A little more advise

[97ttpuller]

In the closed Junk Truck thread I lot lots of very helpful advise. I'm starting think I'm going to have to replace my exhaust system. By what most have told me a partially plugged exhaust would be the most logical reason for my 5.7 being so sluggish and thirsty. I have two more questions:

1) Other than a vacuum test @ 2000 RPM and looking at my O2 sensors in Live Data mode with my scan tool, are there any other test I can do to be 100% sure my exhaust system is partially plugged?

2) I did confirm with a tooth count my gear ratio is 3.42, will that alone cause my truck to be so sluggish and have such poor gas mileage towing my trailer?

A real quick re-cap of last thread, I'm pulling a brand new 18' Stracraft Travel Star that weigh's 3900lbs with all our gear. I'm pulling it with a 1997 K1500 5.7L with 136,*** miles on the clock. The truck is completely gutless pulling the trailer. I'm only getting about 6 mpg with the trailer pulling it mostly down hill. The truck will not even hold 65mph on even the slightest grade without constantly downshifting to 2nd gear. Twice I was on what I guess to be 4-5% grade and even the fully loader semis where passing me, all I could get was about 40mph floored. I thought I had a locked up fan clutch so I put a new one on yesterday and it sounds just like my old one. So the fan clutch was not a factor. Thanks again in advance for the help.

One last thing I was thinking and told by the RV dealer my truck should easily pull that light a trailer at 65mph and I figured I should get 10-11mpg towing it. Without the trailer the truck seems fine and I get 16-18 mpg @ 65mph and around 15-16 at 80+.

 

[BoxerPitMix]

Hmm maybe a clogged cat? I didn't even think about that. Does the exhaust stink like rotten eggs or anything?

 

[Keepinitoldskool]

You could perform what they call a back pressure test. You need the correct gauge and fitting to put it in place of the precat O2 sensor.

 

[TylerZ281500]

3.42s and towing truly arent gonna be great at all thats for sure, a plugged cat or so could also make things very sluggish as you presumed. that is where my knowledge ends i havent played with 02s and cats enough to diagnose

 

[michael hurd]

3.42 and 265/75/16 should be an ok combo, but for towing it would be nice to have either a smaller set of rubber or a change to lower gearing. Going to lower gearing isn't going to pick up the mileage much, but will keep the engine in a better range of RPM for towing.

I got the scale ticket from the scrapyard this morning 5980 was the payload on a 2300 pound trailer (8" channel frame ), and my 305 pulled it OK, but again, 3.73 ratio and smaller rubber.

You could possibly remove one of the post cat o2 sensors, and fashion up an adapter to test the back pressure of the system after it gets hot and you start to step on it. You will need some steel line and a fitting to screw in, then you can use a long length of rubber hose on it to a pressure gauge, preferably one that's got a large sweep, like 270 degrees and low pressures, like 1-10 psi. Mount the gauge temporarily on the cowl where you can see it and go for a drive.

There are many factors that could be contributing, low quality gasoline or a distributor that's not quite set correctly may also be contributing to the lack of power, however with the amount of fuel you burned, it should have been out of the system by now if it was bad gas.

You still haven't said at what altitude you are trying to tow this combo at: I am about ~650 feet above sea level. As you get higher up, you lose power.

If you are changing out the entire system from the manifolds back, I would suggest getting the 8600gvw system for the 5.7, it has larger pipes and larger catalysts, assuming you are keeping manifolds and want a 'bolt on' stock system that is readily available.

 

[michael hurd]

See post # in link for size comparison.

http://www.gmt400.com/forum/showthread.php?27169-454-exhaust-on-my-350&highlight=454+exhaust

 

[97ttpuller]

Thanks Mike your the one that's really got me thinking exhaust. You said in a reply to my other thread your friends truck was a dog and it turned out being the cats overheating. I'm probably going to go with some shorty headers and, Flowmaster direct replacement cats. Does anyone know of a complete system from the heads back? I searched the exhaust forum but no stickies. I was hoping for a few part # for a complete set up. The altitudes I was towing started at 6500' and got home to 460'. Like I said its a 225 mile trip and this struggle was coming down. I don't think this truck would make it back up lol.

 

[97ttpuller]

See post # in link for size comparison.

http://www.gmt400.com/forum/showthread.php?27169-454-exhaust-on-my-350&highlight=454+exhaust

So I think I'm going to look up a HD 2500 exhaust system for my rig. I really want that bigger exhaust. In the engine and performance section there is an AWESOME power upgrade sticky. It strongly suggests upgrading to a 2.5" system from the headers back. All the other normal stuff looks great on this truck, new air filter, plugs look new, TB looks very clean, fuel filter and pump are only a few months old (prev owner had receipts for that), tires are the size listed on door sticker, no codes, so it must be an exhaust problem that rears its head under heavy load.

 

[poncho62]

I would think that if your cats were clogged, you wouldnt get it going to 80 even by itself.
You have the 3.42 gears, but what size tires are on it...If they are bigger than the 245-16s, you are reducing your gearing even more.

 

[97ttpuller]

I would think that if your cats were clogged, you wouldnt get it going to 80 even by itself.
You have the 3.42 gears, but what size tires are on it...If they are bigger than the 245-16s, you are reducing your gearing even more.

It has the factory size tires 265/75/16 that is what is on the door sticker. After pricing an exhaust system for this junker I've decided to just get rid of it. I'm not going to dump close to a grand for freaking exhaust and still have a POS that needs a gear change or tires. Looks like I got hosed on this pile of junk. Thanks for the help everyone.