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Oil in coolent???


NAXJA Forum User
The engine has over heated a couple of time in the past year. Usually because the coolent level was low. It only has the idiot light and one time it over heated the light came on.
I have flushed the system recently.
When it over heated today coolent was leaking out past the cap on the pressure bottle (Yes, 89 closed system).
After the engine cooled I noticed that the coolent bottle had oil on it. And when I added water it looked dirty and dark. Could be some oil or just gunk from the cooling system.
Checked the oil and it looks good.

What prblems would have oil leak into the coolent but no coolent in the oil?
What can I do to better diagnose the problem?
Based on your description, you are using glycol based coolant. In other words, a petroleum based coolant. A certain amount of oil is used in coolant as a lubricant. The oil, like all oils, can leave a build up behind. When you encounter extreme heat, some of that build up will break off and float in the overflow bottle. This is one of the reasons manufacturers encourage yearly flushing. Removes build up and improves cooling.

My thoughts are you have nothing to worry about except for getting your system flushed.
Not sure I would want to "assume" that's the case. Pressure test the cooling system. Possibly do a compression and leakdown test on the cylinders as well. I say this as my XJ is sitting in a shop with a blown motor. With oil in the coolant.

Oil mixed with coolant in the face of overheating usually = blown head gasket or cracked head. I hope I'm wrong on both counts but the only way to find out is to do a compression test on all cylinders. If the compression is low in one cylinder or in two adjacent cylinders, I'm afraid you'll have to remove the head to investigate the cause.
I hope it is not the head gasker ot a cracked head. That is a bit beyond my experience.

OK, what do I need to do a compression test?
Any good how to articles out there?

What is a leak down test?

Also what do I need to pressure test the cooling system?

This is my daily driver so I need to get this figured soon. No over heating problems today but it was cool and raining...
I am having a similar situation. Recently flushed system after overheating.
It looked to be over heating again and was leaking from the pressure bottle cap. I too had oily residue on the bottle.

I thought I might be in your shoes but the oil was clean and I was not loosing any. So I looked, hopefully, for another cause. A couple of times finding the coolent leaking out of the pressure cap when I stopped. It looked like the pressure cap was finished. I found that the rubber gasket in the pressure cap had been displaced. I reset the gasket and it looks good so far. I got lukcy on this one.

If you happen to have pressure release from the cap, for what ever reason, check the gasket seating...

Oh, yes, that oily residue on the bottle appears to have been the build up that was boiled off when the leaky cap allowed the coolent to boil.

A couple of things that have lead to this are:

1) Some of the hoses are 15 years old. So half way up Moab Rim at the XJ meet in april I sprung a pin-hole leak in the heater hose where it is on the pipe coming up by the t-stat. Luckly there was enough hose to cut and stuff it back on:)

2) I still need to replace the stock hose clip on the water pump side of the lower hose with a clamp. Found it was leaking there too. Cheap clips...

I would convert to open but the only time I have over heated are when a leak has reduced the coolent level. Once these leaks fixed it should be good go again.