Another non-starter...

[robin]

Perhaps the cat is blocked? Pressure has to go somewhere and eventually will burst out of one or other of the joints ...


For a quick test you could just loosen off the bolts/nuts on the downpipe side of the cat. If the blowing moves from the engine to the cat flange, then that would suggest a blockage in the cat.


Cheers,
Robin

[Ferg]

Mikie, All gaskets are brand new now. No signed of blowing anywhere.

Robin, That's a very interesting idea, and best of all relatively easy to test. My hands are covered in cuts and bruises after getting that manifold out the bottom of the car, trying to move the engine and extract the manifold at the same time proved interesting... lol I didn't want to do the same job all over again immediately. But I may resign to fitting new Manifold-Head studs at some point.

[Ferg]

Progress.

After checking the cat, it's fine, there's too much gas coming out of the tail for it to be the Cat anyway, and the noise is much worse under load. Which leads me back to the Manifold.

In true BluePeter fashion I made a Stethoscope out of a pipe and tried to locate the blowing. Best I could make out is that it's louder from the top of the downpipe flange of the manifold. Is it possible that it looks ok, but has a big anough hole to be so noisey?

Anyway, just to be sure I'm now sourcing a replacement manifold and I'll replace all the studs when it comes off. If it's still wrong after that, well then I'll start twitching uncontrollably...

[robin]

If you run it without the heatshield fitted (in itself a PITA) you might be able to feel where it blows from with your hand, or squirt A LITTLE WD40 or other light oil onto the manifold when it's hot - you might see the smoke being blown by the escaping gas. How's your valve timing ... (only kidding ).

Cheers,
Robin

[Ferg]

I'm now intimately familiar with the heat-shield. It's not been enjoying getting taken on and off so much, starting to disintegrate a bit but I can't warrant the relatively large cost of a Nimbus replacement yet.

I like the smoke idea, will give that a try. Don't joke about valve timing, it had crossed my mind.

[Ferg]

After Robin's passing suggestion, I found a recent post on SELOC about slipping teeth on the cam belt that worried me. A deep inkling that this might indeed be a timing issue led me to make some time today to do a check. and...



Holy sh1t... The timing is about 6-7 odd degrees out. The bottom pulley has slipped a few teeth on the belt.

I managed to fashion a cam locking tool of sorts, and re-set the timing this afternoon and after several of the usual checks, she burst into life and purred like a kitten.

I'm happy to have found the issue, but a little amazed that cam belt jump is a known fault. Overall though, it's a huge relief that I might just have been lucky enough to get away without damaging the engine. There are some scary pictures of complete engine meltdown from the same fault. Even scarier that I had been driving it in the mean time, never above 2k revs but still. I think I can even recall when it happened, starting after a freeze and then a couple of failed attempts to get out of a rut in the grass must have jolted the engine a little.

I feel a little foolish that I thought it was an exhaust problem . Ultimately though I've learned a fair bit about the car as a result and renewed some worn parts. I'll put the car into someone for a new belt and tensioner asap as it looks like it could do with replacing and after this incident it must have stressed the existing belt. With baby due in 9 weeks I've got my hands full doing DIY and have already commited a lot of the time promised for that on the car so can't do it myself. Any suggestions for places to go would be appreciated.



EDIT: Just wanted to say cheers for everyones input.

[robin]

you need to do a compression test now.

I suspect that you had a tiny valve-to-piston contact when you restarted the engine after being left for a while - whichever valve was left in the open position (there are always some open) could well have just rusted in its valve guide, or the lifter seized in its bore. When the valve hits the piston, the cam belt slips, but the seized part gets a kick up the arse and all is well on the next revolution, except that the timing is now out.

You may now find that there is a bent valve stem and a valve that doesn't quite shut, though that would probably lead to poor running. Hence I would run a compression test and if it comes out OK, job done. If one cylinder is down, I'm afraid you need to remove the cylinder head.

Cheers,
Robin

[Ferg]

Yeah, I couldn't find my compression tester yesterday. I think it may have been lent out and not returned long ago.

The engines never been left for more than a week so hopefully no rusting etc. I noted that when I re-applied the tensioner that it been a little short of creating the proper tension. This may have contributed and also might mean I've escaped any bad valve damage, he says hopefully.

I've had head failures before (some very costly ones) so while I'm happy it's running, I understand the implications if I find any permanent damage.

fingers crossed....