What is broken - thermostat or water pump, or..?

By Grape Ape on Sep 24, 2015 at 12:18 PM
  1. Grape Ape

    Grape Ape Well-Known Member

    Mar 29, 2005
    High Point, NC
    Hey guys,

    Trying to work out this issue while I'm stuck at work. Last weekend my '99 Cherokee XJ 4.0L - started to run hot - "check gauges" dash light came on. I immediately pulled over, and I noticed that there was no coolant in the reservoir. Filled it up and went on my way without issue. This is odd because I checked, and added some the week before. I usually check my fluids on a regular basis. Today the "check gauges" light came on again because it was running hot again. Pulled over again ready to fill up the reservoir, but this time it was full!??? Drove part of the way to work (just a few minutes) with the heat blasting to keep the engine temp down, but then the rest of the way to work I turned off the heat and everything seemed fine. Even while sitting at lights a couple times for over a minute or more, the temp gauge didn't climb.

    My first assumption is that the thermostat is not working, or only working when it wants to. The fan seemed to be working over time this morning too. Could this be a water pump issue?

    Any insights so I can pick up what I need this afternoon? I know the parts are not very expensive, but I can't replacing both this week.

    Thanks in advance for your help.
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    1. Falko
      Water pump failures do not usually present themselves as intermittent issues. Thermostats can, but it wouldn't be my first guess.

      Were there other symptoms of overheat other than the gauge?
    2. Falko
      Though I did just think of one condition that causes intermittent poor water pump performance: Check your belt tension.
      Jack3d_up and Reid like this.
    3. Grape Ape
      Grape Ape
      Hey Falko, Thanks for the reply. Belt tension is seems normal. No other symptoms to note right before the warning. Both times it happened, I accelerated up to around 70 mph, and after a minute or two the light came on. The belt tension could be suspect now too.

      I did notice a coolant leak / build-up at the thermostat housing though, but not enough to drain my coolant in a week. Also, a few months ago I noticed that my oil pressure is not what it normally has been during the life of the engine. It has dropped from 60 to 50 on the gauge. It has stayed that way since.

      This good ol' Jeep does have 205k miles on it, and could probably use a good freshening, but it's my daily driver so really can't let it be down long.
    4. Ron
      1- Id mke sure you have an actual temp problem and not a gauge sender problem. Free to check with a thermometer
      2- Water Pumps dont really go intermittently like stated they are usually good or bad.
      3- If its a mechanical fan a fan clutch could be suspect as well. Since you comment that the fan seemed to be working overtime I am guessing its an e-fan and you could hear it, if so moot point.
      4- Test the thermostat in a pot of water on the stove with the thermometer from step 1
    5. rockcity
      Maybe a head gasket issue?

      Check to see if you have coolant in your oil or coming out your tailpipe (steam), which may indicate a head gasket leak

      That may be suspect for different oil pressure readings
      ckruzer and Reid like this.
    6. shawn
      Electrical issue. Verify that it's actually overheating.
    7. Grape Ape
      Grape Ape
      Thanks guys. I'll try the hot water test and also try to pay more attention to any "other" noises.
    8. Fabrik8
      When you take out the thermostat to test it, buy a new one and test that one too. Then install the new one. They're super cheap and you'll have a known good one for probably the rest of the engines lifespan. You're testing the old one for proper troubleshooting, and you're replacing it for reliability.
      I suspect it's stuck partially open and not opening fully.

      It would be pretty rare to fail an impeller, so if the pump isn't leaking and isn't seized, it's probably not the pump at fault here.
      Last edited: Sep 24, 2015
      GotWood, Ron and Mac5005 like this.
    9. Mac5005
      Fan clutch. Electric fan is still cutting on as it should.

      Another free check. Bring engine up to 200 degrees or normal operation temp. Go drive around a while to make sure heat soak is accurate in engine compartment.

      Stop and shut engine off.

      You shouldn't be able to turn fan, as clutch should be locked up at this point. If you can turn it, replace the clutch.

      YJ, tj, and zj all lock the fan clutch at 170 degrees, not sure about xj, but I would assume it's similar.

      So you need to ensure not only the engine is at 170 and above, but that all the coolant, the radiator, and clutch are above that to ensure your test is accurate.

      Stop and go city driving is a good way to bring it up to temp.

      Check t-stat as mentioned above.

      Also Make sure your radiator cap is holding the actual pressure it should.

      I added a clean out fitting on a heater hose that allows me to pressurize system with a gauge, cold, or even with no coolant, to check everything.
      Jeff B and Ron like this.
    10. Ron
      Great tip. And bingo. Don't just replace it. You won't know if it's good nor bad and won't know if the new one is good, don't assume when trouble shooting it turns you into a parts swapper, but I'd change the $5 part either wayou for piece of mind.
    11. Tradarcher
      My Dodge truck is doing the same thing. On mine the radiator is cold though. Engine is in the red but I can put my hand on the radiator cap and it is cool to the touch. I suspect air in my rad or blocked rad.
    12. Reid
      if the water in your radiator is cool, you probably have a bad pump. no water being pumped through the block.
    13. Mac5005
      Or a tstat stuck closed, or bad cavitation due to air pocket at pump
    14. Grape Ape
      Grape Ape
      Wow, guys, great responses. I didn't have a chance to test / remove anything last night, but I did notice that the radiator hoses were still hot after almost 2 hours of sitting. I will be working on it over the weekend though and will let you know what I find.
    15. jeepinmatt
      Sounds similar to when my wife's ZJ had a split in the upper part of the radiator tank. It would get low on coolant, then the gauge would be all over the place as it intermittently pumped water past the sensor. Only clue was the coolant smell. I had to pull the radiator to find the crack, because it would only leak at higher temps when the water pump was at driving rpms and the radiator was full.
    16. Tradarcher
      So my problem was a leaking water pump that created an air pocket in my radiator when the volume dropped.
    17. Grape Ape
      Grape Ape
      Well, I took the Jeep out for a run this morning to get it warm and do the fan clutch check as Mac5005 had suggested. Looks like that failed, as the fan does spin with the engine running and warm (210-220 degrees), but once turned off, I can spin the fan freely. Also, it looks like the VALVE cover gasket another issue as coolant is leaking out of that.
      Can I just replace the valve cover gasket, or is there other stuff that I need to do while I'm in there? Like I said before, this is my daily driver, and I can't have it out of commission very long.
      Last edited: Sep 26, 2015
    18. rockcity
      You can replace just the head gasket. While the head is off, have it cleaned and tested for cracks. Otherwise if it is cracked and didn't fix, you would continue to have issues
    19. Grape Ape
      Grape Ape
      Hey Rock City, I meant to say valve cover gasket. It looks like I need to take the a lot off just to replace that valve cover gasket too............
    20. jeepinmatt
      If coolant is coming out the valve cover, you've got a bigger problem. There should only be oil in there.
    21. Grape Ape
      Grape Ape
      Uh Oh, like what?

      This was just a visual inspection. It looked like some was coming from the t-stat housing too. Obviously I would replace that too.
      Last edited: Sep 26, 2015
    22. Mac5005
      Sounds like a leak either at upper hose, or heater hoses or tstat housing to cause coolant to leak near valve cover gasket. Only oil under valve cover no coolant..... ?
    23. Mangonesailor
      Have you verified that the radiator is full after you topped off the jug? That's one thing I would check as well as you've already determined your fan clutch is shot.

      Even though you have filled up the jug, coolant can still flow through the radiator with an air pocket in it, or the block. As that air pocket moves around it can make your temp-gage read incorrectly, and therefore cannot be trusted.

      I'd pull the cap with it cold, top it off, squish some hoses to make it fill the voids until you no longer lose any coolant level and then install the cap. I use a lever-vent cap to do this, but the other way works great as well. This should be verified before electrically testing the system or installing a new t-stat.
    24. Grape Ape
      Grape Ape
      Update....drained the oil, looks good / normal with no coolant (I'm glad about that). Tested the old and new t-stats and yes the old one was bad. Replaced with new. Replaced the fan clutch as well. All radiator hoses seemed to be in good condition. Dropped it off at mechanic today for a proper system flush and to verify that nothing is clogged.

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