Hello everyone, I have a curious oil pressure issue on my 1969 427 with new Melling M-77 oil pump (after an engine rebuild). When cold, the oil pressure is just great. After I let the engine warm up and kick down the choke, idle oil pressure is between 15-20 psi (as seen on my aftermarket pressure gauge). From an idle, when I gently rev up the engine, oil pressure increase at a nice rate, right up to about 72 psi at 2000 RPM. Then right about 2200 RPM, the pressure relief valve opens and pressure drops to 50 psi. I continue revving up to 3000 RPM and the pressure increases back to 72 psi. I then allow the engine to slow back down and right at 2000 RPM, pressure is about 40 psi. I continue to drop RPM to about 1500 where pressure is quite low. From there, revving it back to 2000 RPM, pressure is only increases to about 35 psi. I the drop it back to idle, rev it up, and pressure is great once again. I completed this cylce a few times and it is very repeatable. It acts like once the pressure relief valve opens, it then doesn't close until I drop the RPM back to about idle. Could this be the case? Should I remove the oil pump and inspect the innards, maybe even get a replacment pump? Thank you all for your input.

 

exactly which brand and part number Oil Filter ?
what weight oil ?

some filters have an integral bypass valve, OE filter adapter does as well.

 

Rebelyell, I'm running Mobil 1 5W-30 with filter STP S8873. According to ChatGPT, this filter does have an internal bypass valve. However, I hope to learn from you "real" people

 

First reference I rely upon is filter manufacturer, not a sales outlet, not AI.

as indicated, maybe an issue with either filter bypass or adapter bypass.

easiest step is choose another filter, but one with no bypass.
all while closely monitoring and noting behavior.

 

I gotcha, so perhaps the filter bypass (if indeed the STP has one) or adaptor bypass could be opening and then not closing when it should.

 

An Oil pressure relief is just a Spring and a Valve. It is set for a certain pressure depending on the Spring. Do You remember what Color the Spring in the Pump was? The Relief doesn't care what RPM the Engine is, just pressure. A Relief inside a screw on filter? If there is such a thing it would only open with a differential in pressure within the element. In other words when the filter element gets clogged. This is what the Oil Pressure relief that the Filter Screws onto does, so why have another one in the filter element ? That would have no effect on the Oil Pressure in the Engine.

 

None of those numbers would concern me.

 

yes, perhaps one or the other, or both. Or neither.

Seems S8873 is one dang short filter. But I don't have stp specs.

I'm a WIX or BALDWIN OIL FILTER FAN.

Suggest WIX 51060
just over 5 inches tall NO oil bypass

 

Thanks Rebelyell, I will swap my STP out for a WIX. I'll also inspect the adaptor bypass. What's really odd to me though, is why does pressure build nicely to 72 psi at 2K, then open dropping to 50 psig, then coming back down the "bypass" (whichever one) doesn't close? I mean coming back down, oil pressure get really low, but then "resets" at idle then is fine again going back up in RPM. That's a HUGE hysteresis. I appreaciate everone's replies.

 

WAG
the bellows in your mechanical OP gage may have weakened and not producing a linear result. Suggest compare w/ a separate, Known-good gage.

if it comes down to that:
D and M Restorations
gage repair, calibration & more
https://dandmrestoration.com/

 

Regardless if it makes any sense, some filters for sbc-BBC do have an oil pressure bypass valve. I prefer Not. Just as I Do prefer an anti drainback valve for longer term storage. Regardless if it makes sense or not in a motor which mounts gasket-side Up, I Do also believe in existence of capillary action.

 

For what it's worth, the OE dash gauge repeats exactly what my off-the-shelf third-party gauge shows. The pattern is repeatable on both. The pressure builds and very discretly at about 2200 RPM, pressure abruptly drops to 50 psi.

 

The M-77 should be standard volume, standard pressure — given that, 70+psi at 2000 rpm seems high to me unless the oil is cold and/or 50 weight. That it drops to ~40 to ~50 seems reasonable, as does ~70 at 3000, with new bearings and tight clearances.

As suggested by Rebelyell, change to a high quality filter (WIX for me). If the issue still exists, check with another pressure gauge. Such pressure fluctuations doesn't make sense regarding bearings and oil pumps.

 

? a little grit/schmutz in bore of pump's valve ? A tiny scratch impeding smooth action ?

found this recent article about some pros and cons of STP oil filters (seems stp only licenses name to Champion Labs) but stp makes no filters. Seems fair and unbiased. It gets interesting about flow restriction and its non-linear flow effect upon pressures. I could Not find any oil filter specs on official STP site. Not even an OF part number. Conversely, there're mountains of details on WIX site; not always so handy but they're available.

https://cartipsdaily.com/are-stp-oil-filters-good

 

Thanks barkingrats. I'm running Mobil1 5W-30, so I don't think that's an issue. I wait for the engine to kick down on idle before running my tests, temp gauge up towards "normal". I'm totally for replacing the oil filter with a WIX (or other non-bypass) filter. So, it's normal for pressure to build to the pressure I'm seeing (72 psi), the abruptly dropping to 50 when the bypass opens? Then why wouldn't the bypass then close when then dropping from 3K back down past 2k RPM? I'm just a sponge for knowledge at this point

 

72 psi at 2000 sounds far too high to me.
40-50 sounds more normal with a std psi pump.

There is a spring and an internal relief valve, in the pump, that should open SMOOTHLY when the positive displacement pump builds enough flow, more than the downstream bearing clearances can flow, the psi builds slowly with rpm, and then the spring starts opening that relief valve.
But then it takes a large drop in flow to make it reset.
It should open & close much more smoothly than that.

I suspect that piston is binding slightly, perhaps from a piece of grit, or a scratch in its bore.

But is it serious enough to worry about?
That is the question.
It may smooth out / wear in with some excercize.
Or it may get worse.
I would definately keep an eye on it.

Or just change the pump, or polish the piston bore.

The danger is if it sticks open, and does not close down back at idle one day.
You could wind up with almost 0 oil psi at idle if it sticks open.

 

Leigh1322, I think you're onto something. The pump relief valve is "sticky". It remains close upon revving up and should "smootly" open, but doesn't. Then abrupty at about 75 psig, it opens and I see a sudden and drastic drop in pressure. The on the wayback down, the piston doesn't close like it should (maintaining bypass) and the pressure suffer. Then finally at idle (or there abouts), the piston closes and we're good again. More information, this pump isn't exactly new at this point, I've drive the car for over a thousand miles, two oil changes. The behavior has remained.

 

That is what it sounds like. You've apparently changed the oil filter twice, so I doubt that's the problem.

 

Thanks bashcraft. I switch to a no-bypass WIX filter. I'm thinking strongly about replacing the filter adaptor as well just to rule out that bypass valve being a possible problem.

 

Thanks RebelYell for the the article on the STP filters, very interesting on temperatue/flow/pressure differential relationships! Yikes. I only drive my '69 in nice warm weather but I'm definately switching to the WIX filter (for reasons now other than not needing the redundant bypass circuit).

I thnk I may be learning a few things, always a good thing...