Hi all,
working my way through a Cambelt change and while removing spark plugs cylinders 2 and 3 plugs had oil on the thread. Is this the rocker cover gasket failing or something worse ? Also the previous installer of the gasket used a lot of the liquid gasket stuff all the way around, would anyone advice repeating this or just put some at key points ?
thanks in advance
Oil can get on the threads from above (cam cover joint leaking as you suggest) or from inside the cylinders, indicating that the oil is entering the combustion chambers. If it’s the latter oil must be being burned and the sump level will steadily drop with mileage. If not… a new cam cover gasket, carefully fitted to clean joint surfaces (clean being the key word), should cure it. The recommended practice is to apply a small amount of sealant only in the sharp corners where the cover fits over the “humps” by the cam journals.
It’s also possible that some oil was spilled when the engine was being topped up, sometime in the past.
I’d suggest that if enough oil was getting in the cylinder where it was also showing on the plug threads, then you’d know about this.
The engine would be blowing blue smoke and pretty badly too.
More than likely a leaky rocker cover gasket in my opinion.
When the cam cover gasket is breached oil gets into plugwells that are affected pooling around the base of the plug above the thread. When you undo the plug the oil would get onto the threads and into the bore. Was there oil on the plug spanner? If so that confirms it is the cam cover gasket. I don’t think anything else could have caused this assuming it is clean fresh oil. I have found that it is usually the back 2 cylinders(nearest windscreen) that are compromised first.
Spark plug threads are usually coated in the elements of combustion to some degree but fresh oil should not get to the threads from within the bores.
The OP never said it was fresh oil.
A worn engine with poor oil control can certainly result in oil on the spark plug threads. I’ve owned one; a Ford 1.6 Crossflow which had supposedly recently been rebuilt by a professional mechanic. I had to strip it down and replace all the piston rings and one of the pistons, which had snapped across the oil control groove. I’ve seen quite a few others first hand! Two stroke engines very often have some oil on the threads. The oil can easily work its way up threads if there is any wear in the ones in the cylinder head, or sometimes even into apparently good ones. That’s why spark plugs have a sealing washer - the threads themselves don’t form the seal.
There are plenty of photos like this online.
It was used oil, same as what was around the camcover. Thanks for the photo link, they were useful. I think it must be a breached gasket as the threads weren’t burnt and the oil looked Fresh if that makes sense. I changed all the plugs but they didn’t actually look that bad So that makes me thing again it’s just what oil was pooling. Which surprised me as there was a lot of liquid gasket stuff around everything, took some cleaning.
Thank you eveyone for for your help, after a long day the jobs nearly done had trouble with the Cambelt covers at the bottom and middle, the previous person that changed the belt had broken the middle and bottom covers but managed to bodge them back on, unfortunately this has allowed the bolts and sleeves they sit in to rust so when I merely thought about undoing them they snapped. Had to order replacements from mazda which were very expensive but needed to get the job done before the weekend And they could get the parts tomorrow.
just have to get the old studs out now !!
ThRees been no smoking or excessive loss of oil that I have noticed although haven’t used it a lot over winter. I have cleaned the faces up best as I can, as you advise I’ll put some new stuff in and around the cam journals, what about putting a little bit around the middle spark plug gaskets ?
yes there was a little oil on socket, also some dried oil around base of insulator on plug it’s self Which points to the gasket as well I suppose
The OP did not mention fresh oil - I did and for want of a better description ie fresh rather than burnt, that is what it was.
Paul you were wrong about this! Cars are not like mowers or two strokes - oil simply will not transfer from the bore to a spark plug thread in a normally running engine.
As I said the only way oil can get onto spark plug theads in an MX5 running normally is via a breach in the gasket where it surrounds the plug wells.
The sealing washer on a spark plug is to stop the products of combustion not oil. You are correct about the plug threads - they are not a totally effective block against the products of combustion under pressure which is why they are invariably contaminated when removed.
Evo - I recommend you buy and fit a genuine Mazda cam cover gasket to avoid leaks in the future. Aftermarket are a lot cheaper but from my experience do not work reliably.
Thank you for the advice rhino666 I have bought a genuine Mazda Cam cover gasket.
Rhino, I’ve been working on engines since the 1970s. I’m not here to score points but I know what I’ve experienced, so we’ll just have to differ in our opinions and leave it there.