450 Knocking noise, camshaft slap?

[DamnWiseguy]

Getting a loudish irregular knocking noise from what seems to be around the top of the engine at idle on my '98 48k Chim 450. Only seems to appear when the engine has warmed up after a few minutes and I can't seem to find anything in the engine bay that is rattling obviously, although I will put it up on stands and have a closer look this weekend.

Obviously the Cam is in the V of the 8 so could this be camshaft slap? No noticeable problems apart from a misfire under 2000rpm which gets worse as the engine gets hot. Car drives fine however and got 23mpg out of the last tank so seems to be in good health.

Any advice would be appreciated thanks!

[2 sMoKiN bArReLs]

Is it knocking rather than spitting/tapping? If the latter a common fault is exhaust manifold gasket.

[DamnWiseguy]

It's hard to explain the noise apart from to say that it sounds like something metallic rattling. I'll have a look at the manifold and anything else that it could be over the weekend but I have already 'fixed' a couple of things that I thought might be the cause to no avail. I guess the thing to do is leave the engine running and have a poke around.

[gf350]

I heard of this sort of thing being related to one of the trumpets in the plenum comming loose.
Hopefully thats all it is.
GF

[sparky11]

Bonnet up or down?

Oddly, I had this just recently and it turned out to be the bonnet stay popping out of its holder and rattling against the underside of the bonnet!

Hope its that easy!


Mark

[hobbit 4.2]

Check you alternator belt for little (1mm ish) stones wedged in the V's of the belt, as the belt moves over the pullys these tap and make the exact sound you describe.

Good luck

[jonathant]

FWIW, my local garage, after crawling around the car with a stethescope, diagnosed my knocking sound as camshaft slap too. Kinda sounds like you're gently knocking on a desk and only under idle. When the fans kick the knocking stops.

Their solution was to install a modified bracket thing on the end to stop the camshaft floating - not done this yet though.

[DamnWiseguy]

Cheers for the responses, I will check all suggested over the weekend. It happens with bonnet up or down...

If it was camshaft slap then I would kind of expect it to be a regular noise? I also find it strange that it only happens once the engine is warmed up although maybe it is at the right resonant frequency then to rattle something.

I did suspect the exhaust, once it was hot expanding and touching something slightly. The exhaust also rattles under the car.

So annoying, car sounds like a right bag of spanners when idling. I will get to the bottom of it if it takes all weekend!

[dipstick36]

I've just had a similar experience with my 450 - only happened when warm and sounded awful. I checked the usual but have ended up swapping the cam which proved to be well worn after less than 20k - it had it's second cam at 48k and a previous owner also had the followers changed at 58k - it's now on 65k. Apparantly the double valve springs on these engines can be too strong for the cam and can cause early valve train wear. I rana straight edge over the valve tops with the rockers off and there was a 2-3 mm height difference - ie difficult to get any accuracy in the preload setting - so I took the heads off and had the valve seats etc re cut and the heights set properly. I sincerely hope your problem is a lot simpler than this but if you do find it's the cam you may as well check and/or replace all associated parts whilst the cars in bits to save you having to do the job a second time - it'll prove uneconomic to skimp on parts. Give V8Developments a call - I found them to be very knowledgeable and helpful. Best of luck!