Knock only when fully warmed up

September 24, 2018

Hey guys, hope you all are having trouble-free driving!

There has been an issue for the past 3 months and finally I stopped being lazy and documented it somewhat so hopefully someone can shed a little light on what's causing it.

The issue is a metallic knock that shows up ONLY when the engine has fully warmed up. I'm not talking the thermostat opening and temperature gauge reaching normal operating temperature (at this point there's usually no knock, good old diesel purring only). I'm talking about a longer-than-supermarket-and-back drive where the engine fully warms up.

The knock follows the RPMs up until about 1100 RPM, at which point the sound disappears or is drowned out.

The knock has been with me for around 3 months now with no changes. It appeared after around 100km trip on a pretty hot day (30°C sun blasting). The engine didn't lose oil at that trip and didn't overheat. It lost a little bit of coolant (although it might have lost it earlier) but it might be due to a recent radiator replacement at the time (air pocket?)

Here's the video I cruelly put together.

At the start of the video is a completely cold engine, just fired up, with no strange sounds. Then the same for the cabin shot, no sounds. Then at 0:39 is a cabin shot after driving (you can always hear the knock in the cabin, maybe video didn't quite get that). And at the end there's the knock I'm talking about. It's particularly pronounced in the driver's wheel arch, and also can be heard in the exhaust.

Now here's the kicker! A couple of weeks ago the lower end of my driver's side spring broke off, and both front springs were quickly replaced with Kayaba ones. And the kicker is that after the spring the replacement, even after engine was completely warmed up, there was no knock! No knock whatsoever! I was quite happy until about 4 days later the knock came back, same as it was and hasn't gone away since.

So, what could it be? I'm dreading the rod bearing being shot but wouldn't it be heard always? And would it really hold up for 3 months already? And could it really go away for 4 days?

Also at the time of when the sound first appeared, engine oil was in the engine for 15k km already, which was a ubik-up on my part. I then quickly changed it and the knock stayed the same, no changes. Oil is 5W-30 from eni.

So again the rod bearing or something else in the engine is on my mind, but could going 5k over the oil change interval could really screw it all up?

Since it only appears when engine is warm and follows RPMs, what else could it be and how could it tie up to work being done on the springs?

I hope it's nothing serious, this has been driving me mad. Car is at 290k km.

September 25, 2018

So nobody has any ideas what it could be? 😞

September 25, 2018

Does this issue affect your performance? First thing that came to mind was piston slap but idk if it could be. Anyone who recognizes this sound?

September 26, 2018

Hey, no, no performance issues as far as I can see, car rides the same as when I got it. In fact the performance is better on a warm engine but I think that's normal, pushing a cold engine is not recommended anyway.

October 2, 2018

Now that it's getting colder each day down to 6°C in the mornings) naturally it takes a very long time of continuous driving to get to the knocking stage. So it's definitely heat related, something gets up to just the right temperature (oil thins out? some metal expands just right?)

I guess in winter weather I won't be able to get to that stage unless I'm driving hundreds of km in a single trip, with -20°C weather and all.

January 8, 2019

The latest status update of this saga is that the knocking might be the so-called injector nailing.

Mechanic said that it's the only thing that logically makes sense, especially when taking into account that I've been driving with this knock for over 6 months so if it was something in the engine mechanics itself then it's doubtful it would last this long. *knocks on wood*

This knocking also came with poor starting, even in the summer months, but for some reason I never added 2 and 2 together to see that it's related. Even in +30°C weather the first starts after sitting a while were "labored", so to speak. It hasn't been as snappy as it was a year ago since the knocking came about.

Another symptom is that when mashing the gas pedal on acceleration, at certain RPMs the car sort of struggles. There's a noticeable dip in acceleration for maybe half a second, then it picks right up.

Other than that there's no symptoms really, not the usual injector fault symptoms as smoke, poor mpg, or reduced performance.

Will try an injector cleaner in the next week to see if the knock disappears but it looks like I'll have to take out all injectors, send them to the testing lab and we'll see from there. Definitely not happy news since there's one other big repair coming - the right-hand side driveshaft (the really long one going into transmission) is starting to clunk in the middle, and shop says it might be an expensive repair. Also the clutch is slipping when mashing the pedal in 4th or 5th gear, we'll see when that's going to be a problem in daily driving.

Sadly the car looked really good when bought but there's really no way to tell how the previous owner took care of it - or didn't. I guess it's a trend these days to buy a used car, trash it for a year or two, and when your mechanic says some big repairs are coming, dump it to someone else and buy another one to trash.