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crankcase ventilation W116 W126 K jetronic

Started by Randys01, 29 May 2017, 03:55 AM

Randys01

Whilst pulling down a spare K jetronic assy off a 380, I came across a small revelation that I've never read or heard about.
Where the breather hose comes off the lh bank, it fits onto a cast alloy fitting that houses the idle speed screw and 2 other outlet ports plus the inlet idle manifold /cold start
injector etc.
Basically, inlet manifold vacuum purges the cam box/crankcase which is replenished via the hose on the top of the rh cam box at the rear that fits to the aircleaner..and so we have our PCV.

For some time on another project I have speculated how the K jetronic can function at low speed when the PCV is actually providing some "unmeasured 'air.
I discovered that there is the tiniest hole in the alloy fitting which generates enourmous vacuum but little volume..however, it was severely blocked ..no wonder as it is little more than a pin prick.
This blockage means the crankcase is not being purged.....further examination of the air mixture box,  revealed alarming build up of scale which in turn had jammed the little wheel on the end of the air flap fulcrum. This in turn leads to a lazy valve action in the fuel distributor. Further examination determined the valve is stuck in the fuel distributor. The whole area is covered in a nasty crystalline baked goo.  Whether I get the valve freed up is another story but it seems a classic example of a little oversight gives rise to big problems. 

Whether the D Benz Service Manual recognises the potential for this problem and reccos periodic cleaning of the pcv body is uncertain to me. I daresay it was not a foreseen service item but it sure as Hell warrants attention after 35 years..

A bit of background. It is unclear what caused the original blockage but the breather hose was full of nasty crusty shale go which blocked the little hole. Once blocked, there is no vacuum purging. Instead of the nasties being drawn thru the combustion process, the system almost reverses it self and the fumes and pressure build up  start to pump into the air cleaner which is in a modest state of vacuum. So the fumes are being drawn into the air mixture box where over a period of time they condense out on the coolest part being the fuel cooled distributor valve I reckon.
So go check yours and see that she's breathing properly. :)

rumb

'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

daantjie

That little hole is specifically mentioned in the service manual under the engine crankcase ventilation section.
Daniel
1977 450 SEL 6.9 - Astralsilber

Randys01

OK....as a service item it is covered?.    how 'bout that!    ;D   ..and of course everyone diligently attends to it! :P ??

I suspect there is something more sinister going on here with my fuel plunger. I managed to get it out and noticed it was a bit scored. Yet to confirm but I reckon she is a weeper and it is that weeping which has got cooked and caused the cruddy build up. Will test the FD in due course but for a unit supposed reconditioned, a scored plunger is a real disappointment.