Engine/Transmission (1994 - 1998) Oil leak but not when oil fill cap is off?

[CGomulka]

I've had an oil leak for a while coming from the front crank seal so I recently replaced my front crank seal and put on a crankshaft wear sleeve. This cut the leak by about 1/2 but was still leaking. Then one day I was filling it with a little oil and forgot to put the oil fill cap back on. Drove it around a while and noticed that the oil leak was pretty much gone. Any thoughts as to what is going on?

 

[bgilbert]

Your blowby tube might be restricted. All the crankcase pressure was pushing oil out of your crank seal. With the cap removed it vents big time there. As far as what would get caught in the blowby vent/tube, I'm not sure.

 

[BJMarshall]

Check your vent, but also check for a vacuum leak. Does the cruise work? Does the HVAC system change from defrost to vent slowly? These both work off vacuum.

 

[CGomulka]

thanks guys, both cruise and AC/vent changing works fine. Sounds like it could be a plugged blow by, I notice not much if anything drips out of it. Anyone know the ID of the tube and the type of tubing I should buy to just replace it?

 

[Vaughn MacKenzie]

I've heard of blowby tubes theoretically plugging up but I've never figured out how or why that could happen. The only thing I can figure is someone using the wrong oil (non diesel rated) or Pennzoil for years and years which leaves lots of crud. When I pulled mine apart at 386k it was perfectly free and clear. No buildup, zip, nada. Just crawl under your truck with it idling after it's warmed up and look for vapor venting out of it (blowby mist). Or put you finger on it gently and see if it builds pressure fairly rapidly.



What is MUCH more likely than a plugged blowby tube is a bad seal on your turbo. Turbo boost will blow past the seal and through your oil return line and pressurize your crankcase. Had that problem on the '96, swapping to a different turbo cured it.



Vaughn

 

[JAmos]

What is a good way to test for a bad seal in the turbo? I'm working on a pressure tester for boost leaks. Can I hook up the tester to the turbo inlet and remove the turbo oil drain? I'm also suspecting valve guides and I'm going to hook up the tester and remove the valve covers and see if there is any leakage.



Jordan

 

[CGomulka]

thanks Vaughn, any recommendations or links you are aware of that explain how to test for a bad oil seal on the turbo?