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

Asked by Oct 12, 2014 at 04:52 PM about the 1998 Chevrolet C/K 1500 Cheyenne RWD

Question type: Maintenance & Repair

My '98 Chevy Silverado suddenly dropped 4 to 5 mpg so I had a full tune up with new plugs and wires, changed injector syst., changed O2 sensors, had purge valve cleaned in throttle body, changed fuel filter, changed the mass airflow system, with no improvement. My mechanic has run out of ideas. We've made no drastic changes to cause this. Any ideas?

12 Answers


You might suggest that your mechanic run a live data scan, he might pickup on a sensor that is not reading accurately, I had a Tahoe that had lost 4 to 5 mpg, it wound up that the ECT sensor was reading lower than the actual engine temp, replaced it and my mileage came back. It would not turn on the check engine light cause it was changing readings. HTH

1 people found this helpful.

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


I had assumed that you had already replaced it when you stated you replaced the injection system

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 temp gauge sender is not the same as the ECT sensor, did your tech do a live data scan? Have the stations in your area changed over to winter fuel mixture? Have you changed tires? Have you noticed a feel like it is slower on take off, could be a torque converter slippage, check trans fluid color.


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.


I'd go ahead with the live data scan, see if anything shows up, if nothing then it could be engine basics, ie timing chain, camshaft, compression, unmetered air leaks, etc.


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.


I think you just hit on your problem, the garbage they are trying to tell us is fuel has a very short shelf life, dump in a can of octane booster, 103 is some good stuff for todays fuel and if it is going to sit for a while after this treatment put in some StaBil Marine it helps counteract the effects of the methanol in the fuel.

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