Oil Cooling Mystery

[ahendy]

In my 87 with a 3.6L 993 engine, I thought I was having engine cooling issues.

Some background:

My car has two front mounted oil coolers — 1 is the passenger fender "Carrera Cooler" w/ the fan, and the 2nd is a driver fender mounted Mocal 44 row. These worked really well and behaved as expected — temp would rise to the 8 o'clock mark, the thermostat would open, temp would plummet, and then stay steady just below 8 o'clock for most of a drive.

But when the weather started warming up I found some odd issues like the oil temp would go up substantially at freeway speeds. Upon investigation I found, like so many of us, someone had squashed my oil return line at some point. Figuring that was the culprit, I set out to replace the lines.

The lines were seized onto the thermostat so I thought it may be easiest to just replace that as well. After that horrific task, I found that my oil temp was averaging way higher than before — rising nearly to the 10 o'clock mark before dropping, and then it would only drop a little bit — typically back down to around 9 o'clock.

Thinking I maybe had a faulty thermostat, I replaced the insert with a known and tested good one — same result. Seems to be keeping temp stable, but averaging ~20 degrees warmer than before replacing the lines and thermostat. I even replaced the oil temp sender thinking maybe I had a funky one... same result.

So in my mind.. there's only a few possibilities left here:

- Some kind of obstruction in either of my oil coolers or the lines between them
- Everything is fine and it's just summer in Tennessee
- ???

I went for a spirited ~25 min drive earlier, with the temp rising to around 9 o'clock.. otherwise known as 220C or so. When arriving back to my garage, the oil supply line was hot to the touch but the oil return line was cold. Went out for a longer drive, came back, both lines were hot, but oil temp did not change.

My last piece of evidence here — I also have a manual fan override switch installed, and historically engaging that override made a drastic and quick difference in oil temps when stationary or moving. Now it seems to make virtually no difference at all.. so there's clearly not a ton of oil flowing through the coolers for some reason.

I'm stumped. Next step would be to pull the coolers off the car and inspect for obstructions, I suppose..?

[fanaudical]

Couple of thoughts:

Which external thermostat are you running? If the Porsche thermostat, is the pressure relief valve stuck partially open? That would bypass flow to the coolers.

Obstruction in the cooler? Crushed lines?

[ahendy]

It’s a Porsche thermostat, and that did cross my mind, although oil seems to be getting to the coolers just fine.. it’s just not getting back.

Lines are new, but obstruction in one of the coolers is where my mind is at the moment..

[stownsen914]

If oil weren't getting back from your cooler circuit, your engine would self destruct in short order.

Do you know the actual temps you are seeing? If your gauge isn't marked, you can use an infrared thermometer on the cold side of the coolers to get a decent idea. You can also use the IR thermometer to check temps at various places in the oil cooler circuit. This will tell you a couple things:
1. Whether the coolers seeing the expected flow. For example, if they're cold, or cooler than expected, it would suggest thermostat malfunction.
2. If #1 checks out, that you're seeing expected temp drops across the coolers

[dannobee]

This should be fairly easy to diagnose using an IR gun. But first, when you hooked up the second cooler, did you route it in series or parallel?

With the engine at operating temp (or with the gauge at "10 o'clock"), check the four pipes at the thermostat. If it's bypassing, meaning not open, the two going to/from the cooler should be much cooler than the two on the engine side.

If the thermostat is open, the line going to the cooler will be warm and the one on the return side will be cool.

If the coolers are plumbed in series, one will be much cooler than the other. If they're plumbed in parallel, they both should have roughly the same inlet and outlet temps.

Since this seemed to start after replacing/repairing the kinked cooler pipe, I'd look over all of the piping and see if something was missed. Since the pipes also act as a cooler, you can use the IR gun to check for a severe restriction; it'll be a little cooler after said restriction.

Let us know what you find.

[ahendy]

The thermostat is definitely opening — it just seems to be opening substantially later than the old one did, and flow to/from the coolers is lower than it was originally as well. At least that's my gut..

Seems inconceivable that I could've gotten two faulty thermostats — is it possible that the newer production ones just open later than my original (from 87 presumably) one?

Piping all looks intact. I removed and reinstalled the secondary cooler (the Mocal 44 row), and all looks fine there. They're piped in series, fwiw.. to carrera cooler from thermostat -> to mocal 44 -> return to thermostat/tank.

I can say that it's definitely not self destructing, nor do I seem to be able to get it to go into "danger zone" (250+).. but it running ~20 degrees warmer than previously is not a small change and has be concerned.

Andy

[rumlyne]

Just for reference, I have a 993 V-ram in an 81 SC. The oil system is
stock except for a 44 row Mocal in series with the stock 28 row oil
cooler in the passenger fender. The Mocal is in an IROC front bumper.

I'm curious as to what your temps actually are.
This motor has seen a lot of track days in the 12 years it's been in the car.
Bouncing this motor of the chip on a 95 degree day at VIR has never moved
the needle more than an 1/8 inch out of the box. In normal road use it
stays on the upper edge of the box.
I know none of this helps, but I'm curious as to why yours would run that
much hotter in general.
Following the hot with an IR gun is the route I'd take.
Good luck

[stownsen914]

I'd still be interested to know actual temps we're talking about here. A 20 degree change is notable, but it's not out of the realm of possibility that two different thermostats would open at different temps. It's also possible that if your temps are hovering near the thermostat opening temp, that you might see some bouncing around on the temp gauge as the thermostat alternates between open and closed.

It is possible to bench test thermostats, if you'd like to compare them for their opening temps. There are threads on this forum that cover this.

[ahendy]

I'll take some IR gun readings later today and report back. I did do that a few weeks ago when I first observed this, and after 45 mins of driving on a warm-to-hot day, I was seeing 215+ on the oil supply line and on the oil tank itself.. which aligns with the ~220-225 or so the gauge is reading. Which is to say... not absurdly hot, but a little warm for comfort and certainly warmer than it was running before.

@rumlyne - given that we have different gauge types, it's hard to compare exactly (and the working range on the SC gauge is frustratingly small unto itself!) but given that I believe your "box" border is 210, it sounds like your hottest track day was likely similar temps to what I'm seeing in normal road driving currently.

Taking the thermostat out of the car again is my worst fear.. not sure I can handle that kind of emotional abuse. However the pressure relief valve is an interesting point.. perhaps even though the thermostat is opening, there's less than normal flow to the tank as much of it is bypassing the cooling loop.

After all, isn't oil supposed to always be flowing TO the coolers, but it only completes the loop once the thermostat opens? If that's the case.. something is definitely wrong here.. as the lines to the coolers seem to be pretty cold until the thermostat opens.

[ahendy]

Some more information, upon poking around my engine bay..

I noticed that one of the breather tubes coming from the oil tank is seriously kinked.. (pic attached). Need to figure out how to route it better to resolve that, but is there any chance that this could be causing some kind of back pressure substantial enough to engage the pressure relief valve?

[dannobee]

When you say that you replaced the thermostat, did you replace the whole assembly? Or just the thermostat inside the housing?

IIRC, the bypass spring can weaken over time and not allow flow through the cooler(s).

Here's a good description on flow through the thermostat (bypass is not shown though).

How External Oil Thermostat Works - Illustration

[ahendy]

The entire thermostat assembly has been replaced twice — both times with brand new OEM thermostat assemblies. Same results with both.

[ed mayo]

Have you checked the engine mounted thermostat?

[ahendy]

I have not, as hot oil seems to be getting to the external thermostat just fine.

[ed mayo]

I've seen it happen. Need to eliminate all possibilities.

[ahendy]

Do I even have one? I think since I have a conversion into a G-body I’m not sure I have an engine thermostat..?

[stownsen914]

Ed is referring to the thermostat on the top of your engine that controls oil flow to the engine mounted cooler. Yes your engine would have one, unless you went to some effort to convert to a later style setup. That cooler and thermostat are in the pressure side of the oil circuit and are completely separate from the front mounted one coolers (which are in the scavenge circuit), but a malfunction there could cause the fluctuations you are reporting.

[ahendy]

993 engines don’t have engine mounted oil coolers..

[dannobee]

993's have an oil filter housing in place of the engine mounted oil cooler, so no thermostat on the engine.

[ed mayo]

Sorry, forgot you have 3.6