Poor mileage in 98 Silverado, 350 Vortec


Chevy-SS
01-19-2005, 01:51 PM
My 4WD '98 Silverado has a beautiful running 350 Vortec. I have put new cap, rotor, wires, plugs, air filter, gas filter, PCV valve and EGR valve on it within the last two months. I change the oil every 3,000 miles, plus I just changed fluid and filter in tranny. I also bought four new Michelin tires and am running them with about 36 psi. The "check engine" light works, but it is not on, and there are no codes coming up with a check of the OBD.

I bought this truck (with 78,000 miles on it) two years ago and was getting almost 18 mpg on highway. Now the truck has 90,000 miles and is getting maybe 13 mpg on highway.

It runs very nicely, but something has caused fuel mileage to suffer badly. It also seems to be just a little bit down on power. Any ideas?

Thanks

planedude
01-19-2005, 02:13 PM
I have the same truck with the same issue, I'm thinking about putting a mass airflow sensor on it. Changed almost everything else and never helped.

2000CAYukon
01-19-2005, 02:28 PM
You don't list 02 sensor as being replaced. Given your mileage, it would not hurt to replace it. AC Delco replacement (IMO) is the best.

The other thing to look at is what temp is it running at. A lower than stock tstat can cause more fuel consumption since the engine never reaches full operating temp. Something to check.

//2000CAYukon

Thunderbolt
01-19-2005, 03:49 PM
You don't list 02 sensor as being replaced. Given your mileage, it would not hurt to replace it. AC Delco replacement (IMO) is the best.

The other thing to look at is what temp is it running at. A lower than stock tstat can cause more fuel consumption since the engine never reaches full operating temp. Something to check.

//2000CAYukon

I agree it is not a bad idea to replace the O2 sensors. Poor mileage would be caused by the two pre-cat sensors. An O2 sensor can cause poor mileage without setting any codes also. I also believe it was good advice to recomend AC/Delco parts. When it comes to critical sensors and fuel pumps that is all I use. They claim O2 sensors are on borrowed time after 75,000 miles, But I don't know if it is true. I have a 98 and in the summer if I run 70mph or less it will still get 17-18 mpg and 13-14mpg in the winter with warming it up in the morning. I have 200,000 miles on the original sensors and I am not about to change them until one goes out or my mileage drops.

Silverado Brethern
01-19-2005, 04:21 PM
I dont know how to figure exactly my highway mpg, but i have a 26 gallon tank on my '97 and i usually average around 8-9mpg. Figured by my 26 gallon tank gettin me around 200-250 miles. so i would be quite envious of someone who got near 15mpg. But i wanted the power of a V8 and i got it, so im not gonna bitch about the littlest thing like gas.

Chevy-SS
01-19-2005, 04:37 PM
Thanks for comments about the 02 sensors. I was wondering about them myself. Does anyone have a good web site to buy the AC Delco units? Also, how many are there and approximately where are they located?

Thanks

abaird
01-19-2005, 04:54 PM
Chevy-SS you should be able to get them at any gm dealership. I know you can special order some ac delco parts from places like autozone.

Silverado brethren you must have a heavy foot if you are only getting 8-9 mpg. I dont think I have ever gotten below 13 mpg in the winter w/ all city no highway.

2000CAYukon
01-19-2005, 04:59 PM
Thanks for comments about the 02 sensors. I was wondering about them myself. Does anyone have a good web site to buy the AC Delco units? Also, how many are there and approximately where are they located?

Thanks

Goto www.acdelco.com to find a local AC Delco parts store. It should be cheaper than a dealer.

Like Thunderbolt said (and I forgot to mention), it is the pre-cat 02(s) that you should replace. The post-cat 02 sensors are not used for fuel delivery.

//2000CAYukon

Thunderbolt
01-19-2005, 07:00 PM
Goto www.acdelco.com to find a local AC Delco parts store. It should be cheaper than a dealer.

Like Thunderbolt said (and I forgot to mention), it is the pre-cat 02(s) that you should replace. The post-cat 02 sensors are not used for fuel delivery.

//2000CAYukon


ACdelco.com is a good place to get part numbers and then try gmpartsdirect.com for prices. I think they are like $48.00 each.

Chevy-SS
01-19-2005, 08:03 PM
Again, many thanks for help. I went to the acdelco.com site and drilled down to the page shown below. Can someone please tell me which parts I would need? Again, I have a '98 Silverado K1500 4WD, 5.7 Vortec, auto trans. It looks like different trannies use different sensors. My tranny has 16 bolts and uses the thin filter (not the deep one). I know 'cause I just changed the filter and fluid.

http://files.automotiveforums.com/gallery/uploads/275744/Parts.jpg

:rolleyes:

2000CAYukon
01-19-2005, 08:09 PM
AFS105 is the one you need (this is before the cat).

AFS106 is after the cat.

Note that the M30 was the standard autotrans for the 5.7.

//2000CAYukon

Chevy-SS
01-19-2005, 09:01 PM
Awesome, thanks, Yukon. The best price I could find was about $75 per sensor at AutoPartsGiant.com, does that sound like a good price? Thunderbolt mentioned that they might cost around $48 each, is why I ask.

gmpartsdirect.com did not seem to have the part listed. Autozone carries them, but I'll bet they are over $100 each there.

2000CAYukon
01-19-2005, 09:21 PM
www.rockauto.com has the AFS105 at 70.79.

GM Part # is 25312200 and gmpartsdirect.com has it at 61.42 plus s&h (list is 122.83). Also note that the GM part and AC Delco part are the same (and will probably have both part #s on it)

Have you tried any local parts stores that carry ac delco (you can find local stores on their website).

//2000CAYukon

2000CAYukon
01-19-2005, 09:28 PM
Unfortunately, the heated 02 sensors are more expensive. You can buy a non-acdelco brand but in the long run the acdelco will work better and last longer.

//2000CAYukon

Thunderbolt
01-19-2005, 09:30 PM
www.rockauto.com has the AFS105 at 70.79.

GM Part # is 25312200 and gmpartsdirect.com has it at 61.42 plus s&h (list is 122.83). Also note that the GM part and AC Delco part are the same (and will probably have both part #s on it)

Have you tried any local parts stores that carry ac delco (you can find local stores on their website).

//2000CAYukon

After I thought about it I think the AC/Delco website uses different part numbers than the dealer for the same part. Try getting the part number from the dealer and then try gmpartsdirect.com. They don't always have the parts, But it is worth a try. When I was going to replace mine they had them and they were real close to a local guy I deal with and he was $45.00, But I deal with him a lot so I get things pretty cheap. When I did my fuel pump he was $100.00 cheaper than anyone else on a GM pump.

chevytrucks92
01-19-2005, 09:32 PM
Chevy SS, you may also want to raise your air pressure a little. This would help your gas mileage and improve your tire wear.

I dont know what the factory specs for air pressure are for you tires, but I would try around 44 or so psi. Now its not going to raise your gas mileage from 13 back to 18, but it would help a little.

J-Ri
01-19-2005, 09:35 PM
I dont know how to figure exactly my highway mpg, but i have a 26 gallon tank on my '97 and i usually average around 8-9mpg. Figured by my 26 gallon tank gettin me around 200-250 miles. so i would be quite envious of someone who got near 15mpg. But i wanted the power of a V8 and i got it, so im not gonna bitch about the littlest thing like gas.

To figure mpg, take the miles since last fill-up and divide by the gallons the gas pump says you put in. I don't see how you could possibly be in the 8-9 range. I have a quadrajet carb (that is terribly out of tune, black smoke comes out the exhaust under heavy acceleration) on my 350 (3 spd trans w/o overdrive) and 4x4. I also have oversize tires with fairly agressive tread. I get 10 mpg letting it warm up till the temp guage starts to move (have to for it to even run in the cold), and keeping it running for 10-30 minutes for the heater after I get to school/work early.

Chevy-SS
01-19-2005, 09:37 PM
Many thanks to everyone, especially Yukon and Thunder. I will probably just order two sensors tomorrow from gmpartsdirect.com, but I'll make a couple of calls around tomorrow just to check local prices. I gotta feeling the local prices will be higher, but ya never know.

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