puff of blue smoke on overrun (1/2 a stag engine related)

[Ian Durrant]

Looking for some opinions on a minor issue with SWMBO's TR7 which has basically the left hand half of a Stag lump.

I just noticed today, a very slight puff of blue smoke that only happens after its been driven fairly hard in a gear and then you lift off. It puffs just as you lift off but doesn't continue if you let it stay on the overrun. In normal driving it never happens or at least if it does it isn't noticeable.

The only investigation I have done so far is to check for crank case pressuriation and it does chuff fairly strongly from the filler cap on idle but when I pull the breather from the cam cover and put the cap back you can feel little puffs coming from it but nothing like as strong as through the filler hole.

I was thinking valve guides and or stems until I felt the crankcase pressure, now I am wondering if the flame trap in the cam cover being blocked might be a possible cause?

[Ian Durrant]

Oh I also noticed that the new cam cover gasket I fitted about 4000 miles ago is leaking again and I am assuming the crank case pressure may have caused that also.

[GDPR]

Think you were thinking the right lines with valve guides TBH.

Pull the head, do the guides, then re-ring it while the head is off to deal with that cylinder that is chuffing. ..

[Ian Durrant]

Thanks Russ,

I am not too bothered about it at this point as the plan was to pull the engine and box once the Stag is finished and rebuild both anyway (and if I am doing rings and valve guides I might as well do just that anyway). Its odd that on a compression test all reads well, but there is no ignoring the chuffing from the oil filler I guess.

Anyway I wont bother with cleaning out the breather trap if its not a likely cause as it drives just fine and SWMBO thinks 3000 rpm is thrashing it, thought she was going to have a heart attack when I gave it the beans and revved it to 6K

[Matt]

I'd check the breather flame trap - if it's gummed up it will potentially cause positive pressure in the engine which may blow oil to places it shouldn't be - for example into the valve guides on the overrun.... Its a cheap and quick job (AFAIK - not worked on a TR7) and may cure the problem. Leaking oil from the valve cover (and anywhere else) is also a symptom which can be caused by the breather being blocked.