Hi all,

I recently installed a Bosch fluid pressure and temperature sensor in my 99 ITB Miata. It is placed where the stock oil pressure sender used to be. The sensor calibration is prebuilt into the ECU software, PCLink, and the Oil Temperature reads the same as Coolant when the engine is cold and is very close after it has been off for 20 minutes or so, so I'm confident it is accurate.

I've found that the oil temperature doesn't go above 176F (80C) even after a mountain run where I stay WOT for extended periods of time. Since this is a new install, I haven't taken it on the track to monitor temperatures there, but even in the street I would have expected more like 200-210F temperatures under sustained WOT. The car has a Supermiata radiator and a Flyin Miata reroute with a 180F thermostat. Stock high mileage BP-4W otherwise, certainly not a Crusher type engine. No A/C or oil cooler in this car. Coolant temperatures have never gone above 192F even after 3-5 hotlaps at the track. Fan 1 was set to run at 194F, and fan 2 at 205F. I've now increased that to Fan 1 201F, fan 2 212F to see if it helps.

Any thoughts on what may be causing this issue? Things I could try? FM support said they don't think the 180 thermostat is the cause and they haven't registered oil overcooling in their shop or customer cars before. Usually people have the opposite problem :/. I will also note that for the past couple of years I had noticed that the engine oil gets dirty and diluted very quickly, within 1k miles. Because it is very high mileage and modified I just figured it was the tune and fuel getting in the oil. Now I think the overcooling was making things worse?

Cheers,
Ricardo

 

Does it have the factory oil cooler donut on the block, with water lines going to it?

 

The standard oil temp location is the pan. There are hot spots in an engine where the oil temp will be higher, piston rings, valve springs, oil pump etc. NA applications will not warm the oil as fast as a turbo application and may require more high speed use for a longer period to reach it's max. There is no problem running the oil at 180F for extended periods. My 23 Stingray rarely goes over 180F and many times its running at 160F.

 

@curly, yes it still has the stock oil/coolant heat exchanger with coolant passing through it.

@LeoNA, thanks for the input. I'm certainly not comfortable running the engine at high load with 180F oil temps. Does the location I installed it in read lower than the pan? To my understanding oil must go to 212F to burn off contaminants that the combustion will introduce.

 

Your temps sound accurate for your setup. Even in the pan, mine reads ~200-210 with a turbo car after a spirited 16 mile drive home. The stock donut helps keep oil temps near water temps.

 

Understood. So maybe it needs much more time to warmup than a 30 minute blast? Is this currently not something I should worry about unless it never climbs past 200F at the track?

 

Wouldn’t worry about it. If you’re more street oriented, I’d put a hotter temp thermostat in. 195f range. That should also raise oil temps. If you mainly track it, try it as is and see how the temps are.

 

I'm not sure if I understand your concern. Oil at 180F is going to be able to support more load than oil at 210F.


The pan will have sourced oil from several areas and might be different. The oil from the cylinder head might actually be hotter because of the valve springs and dumps directly to the pan. Your measuring the oil post heat exchanger and before it does any work other than the oil pump.

 

Thank you. I would say it is mostly a streetcar, but in recent times its only street time has been 1 mile to the local car meet, mountain blasts and on its way to the track so....trackcar?

Could you educate me on how oil at 180F is able to support more load than at 210? I was under the impression that oil must reach over 210 to burn off water, fuel and sulfur deposited in it by the combustion process, and that it also is able to pass through areas better as it is slightly less viscous, hence the pressure drop at operating temperature? My main concern is that the temperature I saw doesn't reach the boiling point of water, hence the oil may have been getting diluted all this time as a result of it not getting hot enough. I'm trying to verify this and find a solution.


Got it. So maybe reading slightly colder because of the location then?


It seems from both of your inputs that I should not worry about this as much as I have.

 

A higher viscosity will support a higher load. The viscosity can be too high, but we are evaluating the oil well within the normal operating range. An engine will operate for a long time in its life below the target temp. Eg vehicles that are driven short distances for their whole life may never reach the optimum temp. It may not be ideal, but they survive. My point is that it is not going to cause catastrophic failure in a short period.

The more energy that the oil receives the faster the condensates will be evaporated. Energy can be heat and mechanical agitation. There are hotter components in the engine than the oil in the pan. The oil is constantly warming and cooling. A pan oil temp does not tell the whole story, it is a metric. The oil in the pan might be X temp but the oil in other areas might be much higher for short periods. Circulating the oil over the hotter components will effectively evaporate the condensates.

If the oil is saturated it can become acidic. The PH drops because of the excess hydrogen from the water. Modern oils have calcium bicarbonate to neutralize and maintain the PH. The oil has a rating of their total base. I have never seen an oil analysis that had a low base number or a low PH. Higher performance engines typically don’t have extended oil change intervals and most change their oil more often than manufacturers recommendation. The oil temp might be more of a concern for the average car that is used for short trips with a 10kmi/12mo oil change interval. Fuel saturation could be an issue, but is not something that is common with a healthy well tuned engine. Fuel also has a lower boiling point. If you are concerned you can run a lighter oil, maybe a 10-30 instead of a 10-40 and do an oil analysis to confirm the oils state after use. If it’s getting to 180f I would not do anything to raise it because you will lose some capacity that might be needed for high ambient temps and high load conditions.

 

Thank you very much for the detailed response. Noted on all accounts. For completeness, the car already runs 10w-30 and other than trackdays, it sees constant short trips. I'll stop overthinking this and keep doing what I've been doing.

Cheers. Thank you both.