Do you mean blow back or blow by? You can do a search on blow by... you're asking many questions I have recently, so if you follow some of the late responses you'll come up with from your search, you'll probably uncover some more astute, concise, and precise answers than I could give... I'm still learning.

 

You must press the accelerator while you turn the knob to take the tension off the cable.

Blow-by is the amount of air and combustion gasses that get past the piston rings. ALL engines have blow-by, even a 2008 car with 0 miles at the dealership. There is nothing you can do about it except rebuild the engine if blow-by is excessive. The only time to worry about it is when looking at a potential car purchase or if you have a major engine problem.

If your engine runs well and does not consume (not leak) alot of oil, then knowing how much blow-by your engine has is pointless except as a curiosity.

 

blow-by



I was not aware that you had to press the accelerator while turning the idle knob to release tension. Good to know. thanks Zedd

 

Isnt blow back when the egnine rotates in reverse? I know this phenom happens on <240 engines.

 

You can check blowby by obstructing breather hose while engine is running(the hose that goes from from engine valve cover to top of air filter housing). You can use vise grip on the rubber part of the hose to obstruct it. The rule of thumb is that if engine keeps running 15 sec. or more before it dies, you have very little blowby (which is very good). Something between 15 and 8 sec is OK, and anything less than 8 sec means you have significant blowby and perhaps loss of compression. The engine may require new compression rings or complete rebuilding with new pistons, sleeves and crankshaft bearings.

Quick method for assessing blowby is to unscrew oil feeler cap and just place it over oil feeler. observe its behavior with running engine. If if moves barely - low blowby, if it shoots up - high blowby.

 

Never obstruct the breather tube. It's a very harmful test for any engine as it's a good way to blow out oil gaskets!

If you want to know blowby then get the engine warm, remove the oil cap and note the intensity of the fumes coming out.

 

I'll second this. The analogy is....annurism.

Never obstruct the breather tube. It's a very harmful test for any engine as it's a good way to blow out oil gaskets!

If you want to know blowby then get the engine warm, remove the oil cap and note the intensity of the fumes coming out.

 

I made a video for another forum, but it will apply to this thread as well. YouTube - W123 idle speed calbe

 

I've found that you don't need to press the accelerator to notice a difference, but I do anyway because it puts less strain on the plastic piece at the end of the adjuster cable. check to make sure that plastic piece is there and it's moving the metal bracket. that piece busted on my car soon after i got it so that cable was moving but it wasn't attached to anything so it didn't affect the idle.

 

Thank you! It is refreshing to know there is a real following and love for the w123 chassis especially the 300 and 240 series. I owned several audis (coupe quattro, 90) and VW's ('06 Passat, '97 Jetta GT, '82 Rabbit, '63 Kombi) and belonged to several forums per these other nameplates. To be honest I never did receive the same degree of valuable technical information, kind words, or support from the VW or Audi forums. The VW and Audi forums that I belonged to consisted of arrogant, and sometimes smug members.

I also did not enjoy the other vehicles as much as my slow but beatiful 240d.

Thanks again! I am grateful for everyone's help and kind words!