Hi all,

2003 Discovery with 149k miles. Recently started hearing a knocking sound under acceleration and when engine is under load. Ive been meticulous with 3000 mile oil changes with 15w40 Rotella. Engine is all original. Am I looking at rebuilding the lower end soon with the kind of mileage I have?

I should add that the noise disappears over 1500 rpm. And it seems to get quieter as the engine gets warmer. There is no apparent shavings on the dipstick.
Also, the truck feels fine, no loss of power or rough idle. And recently got a P1175 and P1172 code, not sure if this could have anything to do with the knocking?

 

If it gets quieter with RPM then it isn't a rod knocking - at least not due to a bad bearing, though possibly due to oil pressure. Does your oil pressure light ever come on?

Quieter as the engine gets warmer is interesting - usually the opposite is true because the oil thins as the engine warms up.

A cursory glance shows those as fuel related codes - do you recall the description for the code when they were scanned? A lean fuel condition can cause predetonation especially under load which can be heard as a knocking sound and is very bad for the engine as well. I would drive the truck as minimally and as easily as possible until you get fuel related issues sorted out. You can pull spark plugs to read the color and see if it is running lean.

 

Oil light does not come on, I believe the P1172 and P1175 codes are rich codes on each bank.

 

I would hook up an oil pressure gauge, either a temporary one or hook up a glowshift gauge with a pancake adaptor, about $100 in parts, takes about an hour. Monitor the pressure to see if the noise correlates with temp or pressure. A slipped liner gets quieter once the RPM syncs with the liner sliding around. If you cool down the drivers side of the engine with a spray of water, does the knocking go away? If the engine gets hot and you a lose liner it could move around. It generally is less noticable at cooler temps. If you are not having any luck figuring it out, there is a shop on rt 9 in Wall that does excellent engine work. He is on here sometimes, Rover Master Tech or something similar. He did some work for me a few years ago, has been perfect since.

 

Do it only under acceleration, or even while coasting foot off the gas? Quieter on the gas or off? I am thinking exhaust leak, almost no cost to do the shop vac exhaust leak test.

 

I am with Extinct on this one. High load = high exhaust gas pressure = high noise. Steady state or coasting that pressure goes down. At idle there isn't usually other noise competing so even though the pressure is low, it is noticeable.

I also like this one cause it's easy to diagnose and fix, which means if it were one of my cars that would not be it.

 

I think I have narrowed it down to an exhaust leak on the passenger side manifold. I think as the engine warms up it shrinks the gap and reducing the knocking sound. I will be ordering new gaskets soon.