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Thread: Oil blow by through vale cover breather vent

  1. #1

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    Question Oil blow by through vale cover breather vent

    87 MM 4x4 2.6 ltr.

    Started suddenly having oil, smoke blow back through my rocker-valve cover breather vent.

    Checked under cover and found number 2 intake jet valve stuck in it's holder and the rocker arm adjuster snapped off.

    Got all the parts, pieces for the jet valve and left jet valve shaft in place which is stuck in place.

    Checked compression,,all 4 cylinders at 150 and above. Checked piston tops seems all is well. Starts and has good power

    but oil and minor steam like smoke comes out valve cover breather vent.

    Also checked coolant no leaks, brand new radiator also. and oil for coolant leaks no signs. It does not over heat either.

    No holes in pistons and good compression...I'm baffled. Anyone have any idea or have had this happen to you with these engines?

    Thankx for any answers or ideas!

  2. #2

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    been there many years ago with the 1986 4wd 2.6....don't remember how many were broken, had to replace all "adjusters" & "rocker arms". first symptoms were engine tapping, cold running, poor starting (cold), gas in oil, clogged catalytics. make sure to soak all new parts in oil (overnight if possible) prior to assembly & recheck torque after break-in (14-15ftlbs rocker arm "c" & 15-19ftlbs rocker arm "a"). why it happened...only thing i could come up with is "adjusters" wore/got stuck...never happened again. best of luck!

  3. #3

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    Quote Originally Posted by ragragtimetime View Post
    been there many years ago with the 1986 4wd 2.6....don't remember how many were broken, had to replace all "adjusters" & "rocker arms". first symptoms were engine tapping, cold running, poor starting (cold), gas in oil, clogged catalytics. make sure to soak all new parts in oil (overnight if possible) prior to assembly & recheck torque after break-in (14-15ftlbs rocker arm "c" & 15-19ftlbs rocker arm "a"). why it happened...only thing i could come up with is "adjusters" wore/got stuck...never happened again. best of luck!
    just drove it 8-0 miles. ran fine except a small miss in the exhaust tone at idle otherwise runs fine.

    Only the tab for the jet valve broke off the rocker arm. The rocker arm is still operational and working the valve like it should.

    The #2 jet intake valve is fried and the valve shaft is stuck in place in the guide. Until I get the jet valve eliminators I'm leaving it siezed in place cuz nothing to replace it with right now. The rest of the valve train is fine and everything else seems fine too.

    Just the oil blow back through the rocker arm cover breather vent back towards the firewall.
    It makes a real mess so I will make a catch bottle and keep the filtered opening up out of the oil collection area.

    It does not over heat, stayed at 160 degrees all night during my paper route and i drive it everyday so can not afford to be without a vehicle.

    I'm still baffled , does the jet valve passage goto the crankcase some how from the cylinder causing the crankcase to have more
    pressure causing the oil to blow out through the breather vent?

    It seems the pressure is stronger then it was noticeable before this discovery of the broken jet valve and tab busted off the rocker arm itself.

    Anyone have a 2.6 ltr engine for a 4x4 mm or d50 that's good they want to sell cheap?

    Could use a 0991 ratio rear end to, hell how about the whole truck.

  4. #4

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    well seems the crickets are a churpping....

    Going to put another 80 miles on it (hopefully) on the papaer route again tonight only I got my oil blow by catch can in place around
    the air filter coming from the rocker cover breather vent.

    it does not seem to blowing to much oil out but it could get worse. Still waiting for jet valve plug kit to get here. Hope that fixes the internal
    crankcase pressure back to a whisper.

    Comments welcome...speculation even at this point.

  5. #5


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    Oil and smoke is normal to blow out of the breather. Excessive amounts means you have an issue somewhere.

    A busted jet valve shouldn't be a related issue by itself unless it wouldn't seal.

    You should have 2 vents.
    1 - at the firewall
    2 - the PCV valve up front (not sure on the 2.6)

    If your PCV is clogged, or essentially not doing its job, you will be forcing all the excess pressure out of one hole, which will cause higher than normal oil waste.

    Excessive blow by is usually caused by piston rings going bad, or cracking in passages not surrounded by coolant (not common). A compression test alone is usually not enough to test ring condition. A leak down test would give you a better answer.

    I don't normally build the 2.6 engine. Most of my clients like seeing them better used as paper weights, so I can't say for certain there is a definite reason for the excess crank waste, but I can say engines are engines, and the same problems typically apply.

  6. #6

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    Well i drove it again unevenfully thank goodness. Oil catch bottle i made did catch the oil though as designed?

    I did not check the pcv valve at the front so will check that for sure....duh on my part.

    I do not think the jet valve is keeping the passage closed off because the stem was 2 inches higher in the guide then it should have been.

    It broke the adjusting tab part of the rocker arm off and also the spring, retainer and keepers which I found and retrieved. the stem is seized in the guide which I hammered down into the guide to a level I had hoped would close off the air passage. Not so sure it did because the crank pressure is still present. Have a kit on the way to plug all 4 jet valve holes since I have a weber carb setup.

    I'll follow up with a leak down check today to see if it leaks from other then rings. Thankx for the ideas.

  7. #7

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    I take it no one has had this problem then........

    Leak down was nominal and within limits for factory standards.

    Pcv valve is good.

    Going to replace the jet valve and see if it cures the problem that only arose after the discovery of a broken jet valve.

    300 miles more and still runs good..even a little better fuel economy.......

    Tough little trucks...they break and still run.......

  8. #8

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    The jet valves only act as a secondary inlet to the combustion chamber from the inlet manifold so it "shouldn't" cause blow by - unless something scary has happened like a crack between a jet valve gallery and an oil gallery has occurred (don't know if this is even possible...) 4G54 MCA-JET valve heads weren't sold here in Australia - and the few mechanics that have run into them do things like weld the combustion ends of the jet valves shut and cut the auxillary rocker arms off due to their lack of understanding of how they work. Their excuse usually goes something like "they burn out anyway..."

  9. #9

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    I got a new jet valve and jet valve plug kit finally.

    Going to replace the working new jet valve just to see if it fixes the issue.

    The jet valve was burned at the combusion end and not sealing the passage off.

    I'm going to wait and install the plugs at an engine rebuild I plan on doing soon.

    Still driving it and it runs fine just have to empty the catch bottle i made every time i do drive it.

    Wish me luck.

  10. #10



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    Quote Originally Posted by Sgt Mitsu View Post
    I got a new jet valve and jet valve plug kit finally.

    Going to replace the working new jet valve just to see if it fixes the issue.

    The jet valve was burned at the combusion end and not sealing the passage off.

    I'm going to wait and install the plugs at an engine rebuild I plan on doing soon.

    Still driving it and it runs fine just have to empty the catch bottle i made every time i do drive it.

    Wish me luck.

    how is the pcv valve?

    If your getting that much oil through to a catch can then your sucking oil from the valve cover I would think. Catch cans can also catch residual water in engine also. How is your catch can conected. Valve guide seals could be another cheaper fix if oil is seaping into chambers. Bottom line would be to do a compression check also. Check and make sure your compression isn't under spec too far between cylinders.

  11. #11



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    my bet is the mca jet. He figured out the problem on the first post. R&R them and the problems should go away.
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  12. #12

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    I removed all of the jet valve rocker arm adjusters effectively disabling the jet air valves and plugged up the one I found originally broken
    and no more oil blowing out the valve cover breather filter. Now it runs great and all is good. Have a new long block engine on the way and other new goodies to upgrade the suspension with Montera uppers so I can fit some new 31's or 32's.

    Some body work and new paint is in order now. I also have a new tranny and transfer case,stock, and the read dif w/ matching gear set.

    It's almost ready to pull my Gyrocopter to the airport.

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    just don't land on the WH lawn to make a statement - the Secret Service hates that...
    Pennyman1
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  14. #14

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    I'm wandering if I may have a similar issue. I just acquired a '87 4x4 with the 2.6L engine. It has an annoying tapping coming from under the valve cover at low RPM and intermittent blue smoke out the exhaust. Not sure if these two symptoms are related. I'm planning on doing a compression test this weekend and peek under the valve cover to see what can be seen. Did the engine exhaust intermittent blue smoke with your jet valve problem?

    Thanks for sharing your experience.

  15. #15

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    Quote Originally Posted by pennyman1 View Post
    just don't land on the WH lawn to make a statement - the Secret Service hates that...
    Funny thing is I know Doug Hughes personally since he hangs out at our clubs airport where he learned to fly gyro's. Go figure.

    I have NO plans to do this.......EVER!!!!

  16. #16

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    It is possible u r having a jet valve issue. they are adjustable so a check is in order.

    Also the valve guide seals if old and high milage will allow oil to seep into the cylinders and cause temporary blue smoke as it is burnt out then clears up. I had a similar experience once and that was my problem and solution.

    I replaced mine with viton type and have not had any issues for years. Hope this helps. Good Luck.

  17. #17

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    It is possible u r having a jet valve issue. they are adjustable so a check is in order.

    Also the valve guide seals if old and high mileage will allow oil to seep into the cylinders and cause temporary blue smoke as it is burnt out then clears up. I had a similar experience once and that was my problem and solution.

    I replaced mine with viton type and have not had any issues for years. Hope this helps. Good Luck.

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