Second seized engine -- bad oil pump??

langted

This is the second time the 295 cc MCI Robin engine seized. (See previous string February) Happened both times after a slow run in the following sequence (1st for me, then daughter in law):

1) Engine runs fine and gets warmed up
2) Park cart for 5-10 minutes
3) Start cart without revving engine and drive just fast enough to engage torque converter
4) Stop cart for an hour. Will be seized on return.

It ran fine all Summer after the 1st time it seized, but with stepson driving and he usually steps on it and goes fast. Upon disassembly I found the rods are trashed again. The oil pump and case look completely pristine. Can you tell whether a pump is worn by looking at it?

History:

last Spring, I found a single seized connecting rod big end on the drive side of the engine -- pretty clearly due to lubrication failure. It was my stepsons cart, but failed last Fall while I was using it so I got involved. The oil was thickened and very black. There was SOME blockage of the screen, though I didn't think it was serious. I found no blockage in any passage in the cases or crank. After replacing the rod, polishing the crankpin, and reassembly, I installed a pressure switch (none originally installed) but of course my stepson did not hook it up, nor did he measure oil pressure as I requested. He DID run the engine and look in the valve cover oil fill hole. He reported oil was shooting "all over" inside. The second failure was with daughter-in-law driving slowly as I did last year. This time, both rods are affected, but the same one (drive side) is again the one that seized.

This time, the damage looks a little more chronic, but the crankshaft has less aluminum transferred to it. Oil and screen were very clean, and the oil pump and case still look "perfect". I am wondering if there is an excessive clearance in the pump that causes it to be unable to prime itself at low speed if already hot. If so, can I get by with just the pump parts ($50) or do I need a new aluminum crankcase cover too ($250) ??

Thanks
Ted

(Why is this oil pump way above the oil, rather than bathed in it as God intended?)

BGW

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langted

Added note:

I just noticed a little reflection in the oil sheen of the oil pump sheet metal cover and it is bulged ever so slightly outward. The bulge measures 0.014" with a feeler gage slipped under a straight edge. I am guessing this would cause a bypass around the pump and give it less suction as well as reduced discharge pressure. Perhaps when the oil was old and thick last year the pump overpressurized it; maybe the relief was stuck before cleaning as well.

Can anyone confirm that the cover should be perfectly flat?

racin161

If the vanes in the pump are slightly below the mating surface of the pump housing I would say YES the cover plate needs to be perfectly flat bulged would cause pressure loss.

LOU

redcloud

Yes flat is what it has to be for tolerances to give good oil pressure.
You found it.

langted

Thanks, Lou and Redcloud. This sounds like an easy and important check whenever an oil pump cover is removed. If it happened to me, it has and will happen again to someone else, and it was not obvious just by looking at it. I wonder if this is the cause of some of the other posts where pump performance was suspect. It would probably be a variable failure -- marginal performance loss to severe loss depending on the extent of the "bulge".

I am tempted to just tap it flat with a hammer, but I'm afraid it would improve to less than 100%. Alternatively, it would be a cinch to make new from 1/8" plate steel which would be many times stiffer.

Ted

racin161

Ted you may be on the right track making your own but I would advise after fitting it try to find a machine shop with a surface grinder and have it ground flat. Shouldn't be expensive as it's just a magnetic base that holds the part as it's surfacing the top side then it's FLAT. What would be known as a machined surface.


LOU

racin161

OPPS also take a close look at the vanes make sure the is no damage even what appears to be scratches is a sign of wear. When all is well pack the pump with vasoline to give it a prime before the first startup.

LOU

langted

Thanks Lou. Good points. I bet my repaired pump will be the best on the block!

It appears a few folks have looked at this, so I am attaching two pictures, one with the pump parts and "un-enhanced" cover so you can see that the cover superficially looks good unless you look really close. Another with a straight edge and light to show the bulge. I don't see any adverse marks on the pump parts, same in the aluminum case (not shown).

Ted

Attached Images

	295cc robin oil pump parts.JPG (136.1 KB, 0 views)
	295cc robin oil pump cover bulge.JPG (98.0 KB, 0 views)

simple man

Strange that I see no wear marks on the plate at all! I would think that you should. I would also check the gear clearances with a feeler gauge too. If they're not real expensive, just replace them. Your idea of making a thicker plate sounds like a winner to me too. If possible, try and hook a pressure gauge up when you get her going again! That will give you a true pressure reading with no guessing!

langted

Its hard to see on the photo, but there IS a small wear arc on the plate near the top. The "gears" are about $50, but there is just so little apparent wear, even looking at them with magnification, that I can't see any point in replacement unless I still don't get good pressure. However, this time the pressure switch will be hooked up to kill the engine if theres no pressure and I will also install a gauge on the dash. So if the pump pressure is the least suspect, it should not cause damage.

I'm looking at regular auto parts stores and i'm thinking I'll go to an electric gauge to save the risk of a tubing leak. It looks like about $75 at O'reilly's for a 0-80 psi gauge.