Exhaust Back pressure and oil leaks

[Rev]

Looking to fix the 4e leaking seals but repairer says they will leak again.

I am wondering can pressure through valves seals get into the block and push out the block seals?


Seal Symptoms are -

Exhaust back pressure seems to be the cause of oil squirting out of the top breather.
Intermittant idle revs drop now think this may be due to oil dripping on spark plug through seals.

Compression test last year was 160 dry on all cylinders but the wet comp. test showed 200 195 180 170

 

[5e colin]

to me sounds like worn rings or valveguides leaving air pass and build up in bottom block

 

[Rev]

It is a difficult call with old engines I guess it could be too far gone but I wondered if it is worth fixing the valves and not rings as I am thinking of swapping to a 5E later.

My main question then could be is there a guide line for moderate piston blow by that says at what rpm or boost block seals will start to let go? but from what you are saying Colin pressure can buildup so is not just at a certain revs?


PS I have found my exhaust is smaller than I thought 57mm OD , at 4425 RPM (16.5psi) this pipe maxes out and is around the same rpm I have oil from the breather. Also I have found there are new tech valve seals available from SKF they call 2.5, 3.5, 6 and 16bar for higher back pressure available and wonder if these would be suitable for a Starlet head.

 

[5e colin]

the problem with our beloved engines is we dont have any block vents al our gasses escape tru that tiny vent on the rocker cover if you dont want to rebuild i would suggest adding 2 more vents to the rocker cover as you know back pressure is really bad the better is releaves the pressure the longer the engine will last and not damage the seals etc etc ( at a minimum i would change the conrod bearings for peace of mind !!)

i would get bigger bore exhaust really urs is just over 2" i would go 75mm or there abouts bigger bore is better for turbos they dont like back pressure

dunno about the seals ??

 

[Skalabala]

"I am wondering can pressure through valves seals get into the block and push out the block seals?"

Are you talking about a 4 stroke internal combustion engine? Hahaha.

No bud, you are looking at a bore/piston sealing problem or crank case ventilation problem

 

[Rev]

@ Colin thanks always wondered if there were other crankcase vents and @ Skalabala OK piston blowby is the culprit.

So when worn piston blowby is too much for the rocker relief vent crankcase pressure becomes excessive.

The car goes really well otherwise so will have to see what I can come up with.
eg
> was thinking of a second pressure operated exhaust pipe.

because the sound levels have been approved with the current pipe size and 75mm may be too loud.

> Some how put an extra vent on the catch can hose as close to the rocker cover as possible that opens at higher revs and above a certain boost level. [ I guess it will have to remain closed at cruise and idle so PCV system still works effectively ]

 

[weeJohn]

Its more likely to be ring blow by pressure, as stated above.

The stock PCV system is there to alleviate this, but it will only be effective if its connected to the inlet manifold so some vacuum is applied to the valve off power to allow it to open. There will always be an increase in pressure in the block from piston travel causing pressure waves, some race engines will just have a small filter on a hole right through to the inside high up the block to help reduce pressures.

 

[5e colin]

rev that exhaust valve will make no diference 1s the boost builds it willl bypass the piston rings or guides and build up anyways
start drilling holes in the rocker cover

skabla i never knew 2stroke had valves

 

[dac69er]

You have a worn engine.

If the pressure in the crank case gets too high, it will blow the dipstick out. If it doesn't, you Havnt got any issues with excessive crank case pressure. This is if all your oil seals are in good condition of course

 

[Rev]

@weeJohn I have the approved weeJohn catch can setup but am still tweaking it to get pressures right on cruise.

@ Colin Sobering thought boost is the issue especially as boost adds to back pressure.
I read residual chamber pressure in over lap period can be around 45psi and valve stems around work to 38psi pressure so reduction in chamber pressure from exhaust is expected but this is messed up by exhaust back pressure which creates a secondary pressure wave arrives at about -1psi at valve overlap so pressure stays high maybe this causes my worn valve seals to squirt oil?

@ Dac Thanks you have actually quantified blow- by the breather is 10mmID and the oil dip stick is also 10mmID... As my seals are gone I currently have no large push on the dip stick but dont remember this being pushed out before the seals went.

TBH I still dont get blow-by turbo engines have 800psi combustion pressure before the piston goes down yet new pistons pressure test is at about 175psi. What am I missing ?

PS I will try to do some runs tommorrow at high boost and see how much blow by I get.

 

[weeJohn]

You are not missing anything, your wet test results show the rings are fucked on most of your cylinders as the pressure only went up a very small amount.

 

[Rev]

Yea that wet test doesn't look any good at all.
I even had fluid put through the cylinders after the test and got a power improvement but probably was cleaning the cat and a week ago I
fixed the PCV valve so it was more vertical.
As for tonights test no oil squirted out of the breather boost so far looks ok maybe more revs will be different ?
( K is Knock reading mostly when backing off the throttle) )

On a steep hill
15 psi 3600_ K101 OD on
17 psi 3572_ K15 OD off
18psi 3849_ K17 OD off car changed gear 2nd to 3rd dropped to 15psi
20psi 3862_ K20 2nd gear
On the flat
17-18psi 5242_ K23 2nd gear on the flat moderate run

 

[Skalabala]

Valve stem seals has nothing to do with this. Only if your guides are worn a BIG amount then it could cause 1% of the problem.
How many CFM do you think can go through the tiny gap between the valve stem and the guide? Doubt even 1CFM.

Boosting 20psi on a worn engine will for sure have sealing problems! Lots of CFM going to crank case!
Remove both vent pipes from the cam cover and then drive it

 

[Skalabala]

rev that exhaust valve will make no diference 1s the boost builds it willl bypass the piston rings or guides and build up anyways
start drilling holes in the rocker cover

skabla i never knew 2stroke had valves

Hmmmm, makes me wonder about those old hit and miss engines

 

[Rev]

Valve stem seals has nothing to do with this. Only if your guides are worn a BIG amount then it could cause 1% of the problem.
How many CFM do you think can go through the tiny gap between the valve stem and the guide? Doubt even 1CFM.

Boosting 20psi on a worn engine will for sure have sealing problems! Lots of CFM going to crank case!
Remove both vent pipes from the cam cover and then drive it

Excuse my ignorance I was going on the seals having high boost ratings but realise now these ratings are about oil use and emmissions.

The other possibility for oil getting on the rocker cover is from oil blowing/ tracking up from the leaking seals below or elsewhere more investigation to come.

Definitely have extra cfm - if stock is 2% then based on the wet compression test I have about 4% so 2 vents may work.
Because of the high back pressure the VE is less so even at 20psi its not too bad but will know for sure when the seals get fixed.

 

[Skalabala]

Bud, are your crank and cam seals popping out?

Maybe do a leak down test?