Greetings! I just got back from a trip to College Station, Texas. I noticed that my car's engine ran quieter overall and started better when cold while at lower elevations. I assumed it was running a bit quieter due to the small amount of biodiesel Texas adds to the fuel. However, on the way home, still running a tank of Texas fuel, I noticed the same old noises I hear once I got back up into higher elevations (over 5000 feet above sea level).

Here in New Mexico my car's engine does a couple things I've not figured out but have gotten used to. For one, on a cold morning, below about +50F, my car starts well enough but when the afterglow cuts out it runs like crap until it's warmed up. It acts like it's getting too much fuel, runs rough and blows grey fuel smelling smoke. I can work around this problem by using the block heater for a couple hours before starting it up. Works fine when I do that.

The other thing I notice when at higher speeds and elevations is that the engine makes more noise at some power levels, a noise that sounds almost like nailing or clacking. It's a "crinkly" noise, if that makes sense. This does not occur when cruising on flat ground, or when the pedal is all the way down. I hear it between those conditions, like when going up grades or accelerating without flooring it.

The engine runs pretty good overall, plenty of power, decent fuel economy. I have had the injectors rebuilt and balanced. I have replaced the delivery valves with brand new parts. I have cleaned out the intake. I have not checked injection or cam timing. The engine burns zero oil.

We had some cool mornings while I was in CS, in the 40s, and the car started and ran fine. So what I'm wondering is if these cars have altitude compensation as part of the electronic governing system. If so perhaps my car's compensation is not working properly.

What should I be looking at here? Temp or barometric pressure sensor problem? Something else? Feedback appreciated.