Hello everyone. I have what sounds like a knock coming from the engine of my 04 Discovery. It only happens after driving it around town for 30+ minutes. Noise is only at idle, and if I leave it idling for a while, the noise randomly stops and starts again. Coolant temperature doesn’t go over 198 on scan gauge and the truck runs completely normal. I changed the oil just to inspect the oil for any evidence of metal/bearing failure and the oil came out clean. I also installed an oil pressure gauge to verify that it isn’t low oil pressure and the readings are within spec. Below are the cold and warm pressures.



Cold idle - 51psi
Cold 2000 RPM - 52 psi

Hot 2000 RPM - 49/50psi
Hot idle - 18/19 psi (194 coolant temp, 102 ambient temp)



Could this be a slipped liner? The previous owner told me he had the head gaskets replaced. I unfortunately don’t know how much it was overheated. Just odd that it didn’t make this noise when I test drove it for 45+ minutes before buying it. Any input would be greatly appreciated.


https://youtube.com/shorts/2Mlf_YdHIRc?si=YZimJBmaK6OZe5n-

 

If you find the definitive answer to this noise you will be the hero of the community. It's been asked thousands of times and nobody has been able to definitively say for sure. Even after rebuilding my engine to include the pricey top hat liner mod I am getting the same type of noise. So unless the shop did my liners wrong it's still a mystery to me and many others.

 

To get a clue, time the knocking (count how many knocks in 5 seconds for example, and multiply by 12), then compare to engine RPM. If knocks are equal to RPM, then coming from rotating mass. If knocks are about 1/2 of RPM, then valve train related (since cam rotates at 1/2 engine RPM). Since it is hard to count fast for 5 seconds, I count groups of 4 on my fingers (i.e. 1, 2, 3, 4 is one). This will give a clue which part of the engine to focus your attention (or not worry about).

 

Because you checked the oil pressure and it checked good I'm going to say it likely as a slipped ladder, however all is not lost, many times the slip liner problem can be corrected by getting the engine to run cooler. Suggest you switch to the inline thermostat with a 180° thermostat and it likely will never get as hot as 198 again. If you want it definitive answer be sure to notice at what exact temperature it starts knocking

 

Thank you. I went ahead and ordered an inline kit a few days ago. Just waiting on it to arrive. I’m considering pinning the liners. Doing some research on the process.

 

I currently have a 4.0 torn down because of the same issue. Idle she purrs like a kitten cold. Give it maybe 5 minutes to warm up youd hear knocking. Knocking went away, or I should say was drowned out, as rpms went up. The significant other hated going to drive thrus because of this.

I pulled the engine down to find a slipped liner. Just a few mm proud of the deck, wasn't like that when I did head gaskets a year ago.

I tapped it back down and I am now pinning the sleeves and a bunch of other bottom end pre maintenance a 4.0 with over 200k needs.


Hopefully your inline mod works. Ours was tapping before it hit 180 degrees.

 

It sounds like mine, although I would call it a tap more than a knock. I have oil pressure that is identical to yours. I tell myself it's typical push rod lifter noise because that sounds better than the slipped liner. So far, it's worked going on 5 years.

 

I don't see it becoming an issue unless you put a ton of miles on them, more of an annoyance. My better half loves her rover and didn't complain to me about repairing it until a girl at the Arby's drive thru told her to get her car looked at. She puts 20-30 miles on it daily.


And it is more of a tap than a knock but definitely a deeper tone than that of a ticking lifter

 

I think I put over 20k miles on mine with 2 slipped liners. they will run a long time like that as long as you don't have coolant loss.

 

Just wanted to thank everyone and provide a quick update. I installed the inline thermostat with a 170° thermostat and the sound is gone for the time being. I know most people recommend the 180° but with me living in Florida and for troubleshooting purposes, I figured I’d try the 170°. Temps have stayed consistently under 190° and the truck runs great. I knew going with the 170° that the truck would throw a code, but I don’t mind.