I have a 99 DI with 142,000 miles and it really struggles on the highway. Every time there is any slight gradient the speed and rpms slowly drop with foot still depressed and won't maintain speed unless its at 3500rpm. I understand they are heavy beasts and I have a lift with 31's, but on the highway I have to pretty much stay in 3rd gear the whole time which is causing the engine to get way too hot for my comfort.

I have had overheating problems before and have replaced the head gaskets, new fan clutch, thermostat, OEM radiator, hoses.

Just driving an hour an a half yesterday on the highway the ultragauge showed 212 F the whole time going up to 220 uphill. Idling and driving around town its fine and stays at 188-192.

Is this normal to struggle on the highway this much? It feels like I am constantly pulling a trailer.

 

what speed are you trying to maintain? I have no problem with 31s maintaining 60-70, but if you want to go faster on long uphills you need to drop out of overdrive. these trucks really don't like going fast. whens the last time you did a tune up? are you running premium fuel?

 

I had new spark plugs installed when I had the head gaskets replaced, the heads were new as well (machined and cleaned). I have always run 89 and I know I won't be going fast but I never had this much of an issue. It may just be me but never getting below 212F doesn't seem right. A month ago the highest I got was 206F thrashing it up Afton mountain ( long steep stretch) but then it went back down to 192-194F

Thanks for the response btw

 

Maybe bad head gaskets damaged the cats and they are partially plugged?

 

Maybe, when I had my muffler replaced the shop said he checked them and they where fine but who knows....

Do you think an ultragauge would be accurate enough for horsepower? As of yet I have not gotten above 100 hp regardless of rpm. Also is 3750rpm normal for the highest possible rpm?

Also no codes except I had trouble before with the VSS and the governor coming on which threw codes but I replaced the sensor with a new one from British parts of Utah and haven't had any trouble since.
Thanks

 

No. You should easily get 5000 or even 6000 rpm. Does it shift gears at that point, or will it just not get past it?

I'd think your throttle cable or transmission cable or both are too loose.

 

Yeah I have never been able to get above 4000. Basically on the high way with my foot on the floor on a slight gradient it will keep dropping in speed till it gets to 50 mph, downshift to 3rd, get up to 65mph at 3750rpm go back to 4th and start slowing down again.

 

Foot on the floor, it should immediately downshift, even at 65 or 70 mpg. One of those cables is too loose, almost certainly.

 

Ok well thanks for at least validating that this isn't just "how it is" and I just have to deal with it.
I have had the kickdown cable adjusted before but I guess its time again.

And just to be sure 220F is too hot right?

 

under normal conditions, just running down the road, yes, I'd say that's too hot. But it's ridiculously hot, especially if driving up mountains or towing. Since you're having to floor it to go anywhere, I'd take care of that, and see if it helps. Also, higher octane fuel can help lower temps.

Higher temps while driving down the road under a load, makes me think radiator issues or possibly leaking head gasket.

I've also seen weak or stuck open thermostats cause weird problems, that don't always seem to behave like you'd think. I've had a car with a stuck open thermostat that would behave like you'd expect while driving, never heats up fully, no heater, etc, but then when you'd sit too long at the drive-through, it would start to climb way too hot.