal. Returned the Bosh to Autozone and they refunded me my $$$$. DANG, for both sensors even if only one was bad!!!! Guess that is why I keep getting parts from them!

you Won't believe this, getting mis-fires again, then the upstream bosch sensors started reading low????? one has completely failed. Huh???? Not even 6 month old...... I replaced both upstream sensors with AC Delco's as I did the down stream units and all is well again, no-misfires, wow I'm really surprised at these thing failing. Auto zone stood behind their warranty and refunded for the bosch units. What a great company to do business with.