New Oil Pump, Same Low Pressure

[rebelyell]

Thanks for quick replies. The replacement oil pump is the stock Melling (M55 I believe.). I didn't go with higher pressure as the engine is older (so are the seals.). I didn't think it needed more but I also thought after 282k miles the original pump was starting to fail. Apparently wrong as the oil pressure was the same before and after.
I used to run the 10w30 that was spec but swapped years ago when 10w40 kind of replaced 10w30 on the market. It's had the Havoline Synthetic Blend in it for decades. I probably will put something thicker in if nothing else comes up. I ran 30HD in everything back in the 80s and 90s with no problems (deep South.)
There are two oil pressure sensors. The one that reads for the dash gauge is known in parts catalogs as the oil pressure switch, not sure why as it's not a switch. Easy to distinguish as it's the single wire. The double wire one behind the distributor is the oil pressure sensor. I learned years ago to make sure of the correct part as most that are claimed as correct are too large for the heat shield. Anyway I checked the pressure with a mechanical gauge. I wouldn't think much of this if it had happened gradually but it was one day to the next with the across the board pressure drop.
I didn't cut open the old filter as I didn't find anything at all in the pan or else where. I might dig it out of the trash can just to see. I've always used AC Delco filters with no issues. I did put a Fram on this truck once. I always lightly oil the seal before screwing them on. After 5k miles the Fram was stuck so bad I had to put a screwdriver through it to get it back off.

seems you replaced pump w/ New M55, but no change; indicating old pump Not at fault.
? But did you R&R screen & pickup with New as well ? Tube can have small crack/leak or not seated correct; either can suck air & cause low pressure. It's Very difficult for most DIY to adequately clean-dissolve old screen varnish; Melling has published their finding on how great that varnish effect is on reducing pressure. Also, if pan is bent-in/dented on bottom it restricts pickup flow.
FWIW, in early Nineties GM changed & upgraded all OE production sbc's pumps from M55 (5/8") to M155 (3/4") because both GM & Melling engineers determined a larger diameter tube helped low-to-mid rpm performance.

 

[Schurkey]

That other forum I talked about is now up 'n' running. Here's the link to the sending unit thread:
www.automotiveforums.com/vbulletin/showthread.php?t=1136624

Oil Pressure Sending Unit--TBI V-8 dash gauge - Automotive Forums .com Car Chat

Oil Pressure Sending Unit--TBI V-8 dash gauge C/K

www.automotiveforums.com

Doing some more on-line research, BWD S4320 seems to be the correct one. It has a silver case, and green paint around the end with the electrical connector.
http://weblink.carquest.com/acl/?mfg...rtNumber=S4320
The NAPA / Echlin crossover from that number is OP6729. AutoZone sells it as PS150.

in early Nineties GM changed & upgraded all OE production sbc's pumps from M55 (5/8") to M155 (3/4") because both GM & Melling engineers determined a larger diameter tube helped low-to-mid rpm performance.

They also redesigned the oil pump casting to be lighter, for "better fuel economy".

The new-design pumps are/were known for cracking the casting and falling off in heavy-duty use (high rpm, high vibration.)

I've lost track of whether that design defect has ever been corrected, and a quick search of the Melling web site wasn't helpful. There is this, from other sites, though.

Melling M55: Suitability Questions - Don Terrill’s Speed-Talk

Melling M55A pumps - Don Terrill’s Speed-Talk

And LOTS more if a guy does some searching.

 

[rebelyell]

They also redesigned the oil pump casting to be lighter, for "better fuel economy".

The new-design pumps are/were known for cracking the casting and falling off in heavy-duty use (high rpm, high vibration.)

I've lost track of whether that design defect has ever been corrected, and a quick search of the Melling web site wasn't helpful. There is this, from other sites, though.

Melling M55: Suitability Questions - Don Terrill’s Speed-Talk

Melling M55A pumps - Don Terrill’s Speed-Talk

And LOTS more if a guy does some searching.

There are many who've made an informed choice to Not run any HV pump; regardless if it does or doesn't mimic a BBC pump design. Either matter has been discussed ad nauseam.

OTOH, Ya don't suppose they had fixed that lightened but broken design failure long ago ? Things have changed (years ago) and that's No longer an issue. A mere blip on a big screen. Also, that same lite "broke neck failure" came years AFTER introduction of M155.
And, for clarity sake; M155 is both std vol & std press (but has a larger inlet). For those who feel the need, it's also available as a HV pump with that longer gearset.

But, if ya don't want a current M55 or M155, Melling offers several "select series" with a 3/4" pickup.
And, even for those who have yet to resolve psiA versus psiG ... No sir, fear not, the sky ain't falling.

 

[Supercharged111]

Friends don't let friends run Fram oil filters,

AC Delco filters are pretty much bottom feeder as well.

 

[Road Trip]

And LOTS more if a guy does some searching.

Schurkey, thanks for the links! After a brief dive down one of the links, I
was reading about Melling's new helical gear design. Supposedly less 'pressure ripple',
quicker priming, less spark scatter, improved distributor gear wear. I've spun plenty
of the straight cut gears with a speed handle, and after feeling these pulsations at 60 rpm,
I always wondered what these pulsations would contribute to spark scatter at 6000 rpm?

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Maybe a needless optimization, but I would think that the smoother the torque required to turn
the pump, the less wear through the 90° power turn at the cam/distributor shaft gearset?

If I'm fortunate enough to build another engine (before it becomes illegal/impossible to do so) ...I
will take a hard look at the oil pumps with the helical gearset.

Good stuff. So much more to learn than the time allotted. Gotta pick up the pace. :0)

 

[Hell Knight Hicks]

With low pressure after an oil pump change shes prolly wore out. Your tolorances being wider will let more oil out quicker. Getting a high volume pump may bring the pressure up but your still on borrowed time. I got another year out of a motor like that once.

TLDR: Shes wore out ol buddy. Rebuild or Retire.

 

[Oakr11]

I’ve rolled new rod and main bearings in some small blocks with lotsa miles on them with good success. Mostly seems to be the rod bearings that wear the the most.

 

[Road Trip]

I’ve rolled new rod and main bearings in some small blocks with lotsa miles on them with good success. Mostly seems to be the rod bearings that wear the the most.

I have done the same thing with positive results. If you do this while it's still quiet then this seems to be a safe bet.

But if someone has a SBC that already has a steady rod knock (the kind you can make come or go by
temporarily disabling the spark to that cylinder during troubleshooting), + metallic flakes in the oil, and
low oil pressure on the gauge? Not a good candidate, for no doubt the crank journal is now out of round,
too worn, or too rough a surface, and will just wear out the new bearing inserts in short order.

But again, if the oil pressure has dropped but the engine is still quiet, and just as importantly, doesn't
burn that much oil? Then I'd seriously consider a fresh set of main & rod bearings in addition to the
oil pump. It's a judgement thing, but with the current prices of everything else, if the bores, pistons, & rings
are still good, but I'm tired of fretting over iffy oil pressure, then tightening the clearances in the reciprocating
assembly and a fresh oil pump seems like a reasonable gamble for those of us on a finite budget.

YMMV --

 

[old_silverado]

Again, I appreciate the replies. Going to 20W50 gave a few more psi so it stays at 10 psi stopped at a light now while hot. That will do until I get around in a month or so to doing what I've wanted to. For 36 years I resisted doing any mods at all to this truck. I had other toys to satisfy that desire. Now i'm going to rip all hints of computer and sensors out and put in a fresh carb-fed, HEI-fired 350. A few mods and it will be almost as simple as my 62 plus have the proper horses the General should have put in these trucks to begin with.

 

[Schurkey]

Now i'm going to rip all hints of computer and sensors out and put in a fresh carb-fed, HEI-fired 350.

Well. That's a shame.