12 replies to this topic

[dennisthemenace]

Hi, Just wondering if anybody can shine some light on the issue I'm experiencing.

I have an early 1980's 998 which after a number of hours I  have managed to get running very nicely, no hickups .

When I removed the oil filler cap on the rocker cover, the engine slowly died. I noticed when putting the cap back on there was some pressure which when the cap was on again built up and the engine ran sweet again.

I'm thinking valve guide seals or even a head gasket?

Any help would be great.

 

Thanks In anticipation...............

[nicklouse]

Sound like something is not correctly connected.

 

is everything standard and connected correctly?

 

if you are not 100% sure please post a good clear photo of the engine bay complete.

 

host on Imgur so we can view it full size if needed.

[MiniLuke]

Ah, I’m also getting this. Thought it was perhaps normal, however I have silly high oil pressure too…

[sonscar]

Removing the cap may stop the engine as it lets in excess air to the breather system if it is correctly connected.Blowing out is likely piston blow by which is usually a result of wear.Steve

[dennisthemenace]

Nicklouse - The engine is plumbed in correctly.

 

Miniluke - My oil pressure is I believe good at 70 psi.

 

Sonscar - I did notice that when i put my hand over the rocker cover after removing the cap, the pressure felt was "pulsating" for want of a better word, So I'm guessing it may just be wear as you say. I'm a little surprised as it only a 55k engine .... cant vouch for its early life though.

[nicklouse]

Nicklouse - The engine is plumbed in correctly.

 

Miniluke - My oil pressure is I believe good at 70 psi.

 

Sonscar - I did notice that when i put my hand over the rocker cover after removing the cap, the pressure felt was "pulsating" for want of a better word, So I'm guessing it may just be wear as you say. I'm a little surprised as it only a 55k engine .... cant vouch for its early life though.

Pulsating is normal.

[sonscar]

I remember there was a manometer which fitted into the dipstick hole allowing the crankcase depression to be measured?Never seen it done and I suspect many seemingly healthy motors would be out of specification,the value of which escapes me.Steve..

[greenwheels]

Removing the cap may stop the engine as it lets in excess air to the breather system if it is correctly connected.Blowing out is likely piston blow by which is usually a result of wear.Steve

The engine crankcase is meant to be able to 'breathe' to air when running through the cap. Is your cap blocked, in which case clean it with brake cleaner or replace it, and the reture the carb.

[dennisthemenace]

Thanks, I'll check  the cap out and see what happens .

[Ethel]

Positive crankcase ventilation, or the breather hose connected to the inlet/carb in plain speech, should draw air in through the filler cap as long as there's some manifold vacuum and drop the pressure to about 3-4psi below atmospheric. It sounds healthy to me. If it's drawing fresh air in through the open filler & you suddenly cover it with your hand, it will lose inertia & rebound as well as absorb heat & expand. The flow will be proportional to the pressure differential so will virtually stop - and you'll still have a bit of blow by.

[Homersimpson]

Sounds to me like the OP's car is probably normal, the pressure is inevitable in any engine and its just the amount that might cause concern.

 

If its not using a lot of oil (when you put it into use) and its not leaking excessivly at the various seals and gaskets on the engine (which if it were could indicate high internal pressure) then its probably fine.

 

55K miles isn't particuarlly high for a modern engine but for an older engine it could be, especially if its done a lot of short around town journeys.

 

Personally if it were mine I would put it into use, keep an eye on the oil and if its not using a huge amount and its not blowing out smoke when driving I wouldn't worry about it.

[dennisthemenace]

Well further investigation I found the exhaust was very damp , ie water spraying out with the exhaust, which was unusual as the car is kept in a garage. So I began to suspect the head gasket. When I removed the head I found a damp patch around the exhaust/inlet manifold around cylinder 3/4. When I got the head off , the gasket could be seen to have gone between cylinders 2 and 3. The gasket I had used looks to have lifted and allowed the water gallery to weep into cylinders 3 and 4. It may also account for the positive pressure build up in the rocker cover, as the engine was running the compression was making its way across the head and up the push rod guides into the rocker cover.........its just a theory though, my theory.

 

Anyway new gasket going in after a de-coke....lets see what happens.

[dennisthemenace]

Well after taking the head off, found it had gone and was allowing water into cylinders 2 and 3. Also found excess wear on the valve guides and valves not seating properly.
That explains the pressure build up in the rocker cover and the problems with starting, and the reason I had rebuilt the carb..... Problem found now just to fix it haha

Thanks for the help and advice to all who responded.