VW engine failure on Take Off

[jamdfingr]

All, my apologies as I thought the thread on this had gone cold until I spotted the replies today.

So, lots of good areas of investigation and I will try and identify what I found from each to rule out some of the theories.

I managed to replicate the issue on the ground. Tail tied up, engine run up to 3200rpm and held at constant throttle and mixture. As the oil temp began to rise, the manifold pressure kept coming down along with the oil pressure. The CHT's stayed in the green the whole time so I doubt the heads were overheating. Then, at some point, the engine RPM begins to drop, the Manifold pressure drops and the CHT's begin to rise quickly. I bring the engine back to idle before any further damage can occur but it seems to only happen at WOT.

So I check for lean mixture and the plugs are wet and more towards being too rich. This is also evident by some of the soot on the outside of the cowling where the exhaust flows so lean is not the issue.

I checked the fuel vent for starvation but it is clear and there was only half a tank so vaccuum in the fuel doesnt seem to be the problem. There is a gascolator which is gravity fed, but again in using the electric fuel pump gave no change so doubt it is a fueling problem.

Checked the steel inlet manifolds and no obstructions to the air intake.

The oil being used was a 10W 40 and this was changed from a 15W50 which was reccomended until 50hrs which the engine achieved hence the switch. There was sufficient oil in the sump and has been changed now.

I pulled the oil bypass valve and it came out with minimal obstruction so doubting it was the restriction.

Am waiting for a borescope to be able to check the condition of the cylinders but everything in the valve heads seems fine with no evidence of heat or burnt oil there. Even the oil that came out did not have much in the way of metal particles or burning so while it got hot, I doubt it heated the engine. (Note, sustained WOT at that point was causing rising CHT and oil temps but not past their limits).

So I am still scratching my head on this one. Am going to check condition of the jugs, have already replaced oil and will probably give it another extensive ground run to see what is going on or at least replicate the fault with a video of the gauges.

Thanks for all the suggestions so far, I have worked through most of them and agree that until I know what I have got, I am not risking my life behind an unknown defect in the engine. Will see what else I can diagnose first and then will look at pull the cylinders off for a closer inspection of the rod bearings and crank.

Cheers,

Luke.

 

[jamdfingr]

Just thinking about it too, the manifold pressure and lubrication problems are separate problems, but if the lubrication affects the RPM of the motor (eg. loads up the engine as things tighten up) and the RPM falls with the throttle plate in the same position, this is effectively reducing the engines "vacuum" power and would result in falling manifold pressure.

I think my issue is a lubrication one translating into high oil temp and low pressures.

Cheers,

Luke

 

[tls258]

That's what I was thinking reading the symptoms. What brand oil? Synthetic? Oil filter? Oil pressure dropped how much? Could also be a piston->cylinder tolerance issue. When you're running WOT and the MP drops and the temps begin to spike, does the RPM remain constant or is the engine bogging?

 

[bluerocket]

I use to race formula Vee. Not the airplane the car. I had a problem like yours . of course it was on the ground. pulled to the side of the track. car cooled down drove back to the pitts turned out to be a coil getting hot, and breaking up. just a suggestion I don't even know if coils are used in VW airplane engines. I have only owned certified aircraft.

 

[Malovanyy]

Just thinking about it too, the manifold pressure and lubrication problems are separate problems, but if the lubrication affects the RPM of the motor (eg. loads up the engine as things tighten up) and the RPM falls with the throttle plate in the same position, this is effectively reducing the engines "vacuum" power and would result in falling manifold pressure.

I think my issue is a lubrication one translating into high oil temp and low pressures.

Cheers,

Luke

I am not even close to being an engine expert but your theory on the drop of manifold pressure is probably wrong. Check comments on previous page. The air supply system consists of a filter, hoses, throttle, manifold and ending with the intake valves. The pressure in the system before the throttle should be very near atmospheric pressure. Then you have a pressure drop at the throttle and your manifold pressure sensor is located after the throttle. When your throttle is at idle your engine struggles to suck in air through the throttle and you have a vacuum in the manifold. If you turn off the engine with the throttle closed your manifold pressure returns to the atmospheric. When the engine is running with WOT there is no obstruction in the air flow and your manifold pressure is close to atmospheric (a bit lower). The pressure would drop if you have ice buildup obstructing the airflow or stuck air filter. If your engine “reduces the vacuum power”, meaning sucking in less air due to other issues your MAP should rise because the pressure drop between the atmospheric and the MAP is proportional to air flow.
Good luck with troubleshooting!

 

[jamdfingr]

I think you are spot on with that and yes, I would be wrong in that instance. If no restriction and the RPMs dropping as the engine bogs down, the engine would be sucking less but not in a restricted way.

For the other responses, the aircraft is operating on a dual ignition with a Magneto for primary and Coils for the secondary. Unlikely to be related I think.

And for the other question, when the oil pressure drops and temp spikes, the engine starts to drop RPM and bogs down. This is all with the same throttle setting and mixture setting.

I managed to get the bore scope in last night and no evidence of damage in the cylinders, the exhaust valves looked in ok condition from removing the exhausts, the rocker area all looked ok when removed and as noted previously, the plugs were wet/black so more likely running rich than running lean.

Aside from pulling the head and cylinders off to remove a couple of con-rods and check the rod bearings, I am having trouble sourcing this fault. I will go back to the induction system from start to finish again and see if there is something I have missed. Will also do a fuel flow test to ensure I am getting the full flow.

Thanks again for all the constructive feedback and places I should be looking.

 

[tls258]

If you wanted to spend around $500, you can install a fuel flow meter that would show you what is happening to fuel flow during the event. If your flow remains constant or increases as your CHT is elevating and RPM is decreasing, then pretty good chance it's a friction-caused event and not a too-lean event, so that could eliminate fuel/air as an avenue of inquiry.

 

[Steve Stephenson]

Here's version two. I'll try to hook up the atv vent later. Right now I'll take some positive pressure at the top of the tank.

 

[jamdfingr]

So I checked the fuel flow last night and it was:
Gravity fed: 40L/hr
Electric Pump: 80L/hr
Hence, I don't think I have a fuel issue.

I re-checked through the intake manifold but there was no restrictions anywhere I could find. The fuel breathers are all clear and all of the manifold pipeworks are clear.

With having checked everything over and with an oil change back to the 15W 50 oil recommended for engine run in, I am going to try and run it up on the ground again to check the oil cooler function and see how the temp/pressure of the oil goes.

CHTs were never a problem and stayed low when it happened the first couple of times so am wondering if there is a blockage in the oil cooler somewhere preventing the cooling circuit from functioning properly. If so, that might just be a case of adding an external cooler with filter from a full flow pump. At least I can confirm the oil cooler working with a thermometer on the back of it first to ensure I am heading down the right pathway.

Will see what happens over the weekend with a test run.

 

[DeadRinger]

Man you probably have two or more issues there. Make sure you’re using oil with high zinc like brad pen or amsoil. Lots of vw guys (and aero vee sonex) like brad pen. In a pinch in a car I’d run VR1. No synthetics I’ve seen play well with the solid lifters. So at this point I’d pull your oil bypass valves and check for fitment. They can stick or not seal well dumping oil when hot ant not building pressure. Also oil pump seal/fitment can do that. Next intake heads and ignition system for your loss or power. Although loss of power could be a bearing sticking I’d also send the oil to Blackstone.

 

[moosepileit]

About fuel- this year's winter blend no ethanol mogas seems to have been a very off-norm RVP and more keen to allow vapor locks.

It is a spring issue as the weather heats up but folks still have winter blends.

By Sun n Fun someone runs into it.

Any chance you had this?

 

[Chucker]

...just asking again... Is the engine built with a new case? Or, has the case been align bored? Magnesium has memory and your case may be distorting once it gets up to temperature. The distortion will cause a loss of oil pressure and increased oil temperature. Lack of lubrication would cause the drag that is bogging it down.

 

[aztrooper]

I got vapor lock using mogas in an Aeronca Champ I had once from using auto gas that has alcohol in it. That would be an easy switch for you if that is the case. Go to a filling station that can show you they don't have alcohol in their fuel. Dump your old gas and use it in your car or lawn mower. If you can't find a filling station that can't certify it doen't have alcohol in it's fuel, use avgas with an additive.

 

[jamdfingr]

Sorry, it was a new case however previously owned by someone else so can’t be 100% they didn’t heat it up. As for problems like vapour lock and fuel, the gauges were pretty clear that the oil pressure dropped to a certain point before the engine bogged down. Didn’t get a chance to run up the engine this weekend with kids around the place but will try again soon.

Cheers

 

[tls258]

Even the oil that came out did not have much in the way of metal particles or burning

Luke-
What does "not much" mean when it comes to metal particles in the oil?

Here's an article on break-in oils and properties.
http://11hc.44rf.com/manuals/engine-prop/Break-in summary.pdf

 

[jamdfingr]

Thanks mate, was a good read.

By not much I mean nothing of significance to report. Pretty much nothing but a spec here or there.

 

[Flyingfool]

Could you temporarily remove the oil cooler at least for a test on the ground to rule out some sort of restriction within it?

I though carb ice as a possibility. But that would not cause a loss of oil pressure and increased temps. Especially if the oil pressure reduction is PRIOR to the RPM loss and engine bogging down.

 

[DeadRinger]

Common problem used to be sticking oil bypass plungers. If it was my motor I’d tear it down and triple check everything and wouldn’t stop till I found all the problems.

 

[Chucker]

Common problem used to be sticking oil bypass plungers. If it was my motor I’d tear it down and triple check everything and wouldn’t stop till I found all the problems.

A stuck plunger makes great sense. The cold oil would make sufficient pressure with the plunger open. As the oil warms and thins the open plunger would cause the pressure to drop. Low pressure would increase the temperature and further reduce the pressure rapidly. Friction would reduce RPM. The only thing not explained is the drop in manifold pressure.

It may be a little pricey (about $125 if I remember correctly), but there is a ball and spring replacement available for the plunger. There is no need to split the cases. You remove the plunger and spring, drive a sleeve up into the passage, and insert a steel ball and spring. If you are using a top mount oil cooler, it becomes a full-flow system....cold oil will not bypass the oil cooler. The benefit is more precise pressure regulation and no chance of sticking. You adjust your maximum oil pressure by trimming the spring.

 

[shocktrooper]

A stuck plunger makes great sense. The cold oil would make sufficient pressure with the plunger open. As the oil warms and thins the open plunger would cause the pressure to drop. Low pressure would increase the temperature and further reduce the pressure rapidly. Friction would reduce RPM. The only thing not explained is the drop in manifold pressure.

It may be a little pricey (about $125 if I remember correctly), but there is a ball and spring replacement available for the plunger. There is no need to split the cases. You remove the plunger and spring, drive a sleeve up into the passage, and insert a steel ball and spring. If you are using a top mount oil cooler, it becomes a full-flow system....cold oil will not bypass the oil cooler. The benefit is more precise pressure regulation and no chance of sticking. You adjust your maximum oil pressure by trimming the spring.

Perhaps the MAP is 'chasing the RPMs downward...? I usually found that if not easily ID'd, several small problems can combine to make one big problem ...especially concerning airplanes. A small, to incrementally larger; 'process of elimination' inspection would be industry standard. Keep that bird AOG status until you find and fix (all) of the 'flight safety' 'squawks'. I've seen (worn) magnetos reduce output or fail at high power to WOT...when was your mag last inspected/overhauled and tested? Maybe (intermittently and/or at WOT) part of your ignition chain is breaking? In the end it all has to work together and "timed just right".