Wild & Rapid Oil Pressure Fluctuations

I recently installed a panel mounted manual oil pressure gauge to back up the electric Rotax oil pressure sensor. In the process, I relocated the original sensor to the firewall to help improve the life of the sensor.

Everything worked great until the engine warmed up to operating temperature. Once the oil was hot, the pressure reading started fluctuating rapidly when viewed on the manual gauge. It was oscillating so rapidly that it made the gauge impossible to read. The Skyview also showed the pressure changes, but the fluctuations were dampened a lot, so it was not a big issue. I had observed the Skyview doing this in the prior 16 hours I have on the plane, but I chalked it up to "normal", thinking it was caused by sensor inaccuracies.

After spending a few frustrating days trouble shooting, I zeroed in on the oil pressure relief valve near the oil pump as the culprit. I inspected the spring and ball but found them to be in good shape. Some research on the Internet suggested that this was a known issue. In fact, I found out that Rotax sells an optional "piston valve" to replace the check ball to cure this very problem. Sure enough, I replaced the ball with the new piston valve and it cleared up the issue. Now the pressure readings are rock solid on both the Skyview and the manual gauge.

In case you guys run into similar issues, take a look at Rotax part number 857-230, "valve piston". It sells for a pricey sum of $30.00 (for what looks like a 25 cent part) but in this case I can say the investment was worth it.

After observing the wild and rapid fluctuations in the oil pressure, I have to wonder if it has been partially responsible for the early demise of the Rotax oil pressure senders that many have encountered.

Interesting that anyone would design a new engine with a poppet oil relief valve. The area on the plunger ( and thus the force on the spring) is different when open than when closed, so it can not find a happy home. At best hammering oil pressure, at worst, hammering metal too.

One of the first engine design issues I was assigned was a poppet that hammered the seat shed metal chunks. It had been replaced with a piston in a sleeve with a spiral of holes. The holes caused a side load and it scuffed. A little plotting and a weird, but successful hole pattern.

A piston style with an edge valving feature is the best. It is balanced and stable. They are sometimes call an "H" valve.

The old oil pressure plug screw was too short and allowed he pressure spring to rub on the sides of the casing. The round ball bearing also had some surface tension so it would make the oil pressure chatter. The new style plug screw has taller sides, it's the same spring, but should be replaced when changing out the old parts. The ball bearing is now replaced with what looks like a steel mushroom. It has a mushroom head with a hole drilled in it to stop any surface tension and a shank that goes down into the spring a ways to keep the spring straight and it also goes down into the plug screw. All this keeps the spring from rubbing and the pressure constant. Some people have noticed a 5 psi increase.

You can tell the old style plug screw because the oil pressure plug screw located down by the oil filter has a flat 17mm head on it. The new style has a recessed head. The old plug is gold in color and the new one aluminum colored.

All three new parts are in the neighborhood of $75 and is worth doing and only takes a few minutes to swap them out.

p.s.
Don't forget to safety wire the plug in place when done.

Sink,
I have a fairly new engine, purchased late in 2014. It had the new style plug screw, but came with the ball instead of the mushroom valve. That was the cause of my issue. Why do you suppose Rotax changed back to the ball?
Alex

Hi Alex,

That's a darn good question. A 2014 engine should have had the new style oil pressure reg. mushroom and not the ball. Sorry I don't have an answer for that.

I think you are on to something. I had similar wild oscillations on last flight and posted on a separate thread earlier. Thought engine mounted Honeywell had turned up its toes after 50 hrs. Ordered a new sensor and firewall relocation kit, about $500 all up. Saw your solution and decided to try mushroom valve before fitting new sensor. Started the engine and was initially disappointed - constant excursions into the red but still fluctuating. As oil warmed up gradually settled back, still fluctuating but only into caution zone. Once oil reached temperature, tried a runup - looked pretty normal, oscillating but in the green. Took off, no dramas. As oil got hot pressure settled rock solid on 57psi - never seen that before. Flew around for a while. Came back for touch and go - constant 57psi all around circuit regardless of revs. Only deviation was when go round power applied - slight blip but settled back to 57 in less than a second. Need to start again from cold to confirm (can't do until next week) but looks like a $29 part may have fixed the problem. Still question of wild oscillations while cold but wonder if there was air in system, although engine was burped before startup. I'll let you know what happens on next flight.

Jack Moore

Hi Jack,
My problem was a bit different than yours. I only observed the fluctuations once the oil warmed up. In other words, it occured when the relief valve was starting to shut. Its possible that you also have a bad sensor. Since you have a new one on hand, swap it out and see what happens. It only takes a few minutes.
Alex

Crazy oil pressures

I just had this same thing happen a while back and found that it was my oil pressure sender that was the problem. Afterwards I replaced the oil pressure gauge with one that has a pressure hose, I used copper to go through the firewall for fire safety reasons.

I had a previously reported momentary wild fluctuation on one run up. Maybe I'll invest in the new pressure regulator valve. 350 hours and my Honeywell sensor is hanging in there.

I have a newer engine and have noticed the same thing - oil pressure is pretty steady till 120+, then it chatters up/down - no more that 5-10 psi I think, just annoying to look at more than anything else.

Looking at my Savvy data it seems to fluctuate between 55-61, with the chattering around 2-3 psi within 58 psi generally.

John,

The sender and Skyview seem to dampen the real amount of the "chatter". I think if you hooked up a quality mechanical guage you would be surprized by how much the true chatter is. I was seeing around 20 lbs variance up and down, wildly oscillating between the two. It's still my opinion that these erratic fluctuations are partially responsible for the premature failures of the electroninc senders. It appears to be getting beat up pretty hard by the constant large variations in the oil pressure.

Anyway, just my thoughts above and no real facts to prove it. The replacement of the ball with the mushroom valve has solved my problem.

Alex

If you have oil pressure fluctuations I would make sure you have the newer style oil pressure plug, a new spring and an oil pressure regulator cone (mushroom) in place and not the old ball bearing. The old style with the ball bearing allows the spring to rub on the case sides and the ball bearing can have increased surface tension due to its design. The new mushroom and plug screw prevent this from happening.

The old oil pressure plug screw has a flat surface and the new style has a recessed head. If you are having oil pressure issues and you have eliminated the oil pressure sender I would change to the new style pressure parts. Actually I would change them anyway.

Just realised I never got around to providing the additional information alluded to in my last post. On the next flight I found that the Honeywell had in fact failed. It was rock solid on 57 psi - but all the time! I then fitted the new sensor with remote mounting kit. First flight it was still fluctuating fairly rapidly but only over a range of about 5 psi - much steadier than previously. Even though I had purged the remote oil hose, I think there must've been a bit of air in the system as it has gradually got better. It still varies a little but at a very slow rate. I might see 57 psi for 3-4 seconds and then it will move to 58. I'm convinced that the mushroom relief valve is a definite improvement. One of my mates has a Rotax in a different airframe, and he also found a similar improvement after switching to the new valve.