Cracked Manifold or leaking gasket

[bluebottle]

I have a definate leak from somewhere near the second cylinder back on the nearside which is causing a bad missfire at high revs.

Using a metal tube to listen the clicking sounds loudest where the outlet pipe joins the main manifold than it does at the cylinder head gasket end.

As there does not appear to be any obvious visual signs, is there a method of confirming whether the manifold is cracked without the trial and error method of replacing the gaskets?

[GreenV8S]

I wouldn't expect a manifold leak to cause a misfire at high revs, just lots of noise.

I don't know anyone who has had a manifold crack at the engine end, the cracks normally appear in the region of the collector. If you hear noises at the engine end I think it's pretty likely you have a leaking / blown gasket.

If a new gasket doesn't fix it then perhaps you could do a cursory check for cracks using a bright light and small mirror but I doubt it would be conclusive. Your best option would be to take the manifold right off and inspect it.

[v8 racing]

seconded what peter says i doubt very much it would cause a misfire?

[bluebottle]

thanks for the swift reply chaps,
When i say missfire, what i actual mean is a loud slapping noise coming in at around 4k rpm and engine starts run rough, and by 4.5-5k rpm causes a backfire. This i was assuming to be detonation due to air being sucked into the exhaust?
When you say cracks normally occur at the collector end, do you mean wher the 4 pipes go into the pre cat housing? My manifold is lagged with heat insulation wrap, i was getting the loudest noise at the location where the second pipe enter the lagging and there is a gap in the wrapping.

>> Edited by bluebottle on Tuesday 18th October 18:37

[GreenV8S]

bluebottle said:

thanks for the swift reply chaps, When i say missfire, what i actual mean is a loud slapping noise coming in at around 4k rpm and engine starts run rough, and by 4.5-5k rpm causes a backfire. This i was assuming to be detonation due to air being sucked into the exhaust? When you say cracks normally occur at the collector end, do you mean wher the 4 pipes go into the pre cat housing? My manifold is lagged with heat insulation wrap, i was getting the loudest noise at the location where the second pipe enter the lagging and there is a gap in the wrapping. >> Edited by bluebottle on Tuesday 18th October 18:37

Yes, I was thinking of the 4:1 collector where the primaries go into the big pipe with the precats in. It seems fairly common to get cracks in the primaries close by this collector.

Loud tapping/slapping noises under power could well be an exhaust manifold leak, and this could also cause pops and bangs in the exhaust when there is unburnt fuel going into the exhaust. However, there shouldn't be enough unburnt fuel at high revs / full throttle to cause this, it would most occur when you lift off the throttle I would have thought. It also wouldn't cause a misfire. But if you have some other fault causing the misfire, the unburnt fuel from the misfire could cause pops and bangs especially if you also have an exhaust leak letting air in.

Since you can hear the exhaust leak it is worth fixing this, it is easy to do and may be the only problem you have. But it's possible you still have some other problem causing the misfire.

[The Surveyor]

If you have to take the manifold off to do the gasket, you can check for cracks by filling the manifold full of water. Lay it on the ground, fill it with water and let it stand for a short while, if it's craked at the collector (where my Chimaera ones went), then you'll see the leak.

The water won't harm the pre-cats or lambda sensors if you have them.

Paul

[Yellow Griff]

This sounds awfully familiar

If you only get the "slaps" when the engine is under load (i.e. when driving but not when stationary) then it sounds like the problem I'm having fixed at the moment.

Initially we thought it was a weak spark or something causing a misfire. Then we looked at fuel igniting in the exhaust but actually showed that fuel was igniting in the inlet instead.

A compression test showed that No 4 cylinder was low and when the heads came off it was apparent that the valve seat had receded and that the inlet valve wasn't closing properly. Looking over the other valves showed that several were pretty badly coked, so not sure what had been done to it by previous owners.

Now I wouldn't recommend you jump straight in and take the heads off, but you might want to investigate a few things along this route.

Simon

[bluebottle]

Thanks all for your thoughts.
Simon, your problem sprang to my mind to re: the possibility of valve stem or seat failure. I guess a compression test tonight will confirm if there is a head problem or not.
I do have an exhaust leak somewhere which could be masking a more sinister problem, which only started after the car was returned to me after being sat at Peninsula's body repair shop for 9 weeks.
The pops and backfire only occur under load(or gassing can be repeated with car stationary) and not when at constant throttle or backing off, and only above 4k rpm.

[GreenV8S]

Yellow Griff said:

the heads came off it was apparent that the valve seat had receded and that the inlet valve wasn't closing properly.

That's interesting. Do you happen to know whether this was caused by the valve seat wearing (the normal cause of valve seat recession as I understand it) or by the seat physically moving up within the head as siwes suggested on another thread?

[Yellow Griff]

Peter,

I'm rapidly approaching the limit of my technical knowledge here so please bear with me if this description isn't totally accurate.

The garage showed me the head that was causing the problem. When you run your finger around the inside edge of the valve seat normally its completely smooth and similarly when running your finger in and out of the "hole" (I'm sure there's a technical term for it) its also smooth.

However, with this particular one there were small ridges that had formed running around the bottom edge of the hole. Hence when you rubbed your finger in and out you could feel the ridges.

As it was explained to me, the actual metal of the seat that the valve meets had been compacted. I don't believe the seat itself had moved within the head.

Hope that makes some sense!

I'm due to pick the car back up again tomorrow so I'll see it I can get a better explanation. The garage spoke to the factory, but they claimed not to have heard of this particular fault before - maybe that by the time is happens the cars are so far out of warranty that the specialists/dealers just deal with it.

I can only assume that someone previously had run the car with no oil in, or thrashed it before it had warmed up properly, etc. to cause this to happen.

Simon

[griffter]

Interesting. The RV8s built in the days of leaded fuel were reckoned not to need special valve set inserts for unleaded fuel - and of course catalysed cars were designed to run on nothing but. I wonder if certain or even all RV8 cars are in fact susceptible to valve seat recession? Perhaps more so TVRs given the relatively high stress.

[GreenV8S]

Yellow Griff said:

However, with this particular one there were small ridges that had formed running around the bottom edge of the hole. Hence when you rubbed your finger in and out you could feel the ridges. As it was explained to me, the actual metal of the seat that the valve meets had been compacted. I don't believe the seat itself had moved within the head.

That's very interesting, hope you'll let me know how that turns out. The actual valve seat is just a thin steel ring a few mm thick that is pressed into the alloy casting of the head, and you should be able to see the boundary between them if you look closely. I'd be particularly interested to know whether these ridges are in the valve seats themselves or the head casting just behind them.

I's hard to imagine that the steel valve seat could have been hammered out of shape, but that seems to be what you're describing.

[Yellow Griff]

OK, have got the car back now - better but not completely cured.

The explaination I was given would seem to support some of siwes theories on his recent thread about ticking griffs. The valve seats had definately receded rather than being worn and it was suggested that this was due to the way the heads had been ported. This, coupled with larger valves, weakens the heads on the 500 cars. Interestingly the most severe case for me was on an inlet valve whereas siwes implied that exhaust valves were more likely to suffer.

Everything has now been machined, cleaned and reseated and whilst everything goes ok, the lifters are at the end of their range and they can't get a perfect set up and tune on the engine. [Still better than before as I'm now generating power from all 8 cylinders, rather than the 7 before .] We're currently looking at the cost of adjustable push-rods to see how that might help solve the problem.

We're still none the wiser as to what caused this in the first place but the garage have spoken to a number of engine guys who confirm that this has happened before. Worse still is that you can get into a viscous circle here as by curing the problem you reduce the amount of tolerance that can be handled by the lifters. It also seems that on my car someone had been steadily advancing the ignition timing to overcome some of the effects but not addressing the root cause.

As before I apologise if there are any technical inaccuracies in here but that's about the limit of my knowledge of the inside of engine heads

Simon