Whenever I start the car xj x351 5.0 n/a 2012 a bad knocking sound comes from the bottom of engine but it goes away after a second and then it is absolutely normal only happens at start up any idea will be appreciated

 

If you're lucky, you might simply be the victim of restrictions in the oil passages or a weak oil pump that starves the engine for a few seconds upon cold start-up. But you certainly haven't been lucky thus far with this car.

What you are describing is often a malady called "piston slap". With modern aluminum pistons and their short "skirts", any excessive clearance between the piston and cylinder wall will cause the piston to wobble in the bore. Early manifestations include the noise you're describing until oil pressure builds up and/or the piston expands from heat to close the gap. Piston slap sounds like, but is differentiated from "rod knock" (excessive clearance in connecting rod bearings) by engine temperature. Piston slap noise disappears after the engine is warm, rod knock gets worse as the engine heats up.
A mechanic can diagnose piston slap by removing all spark plugs except one, then rotating the cold engine by hand until feeling resistance on the compression stroke. Then he rotates the crankshaft slightly back-and-forth. If there is excessive clearance, he will hear it. He repeats the procedure on the other cylinders. Before you start replacing parts, have an expert diagnosis.

Piston slap is frightening but often harmless, although it definitely indicates that at least one of your cylinders is worn. It won't get better. The only cure is to rebuild the engine, but you've been down this familiar road earlier. When I re-read your earlier posts, I think we can safely assume that the engine in this car has experienced significant trauma in its lifetime. You've already replaced the cylinder heads and many other parts, but believe me, there are still many more bits in there to go wrong. I know you are way underwater with this car, but if you can unload this sick kitty, do it.

 

Thank you for the reply. I know this kitty is just causing me to loosee my hairs 🤣 however just stuck with it for now.

Ok timeline of the issue

Recently change the intercooler on the engine because of oil getting mixed with the water however there is not water on the engine side.

Same thing happened and now it is making this sound even if the car was running at norm temp if I start it again makes the same sound everytime on the start up.

So now I am going to change the engine cooler again however recently change the tranny of the car aswell.

Can be the possibility of low oil pressure or something to do with coolent getting mixed with oil.

And just clarify the car is stationed as soon as I found out about oil and water

And here UAE I can't even part it out and invested just alot of time and $$

Last option is to just get a new to the car. Complete power train (engine and trans)

I have an issue in identifing the right one for it
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Once again I made assumptions which subsequent posts invalidate my suggestions.

I assumed in your original post that the abnormal sound occured only at start-up with a cold engine. Now it seems that it's happening every time you start the car. I'm sorry, but I can't offer any other suggestions

 

I am bit confused?
You mention having a 5.0L NA but then talk about inter-cooler leaking oil? The 5.0L NA engine is NOT super charged and does not have an inter cooler?

Next the inter-cooler is an air to water cooler? If the IC is leaking you should see it leaking on the ground? If not then it's being sucked into and burned inside the engine?

Can you pressure test the cooling system? Maybe you do have a leak somewhere? Are you losing coolant?

If you can give us more details?
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He may be talking about the water cooled engine oil cooler in the "V" of the engine.

 

Yes six is right apologize for the word intercooler this is what you will hear in this part of the world what I am trying to refer to is oil cooler on the v of the engine

 

Ok sorry I was thinking of the wrong thing!

Yes the oil cooler brick can leak? Now has the water pump been replaced? Because there is a plastic transfer tube between them that is a known problem. The installation is a bit difficult and it's plastic. There is a metal replacement tube out too if you want to replace it. But that only causes an external coolant leak? No mixing of oil and coolant.

You have replaced the oil cooler brick already? Not a common leak? How did they determine that was the problem?
By chance do you still have the old one?




This picture is from a NA XK and should be the same as your XJ. Still need to remove the intake to get to it so a good bit of work.
You have oil in the coolant but NO water in the oil? Correct?

Sounds like you will be replacing the oil cooler again? Maybe pressure test it? As I said we just don't see these leaking so not a common or normal repair.
There are aftermarket versions out there if your interested? Not very expensive at around $80.




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Here this part is for like 300 usd I have change one a week ago and before that around 2 months ago.

The issue is can it cause a sound at the start of the car.

 

Remove the accessory drive belt and see if the noise changes. If it's still present, have the timing chains and tensioners inspected for wear.

Here is the front of an AJ126 with the timing chains and tensioners removed for replacement. Note the separate chain for the high pressure fuel pumps. The AJ133 is similar.

 

I don't see how the oil cooler can cause noise? It's a passive device and just has hot oil flowing thru it?
I would follow NBCat's suggestion. Those problems are more likely and have been reported often.

Report back if the noise changes without the drive belt?
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