When warm, my truck exhibits a rattle-y knock under low-rpm, high-load conditions.

It only happens between 1100-1400 rpm or so when I give it a moderate amount of gas.

I've taken to using the O/D off button on the shifter column to keep it out of high gear until the road is level and the truck is cruising.

4.6l, '99, 2WD, 240K. Green.

I normally put 87 in it, but just for kicks I filled it up with 93 yesterday and the knock is gone, which would seem to rule out things like a loose spark plug or cracked exhaust manifold.

I don't want to keep putting 93 in it, 1) because it's more expensive and I'm driving a truck and 2) because that doesn't actually address the problem.

Any theories? I'm thinking dirty injectors at this point, but any other input would be great. Thanks.

 

Bad knock sensor. You must pull the intake manifold to change.

 

Wouldn't that throw a code though? I have no CEL; not even a flicker.

 

It probably would but not always. Maybe you bought a bad tank of gas. Since you put in 93 octane I would also add a big bottle of techron and run that tank till almost empty. Refill with 87 and report back the results.

 

No it's been doing it for a month or two now. Long enough to rule out a bad tank of gas I think.

I'll try some fuel injector cleaner, but in my experience nothing really solves injector problems other than pulling them and having them cleaned and balanced or replacing them outright.

The knock sensor would be cheaper... RockAuto sells reman'd injectors at ~$30 apiece and RC charges $24 each to clean---and the truck would be down in the meantime, so that's a no-go. V8s are expensive...

Thanks.

 

I would think if it was bad injectors the 93 octane would have done nothing and the truck would still run bad. But I am not a professional, just trying to help.

 

Bad Knock sensor, Poor flowing injectors, partially clogged fuel filter, carbon deposits. There are many things that can cause pre-detenation. My vote is a bad knock sensor an the recent change to winter blended fuels for most of the country. Try replacing the fuel filter because its cheap, and try 89 to see if the issue persists.

 

If it were poor flowing injectors, clogged fuel filter or carbon deposits I dont think the 93 octane would have helped. Right? Again Im no expert. Just using logic and past experciences. Carcrazy, I think you have more experience than I do. Im not questioning your advice, just asking.

 

Thanks guys. I should mention that the truck runs perfectly fine otherwise. Starts good, has good power and no hesitation except when it's knocking, and that's only in 3rd or 4th gear, under load, between 1100-1400 rpm.

The fuel pump was replaced about 18 months ago, according to the records I got when I purchased the truck last December. Dunno if the fuel filter was replaced at the same time but I assume it would have been.

Maybe I'll try fuel injector cleaner and new fuel filter and see where that gets me.

 

Like carcrazy said that is your least expensive option and a good place to start. I like techron additive but there are other options. Let us know.