What the hell is happening to my exhaust valves?!

Dave3066

Dave3066

Well-Known Member
Folks

As the title suggests, I'm having a bit of a 'mare with exhaust valves on my good lady. You may recall back in February I posted that she had burnt out a valve on No 4 cylinder, 40k miles after doing the same on No 2 cylinder. Well, 4 months and just shy of 6,ooo miles later she's done it again! Same cylinder, same damage in the same place.

This is the exhaust valve



This is the previous valve



Look familiar? The damage is in pretty much the same place and has a similar "nibbled" look to it. There's no sign of anything fouling at the top of the cylinder and this position corresponds roughly with one of the gaps in the piston crown. There's a bit of carbon deposit build up on the edge of the cylinder, but surely the valve material would not be mechanically affected by this?

In terms of my driving style etc I don't hammer the car very often. The engine never gets too hot either and that valve was brand new when fitted to a reconditioned head. I can't have this happening every 5k miles so I'm open to opinions on what might be happening here. Some have said I should try to source some TC valves, but I've tried and can't find any out there. I did manage to source a good used set of valves but it turned out that the exhaust valves were too big (2200 perhaps), so that kinda annoyed me when I went to fit one. Hey ho, that's eBay for you.

Dave
 
Just looking at that last picture again something springs to mind. Do the valves rotate during operation? Could it be fouling against a spark plug that's too long? The plugs all looked okay and I couldn't see any corresponding damage on No 4 plug, but I'll fit the plugs with the head off and see how far they protrude. Hmmmm!

Dave
 
ghce said:
Cam lobe opening the valve far enough?
Click to expand...

Oooh that's an interesting one. So if the valve isn't opening wide enough the heat from the exhaust gasses might be increased by having a smaller space to exit from? Is that what you're thinking? Care to expand on that theory? I have a spare NOS camshaft at home so I might compare lobe heights.

arthuy said:
I have a spare 2200tc head if any valves would suit you.

Colin
Click to expand...

If the 2200SC has larger exhaust valves then wouldn't the 2200TC also have larger exhaust valves?

Dave
 
Hi, Is the seat damaged? It doesn't need much to start to blow and will take the valve first because the seat is harder.

Colin
 
I would think it's more likely that the valve clearances are too tight, so the valve isn't closing completely, or maybe the valves are seizing in the guides due to poor coolant circulation.
 
Hi Dave

Thinking out loud...

Valves normally burn out when they aren't in contact with the valve seat for long enough. Heat is taken away when the valve is in contact with the seat which is cooled by the flow of water through the cylinder head. Two possibilities spring to mind. A blockage in the coolant channels in the cylinder head or the exhaust valve being held off its seat by the heel of the cam due to insufficient clearance.

Another possibility is incorrect ignition timing or advance curve although I would have thought you would have notice a rise in temperature of the engine and maybe some knock.

What fuel grade do you use Dave ? Do you use an additive ?

Pat
 
pat180269 said:
Valves normally burn out when they aren't in contact with the valve seat for long enough.
Click to expand...

Which is exactly what happens when you have a tight tappet clearance. The valve closes, but for a shorter time because the lack of clearance means it opens earlier and closes later.

I'd be looking for weak mixture, either carb settings or an air leak, (after checking all the ignition side) but if those were OK and the clearances were correct (which in the OPs case I'd guess they were) I'd begin to suspect faulty valves. It does seem to be happening a bit too regularly. IIRC the TC valves were sodium filled to counteract this problem, but if you buy valves nowadays, they may look OK but you have no real way of knowing if a little chinaman made them last week.....
 
colnerov said:
Hi, Is the seat damaged? It doesn't need much to start to blow and will take the valve first because the seat is harder.

Colin
Click to expand...

The seats were re-cut last time I had the head off in Feb.

suffolkpete said:
I would think it's more likely that the valve clearances are too tight, so the valve isn't closing completely, or maybe the valves are seizing in the guides due to poor coolant circulation.
Click to expand...

Valve clearances are spot on and the head was chemically cleaned when it was last off in Feb so coolant circulation should be good. A sticky valve also crossed my mind but they all seemed to move smoothly when the engine was turned slowly. I wondered if the valve might only be sticking at speed, which would give the lack of contact with the seat problem? I would've thought that might show up on a compression test, unless the starter can't turn the engine quickly enough to replicate the symptoms.

pat180269 said:
Hi Dave

Thinking out loud...

Valves normally burn out when they aren't in contact with the valve seat for long enough. Heat is taken away when the valve is in contact with the seat which is cooled by the flow of water through the cylinder head. Two possibilities spring to mind. A blockage in the coolant channels in the cylinder head or the exhaust valve being held off its seat by the heel of the cam due to insufficient clearance.

Another possibility is incorrect ignition timing or advance curve although I would have thought you would have notice a rise in temperature of the engine and maybe some knock.

What fuel grade do you use Dave ? Do you use an additive ?

Pat
Click to expand...

See above for coolant and valve clearances. Since the head came off last I 've been using Tesco Momentum which is reputedly 99 octane, plus an additive. The engine used to run quite hot (top of the green) until I replaced the side plates last year and cleaned out all the crap around the cylinders. Since then the engine temp has been spot on in the middle of the green, but that wouldn't identify any localised hot-spots if coolant wasn't getting there. The engine has been running quite cool (bottom of the green) since this problem started. A new SimonBBC distributor was fitted recently.

harveyp6 said:
Which is exactly what happens when you have a tight tappet clearance. The valve closes, but for a shorter time because the lack of clearance means it opens earlier and closes later.

I'd be looking for weak mixture, either carb settings or an air leak, (after checking all the ignition side) but if those were OK and the clearances were correct (which in the OPs case I'd guess they were) I'd begin to suspect faulty valves. It does seem to be happening a bit too regularly. IIRC the TC valves were sodium filled to counteract this problem, but if you buy valves nowadays, they may look OK but you have no real way of knowing if a little chinaman made them last week.....
Click to expand...

I think a weak mixture might be closer to the problem. However, I did make the mixture richer after the last time this happened and even switched to a richer needle. I've since switched back to the original needle after a period of sustained cruising at 70 fouled all the plugs with soot. I think the replacement needle (RE as opposed to RN) was too rich at higher engine speeds.

It could perhaps be a coincidence that it's No 4 cylinder again. It could be crap quality valves. It could be a weak mixture. It could be a loss of localised coolant in that area. It could be all of those things together combining to create the "perfect storm". Time will tell with another new valve and a bit more tweaking.

Thanks for your comments gents. Plenty to think about and check.

Dave
 
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Dave3066 said:
A sticky valve also crossed my mind but they all seemed to move smoothly when the engine was turned slowly. I wondered if the valve might only be sticking at speed, which would give the lack of contact with the seat problem?
Click to expand...

If a valve was sticking it would give you a misfire.
 
thats a typical burnt valve. caused by hot exhaust gases rushing past a non sealing valve and valve seat. molten blobs of melted metal can get blown around and cause cylinder and pistol damage too. when changing valves where they are 'burnt' its imperative to have valve seat changed too or head refurbished as the heat for melting valve/exhaust softens and weakens seat. bedding in with grinding paste etc can if in excess actually reduce seating pressure by having a larger surface area for valve to seat on.
if valve was? ground in to seat.. its important to have it done correctly and no 'tramlines' and a 'full' contact area around valve with seat surface. ( often easily seen when grinding in by a 'grey' not shiny..band ) ANY minuet grinding paste can and will cause issues such as burnt valves so cleanliness is essential. it only takes a small particle of grit or even carbon to get stuck on seating area for a small gap to be created thus allowing valve 'blow by' and to burn valve very quickly.
bit of a concern that its exactly same place ..unless seat was not ok? or valve isn't closing correctly ? a compression test is a very rough gauge and won't easily pinpoint non seating if very minor. if we have hydraulic tappet? then change that one for sure. we may want to look at cam lobes too. turn engine over slowly by hand and see if we can see what is occurring with valves. do try adding a small ! amount of engine oil to the cylinder before doing compression test. without oil added record figure add oil. should rise by at least 15-30psi. if NO change ? then check other cylinders doing same . oil adds a hydraulic oil 'seal' around piston rings so ALL cylinders ought to get a rise in pressure after adding oil ( do crank over few seconds Before doing compression test to distribute oil around cyl/piston walls/rings)
other factors that can gibe rise to valve bring are wrong ignition timing. weak fuel mixture and over heating . got spots in a cylinder ..( carbon build up) can cause preignition in a one or more cylinders . this ought to be heard as a sharp knocking . this can do the same damage .
 
As mentioned previously, the valve seats were re-cut after the last time this happened. I should've mentioned that the machine shop vacuum checked the exhaust valves after re-cutting the seats to ensure a good seal. The tappets on the 4 cylinder are not hydraulic, but I will be sure to grind the new valve in properly and maintain cleanliness throughout. I don't think I can see what the valves are doing with the head fitted without a borescope of some kind.

Dave
 
One other thought I had was the valve guides. If there is excessive wear in the valve guide on that exhaust valve, the valve might not seal properly on the seat leading to it eventually burning out.
 
pat180269 said:
One other thought I had was the valve guides. If there is excessive wear in the valve guide on that exhaust valve, the valve might not seal properly on the seat leading to it eventually burning out.
Click to expand...

Another good one Pat, but all the valve guides were replaced with new last time the head was off.

Dave
 
Hi Dave,

For some reason, the head area and valves in your photo look way too dirty after 4 months and 6 K miles.
You already said that it was runnning rich, but how's your oil consumption? In any case, try to avoid carbon build up like this, it would only cause problems.
It is not a coincidence that it happened again on number 4 cylinder. This cylinder tends to run hotter than the others, a problem that could be amplified under certain circumstances.
Regarding valve clearances, i tend to adjust the exhaust ones at the upper end of their gap range. This makes the engine somewhat less quiet than it should be, but it keeps the valves happier. I have around 110K miles with the current engine in my hands, and i never had to remove the cylinder head, but i had to adjust the tappet clearance a few times.
 
Demetris said:
Hi Dave,

For some reason, the head area and valves in your photo look way too dirty after 4 months and 6 K miles.
You already said that it was runnning rich, but how's your oil consumption? In any case, try to avoid carbon build up like this, it would only cause problems.
It is not a coincidence that it happened again on number 4 cylinder. This cylinder tends to run hotter than the others, a problem that could be amplified under certain circumstances.
Regarding valve clearances, i tend to adjust the exhaust ones at the upper end of their gap range. This makes the engine somewhat less quiet than it should be, but it keeps the valves happier. I have around 110K miles with the current engine in my hands, and i never had to remove the cylinder head, but i had to adjust the tappet clearance a few times.
Click to expand...

Hi Demetris

That picture of the head is from the previous removal. I forgot to take a picture of the head before I removed the valve this time. The head is pretty clean, with minimal carbon deposits.

It only ran rich with the RE needle fitted. I'm back on the standard RN needle now.

Your car being a TC should have valves made from harder steel that give a higher tolerance to heat. I've not been able to source any to fit so I'm stuck with standard SC valves. Oil consumption is not any higher than usual so no worries on that front.

Dave
 
New valve arrived mid-week and has now been fitted. I went to a different supplier this time and the valve appears to be of different manufacture. Note the dimple in the centre, which the others do not have.



I re-lapped the other valves whilst I had the head off. The exhaust valves all showed quite bad pitting and took a while to re-lap, I'm interested to see how this different one lasts. All the inlet valves were already showing a slight build-up of carbon on their shoulders. I also checked the camshaft lobes against a NOS spare that I have and there was next to no difference in measurements. The valve clearances were all within tolerances when everything was back together and the engine is running sweetly, albeit a little rich, again. I'm going to let it run rich for a while to see how things go.

Hopefully I won't have to do this again for a while, but I'm sure I said that last time.

Dave
 
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