VW engine failure on Take Off

[jamdfingr]

So I ran up the engine on the weekend after having changed out the oil, checked every passage, valve, spring, filter and pipe possible. The engine started up with no issues at all with good cylinder pressure. I let it warm up for a bit and then started some higher RPM runs (with it restrained on the ground) around 3000rpm. The temps started to come up on the oil slowly with a resulting drop in pressure in the oil. The CHTs came up but remained very low throughout the testing. I let the oil come up to about 90-100 degrees C when the resulting pressure drop was down to about 10 PSI. I used a temp gun to check the oil in the cooler and it was reading the same as the gauge. The cylinder heads were all reading fairly consistently around 150 degrees C and this was the same on the gauge.

So given this test was on the ground and tethered, I expected to get a hotter engine as the cooling flow through the plenum is lower than when flying. What I did notice however was that the oil temp kept climbing and the pressure continued to drop while the CHT's remained pretty stable.

So the issue seems to be lack of oil pressure. The OEM suggests a minimum 25 PSI is required and I was getting well below that. The Oil temp seems to be in range between 50C and 120C but the amount of drop in pressure is significant. It seems that even with the recommended oil in the system, that the oil pump cannot keep up the pressure once the oil warms up. I think I will need to pull the oil pump itself and inspect the gears to see if there has been wear on them as this just doesn't seem right. If that doesn't work, then I guess the only conclusion is that the bearing clearances have opened up enough to increase oil flow and reduce pressure which will require a total strip, assess and rebuild of the engine.

This might get expensive soon... (but still better than falling out of the sky).

Thanks for the other comments and suggestions too.

 

[M61A!]

There's an inexpensive 912 on Aviation buy swap sell page. Close to TBO but since you're experimental , you can run it on condition. The rotaxes just keep going.

 

[eschrom]

I think I will need to pull the oil pump itself...

Make sure the gasket under the cover is thin. I eliminated the gasket completely and used liquid gasket after verifying a few thousandths of an inch clearance between the gears and the cover.

My 2180 wouldn't make enough oil pressure and when I stripped it down I found the lifter bores had become bell mouthed. Still scratching my head about how that can happen. The case has been repaired with brass sleeves but I haven't reassembled it yet to confirm the problem has been fixed.

Ed

 

[DeadRinger]

Check the oil bypass plugs first way faster and needs to be done. http://www.headflowmasters.com/images/oil-valve-body/case.jpg
http://www.headflowmasters.com/images/oil-valve-body/step1.jpgIf you need the you have an issue
https://www2.cip1.com/v/vspfiles/photos/ACC-C10-7010-2.jpg?v-cache=1553085655
After that I’d check the pump oil passage alinement with the case whole by pilling the oil pump and measuring the case and comparing it to the pump.
After that is all checked good tear it sounds and look inside. It’s probably lifter boars like was said before.

 

[John B]

Have you checked the clearance between the pump gear and the pump cover. If too loose you will never get good pressure. If I remember right it is supposed to be 0.003". The oil pumps have 2 gaskets. One between the pump body and the engine case, and one between the cover and pump body. These gaskets are different thickness If they got switched around during install it will cause low oil pressure. Like mentioned above, if you have enough clearance, just use Aviation Permatex.

VW's when in the cars had low oil pressure to begin with and the oil pressure switch that lights the idiot light in the car comes on at a measly 3 PSI. What size pump gears do you have. Stock is 26mm oil pump gears. Also do you have a stamped metal cover, a cast cover, etc. I like the cast iron ground pump covers. Have had the stamped steel ones like came stock wind up warped. If the bearing/lifter/cam clearances are on the loose end of the allowable range on a VW you will have low oil pressure and will need a larger pump. Maybe 28mm or 30mm pump gears will bring the pressure up. The stock pump is a bit marginal in my opinion.

Also, check your ignition timing. If just a bit too far advanced it puts more pressure on the working bits and heats up the oil.

 

[Pops]

Oil pump gear to face of oil pump body is .0005. I measured about a dozen new oil pumps. The stock oil pump gasket for the face plate is .004. So that is a total of .0045 gap between the face of the gears to the inside surface of the face plate. Did the Bob Hoover mods to my oil system for more flow to the heads for extra cooling and lost 10 psi on the oil pressure. So I made a .002 thick gasket for the oil pump cover for a gap of .0025 and gained the 10 psi back for a hot oil pressure of 38-40 psi at cruise of 2650/2700 rpm and a hot idle of 20 psi at 950 rpm.

 

[John B]

Another thought. Are your pressure relief springs collapsed or too tight? The longer spring and plunger controls the oil cooler bypass, so that when cold it bypasses the cooler, and is near the prop end of the engine. The shorter spring and plunger are what controls overall system pressure. Many aftermarket kits for these have heavier or longer springs to increase system pressure. If you have one of these heavy springs in the relief valve by the prop, it will result in all of the oil going through the cooler. If the spring is too weak, it could be bypassing too much oil past the cooler resulting in higher temps.

On the other hand if your spring is too light on the other end of the engine, then it will dump oil at too low a pressure and cause low oil pressure when hot.

There are companies that make kits with heavier springs to raise oil pressure. Way back in the day I went this route on the shorter spring on a VW car I had that had low oil pressure. Didn't want to tear the engine down. It did work and raised oil pressure. I did not chance the bypass spring. I reasoned that I would rather have the oil directed to the cooler when the factory said and I didn't have an oil temp problem.

If you put a heavier spring in the flywheel end of the engine and the hot oil pressure goes up that will tell you that the pump/bearing/etc are ok and the pressure control valve was the culprit. I say this because if it is excessive clearances in bearings, cam bores, lifers, etc., then you could plug the control valve shut and hot oil pressure would not change.

 

[DeadRinger]

check this pic out.