Rotax 912 oil pressure fluctuations

[raizo]

I am experiencing very erratic oil pressure readings on my 912,it is fitted with the new style sender,and have fitted the new mushroom valve and spring, the pressure gauge is an MGL TP1 dual oil pressure and temperature type gauge. at low oil temperature from start up to app 55 degrees the pressure is fairly steady but once it has reached app.70 + the readings become VERY erratic,impossible to read. gauge is directly earthed to engine block ,oil temp. reading is steady .I think it may be an earth loop problem but the change from reasonable reading when cold to very erratic when hot is throwing me.

 

Any suggestions would be appreciated. I have run this by the guys at Bert Flood and they are scratching their heads also.also MGL just tell me to check the earth which i have done

 

Cheers John

 

 

[cscotthendry]

John:

 

This is just a guess but...

 

Try fitting a new oil filter. The oil filter has a temperature activated bypass in it which is why Rotax say don't go over a certain RPM until the oil reaches 50C. It's possible that the valve mechanism is defective Or something is jammed in it. Unlikely, yes I know, but an oil filter only costs about $25 so it's cheap and easy to try.

 

Edit:

 

If this fixes the problem, cut open the problem oil filter and have a REALLY good look for anything that may have jammed the bypass valve. It may be chips coming off the PSU gears and that's very serious stuff!

 

 

[raizo]

Thanks Scott,this has been a problem since the motor was new ,and has had 3 oil fliters ,i have a gentlemen coming this afternoon to check the electrical side of things ,as Matt fromm MGL seems to think this is where the problem is,

 

Cheers John

 

 

[skippydiesel]

Hmmmm - my 2000, 912 (100hp) operating on 98 RON Mogas has 720hrs. Very recently (after the last service) I noticed a small drop in operating oil pressure. So small it could be my imagination, but its consistent. Start up (cold) pressure is as normal, dropping as temperatures rise - all expected. Engine oil and coolant temperatures all normal. I have noticed the occasional rapid fluctuation of the needle and an occasional drop (below normal) as RPM rises for takeoff, followed by a return to near normal pressure readings. Any cause for concern ?

 

 

[geoffreywh]

do you have the Honeywell oil pressure sender with the Avia gauge? That seems to be the duck's guts as far as oil pressure goes.

 

 

[skippydiesel]

Hi Geoff - Will check out the sender, next visit to aircraft - gauge definitely not Avia (think Speco). With exception of calibrated "Master Gauges" I have always understood that you read the gauges as fitted as indicative only and be alert to changes/deviations from normal/expected readings. I have had the privilege of owning my aircraft for about 7 years and am the third owner. I am fairly sure sender & gauges are as fitted in 2000 by original owner/builder, so are 16 years old. It could just be the sender and/or gauge are a but tired or it could be the beginnings of something more concerning.

 

 

[Downunder]

I think in 2000 they were still fitted with the VDO senders with the brass anti vibration ring?

 

If it has lasted this long, you're doing well.......NOTE* remotely mounted senders lasted far longer due to the reduced vibration which was the killer.

 

Rotax then moved to the Honeywell senders which still didn't fully cure the vibration problems.

 

Honeywell ceased production and now Keller sender units are used.

 

I don't even want to think about how much Rotax would be asking for one of these latest senders. It more than likely involves the gifting of you're first born and thinking of an outrageously expensive figure, then quadrupling it.

 

 

[Ayecapt]

I am experiencing very erratic oil pressure readings on my 912,it is fitted with the new style sender,and have fitted the new mushroom valve and spring, the pressure gauge is an MGL TP1 dual oil pressure and temperature type gauge. at low oil temperature from start up to app 55 degrees the pressure is fairly steady but once it has reached app.70 + the readings become VERY erratic,impossible to read. gauge is directly earthed to engine block ,oil temp. reading is steady .I think it may be an earth loop problem but the change from reasonable reading when cold to very erratic when hot is throwing me.Any suggestions would be appreciated. I have run this by the guys at Bert Flood and they are scratching their heads also.also MGL just tell me to check the earth which i have done

Cheers John

Just a quick thought i had a 912 with very erratic gauge operation during engine cranking. Not exactly your prob but... It turned out to be a very poor earth connection to the engine mount. (The big cable)

 

The frame where the lug was bolted had not been cleaned of paint and the nut and bolt was loosening, so it was a lousy connection . Fixing this restored everything to normal.

 

 

[skippydiesel]

I have long recognised that 12 volt automotive systems are prone to earth return problems, so much so that I have fitted my 912 with additional earthing cables to ensure good continuity . I have also done this with all my older (pre computer) vehicles. The small additional cost has resulted in more efficient electrical & longer component operational life. Lights shine brighter, globes contacts/switches & batteries last longer. In short I don't think I have a earthing problem (but will check anyhow).

 

 

[Kyle Communications]

I have just had a similar issue although mine is the honeywell 4-20ma sensor. The oil pressure reading on the MGL always fluctuated a little bit as it s the way it is read also the sensitivity of the ADC in the MGL. My engine is 2011 and has 170 hrs the oil pressure started bouncing between 55 and 20 psi when I was taxiing to take off so I turned around to investigate. I deemed that the oil pressure sender had become erratic so I have done what another Sav owner has done recently when his older VDO style sender when tits up as well. We have both remote mounted the sender with some pressure pipe. I will take some photos and post them. i have gone back to the older VDO style and set the MGL accordingly. I ran the engine on sunday and all seems fine now. The earth is simply done and it is working fine but earthing in these style of sensors is pretty critical for long term accuracy.

 

The say the engine vibrations are what wreck the sender units. Danny's was only 100 hrs old and the older VDO and mine the later honeywell electronic version and 170 hrs...so go figure. The remote mounting should take most of that vibration now out of the mix

 

Mark

 

 

[IBob]

Consider the possibilities. It is:

 

a. The oil pressure fluctuating.

 

b. The sender.

 

c. The wiring.

 

d. The gauge.

 

e. Or a combination of those.

 

I would be getting a manual gauge onto the engine to check for oil pressure flutter, and if so, how much.

 

I would then be getting a meter onto the wiring to see if the signal from the sender reflected what the gauge was saying. To measure mA you have to get your meter in series with the circuit, however initially I would just look at the voltage: it's simpler and will reflect noise or fluctuations just as the mA will.

 

You now know if you have oil pressure flutter and if so approx how much. And you know if your sender is reflecting the oil pressure.

 

If the sender signal is steady, then clearly the gauge should read it correctly.

 

If the sender signal is not steady, it gets messier:

 

Your digital gauge is not reading continuously, but sampling the signal at a set rate (the MGL spec doesn't say what that rate is). If there is some flutter in the pressure, it will be getting a rapid series of different readings, which it will display. It doesn't take much flutter in some instruments for the readout to be effectively meaningless, as new values constantly overwrite the old.

 

Some instruments offer adjustable sample rates and filter settings to help overcome these sort of problems. I don't see those in the MGL setups.

 

Finally, the reason your temperature reading is stable is that temperature where you are reading it just cannot change very fast. So the temperature sender will be sending a steady or slowly changing signal.

 

 

[skippydiesel]

All the excellent advice has focused on the sender, gauge and integrity of the wiring - no one has commented on the idea, promoted by John, that the oil filter may be causing the problem. For myself I note that my aircraft's change in oil pressure reading seems to have occurred/first noticed, after an oil/filter change. My oil filters are supplied by Floods, the oil is AiroShell Sport PLUS 4 - all by the book. If the filters are known to occasionally malfunction in this way, it's a fare easier/cheaper fix than anything to do with the oil pressure read system. Anyone care to comment!

 

 

[IBob]

Sure.

 

Stick a gauge on it and you'll know if you have fluctuating oil pressure. I prefer to know...

 

 

[facthunter]

I can't see how the filter can fail. If it blocks (usually after a looong time) it will bypass and you still have (unfiltered ) oil. (at a slightly lower pressure) Some poor quality ones have had the element disintegrate due water in the oil, but the paper is usually treated to stop that happening. Nev

 

 

[IBob]

There was mention somewhere of an overpressure bypass valve in the filter, though I've no idea if this is the case?

 

 

[facthunter]

It's a pretty universal feature. It's just a spring holding the cartridge against the case. If it's blocked it lifts off the seal and bypasses. It can happen if you rev a cold engine with thick oil . Some have an anti drain flap valve too. Nev

 

 

[skippydiesel]

My apologies, it was Scott, not John, who put forward the oil filter idea. Anyhow - in my long life, playing with all sorts of machines/equipment I have come to the conclusion that we humans are most often attracted to the complex/costly solution to any given problem. We should (but usually don't) adhere to the KISS principal and start any diagnostic effort with the simplest (often least costly) fix. "If- it -don't- work" move to the next level and so on. Facthunter's comment would be my thinking on oil filter failure, that is this is UNLIKELY sceneriao. However the idea has some merit as its cheap and easy enough to be attractive IF there has been a history of failures in recently installed filters causing oil pressure reading anomalies. I think I might just check this out with Flood's.

 

IBob - I don't have a calibrated master oil pressure gauge. The rapidly fluctuating needle is not a regular (predictable) occurrence so it will be pure chance that I get any useful information on this but the very slight drop in oil pressure reading concern (still within Rotax specifications) may be put to rest by using such a gauge - will see if I can borrow a suitable master gauge.

 

 

[IBob]

Hi Skippy.

 

I'm one huge fan of the KISS principle.

 

I also mostly troubleshoot rather than shotgun (swap parts until the problem vanishes) because that is my background.

 

I'm also prepping and painting right now, so the fumes may have something to do with my periodic outbursts!

 

 

[IBob]

Ah right...and yes, I am confused:

 

this thread began with a report of erratic pressure readings from raizo,

 

but your query, skippy, is with regard to a recent small change in your normal readings.

 

And we have comments about both these situations, intermixed..

 

Or I do.........might be time for as lie down!

 

 

[skippydiesel]

IBob - I stray badly and am probably far to late but I had good results with the Stewart (water born) paint system - not much chance of the fumes getting to you.

 

You are correct the subject has migrated somewhat but all comments still relate in some way to anomalous oil pressure readings - so probably all valid.

 

Have written to Floods - will pass on any pertinent comments from the oracle

 

 

[geoffreywh]

The diaphragm type of pressure sender are notoriously unreliable, especially if mounted on the engine. Personally i have a full scale mechanical gauge that reads in individual pounds 0-60. So ANY change is immediately visable. BTW I believe the faulty filter therory is just wishful thinking. AS other people have written Just fit a mechanical gauge and all will be revealed....

 

 

[facthunter]

I agree with the avoid complexity concept. The older indications were just a capillary tube (fine Bore) with a bourdon tube that straightens out a bit under pressure, and you magnify the movement on to a needle. Electrical senders appear to be among the most unreliable pressure sensors about, in my experience. Oil pressure is nice to have, so would a reliable indicator be also. Nev

 

 

[skippydiesel]

Conversation is straying a bit - no matter - I have always been told that all but calibrated master gauges, are suspect. Further they should only be used to determine whats is a "normal reading" for that engine & gauge combination. For example:

 

Rotax specify a hot engine oil pressure normal range between 22(1.5 bar) - 73 psi (5.0 bar) @ 3500 rpm.

 

When I first acquired my aircraft, about 7 years and 300 plus hours ago, I was concerned by the apparent low oil pressure reading of 30 psi on my Speco (cheap) gauge at between 3000 - 5000 rpm.

 

I contacted B Flood (BF) - they reassured me that all was normal & within acceptable limits.

 

Without a calibrated oil pressure gauge to check the actual pressure at various rpm, I accepted the BF reassurance and have monitored my, as is, cheapo, oil pressure gauge aver since.

 

All was well (no deviation from the indicated 30 psi) in the intervening years and hours of flight until after the last oil change.

 

I noticed a very slight, 1-2 psi, drop in operating pressure (This is a small faced/scale/electrical gauge with increments of 10 psi). Still within Rotax limits, a very small change, on an inaccurate gauge, but I have been trained to view any deviation from "normal" with suspicion.

 

This is the value of your typical engine gauge - giving an indication of a deviation from what constitutes normal for that gauge/engine combination - not an absolute accurate reading.

 

No offence intended Geoff, unless you have your undoubtedly more accurate, small increment, larger faced, capillary tube, gauge regularly calibrate (Borden tubes and multiplier mechanisms ware) is unlikely to give much more usable information than my el cheapo.

 

HOT NEWS - BF have just got back to me with some thoughts on my oil pressure changes. The conversation continues but the stand out idea at the moment is the change from AeroShell Oil PLUS 4 (Black Bottle) to AeroShell Oil PLUS 4 (Red Bottle). Apparently the formulation has changed and this may cause a small drop in indicated oil pressure.

 

 

[facthunter]

Bit like economics. In the black is OK but the red is worse for you. (Or whoever lent you the money). Nev

 

 

[Downunder]

AeroShell Oil PLUS 4 (Red Bottle)

 

Rotax-Owner.com - Shell Aviation introduces the new AeroShell Oil Sport Plus 4, developed in collaboration with BRP-Rotax