Oil in intake manifold

arisLgr · Post by **arisLgr** » Thu Apr 07, 2022 9:55 am

I found some oil residue in the intake manifold right under the carburetor, and now I realize that my spark plugs latelly are black and dirty.
My M38 has the original dipstick metal tube, with the rubber hose throwing the crankcase fumes into the carb.
From the side valve cover I have the fording brass tubes etc connected to the intake manifold below the carb, but I keep the valve in the off position so I beleive no oil fumes go in the intake manifold from there.
Am I missing some additional crankase fumes exit on my setup?
Thanks
Aris [/img]

Xamon · Post by **Xamon** » Thu Apr 07, 2022 10:07 am

Sounds more like some blow by or a bad head gasket. Have you checked your coolant to make sure it is clean and has no oil in it?

Post by **wesk** » Thu Apr 07, 2022 12:33 pm

Is your PCV valve oriented in the correct direction and clean and free to operate as it is designed to operate?

TM 9-1804A offers a good description of the PCV valve and it's associated plumbing. Fig 4 and pages 109 thru 112. This is a time when the civilian jeep factory manual can also help.

Early M38 crankcase ventilation system.

Late M38 crankcase ventilation system.

And if you use google check these:

PCV Valve Question - G503 Military Vehicle Message Forumshttps://forums.g503.com › viewtopic
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4x4M38 · Post by **4x4M38** » Fri Apr 08, 2022 8:28 am

Keeping your fording valves closed will pressurize your crankcase unless I'm mistaken.

arisLgr · Post by **arisLgr** » Fri Apr 08, 2022 10:49 am

Thank you all, I did recently a complete cooling overhaul, no oil in water, will do a compression test when everything goes back on the engine because It’s stripped now and will confirm the PVC valve is properly oriented.

But as Brian noted, since I installed it, I have been operating with the brass shut-off valve always closed! Whow !!! This must be it and I have no excuse, only that when I installed the fording knob on the dash I did not attached it’s cables in the engine, otherwise I would have noted (hopefully) the shut off valve’s proper operation!

I have the M38 in operation since 1972 and back then I had no way out here how to make it right, so better late than never with the forum and all your help it is slowly getting more original as I always wanted!

Again thanks !!!!!

Eades · Post by **Eades** » Tue Apr 12, 2022 11:56 am

If you still have the underwater fording set up, note that the vacuum pump return line goes into the intake. I get oil past the diaphragm seal and it intermittently gets sucked into the intake. Stinky at times.

arisLgr · Post by **arisLgr** » Sun May 01, 2022 9:57 am

Eades wrote:If you still have the underwater fording set up, note that the vacuum pump return line goes into the intake. I get oil past the diaphragm seal and it intermittently gets sucked into the intake. Stinky at times.

Eades you EUREKA (meaning in Greek I -you- have found it) You are so right, I removed the vacuum bottom cup and the whole thing was filled to the brim with oil! Lots of oil was also on top of the diaphragm.

With all your help now I know that oil in my intake manifold probably came both from the vacuum pump and the oil breather hose into the air intake tube, because of the closed PVC valve. Never late to learn!!!

I am ready to modify my M38's fuel/vac pump as I have done with the same type pump on my '54 Mercury driver, after repeatedly having it's fuel diaphragm leaking. I have inserted an aluminium plate in the diaphragm’s space while disabling completely the pump's mechanical operation; the fuel goes through and the fuel/vacuum lines are for originality in place.
I rely only on my eclectic pump that properly primes the carb after long layoffs, while giving me audio assurance that I have fuel supplied.

In the M38 I can do the same both in the fuel and vacuum diaphragms and for the wipers, intake manifold vacuum will have to do, anyway I use it in the summer, open with the windshield down

How do you guys feel about this? Has anyone tried it?