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No Power From '98 Gmc 2500; Terrible Mileage

mlewis73

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mlewis73 Enthusiast

mlewis73

Posted
Posted

With our '98 GMC 2500 with 350 (5.7L) engine and automatic transmission, I note a significant lack of power when pulling a 2000 pound horse trailer (empty) up the slightest hill. The speed drops off quickly if the truck is in overdrive (about 1800 RPM); not quite as quickly but still drops if out of OD (about 2500 RPM at 55-60 mph.)

 

I have talked to folks who own similar trucks and it sounds like they have no trouble pulling heavier trailers. It sounds as though our 350 is weak.

 

Mileage is only 114,900. The engine uses no oil and shouldn't be worn out.

 

Notably, fuel mileage is terrible.

 

What might be the trouble? Would spark plugs/wires cause such low fuel mileage and power? That is the first thing I intend to check.

 

 

Thanks,

 

Mike

StoneyPgh Enthusiast

StoneyPgh

Posted
Posted

Losing speed in OD is normal. The owner's manual probably advises against it.

 

As for the reason it lacks power probably has more to do with your differential gear ratio than anything else. If it has 3.42s instead of 3.73s or 4.10s, there's not much you can do to improve things.

mlewis73 Enthusiast

mlewis73

Posted
  • Author
Posted
Define "terrible" mileage.

 

 

Trip to the dump and back, with trailer in tow, was about 9 miles round trip. Went through 1/8 tank of gas in that distance.

 

Just seems excessive. I'd estimate 10-12 mpg, tops.

 

Did GMC actually spec this truck with such a ratio as the 3.42? Seems like a 2500 would have come with a gear ratio actually useful for doing work (actually, 3.42 shouldn't be too bad...)

 

How can I tell which ratio I have? The door sticker has long since worn away.

 

Mike

Zembonez Senior Enthusiast

Zembonez

Posted
Posted
Define "terrible" mileage.

Trip to the dump and back, with trailer in tow, was about 9 miles round trip. Went through 1/8 tank of gas in that distance.

Mike

 

An eighth of a tank is no way to really determine fuel economy... those gages move a lot. 10-12 MPG towing is excellent mileage for that truck. I'd be shocked if you ever see anything north of 14 in its lifetime. I'd bet yours is a 3.73 or 4.10. That seems to be what most in here are.

2000DV4.8 Senior Enthusiast

2000DV4.8

Posted
Posted
Define "terrible" mileage.

 

 

Trip to the dump and back, with trailer in tow, was about 9 miles round trip. Went through 1/8 tank of gas in that distance.

 

Just seems excessive. I'd estimate 10-12 mpg, tops.

 

Did GMC actually spec this truck with such a ratio as the 3.42? Seems like a 2500 would have come with a gear ratio actually useful for doing work (actually, 3.42 shouldn't be too bad...)

 

How can I tell which ratio I have? The door sticker has long since worn away.

 

Mike

 

 

Check for the RPO sheet in the glovebox. Here are the different codes:

 

GU4 - 3.08 rear end ratio

GU5 - 3.23 rear end ratio

GU6 - 3.42 rear end ratio

GT4 - 3.73 rear end ratio

GT5 - 4.10 rear end ratio

mlewis73 Enthusiast

mlewis73

Posted
  • Author
Posted
Define "terrible" mileage.

Trip to the dump and back, with trailer in tow, was about 9 miles round trip. Went through 1/8 tank of gas in that distance.

Mike

 

An eighth of a tank is no way to really determine fuel economy... those gages move a lot. 10-12 MPG towing is excellent mileage for that truck. I'd be shocked if you ever see anything north of 14 in its lifetime. I'd bet yours is a 3.73 or 4.10. That seems to be what most in here are.

 

 

That mileage estimate is unloaded. Loaded, I'd bet we're around 7mpg. Never pull far enough with the truck to get an estimate. Next time we go somewhere with the horses, I'll determine our mileage towing over a reasonable distance.

 

DV2000NJ, thank you for the information regarding axle codes. The truck is a 3.42. I'd have spec'd a 3.73 if I'd bought the truck new, but think we can deal OK with the 3.42.

 

Surprisingly, the sticker says payload is only about 1400 pounds. I had a '98 Ford Ranger rated at 1750 with the payload package. 1400 seems low for a 3/4 ton truck... this is evidently a very BASE MODEL 3/4 ton that carries the badging only. I understand the REAL 3/4 ton trucks have 8 lugnuts per wheel. :-)

 

 

Mike

Zembonez Senior Enthusiast

Zembonez

Posted
Posted

I'd bet you would get 14 or so in normal daily driving.

 

I thought ALL 2500s were 8 lug? You sure somebody didn't just add some 2500 emblems? (That would be the Redneck 2500)

 

Maybe you should get somebody to run the VIN.

thecraft Senior Enthusiast

thecraft

Posted
Posted
You sure somebody didn't just add some 2500 emblems? (That would be the Redneck 2500)

 

Maybe you should get somebody to run the VIN.

No kidding, I never heard of 3.42s in a 3/4-ton.

2000DV4.8 Senior Enthusiast

2000DV4.8

Posted
Posted
You sure somebody didn't just add some 2500 emblems? (That would be the Redneck 2500)

 

Maybe you should get somebody to run the VIN.

No kidding, I never heard of 3.42s in a 3/4-ton.

 

 

 

 

I'm pretty sure they made a 2500 LD truck back then, or basically a heavy half ton.

s-ten Enthusiast

s-ten

Posted
Posted
You sure somebody didn't just add some 2500 emblems? (That would be the Redneck 2500)

 

Maybe you should get somebody to run the VIN.

No kidding, I never heard of 3.42s in a 3/4-ton.

 

 

 

 

I'm pretty sure they made a 2500 LD truck back then, or basically a heavy half ton.

 

 

I have seen light duty 2500 from the early 90's and have heard of them from later 90's as well. I am sure that it is entirely possible that he has on.

sonjaab Senior Enthusiast

sonjaab

Posted
Posted

Plugged cat converter.........................

 

No such thing as a LD 2500 in those years

 

That truck could have 3:42s only if it is a 305 2wd 2500 according to my 98 GM brochure.

 

If so no wonder it has no power !

mlewis73 Enthusiast

mlewis73

Posted
  • Author
Posted
Plugged cat converter.........................

 

No such thing as a LD 2500 in those years

 

That truck could have 3:42s only if it is a 305 2wd 2500 according to my 98 GM brochure.

 

If so no wonder it has no power !

 

 

Pulled a Carfax before buying; the truck is a 3/4 ton, maybe "heavy half". I'll relook the axle code, but am fairly certain the code was for a 3.42. Hmmm... This truck was built before June of '98, which I understand was a changeover point, so perhaps they had a heavy half during the '97 model year and this is a combination of leftover parts for that model year. Just a guess...

 

PCV valve and cat sound like possibilities. Can the cat be gutted?

 

 

Mike

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