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Oil pump ?

3K views 9 replies 5 participants last post by  70gmcer 
#1 ·
I want to change my current oil pump because I believe it is defective. I have a Melling M55 oil pump now in my sbc and am thinking about going to a M55A pump. At a cold start, the pressure is about 60 psi at 2400 rpm and 30 psi at idle. After warm up, oil pressure drops to 0 psi at idle and around 15 at 2400 rpm. And yes, I have tried different gauges. This is a fresh rebuild, with all bearings replaced and all clearances checked. All within specs. After warm up, I can start to hear the lifters tap, but no bottom end noises are detected. Someone told me that the by-pass spring is bad in the pump. Is this possible? Also, the M55A pump is supposed to be standard volume and high pressure. How much oil pressure is added, using the M55A pump?

any feedback on this??

steve
 
#2 ·
Was your readings the same when you tried different guages? Yes it could be a weak by-pass spring in the pump. Did you install new cam bearings during the engine build? Most of your oil pressure comes from the cam bearings. I have seen fresh engines with low oil pressure caused because the new cam bearings were reamed too much upon installation. Also if your guage is electrical instead of mechanical you could have a bad oil pressure sending unit. Always check your oil pressure with a mechanical guage when you are troubleshooting oil pressure problems. Remove your sending unit and screw in a short hose with a mechanical guage srewed on the other end.
 
#4 ·
Well I would put another new pump on it and then if your pressure is too low.... then the guy at the machine shop might have went overboard when he reamed your new cam bearings. It happens...not often, but I have had it happen.
 
#5 ·
Oil pressure loss at the cam bearings is rarely a problem. Most guys don't know it, but clearances at the cam bearings can be as much as .004"to.005" clearance.

Modern cam bearings are built on-size. One does not need to "ream" cam bearings, especially in the SBC. Occasionally, they need to be fitted individually, but that's usually an alignment issue.

Did you check the rod and main bearing clearances when you assembled the engine?

I have seen more than a few funky Melling pumps lately. Some have had sticky relief valves and some have had the wrong pressure springs installed in them. Some have had too much gear end-play.

Check you bearing clearances. If they are in spec. (no more than .002" on the rods, .003" on the mains, take a good look at the pump.

An M55A will increase you cold pressure 25-30 pounds and the warm pressure will depend on bearing clearance and rpms.

tom
 
#6 ·
Thanks, tom.....my bearings clearances are within specs as you mentioned. So, it is possible to be in the pump. I keep hearing from others that new pumps aren't bad out of the box. To me, anything is possible. That's why I keep checking around for possible solutions before digging into the engine.

That being said, is it worth using the M55A pump that might make up for the low psi problem, if it is something other than the oil pump....

thanks again, steve
 
#7 ·
If there is something other than the pump causing the low pressure, then a pump won't help, high-pressure, high-volume or othewise. If the bearings clearances are good and there aren't any galley plugs leaking too much, then the problem is probably the pump.

I always take the new pumps apart and at least clean them thoroughly. Try this sometime: Take the cover off a new pump. Take a white shop or paper towel and dip in in thinner or carb cleaner then wipe the inside face of the cover. That dark grey or black stuff is METAL particles.

tom
 
#9 ·
Your engine assembler screwed up. Period. PERIOD.

Don't try to whitewash it.

Either the engine clearances are bad,
there is an internal pressure leak somewhere,
or he did not check the oil pump before installation.

Take it back and make him fix it.

And no, duct tape and WD-40 will not fix it.
 
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