V8 3.5 - unidentified noise

[2 rocks]

This should be interesting...how do I describe it?

Background:
Stock (I think) 3.5 with carbs
LPG (don't believe that's a factor)
250k kms
No coolant in oil, no oil in coolant.

There's a noise that is intermittent while running, for want of a better description it kind of sounds like the noise you get when kids used to peg a piece of cardboard to their bike frame so it flaps through the wheel spokes.

It seems to be coming from the rear of the motor on the left hand side of the car and is matched to revs.
At first I thought it might be a really badly adjusted valve, or maybe a broken rocker post or a cracked rocker shaft, but there seems to be no effect on performance.
It's not there all the time, when I start off in the morning it's not there, after a few kms it comes on, then after maybe another 10kms it may go away again. But then again it may come back.

It's there under power and on over-run. I also thought it might be 'blow-by' and perhaps it was the extractors changing shape as they heat up and then closing the gap again at full temperature.

I'm not convinced it's mechanical, as I would have thought it would be there all the time, but I just can't tell. Whatever it is, it's disturbing at times as it sounds like it's going to put a leg out of bed. :(

Any ideas from the wise?
Cheers
Mike

[sheerluck]

Exhaust manifold gasket (or header gasket) I would guess, judging by what you are hearing.

Mine blew on the right hand rear and made a strange rattle noise. Found that the bolts were a little loose after 1500km from replacing the head gasket.

May even be a HG blowing in the middle, where there are no coolant channels.

[Homestar]

What is your oil pressure like? Could be dropping as it heats up?

My 101 has a small exhaust leak that sounds like that too, but it is there all the time. Hopefully it is just that though.

[justinc]

Could also be compression leak from head gasket fire ring failure, external out under exhaust manifold. I have seen a few on LPG like this and yes it sounds very like a knocking rattle.
Also check you don't have a loose spark plug...

JC

[2 rocks]

Thanks guys!
I will check the extractor manifold nuts on the weekend, when I have a chance (and daylight) and also the loose plug option. They would really be my preferred option. ;)

...Followed by exhaust gasket and head gasket in a distant last place :(.

Without a "proper" gauge, I believe oil pressure is ok, well the tell-tale is not lighting on the the dash...

Cheers & thanks
Mike

[justinc]

Mmmm, extractors have a habit of cracking at the collectors, and making 'tapping' noises sometimes too.

Sorry I hadn't noticed it had extractors, the devils spawn them things:D, you would do better with a set of RRC or D1 EFI manifolds and engine pipes IME.

JC

[d@rk51d3]

Does your engine have the air injector lines?

One engine Dad had was fitted with them, and the previous owner had cut and crimped each inlet. We had a disturbing rattle coming from the engine, and found ball bearings inserted in the tubes.......maybe to make a seal? Either way, they didn't, and removing the remnants of the air lines got rid of the tapping.

[2 rocks]

Does your engine have the air injector lines?

One engine Dad had was fitted with them, and the previous owner had cut and crimped each inlet. We had a disturbing rattle coming from the engine, and found ball bearings inserted in the tubes.......maybe to make a seal? Either way, they didn't, and removing the remnants of the air lines got rid of the tapping.

Hmmm, to be honest mate I have no idea ;)...however I'll go looking!:)
Cheers
Mike

[BilboBoggles]

Loose spark plugs can be spectacular...

Does the sound increase when the engine is under load? For example if you floor it up a hill in 4th. IF it is much louder under load then probably an exhaust leak somewhere, Manifold or Head, or spark plug!...

If it does not increase under load - another possibility - it's done 250,000k's? at that age the camshaft probably will be fairly worn. Could it be lifter noise? It's also possible that there is a bit of crud floating around in the lifter and causing it to leak down randomly. I had a V8 with a light tapping sound, no really noticeable performance issues, but when I popped the rocker cover off 2 valves were no longer opening.... The cam was completely round!.

Could it be a stuck valve, Iv'e read that on some versions of the 3.5 v8, the valve guides don't get enough lube, and especially on LPG are prone to seizing up. A stuck exhaust valve might make an interesting tapping sound, just before it roots itself. Might be visible if you pop the rocker cover off.

[2 rocks]

Yep, I finally got the chance to lift the bonnet today and thought I'd solved the problem.

Waggled the plugs to find that numbers 1 & 7 were loose - number 1 almost falling out! :eek: Out with the plug socket and about 73 extension bars plus a wobbly :p:p and they were snug once more, ran the socket over all the rest to be sure and all was well.

Impressed with my mechanical prowess, I fired it up and it purred like a kitten, so off for a test drive. It took I would guess, about 2 to 3kms before the noise returned. Re-checked pugs - all tight.

I used my "bush stethoscope", my largest flat blade screwdriver and tried it on the rocker covers, but it seemed most apparent when I had it on the inlet manifold.

So no wiser, still don't know if there will be a catastrophic failure of something, but it certainly runs smoother and has a bit more poke - who'd have thunk it ;):D.

Next...maybe rocker covers off, but the A/C and LPG plumbing looks like making a bog-simple job a PITA...:mad:
Cheers
Mike