Vehicle/engine is a 1997 Jeep 4.0L inline-6 Cherokee Sport.

I just got my oil sample analysis back from Blackstone Lab. Here it is.

They say there's coolant in the oil, as indicated by the presence of potassium and sodium.

I'm dumbfounded though. Because:

  • I checked compression and all cylinders are great! 140-150 psi
  • There's no visible coolant in the oil, no visible oil in the coolant, no exhaust smoke, etc.
  • I had a local garage test for combustion gases in the cooling system. They used their emissions equipment and they didn't find any exhaust gases in the coolant
  • No bubbles visible in the radiator filler neck when the engine is running

Could the head or head gasket be cracked between a coolant jacket and oil passageway? Is that possible? And, can that itself cause overheating issues?

This engine (all cast iron) has an issue with running hot and has boiled over a few times, though not severely or for long periods of time. It all started last summer (first summer after buying it, so I don't know if there was a problem before or not) when the radiator exploded when I pulled off the highway to get gas. I was towed home and I put in a new radiator, but I found that it was now overheating on the highway after 20 minutes or so.

I replaced the rest of the cooling system and started aggressively flushing the rust buildup out of the coolant passages and it got better, now it only overheated after several hours of highway use. I figured the new radiator was clogged with rust scale and stuff, so last week I swapped it out. I cut open the old one a yeah there was some black crud in it. But a few days ago I went on a 30 minute highway drive and it started getting warm again, like 230*F. I used the heater core to lower it back down to normal temps and drove slow on the way back home.

What do you think?

  • 1
    Yes, depending on size of crack or leak in head gasket. It may not leak during a compression test but when the cylinder fires it will leak under much higher cylinder pressures.
    – Moab
    Jun 14, 2018 at 17:54
  • @Moab hmm I didn't think about that Jun 14, 2018 at 18:53
  • I just went on a test drive in brutal heat today (97*F). After 30 minutes of 75+ MPH, it started running hot and the temp gauge slowly went up. I slowed down to 65 MPH and turned on the heater and the temp stayed the same - about 230*F. When I got into the city, the temp slowly started to drop, even without the heater on. Jun 14, 2018 at 18:55
  • Sounds like a clogged radiator possibly.
    – Moab
    Jun 14, 2018 at 21:04
  • @Moab yeah it does, but I replaced the radiator less than a week ago! Jun 14, 2018 at 21:10

2 Answers 2


It's entirely possible for there to be a headgasket failure between a coolantway and an oilway, without it breaching any of the cylinders (or indeed between an oilway or coolantway and the outside).

Have you done a coolant system pressure check?

Do you have an oil cooler? That's another common cause of oil/coolant contamination.


It’s a Jeep.the head may be warped a little,my first thing would be to retorque the heads.its worth a try.youve done just about all you can do without a total top end tear down.

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