'98 GMC 2500--low power and terrible mileage

[Mikes91]

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

 

[Captain Morgan]

A good tune up should help. Is there any check engine or service engine soon light on? O2 sensor failure causes similar symptoms, it will set a code and there are 4 of em on your truck. Curious to know what you come up with.

 

[RustyBolts]

CMP Sync

When you take it to a shop, have them check if the CaMshaft Position sensor is in sync with the camshaft, and it's usually tester on the computer scanner as cam retard. The CMP is in the Distributor, and if it isn't within 2 degrees of 0, it'll turn it into a DAWG. My Grandpa ran into this on his '98 K3500 Crew Dually 454, once. Said at times a Briggs felt like it had more power. He took it to the shop, had them Sync the sensor, and he said after that it drove like a totally different truck. Better power and more torque in the rpm ranges you needed it to be in. before the fix, he got about 7MPG pulling an empty tandem axle flatbed gooseneck (he transports them from the manufacturer to the dealers around the country). Nowdays he gets 12.2 with the same empty tandem axle flatbeds. Bobtail he has seen up to 15 MPG if he stays delow 65 on the highway.

 

[RustyBolts]

CMP Sync

When you take it to a shop, have them check if the CaMshaft Position sensor is in sync with the camshaft, and it's usually tested on the computer scanner as cam retard. The CMP is in the Distributor, and if it isn't within 2 degrees of 0, it'll turn it into a DAWG. My Grandpa ran into this on his '98 K3500 Crew Dually 454, once. Said at times a Briggs felt like it had more power. He took it to the shop, had them Sync the sensor, and he said after that it drove like a totally different truck. Better power and more torque in the rpm ranges you needed it to be in. Before the fix, he got about 7MPG pulling an empty tandem axle flatbed gooseneck (he transports them from the manufacturer to the dealers around the country). Nowdays he gets 12.2 with the same empty tandem axle flatbeds. Bobtail he has seen up to 15 MPG if he stays delow 65 on the highway.

I have a '96 C3500 Extended Dually with a 350 that needs the CMP reset on it right now. It does good to get 14 MPG on the highway and has no low end pulling power whatsoever. Knocking around town I do good to see 10 MPG. For a 350 in a 6200LB truck, it should be getting alot better mileage than Grandpa's 7700LB 454, Don't you think?

 

[rthomas]

have you checked the fuel pump? my 98 tahoe was super gutless until i replaced the pump. If it is a bad pump buy a factory unit, after market pumps dont last.

 

[RustyBolts]

Another thing you might try is SeaFoam. Every year a run a can through the gas tank, and a can through the intake in 2 shots. If your truck has Vacuum brakes, then just disconnect the brake booster line. If it's Hydroboost(like mine) pull the PCV valve out of the driver side valve cover and let it suck the SeaFoam out of a shallow cup. I put 1/2 can through it, then drive it for a day, then send the other 1/2 can through it. I never put any in the oil because i've heard horror stories about putting SeaForm in the crankcase. Whenever I change my oil i put in a Quart of Marvel Mystery Oil, to keep the seals and whatnot conditioned. Instead of SeaFoam, every 10,000 miles, I put a quart of Kerosene in the oil the morning before I change the oil. I know this all sounds Old School coming from an 18 year old, but my dad and grandpa have sworn by these things (SeaFoam, Marvel Mystery Oil, Kerosene) for decades. The way i see it, If their system ain't broke, don't fix it.