Gas mileage dropping on my 98 5.7L - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Tom · 08-24-2014, 04:15 PM

My 98 has always been excellent with the gas mileage. When I first got it about 8yrs ago at 75,000 miles, I would do about 17mpg city and 22mpg freeway. The city has slowly dropped till it averaged 15, the freeway I was still managing 22 [remember its all flat out here, no hills anywhere]. Over the last year, my freeway has been falling as well. I'm down to like 17 freeway, which means something isn't right. My last tank average was only like 14mpg.

I maintain the crap out of this truck. Fuel filter, wires, cap+rotor, wheel bearings, brakes, trans fluid, spark plugs, air filter, u-joints, intake gaskets and fuel pump are all new or recently new. The truck still runs great. Pulled all the plugs and they are evenly worn and look good. Checked compression on a cylinder from each bank just cause and they were excellent. My brake booster went out about a month ago and leaked vaccum so I was hoping that was the cause. New booster and mileage is still getting worse. So now I'm left guessing what could cause this. Cat convertors? O2 sensors? I would think they would trip a CEL eventually though. What can I check at this point?

speedygonzales · 08-27-2014, 06:59 AM

2 things that most people don't realize are normal tune up items are the front O2s and the ignition coil. They should be replaced every 60K miles.

However, I'm going with fuel injectors as your major cause of your problem. I switched to the new style quite a while ago.

95 S_Trucker · 08-27-2014, 02:14 PM

I would say that oxygen sensors and coils are not regular maintenance items. There's no sense in replacing parts when you don't have to.

The problem with the old style fuel injectors(poppet style) they will stick and gum up. They will cause hard start issues from them dripping fuel when they shouldn't. They also will cause performance problems when the poppets don't open the entire way.

See if you can use a scanner that will show live data, Check fuel trims, both O2 sensor voltages and MAF(g/s)

You can do an injector balance test to test the injector assembly.

It sounds like the problem on your truck isn't quite bad enough to turn on the mil.

08-24-2014, 04:15 PM	#1
Tom driving is in my blood Join Date: Dec 1999 Location: Mesa AZ Posts: 5,763	Gas mileage dropping on my 98 5.7L My 98 has always been excellent with the gas mileage. When I first got it about 8yrs ago at 75,000 miles, I would do about 17mpg city and 22mpg freeway. The city has slowly dropped till it averaged 15, the freeway I was still managing 22 [remember its all flat out here, no hills anywhere]. Over the last year, my freeway has been falling as well. I'm down to like 17 freeway, which means something isn't right. My last tank average was only like 14mpg. I maintain the crap out of this truck. Fuel filter, wires, cap+rotor, wheel bearings, brakes, trans fluid, spark plugs, air filter, u-joints, intake gaskets and fuel pump are all new or recently new. The truck still runs great. Pulled all the plugs and they are evenly worn and look good. Checked compression on a cylinder from each bank just cause and they were excellent. My brake booster went out about a month ago and leaked vaccum so I was hoping that was the cause. New booster and mileage is still getting worse. So now I'm left guessing what could cause this. Cat convertors? O2 sensors? I would think they would trip a CEL eventually though. What can I check at this point? __________________ -78 c10 short/step: 388cid, M20, 5/5 drop, lots more. Playtoy and first vehicle. -98 c1500 x-cab: 5.7L, 17" rims, 5/6 drop, flowmaster, helper bags,NBS rear disk brakes. -02 Suburban 4x4: leveled front -CBR600F4i, CBR600RR, CBR1000RR, and standup skis DISCLAIMER: I cant spell for the life of me.

08-27-2014, 06:59 AM	#2
speedygonzales Registered User Join Date: Jan 2006 Location: Smithfield, VA Posts: 1,501	Re: Gas mileage dropping on my 98 5.7L 2 things that most people don't realize are normal tune up items are the front O2s and the ignition coil. They should be replaced every 60K miles. However, I'm going with fuel injectors as your major cause of your problem. I switched to the new style quite a while ago. __________________ 99 Z71 (98 body style) Pride and joy

08-27-2014, 02:14 PM	#3
95 S_Trucker Registered User Join Date: Oct 2009 Location: . Posts: 2,187	Re: Gas mileage dropping on my 98 5.7L I would say that oxygen sensors and coils are not regular maintenance items. There's no sense in replacing parts when you don't have to. The problem with the old style fuel injectors(poppet style) they will stick and gum up. They will cause hard start issues from them dripping fuel when they shouldn't. They also will cause performance problems when the poppets don't open the entire way. See if you can use a scanner that will show live data, Check fuel trims, both O2 sensor voltages and MAF(g/s) You can do an injector balance test to test the injector assembly. It sounds like the problem on your truck isn't quite bad enough to turn on the mil.



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