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Crankcase Pressure


Guest Gargoil

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Guest Gargoil

Further investigation after Bettys disappointing show on the Dyno has shown that for some reason there is excessive pressure in the crank case. It manifests itself by bubbling oil up through the dipstick tube. I have also had a few problems with the oil catch tank smoking too much. Today I changed the oil, filter and spark plugs, using Halfords classic car oil. It has made a difference to the catch can, not noticing any smoke until the engine was hot (phew) but I am still loosing oil through the dipstick tube. A compression test revealed all cylinders around the 125 mark. Low I know, so it looks like I may be getting a bit of blow through, hence the pressure in the crank case. Not wanting to think that it is piston rings, could the low compression be caused by an incorrect cylinder head gasket or the cylinder head itself? It has just been converted to unleaded, so it's been off and put back on again by a local garage. Could incorrect cam timing be a cause, too?

 

I may have to live with it, the lack of power doesn't worry me, it's reliability that I want, so is there a way of relieving the pressure, such as drilling the catch tank and venting it to the underside of the car?

The present setup, is one feed from the crankcase to the can one feed from can to cam cover and one from can to the sump as a drain. The can already has a vented cap, but it's not coping at high revs.

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Guest Gargoil

Yes, throttle wide open, wish that was a mistake I had made but no. Strangely, the compression test done before the head was altered showed that cylinders one and two at about 125 with 3+4 being 190. That was the original reason for the head work.

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Catch tank should breath to air via a filter if its just crank case and rocker to sealed tank there's your problem, the pressure has to go somewhere

Some run crank case to rocker cover with seperate outlet from rocker to tank

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Regards your low comp readings; The best way to decipher worn rings would be to put a teaspoon of oil into the cylinder through the spark plug hole, allow 5 mins for it to 'settle' and then do another compression test. If you see a marked improvement on pressure then it does point to your rings....if not, then the problem lies elsewhere.

 

Regards the catch tank pressure, check out what Paul has highlighted as not venting your tank to atmosphere will cause pressure build up and the easiest way out would for the pressure would be the dipstick tube.

Edited by steamer
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Guest Gargoil

Thanks, gents, that's given me something to go on.

The catch can is vented through the cap, what I might try is leaving the cap off to see what happens. If it alleviates the crankcase pressure, I will do a more permanent job. Oil into the combustion chamber, what a good idea, that's something I'm going to try tomorrow.

I'm also going to get a set of feeler gauges as the mechanic who put it back together was a little wooly when commenting on the tappet adjustment. Not entirely sure about his way of putting the timing belt back on. I could see the marks someone had put on the various cogs for a straight one to one swap, no alignment needed, it's just that it was two different people over a three week period that did it. Mistakes can happen.

Also a call to the engineer who fettled the head, because he was being rushed (everyone was on leave and he was holding down three jobs) the grinding of the valves might have been missed.

Thanks again though, what a great club.

:good:

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the "engineer" might have missed grinding the valves in? could see marks on the cogs for a straight one for one swap? and you have to obtain feeler gauges to check work you paid for? Am I the only one a little horrified by this tail?

 

my theory, two cylinders were at 190, with head work they are now at 125. the rings will not have worn out while the head was off, so should assume problem has some bearing on recent work, or previous compression check was inaccurate.

 

jmho

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Guest Gargoil

Dan, the headwork was done by a company in Doncaster.

I talked to them today and they were helpful with my questions. The valves will have been fitted correctly according to them, but if I want, I can return the head to them for further testing.

Something I am trying to avoid at this moment in time. The work of removing and refitting the head was done by a local firm to me. I don't want to name names at the moment, as nothing is proved.

I hope to try the compression test with a teaspoon of oil tonight, if the compression doesn't leap up then it's looking like setup problems.

If I was a betting man, cam timing and/or tappets would be favorate.

Edited by Gargoil
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Guest Gargoil

Well, I think that was good news. Compression test on no 1 and three cylinders almost identical, pre oil added 118 lbs/ft, post oil 125 lbs/ft and dropping after a few seconds.

doesn't look like the bottom end....feeler gauges ordered, because Halfords really arn't trying.

:acute:

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Guest old_timbo

I find out that a leak-down test is the best way to find out what's up with an engine. You need a compressor and an airline fitting on a spark plug. Take all the plugs out. Turn the engine to TDC on the compression stroke for the cylinder to be tested. Set the airline regulator to 100 psi and connect-up. Slowly open the air valve and pressurise the cylinder - the engine will turn over if it is not at or near TDC. All being well, you should see the airline gauge holding at 90-95 psi, and hear a gentle hissing of air down the bore. If not then its obvious where the problem lies ie leaks through the exhaust / inlet / breather. If the head gasket is blown between cylinders you get a blast out of the neighbouring plug hole!

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Guest mower man

Been away so out of touch but I would be seriously bending some ones ears over this ,you have paid for a service and recieved a poor one , my guess is the cam timing is out and valve clearances incorrectly set, also ign timing will be out . Get intouch with the Garage?who set it up and be a pain in the bum till its corrected mick :diablo: :crazy: :aggressive: I get upset when a so called mechanic cannot do a basicly simple job :sorry:

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Guest Gargoil

I try and look on the bright side of things, Mick. I'm going to set my first set of valve clearances (when the feeler gauges arrive). It looks like an easy enough job, not sure where I put the hacksaw blade but I'll find somewhere.... :fool:

 

If setting those don't work, then the cam timing it is. I will be speaking to someone about that and being a pain in the rear.

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  • 2 weeks later...
Guest Gargoil

Ok, a rather sad update.

The car went back to the garage that installed the head, to see if they could find out what is happening.

They tested the cylinders using compressed air and demonstrated this to me. On cylinder 1 which was the worse one of the lot, it failed miserably. The air just went from cylinder to crank to the vent in the oil catch can. Compression for this cylinder was 70 (yes just 70) cylinder No2 was fine, it held air pressure and the compression test was 160. They did test cylinder 3 but I can't remember exactly what the results were, I think they were somewhere between the two extremes. This is all totally different to what has been measured before and I can't quite get my head round it. I spoke to the people who did the head work and they mentioned that the cylinder head gasket supplied might not be good enough as it was for a a 2ltr and although in theory should work, it might give problems. Has anyone had experience with with head gasket failure and could it provide the symptoms already mentioned? I still can't understand how all of this can happen, just because of a cylinder head conversion, there has to be more to it, than piston rings failing. The car doesn't even smoke (except through the oil catch can), how can it degrade so fast and with such little mileage?

Edited by Gargoil
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