Oil Pump

·         Steve Bennett

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I've been investigating an oil pressure issue on my solid lifter 3300 (#556) and when I had the oil pump cover off to check for wear or other issues I've come across an anomaly. I was under the impression that the oil pump internals were two gears of equal thickness, with the difference between the 2200 & 3300 pumps being the thickness of the said gears,

What I have is a single inner gear and two identical outer gears that together are the same thickness as the inner gear. I've not measured the gears but the total thickness appears to be 20mm as per the 3300 pump, meaning the two outer gears are 10mm each. At first I thought someone had bodged it using two 2200 outer gears but I have since realised they are 14mm so that's clearly not the case. I can't seem to find any reference to an earlier version of the 3300 pump and I'm questioning whether this set up would contribute to getting lowering oil pressure on idle when the oil temps are in our normal operating zone (85-98°c)

Any thoughts?

Steve

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·         Doug Smith

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Hi Steve,
I'm told by the engine shop gurus that early 3300 engines had the two piece outer oil pump up to approximately S/No. 33A697 and that the 2-piece outer shouldn’t have any effect on the oil pressure provided the clearance measures OK

Section 5.10.4 of the engine Overhaul Manual (JEM0001-8) shows how to check the pump gear clearances and that would have been my third suggestion for things to check (you've probably already looked at the first two which I'll put below). Just be careful that you follow the procedure in Section 7.8.17 of the Overhaul Manual when you reassemble the pump housing as it's possible to get a misalignment there that can eventually cause a crack in the end of the cam.

My first suggestion for things to check would be the relief valve: the piston has to seat properly onto the washer or you'll have issues. It's worth checking that the piston is made from steel and that it's seating properly onto the washer - check the seal face for damage/wear.

The second step would have been to confirm the accuracy of the oil pressure reading - failures (or blockages) of the oil pressure sensor are not unheard of and the gauges can sometimes go too - we have a mechanical gauge which we can plug into one of the spare ports at the front of the engine as a check on the regular gauge.

The way the issue presented itself might also give a clue as to what happened - the troubleshooting might vary depending on if it happened suddenly or slowly. How many hours in service has the engine?

Regards,
Doug.

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·         Steve Bennett

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Thanks for checking that for me.

I think the problem was the relief valve as after giving it a clean the oil pressure seems ok.

The problem was a slowly lowering oil pressure in cruise as the temp increased, although the temp never increased above normal. Our usual pressure tended to be 40 - 45psi but when we were experiencing a problem it would slowly lower toward the lower limit of 32psi. I do not know if it would have continued below as we tended to be back on the ground by then. At this point the idle pressure was well below the idle limit of 12psi, sometimes as low as 4 or 5psi. Once the oil temp dropped under 70°c the pressure would increase.

We had fitted a TOCA at the last oil & filter change approximately 3hrs before the issue started to occur, and it seemed to come on quite suddenly on final approach with the pressure dropping lower than expected. It was on subsequent test flights that the slow drop in pressure at cruise as temp increased was noticed. Obviously the first thought was to suspect the TOCA and new hoses so they were removed from the oil circuit with the filter attached direct to the engine block and the symptoms were still there. As we wanted to limit flight time we cleaned the relief valve, checked the pump and changed the filter (again),before testing again and something seems to have sorted the problem. The idle pressure (850rpm) is still under 12psi, at around 9 or 10 when the temp is in the 90's which I'd like to see higher but I'm unsure if that is how it has always been as we've never micro analysed the pressure at that particular point before as it would be just after landing. Idle pressure before take off, even with a hot engine is nearer 20psi, but the oil temp will have cooled. The pressure sender has been changed as well, just in case.

The engine has a total of 860 hours and 220hrs from having a top end rebuild just before my group became the owners. I need to get a couple of hours under my belt before I'm sure the problem is sorted but at the moment it's looking ok.

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·         Doug Smith

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No problem.
TOCA - that's a new one to me. From what you've said it sounds like a thermostat? Sometimes they've got a bypass which can affect the pressure a little. The other thing I've seen is if the fit of the oil filter adaptor (or whatever is being used there) to the oil filter thread/spigot is not right you can get some leakage which can give you some strange effects.
Anyhow, hope it behaves for you now.

Doug.

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·         Steve Bennett

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Yeah sorry, I forgot that some acronyms are not international 🙂

It's a Thermostatic Oil Cooler Adaptor, that doesn't send oil to the cooler until the oil temp gets to 80degrees. It's an official mod for the UK that helps keep the temp good in the colder months.

Cheers

Steve

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·         Doug Smith

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Ok, makes sense. I ran an automotive one on my (kit) plane a few years back but ended up removing it. I can certainly see the use in colder climates, particularly to get the oil temp up to the operating limit before first take-off.
Regards,
Doug.

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·         Rob Turk

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Steve Bennett wrote: I've been investigating an oil pressure issue on my solid lifter 3300 (#556) and when I had the oil pump cover off to check for wear or other issues I've come across an anomaly. I was under the impression that the oil pump internals were two gears of equal thickness, with the difference between the 2200 & 3300 pumps being the thickness of the said gears,

Steve

Hi Steve,
My engine is right in your range, I have S/N 551. I too have the two 10mm outer pump gears and was puzzled at first. And I too had low oil pressure when hot and idling. Roger Lewis in the UK has updated my engine with new through bolts, and while at it he replaced the original high-leak bearings for the current type. He also was meticulous in re-sealing between the case halves. This has really helped my hot-and-idle pressure, it is now well in the green.

On the bright side, the engine had been like yours for 200 hours and showed no internal damage whatsoever from that low oil pressure.

Rob

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·         Steve Bennett

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Cheers Rob