Pressure Relief Valve or Exterior Line Blockage?

[Jack Olsen]

1972 911 with exterior circuit. Symptoms were high oil temps on a long drive through a cold desert. Temps went to 240 before the thermostat finally kicked in -- then normal. But then it happened a couple of times again, getting up to 250 before I shut it off -- then tried again, with it eventually kicking in. Sometimes it worked completely normally. So I ordered a new thermostat, pulled the console (1972 has the thermostat as part of the oil filter console) and replaced the sticking unit.

Nothing changed. Two tests had it go up toward 250 without the thermostat kicking in. Or so I thought. I saw heat in the exterior oil line for the portion before it went into the rockers (braided oil lines in a custom, through-the-rockers setup, two oil coolers up front). But no heat in the lines forward of the rockers. Old thermostat worked fine in hot water test. Hard to believe now that it's the thermostat.

My thinking is that the problem is one of two things: there could be an obstruction in the front cooler loop. Or the pressure relief piston in the thermostat assembly is stuck and sending oil back to the tank. But if it's an obstruction, it's weird that it's intermittent. And if it's the pressure relief valve it's a little odd that I'm getting hot oil through part of the exterior loop plumbing.

So here's my situation, as I see it: I can pull the console out again and disassemble and inspect the pressure relief piston/valve. That's a pain because it means disconnecting all the oil lines again -- and I'll have to fabricate some kind of tool for the coin slot groove in the cap that gets me to the pressure relief valve. Or I could attack the lines, by running mineral spirits through the exterior loop and coolers to get 20 years of crud (maybe?) cleaned out. I had the coolers themselves professionally cleaned out a couple decades ago. There's no apparent pinching or kinks anywhere. In any case, that'll be messy and also a pain.

Both paths are less than fun. But does it make sense to attack one before the other?

If you're curious, here's what the 1972 console looks like:



And here's the coin slot I'd have to crack open:

[3rd_gear_Ted]

A flow sensor for diagnosis would sure help, they can be rented.
PTFE coating on the hose interior is used in high grade industrial applications.
Robots & aircraft don't like motion surprises so hose grade is a reliability factor
What internal coating type does your hose have?

[r lane]

I am guessing this is a new problem, was working correctly before it didn;t. I think only the 72 S had the thermostat in the filter console. And that console was slightly different to accommodate the thermostat. You still have the other thermostat in the top of the engine I believe. Never felt hot water test for a thermostat was reliable. I will look at a schematic to get an idea of how the flow works, and at a housing I have that has been modified to handle a thermostat, but fairly straightforward I believe. Bob

[Jack Olsen]

Originally Posted by r lane

I am guessing this is a new problem, was working correctly before it didn;t. I think only the 72 S had the thermostat in the filter console. And that console was slightly different to accommodate the thermostat. You still have the other thermostat in the top of the engine I believe. Never felt hot water test for a thermostat was reliable. I will look at a schematic to get an idea of how the flow works, and at a housing I have that has been modified to handle a thermostat, but fairly straightforward I believe. Bob

Thank you! I don't have a cooler on the engine, so I've been assuming I don't have a thermostat there. It's a 1995 3.6 engine.

Yes, the 72T didn't have the external loop -- but like you say the console could be modified for it. Mine definitely has the thermostat in it -- it dropped right in. That said, there were some differences between the new (superseding) part and the one I pulled out. But the openings were in the right place == and the behavior with both -- appears to be the same. Maybe one thing to try (now that I've tested the old one) would be to swap it back in and see if anything changes.

Old vs new:

[Jack Olsen]

Originally Posted by 3rd_gear_Ted

A flow sensor for diagnosis would sure help, they can be rented.
PTFE coating on the hose interior is used in high grade industrial applications.
Robots & aircraft don't like motion surprises so hose grade is a reliability factor
What internal coating type does your hose have?

It's the standard aeroquip stuff -- they describe it as an elastomer smooth bore inner tube. The on/off nature of the cooling behavior, and the fact that sometimes it worked just fine -- makes this especially frustrating.

I could put together a short loop to bypass the whole cooling circuit to see if the thermostat and pressure-relief valve are doing their thing. It wouldn't give me a flow rate, but it might confirm one thing or the other.

[boyt911sc]

Jack,

Have you tested the oil thermostat in an oil/water bath? A simple test will show you if the slide valve is fully opening or not. A visual test will help you diagnose the culprit. Keep us posted.

Tony

[Jack Olsen]

Originally Posted by boyt911sc

Jack,

Have you tested the oil thermostat in an oil/water bath? A simple test will show you if the slide valve is fully opening or not. A visual test will help you diagnose the culprit. Keep us posted.

Tony

Thanks.

The answer is yes -- or, sort of. I took out the old one and put in a brand new one. When the brand new one did not change the symptoms, I decided to test the original. It worked great. So my guess is that the problem is not the actual thermostat. New and old both sticking kind of defies the odds.

I believe the two possible causes are 1) a possibly-stuck pressure relief piston might be sending the oil right back to the engine, regardless of thermostat activity, or 2) there might be some kind of obstruction in the line (how? how could it be intermittent?) and that is producing enough actual pressure in the outside look to correctly trigger the pressure relieve piston to do its thing and send the oil back to the engine.

I'm hoping to find the the most time-effective path to figure it out.

[HaroldMHedge]

Reviewing your other thread (Sticking Thermostat -- 1972 911) and this one, it appears your old thermostat is a 901-107-751-00. John Walker recommends against using the newer thermostat in the older housing in Oil thermostat - can I use the later type in my swb?. If your housing has been modified then it may not matter.

Since the problem is intermittent and your hoses are 20 years old, it might be your hoses are failing internally and causing binding of the thermostat. This is an SWAG. I think an easy check for rubber failure would be to filter the drained oil through a fine strainer and cut open the oil filter and check for rubber bits.

[Jack Olsen]

Thanks. That's a good idea about filtering the oil from the lines. I've had clean oil and filters in all my oil changes, but it can't hurt to look.

I'm not sure what the machining is/was for making a 911T console work with an external cooler, but the newer model fit right in without any clearance issues for me. If the next thing I do is to look at the pressure-relief piston, I'll probably swap back the old adjustable early-model thermostat at the same time.

[Jack Olsen]

Does anyone know what the travel distance is for the pressure-relief piston in the external thermostat assembly (not the thermostat movement)?

​​​​​​​Removing the coin-type cap after 53 years in one position looks to be a headache, and I can move the piston (against its spring resistance) with a probe -- but it's hard to see exactly how it works and how far it should travel. The fact that it does move makes me less inclined to try and disassemble the thing.

So my next step will be to pump mineral spirits through the external lines next to see if there is an obstruction.

[Bill Verburg]

Originally Posted by Jack Olsen

Thank you! I don't have a cooler on the engine, so I've been assuming I don't have a thermostat there. It's a 1995 3.6 engine.

Yes, the 72T didn't have the external loop -- but like you say the console could be modified for it. Mine definitely has the thermostat in it -- it dropped right in. That said, there were some differences between the new (superseding) part and the one I pulled out. But the openings were in the right place == and the behavior with both -- appears to be the same. Maybe one thing to try (now that I've tested the old one) would be to swap it back in and see if anything changes.

Old vs new:

Jack there is no t-stat in the 993 block, it's a big hole w/ openings to the appropriate oil galleries, there are 3 or 4 different covers depending on which specific engine 964t w/ block filter 964 w.o block filter, 993 w/ block filter or 911 w/ block cooler , a 993 will have 993.105.135.00



here are 2 of the different covers

[stownsen914]

One thing you can do to try to localize the problem is probing with a hand held IR gun. See what's hot and what's not. For example, from cold, get it up to 200 deg (thermostats should be open by then), and see if your external cooler and engine mounted cooler are hot.
On the external cooler, check the inlet and outlet of the cooler to see if you see an expected temp drop.
You can also check that the external cooler is evenly hot, i.e. if there's a blockage maybe that part will still be cold, etc.

[Mark Salvetti]

Originally Posted by Jack Olsen

Does anyone know what the travel distance is for the pressure-relief piston in the external thermostat assembly (not the thermostat movement)?

​​​​​​​Removing the coin-type cap after 53 years in one position looks to be a headache, and I can move the piston (against its spring resistance) with a probe -- but it's hard to see exactly how it works and how far it should travel. The fact that it does move makes me less inclined to try and disassemble the thing.

So my next step will be to pump mineral spirits through the external lines next to see if there is an obstruction.

Start with gentle air pressure. Use some kind of rubber stopper with an air fitting attached. You'll want to do that first to get out as much oil as you can anyway before you start pumping in mineral spirits. That might also be enough to tell you if you have a blockage of some kind.

Agree the intermittent nature of the problem is a real head-scratcher. I would believe an intermittent thermostat issue before an intermittent blockage, but who knows.

Mark

[fanaudical]

Perplexing - Here a are a few random thoughts:

- I would open up the cap on the relief valve side just to be sure of what's in there. There are aftermarket caps available with a hex fitting on top.

- You mention the new-design thermostat drops right in, but do the ports in the thermostat align with the ports in the housing?

- I have seen inner hose liner collapse and block flow on hydraulic systems so it does happen.

[Jack Olsen]

Originally Posted by stownsen914

One thing you can do to try to localize the problem is probing with a hand held IR gun. See what's hot and what's not. For example, from cold, get it up to 200 deg (thermostats should be open by then), and see if your external cooler and engine mounted cooler are hot.
On the external cooler, check the inlet and outlet of the cooler to see if you see an expected temp drop.
You can also check that the external cooler is evenly hot, i.e. if there's a blockage maybe that part will still be cold, etc.

Thanks. I did that once the engine was pretty hot (230°+) and I got heat about 30" into the external line. That could have been radiant heat from the engine bay at that point, or (I thought) it might be a trickle of hot oil coming out if the pressure relief valve was sending the overwhelming majority of the oil back to the tank.

I haven't been able to IR-temp-check the cooler lines because -- on the lift -- I haven't been able to get the thermostat to open. But I'm slowly getting through more testing -- if I get the same behavior with verified-clear-and-open external lines and two different thermostats, I'm going to look much more closely at that pressure relief valve.

Originally Posted by Mark Salvetti

Start with gentle air pressure. Use some kind of rubber stopper with an air fitting attached. You'll want to do that first to get out as much oil as you can anyway before you start pumping in mineral spirits. That might also be enough to tell you if you have a blockage of some kind.

Agree the intermittent nature of the problem is a real head-scratcher. I would believe an intermittent thermostat issue before an intermittent blockage, but who knows.

Thanks. I've done that so far on the line that goes to the first external cooler, and I did get one surprise where it felt like pressure built up and then 'splatted' out. I haven't put that oil through a filter yet to see if there is actual debris. I've then checked the first cooler and the line together, and it seems clear. I was waiting for a fitting, but next up I'll check the remaining cooler and the lines running to, and from, it -- all the way back to the tank.

I agree 100% on the weirdness of the past behavior. It was exactly like the thermostat had been sticking, and then -- in an instant -- opening back up and the temps dropping as you would expect. But then I had a couple of instances of that not happening for awhile, and then -- up on the lift -- the tripping point not happening at all. And again: the odds of this happening with two different thermostats borders on the impossible.

All my local hardware stores are out of mineral spirits -- but I have lots of acetone on hand. I don't know for sure if my aeroquip lines are PTFE coated or rubber, and my worry is that acetone will harm them if they're rubber. Does anyone reading this know better than me?

Originally Posted by fanaudical

Perplexing - Here a are a few random thoughts:

- I would open up the cap on the relief valve side just to be sure of what's in there. There are aftermarket caps available with a hex fitting on top.

- You mention the new-design thermostat drops right in, but do the ports in the thermostat align with the ports in the housing?

- I have seen inner hose liner collapse and block flow on hydraulic systems so it does happen.

Thanks! Do you know specs or a source for a repacement cap? I worry that with 53 years of heat cycing and such a thin edge to grab that I might destroy the whole assembly in trying to get it apart. But I'd feel better if I knew there was a hex-capped replacement out there.

Yes, the ports line up on the new thermostat, and so far the behavior has been the same with both of them. I'm going to switch back to the old one next, since it bench tested fine, and then bench test the new one just to see if the behavior is the same.

Collapsing liner on the aeroquip lines is somethign I lose sleep over -- if it could somehow cause the intermittent failure/blockage I've maybe seen. It's obviously a very big job to replace it all. I can get a camera up it to some amount of length, but it might not be an easy thing to see and identify.

I'll keep testing.

[dg567]

Jack, Jim Tidwell makes a hex replacement:

https://www.early911sregistry.org/forums/showthread.php?122686-FS-911-Remote-Oil-Cooler-Thermostat-Cap

[fanaudical]

Sorry - I may have spoken out of turn on the hex caps. I thought Elephant Racing offered those caps but don't seem them on their catalog. I thought that Pelican used to offer them but don't see them here either.

Most elastomers are attacked by acetone - i would not use that as a test fluid. Couple of other thoughts for compatible fluid:

- Water - Not exactly compatible with oil but if you blow the lines out you shouldn't have much more remaining than what you might find on a moisture-laden cold start. Get the engine warm and it boils off.

- Mineral-spirit-based paint thinner?

- Kerosene?

- Stoddard solvent? (Not sure you can find that in California.)

If testing t-stats in a heated pan, I recommend doing several (7+) full thermal cycles - Not just one. Maybe have a hot pan and a cold pan - Move the t-state between the two every 10 minutes. Make sure they are moving every single time.

[Jack Olsen]

fanaudical, dg567's link worked. Jim Tidwell at the Early S Registry is still selling the caps. Seems like a great guy.

I've got (semi-legal) mineral spirits on the way to me, and I'll keep plodding through the external lines to see if I can locate an obstruction.

I'll test both thermostats again, but it just seems unlikely that both the old one and the new one would fail in the same way. Still, anything's possible.

[Showdown]

No acetone!

You can use VM&P Naphtha as an alternative to Mineral spirits. Odorless mineral spirits is best (OMS) but the stinky non odorless stuff will work too.


Sent from my iPhone using Tapatalk

[Jack Olsen]

Well, here we are a short while later. I've run mineral spirits through the external lines and blown them out. I've changed the oil and the two filters. I've re-checked the functionality of both thermostats, side by side, and they both pass just fine. I buttoned everything back up and five consecutive tests produce no problems at all. The thermostat passes oil to the external loop, and everything cools down the way it should.

It's frustrating, since nothing has really changed -- so it will be a while that I'm still waiting for the second shoe to drop. My best guess remains the pressure relief valve sticking, but I moved it back and forth with no resistance and haven't been able to make it stick in the last five heat-and-cool cycles, so I'm going to hold off on the (probably destructive, probably curse-inducing) uncapping of that component. In anticipation of it possibly repeating the issue in the future, I have a replacement cap and a source for the internals. But if the solution was just taking everything apart, cleaning and checking, and putting it back together? Well, I'll see how that goes before I go in with the mapp torch and a pipe wrench to get that crazy coin-screw cap off of the filter assembly.

That's the update.