1998 Chevy Silverado has a drastic drop in mileage that my mechanic and I can't find the reason for

good idea tennisshoes. someone once told me a fuel pressure regulator could also cause something like this.

Thanks for your feedback tennisshoes. I appreciate you trying to help me out. Yes, I replaced the thermostat, the temperature gauge sensor, O2 sensor and my mechanic was thinking the same thing as you, but it didn't change anything mileage-wise. I would appreciate any other ideas you have. I've had the truck since it was new, and have recently replaced everything we could think of that could be causing this.

The fuel pressure regulator is the usual culprit. Remove the vacuum hose from it and if any gas comes out of it, its shot.

This all began when I noticed that when deaccerlating the engine wouldn't decompress and when going down hill I had to brake a lot. He cleaned or replaced the purge valve which fixed that problem, but not the mileage. As for winter fuel, we are in the Phoenix area, so I'm not sure that there is a winter fuel here. My tires are 10 ply and in good shape, but are approx. 12 years old with probably 40 or 50,000 miles on them. No noticable change in accerlation or power. I did have transmission serviced at about 90,000 miles and nothing appeared to be wrong. The mechanic had some suspions about that, too.

Oops, twelve year old tires and living in AZ hot weather. Any bets when the tires will let go and the tread comes apart from the casing? That just may be part of the problem. Too much drag and less rolling resistance on the tires. It is not safe to drive on tires over ten years. I replaced the tires on my seldom driven Corvette after 10 years even when there was little wear on the tread. No only did I get rid of the vibrations, but got better handling, traction and mileage. You may have a transmission slippage also, not knowing the loads you haul or how the van is driven. As good old tennisshoes said, a full scan should help find any sensors acting funny. Is there a chance someone is taking gas from your tank a little bit at a time?

I'll go ahead and ask for the scan. I guess I will look into new tires. I talked to a tire dealer last spring, and he didn't think the tires would be a problem with fuel effeciency. Obviously everyone else says differently. The truck doesn't get used much and sits in the garage most of the time- so no, the gas isn't being taken out, though that would be an easy fix! I'll let you know what happens.

The tires are more of a safety issue, not so much of an economy robber. It'd be really simple if it was someone helping themselves to gas for their motor cycle or lawn mower.

dirty EGR valve