Power problem
 

I have a 2001 Isuzu NPR (16' box truck), it has a chev 5.7L Vortex in it.

It has always ran strong/good until a few weeks ago. So we did a major tune-up on it by putting in new plugs, wires, rotor, cap, pcv, hoses, thermostat, oil.

The problem we are having is it runs great while it is cold, after a few miles and at operating temp, it looses power doesn't want to go over 35-40 and you have to really romp on it to get it to do that. A local mechanic said he thought one of the catalytic converters was plugged so we punched it out (got a new one on order), but no difference. We went from 12-14 mpg to about 7.5 mpg overnight.

The truck is bifuel but we have unhooked the LP because of problems there and the cost of LP vs gas.

Has me scratching my head!!!!!! Any suggestions????

You don't mention the fuel filter under your listings of items replaced with the major tune up. If not done I would start there and also check the fuel pressure and volume after warming up. Also check the exhaust, any whistling at the tailpipe could indicate a plugged exhaust. Another way to check exhaust is to hook up a vacume gauge to the engine cold then drive vehicle until it starts to act up. Compare readings-lower would indicate an exhaust system plugging after getting warm.

I am assuming that the truck has fuel injection.

yes - fuel filter was replaced, and it does have injection

Is it something that happened "all of the sudden", or something that gradually happened?
Did you check your airfilter? If a car breaths too much hot air, it will act pretty much the same way, i think. And if it catches not enough air, its also something like that.

You also have an LPI installation,right? If I'm not mistaking, then the injector of those kind of cars, are electronically steered. So maybe if you tell us what the LPI problem is, this could also cause the strange driving conditions on your car.

So it started running poorly BEFORE you ever did anything to it? Just on it's own?

If it runs ok when cold, but runs worse and worse as it warms up, and also gets much worse MPG than before, it may be running too rich. Possible causes include broken coolant temperature sensor or a wire leading to it. I would check those first.

If you have a multimeter you can test a coolant temp sensor by measuring the resistance when cold, and then again when warm. Resistance should drop as it warms up.

Have you looked at the engine temp gauge on the dash when it is fully warmed up? Does that act normally? In many cases it has it's own separate sender, and the computer reads coolant temperature from another sender somewhere. I suspect the latter has failed.