I have a 2013 Mazda CX-9 with 58k miles. It overheated previously, assumably because of a bad water pump and coolant got into the oil. I drained the bad oil, drained the coolant, and then deconstructed the engine to the water pump (it's inside the engine in this car). While I could see the pump, I performed a pressure test, and could physically see coolant was leaking from the pump into the oil (just like I expected/hoped I would). I replaced the pump and then did a pressure test again while I could still see it, and the pressure held this time. Perfect, that basicially confirms it was the reason for overheating. I put the whole engine back together, added new oil, added new coolant, and ran it. It ran fine, no odd noises or unexpected hiccups.

However, after about 10 minutes it feels like the engine is getting too hot, even though the coolant temperature stays at the midway point on the gauge. I've benchmarked it against other cars and it definitely feels too hot. I could continue to run it but I'm worried it'll overheat and do more damage. After turning the engine off, I can see that some oil has somehow made it's way into the coolant, but there is no coolant entering the oil. I'm stumped...

• I've looked inside the engine; No coolant/"chocolate milk" which is a great sign at least.
• I've performed a coolant reservoir pressure test; It holds perfectly at 16psi, just like it should.
• I've performed a coolant reservoir emissions test; No signs of exhausts fumes, which rules out a headgasket failure
• Getting no codes on the ODBII reader

The only conclusion I can come to it that the oil cooler would be bad. Is that likely? Could it have broken from the original overheating episode? Is there anything else I should check? Any ideas at all?

I'm absolutely happy to pay anyone who can help me diagnose this issue successfully.

 

How do you determine that the engine is too hot? What spot on the engine is too hot? (Every spot on an engine has its own temperature; the temp gauge shows the temp at only its location.)
Does the oil temperature seem OK? Normally warm engine oil might be as hot as 180°F (that's the set point for a thermostatic valve to send oil to an aftermarket oil cooler.)
Could that oil in the coolant be residual that was left in the engine or radiator from before and now coming free due to the heat in the engine? Look for a cooling system cleanser to remove oil residue. (I've used machine dishwasher detergent in the past, then flush, flush, flush.)
Visible milkiness in the oil would indicate a fair amount of coolant in the oil.
In your penultimate line do you mean "oil pump" or did you intend to write "water pump?" I'm unsure how an oil pump would be related to this problem.

 

Really appreciate the response. Big up the Washington massive.

Honestly, simply by feel. I'm benchmarking it against other engines and stuff I've worked on. Stupid, I know. I could be wrong, and it could be running fine, but it definitely feels too hot. I do have thermometers I could use to test the temperature, I'm just not sure on the best way to do it. The worry too, is that I don't want to run it too long for it to actually overheat in order to know if it's overheating, if it's already feeling too hot. Does that make sense? Actually causing the engine to fully overheat in order to prove myself right seems like a "if half a burger already tastes bad, you should eat the whole thing to know if it actually is" kind of logic.

Firstly, it's not an aftermarket oil cooler. This car has an oil cooler by default.

Secondly, this is probably such a noob quesiton, but what would be the best way to go about measuring this? I obviously don't want to open the drain plug with scolding hot oil, and there simply wouldn't be enough oil in the valve covers to reach any through the oil cap. I can't think of a way to access an oil sample to measure the temperature without actually burning myself.

I don't know how that would be possible. Remember, coolant dripped into the oil originally, not the other way around. The way the water pump sits in this car would've only really allowed coolant to leak out rather than oil to leak in - which is ultimately what caused the engine to fail. So there shouldn't be any "residual" oil in the coolant, because there was simply never oil in the coolant to begin with. I could flush it entirely, refill with new coolant, and run it again to see if it comes back, but that would be a ton of money to waste for it to just immediately come back and contaminate a whole new batch of coolant, for me to flush it and then refill it again after I've fixed it.

Absolutely no milkiness in the oil. Oil seems to be fine and the issue with coolant getting into the oil is fine. It's actually helped me rule out some things. If oil was getting in the coolant AND coolant was getting the oil, that would be one thing, but oil simply getting in the coolant and/or not the other way around, seems very odd.

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Sorry, I meant oil cooler. Oil cooler seems to be coming up a lot in my research on how oil can get into the coolant, and it would line up with how the engine seems to be getting too hot.

 

This is a must have you should have in your tool box!

MiniTemp MT4 Automotive Handheld Thermometer RTK-MT4



As for *coolant style automatic transmission coolers most have only 2 rubber o-rings that seal the transmission fluid only that could if the integrity of the cooler (corrosion or internally damaged) can leak oil into the transmission fluid if the failing conditions are right.

These type are design for 2 jobs, to help regulate the transmission fluid temperature to the same as the coolant and to help during cold starts to heat up the transmission fluid to operating engine oil temperatures.

 

I'm an idiot, of course laser thermometers exist. How hot should the engine be from the outside? I've heard 200 degrees fahrenheit is normal?

I could be completely misunderstanding how oil coolers work but from the 100+ pages I've been through talking about oil coolers, they can very well cause oil from the engine to mix with coolant. And every article I could find regarding oil getting into the coolant tank mentioned the oil cooler. In this car, the oil cooler is mounted right next to the oil filter. I don't think it's connected to the transmission fluid at all. Again, I could be wrong. Here's a picture taken directly from the service manual for this car:

 

I never said or indicated you were! I simply suggested that this is a vital tool when diagnosis heating issues. I personal own 3. Engine temperatures can vary depending of the ambientes and if you use AC. Also towing or loading the vehicle ,going up a mountain ext., etc. etc....
The engines thermostat are normal 195F factory in most engines. This means operating temperatures are between 190F-210F normal. They can run up to 210F safely if your coolant is the proper mixture. That means that you could also see under hood temperatures equally the same coolant . It all depends on where you are taking your temperature readings. They can be over 600F on the back of the head exhaust side and exhaust manifold. Just normal driving your CAT will run from 500F to well over 1000F and it can heat saturate somewhat into the engine compartment when you are in traffic and no or very little airflow is running through the engine.

There is no way unless you are exposed and can verify what temperatures are under a hood. They are very deceiving to the mind. I open my hood EVERY DAY on my Mazda because ZI connect a battery tender to it. The last thing I look at before shutting of the car is my transmission temperature and coolant temperature from 2 areas, factory and aftermarket sensor to my temperature gauge.


As for the cooler you can replace my transmission fluid and transmission info with engine, and it is the same. Basically, if the core corrodes or is damaged it can leak into the coolant. Sorry about the confusion but there are more failed or damaged transmission coolers then engine oil coolers so that how I posted the information. However, as I said they are the same type of coolers.
oil cooler type ... notice the similarities in the transmission cooler I posted earlier?
I do replace twice as many factory transmission cooler vs oil coolers.

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Oh I wasn't being sarcastic, man. I was genuinely just berating myself for foolfishly forgetting something so simple. You made an excellent point, and I will definitely be getting one.

That's wild. I didn't realize it could get so hot. I've just been basing my observations off how the front/top of the valve covers feel and how much heat is generally radiating out of the hood when it's open. Compared it to a couple of other cars and I genuinely believe I'm bordering overheating this Mazda based on the comparisons.

I do see the similarites yes, and I suppose that could be the cause. I guess my reason for being doubtful it's that though are because of the general symptoms; For example, the coolant right now looks identical to when coolant mixed with my oil before, so I'm certain it's coolant mixed with oil as opposed to coolant mixed with Transmission fluid, or any other fluid for that matter. Also with the engine feeling like it's overheating (which again, I know isn't proven until I've actually measured it, but let's just assume I'm right) that would line up more closely with it being an oil cooler rather than a transmission cooler - I think. I will definitely look into it though, I'm dying to figure this out so I can finally drive this beautiful car...

 

The thermal plastic valve covers will be cooler than the aluminum head.
When.... you get a handheld thermal device you will be very surprised all the different temperatures under the hood of your MAZDA or any other vehicle for that matter. I even use it around the house. I am always checking the HVAC system for proper heating and more so AC cooling between the exit air vents and intake main vent.
If you ever bored go look at my thread and you will see temperatures are a very concerning element of many things I watch and decide for upgraded projects.

I can say however that in every case that the oil cooler was the issue it was apparent that corrosion from mostly road treatment had cause there failures. And in one case the coolant had been in the engine for over 15 years and caused not only the cooler to corroded but other cooling system parts.
One way to get an indication is to open the drain valve on the radiator drain only the radiator coolant. Then take a clean WHITE rag and carefully wipe as far as your figures can wipe around that area. Look at your rag. There should only be coolant that will easily rinse out of the rag with water. but oil residue will not rinse out and also will discolor the rag. Long shot and some work but you seem to be a experienced DIY!
One thing to consider which was already mentioned that if your coolant at any time was contaminated with engine oil and even if the cause was corrected unless the cooling system is completely flushed and cleaned with a professional cooling system equipment or a few hours with cleaners and your garden hose the residue will remain there indefinitely.

 

Thanks for the information. I will look into this. I don't think it solves my issue, but it gives me some stuff to think about/test.

It wasn't. To reiterate, there has never been oil in the coolant.

 

I have about 8 forums I am on activity. Friday, I get slammed with p.m.'s as well I like to post on my favorite forums some threads. .not to many as some other like to think. haha! No excuse for not being more careful about reading your detailed first post. And all the follow-ups. My general information stills good just And I S-L-O-L-Y read the first post again. lol I think I got caught up in your testing methods which were better than most I see on forums for this kind of issue or any other issue for that as a matter of fact!!! lol . So if coolant got into the oil, then it may also be very likely that oil also got into the coolant even small amounts can cause issues. ?
The radiator does not function well and it get reduced effectiveness when any film start to build up on the cores. This is measurable on older radiator by 1-5 degrees from measuring a new one when replaced. I see you wrote your engine over heated but I did not see how you determined that?
couple things. pressure testing the cooling system you must got over the highest limit pressure and test not just the stated cap psi.
Just because your factory dash is in the same position is really useless unless you can get a mechanical reading or basically not using the ECU or the gauge as an indicator. If the sensor was damaged it may be giving you a false reading.
The chemical test unless you are experienced using it often times may not be accurate no matter how ell you followed the direction. Nothing personal I dont always get a positive reading and often have someone redue the test to compare results.
You said you used an OBDII reader. If it was not connected directly to the OBDII diagnostic connector and it is at least a mid-range quality or better diagnostic tool then you may not be getting true or all the information. YOu did not mention checking for pending or permeant DTCs and what about the monitors are they all in readiness? I can tell you how many times customers tell me there are no codes they checked and right in front of them I go into the ECU and ask what this... LOL
I took some picture and a short video just for you.

Today I made a short run to pick up the wife. The ambient temperature outside was 105 F. If you look there is a lot of information in the pictures about temperatures. You may also notice my transmission temperature which is not like the OEM temperatures normally associated with a stock MAZDA and what I posted earlier.
I also run a 180F thermostat to replace the 195F factory unit. I do not run Mazda FL@@ and I mix my own coolant to water @ 60/40 . I do have my own calibrated timing cycle for the radiator fans but also and more so then letting the ECU run the fans manually turn them on and off.


My transmission cooler has a fan on it and it did run for 10 minutes after I shut the engine off.