Need help. Too much oil pressure blowing up coolers

[Chuck Moreland]

I've got a freshly built 2.7. '77 7R case, turbo oil pump.

Upon initialy firing it up, it promptly developed a leak in the on-engine cooler.

Inspecting the cooler, I found it split along the welded seam. This was a new-to-me oil cooler. So I suspected it may have been faulty.

I swapped in a used but known good cooler.

I checked both pressure release valves to confirm the correct springs are installed. They are correct; long 86mm spring vertical, short 70mm spring horizontal.

Figuring all was good I fired it up again. And promptly pegged the oil pressure guage and developed another leak in the second cooler.

I haven't yet removed the second cooler, so I don't know if the failure was the same as the first. But it is clear the leak is in the cooler.

I'm more than a bit frustrated, having now blown up two $700 coolers.

Another cooler will be had, but I need help finding the root cause of the problem.

My understanding is both valves serve to limit pressure and protect the cooler. A dual failure seems unlikely, all parts are brand new.

Engine Experts, Now what?

[Early_S_Man]

Chuck,

Did the case get the 'Bypass Mod' ... and, if so, did the proper pressure-relief pistons get used?

[Chuck Moreland]

It is a 77 case. My understanding is all motors '76+ have the "bypass mod" from the factory.

I did not remove the pistons to inspect them, so I don't know the answer. But according to Wayne's book, using the older perferated pistons in a "bypass modded" engine produces no oil pressure, not high pressure.

[Chuck Moreland]

Tomorrow, will be removing the pistons for inspection. I have to believe the problem is in the relief pistons assemblies.

After all that is their function, to limit pressure. And I have unlimited pressure.


Did I mention, I did not assemble this motor. But it was built by a well respected pro.

[Tyson Schmidt]

Chuck, it seems strange to me that your oil cooler is getting pressure when it's cold. Maybe take a look at your thermostat?

[Chuck Moreland]

That was my first thought too. But it seems that the cooler does see pressure even when cold, it just doesn't get flow.

The thermostat valves-off one side of the cooler only. The oil is still pressurized by the other side. Right?

This diagram is not real clear, but you see my point.

[ischmitz]

Chuck,

Could it be that the piston of the pressure reliev valve jams in the passages and won't open once the engine builds up pressure?

I remember that my 2.7 when cold showed clear signs of the pressure reliev valve opening. The needle of the pressure gauge went up and at around 3k RPM suddenly dropped. The RPM level where that happened went up over the next 10 minutes until it fully disapeared once the engine got up to temperature.

Ingo

[ischmitz]

Well,
the diagram tells me the oil cooler sees the same oil as the pressure/temp port on top of the block. So yes it sees pressure right away

[Tyson Schmidt]

Well, according to that diagram, the thermostat blocks off the inlet to the cooler, not the return. Still seems to me like it should keep pressure from the cooler when cold.

I had a cooler leak on a motor of mine. It was bone dry until the thermostat opened up, then it leaked like seive.

[ischmitz]

you just forgot to sprinkle magic power onto the engine Mr. T

[Tyson Schmidt]

Hmmm, that is a very deceiving pic. I'm going to go look at the Factory manual to see if they actually intersect.

[Tyson Schmidt]

O.K., I just went and looked at a bare case. The cooler does get full engine oil pressure on one side, regardless of the thermostat.

It's got to be a pressure relief valve problem, or a big coincidence.


You know you can shim the pressure relief and safety valve caps with extra sealing washers if you need to get the spring pressure down.

[Chuck Moreland]

I have to believe it's the relief valves. But both failing? Just seems highly unlikely.

I see how shimming would reduce spring pressure, but I shouldn't have to do that. There has to be another problem here.

Do these pistons sometimes get frozen in place? If yes, why/how?

[ischmitz]

Chuck,

one opens at 5bar (reliev) and the other at 10bar (safety) per design. Maybe just the 5bar reliev failed while the cooler wasn't good for 10 bar.... just a thought

[Chuck Moreland]

Ingo, where do you get those specs?

The guage read 140 psi, 10 bar. But I think the guage was pegged so I don't know what the pressure really was.

It seems that, by design, the safety valve should open before the cooler blows apart. Otherwise the cooler IS the safety valve, just as I experienced

[ischmitz]

The spring and the plunger are the same part# for my 964 engine as they are for earlier engines. I know that my engine has those specs 5 and 10. They are somewhere in the factory manual. I simply assumed that the same spring will roughly lead to the same pressure.

I also remember that I could not get my gauge much above 60PSI before it jerked down (opening of the reliev valve) with the engine cold.

[ischmitz]

Pegging the gauge simply CAN NOT happen with a properly working reliev valve. No way. Have you taken the cap off again. The plunger should slide out with slight tapping or the help of a small magnet on a stick. Check if it is jammed up there. Maybe there is debris or grooves in the valve area

[obrut]

A mate's 914/6 did this - split the front oil cooler twice. I think it was the oil filter mount or something simple like that??

Whatever it was could be assembled in 3 positions yet only 1 of these postions had all of the oil passages lined up correctly. Hope this helps.

[911pcars]

Chuck,
I too think it's a pressure relief valve issue (that's the purpose of the valve).

I would suspect it's either not moving or it's installed 180º off with the spring and thus not operational. With the pressure relief valve completely removed, you should be getting 0 pressure (oil dumps back to sump). Try that first and see if it makes a difference.

You could remove the spark plugs, then crank the engine to verify the oil pressure isn't maxing out. Maybe this strategy will prevent another trashed cooler.

Sherwood

PS: After rethinking the above possiblities, I'll retract the 180º theory unless the incorrectly installed piston is able to block the dump port to the sump. If so, there would probably be 0 pressure. The most likely is the PR piston that's stuck in the bore, but I guess we'll see.

[Joeaksa]

Had this happen on an old 2.4 engine. Piston was sticking in the bore and letting full oil pressure into the system.

Fixed it and pressure went back down to 4-5 bar at startup. 10 bar is way too much and will blow out seals and ruin coolers like you have found out.

JoeA