Blow By In K Motor

[Kirill]

When I rebuilt the motor the bores were good so I honed it, did not do a rebore.

 

Car ran great first year but now when I give it the herbs the oil cap pops off and I have to drive back and look for it. I put it on a piese of wire now.

 

Compression in all cylinders is still good but at 5500 it not only pops it off it pops out dipstick too!

 

Crankcase vent goes to catch can then dumps under the car . So there's little resistance , this thing just generates so much pressure it's unreal.

 

Is the motor cooked ?

[ke70dave]

Did you replace the rings? If so did you gap them correctly?

 

Did you get a valve grind too?

[rebuilder86]

hang on, the oil cap shouldn't blow off. the threads on the cap are probably just toast. the pcv hose should blow off before the oil cap. Unless u really mean the dipstick.

[Taz_Rx]

Some old motors (or more so tappet covers) had a rubber push in 710 cap. ;)

[Kirill]

Yeah it's push in cap. Yeah rings got changed I did put them in bore and gap was within spec. I think the boresare worn . Just wondering if anyone else had same issue with oil cap

[styler]

Screw in cap for sure, ditch the push in cap. PCV stands for positive crankcase ventilation, basically the PCV system should

suck fumes out via vacuum on the intake manifold. The PCV valve regulates this and should be in good working order, not seized.

Replacing this system with a simple vent to atmosphere catch can with no vacuum or PCV valve is a problem for flow of blowby.

Although I would say its probably more the 40 year old bores needing a overbore/hone to suit but you say compression is good,

whats are the current readings like? Bad Ignition timing may possibly cause abnormal cylinder pressures when firing, food for thought

anyway just another factor in the mix.

[Kirill]

Thanks for replies , I just put wire leash on the cap, so I don't have to look for it next time haha

[altezzaclub]

I haven't heard of oil caps being blown off!

 

Take a look under the tappet cover vent inside the cover. If the plate leading to the vent is 90% blocked with shit it would explain your problem. Do you want to drill a second vent into the oil cap itself? Make a 1/2" hole and fit a hose barb to it, run both to a catch tank.

 

The best solution would be a dry-sump scavenge pump!

 

It must be blowing oil vapour past the crank seals too, so it will eventually slip the clutch.

 

If you hunt the web enough you'll find a figure for how much blowby you should have in a car, measured by fitting a hose to the tappet cover vent and leading it under a jar in a tub of water. Its a few litres/min, but I can't remember the figure. Like a compression test, its a bit meaningless as the blowby when the car is stationary, even at 3000rpm, is nothing compared to full throttle when driving.

 

No tapping sound of a broken ring?? It doesn't sound like normal ring wear to me.

[Kirill]

Nah motor pulls hard sounds like a fiat I recon , must be the Weber . I will make crankcase vent better flowing

[Big G]

Years ago my parents had a honda civic that was doing the same thing but not at such high revs. It turned out to be the head bolts being stripped. when a compression test was done cold at all looked ok but when the engine heated up the head would expand raise of fthe engine deck and let gas into places it shouldn t be.

[Kirill]

That's pretty interesting actually. That could explain why after about 5 minutes of going 150 km /hr one night the motor overheated . It was 2 am at night in winter so it was cold . Radiator was flushed etc etc. 4k has a cavity where push rods travel so it could be that the head is lifting up