Enemies of Liberty are ruthless. To own your Liberty, you'd better come harder than your enemies..

Saturday, August 31, 2013

Of Radiator Caps...

Can anyone give me a good reason why someone would replace the OEM (16psi) radiator cap on a Jeep 4.0 inline 6 with a 4psi cap that likes to spit radiator goo on a hot highway?

K

12 comments:

  1. No. Bad juju to have the wrong cap. One of the first things i replace when cooling systems start to go sideways cuz they is cheap and so am i...

    ReplyDelete
  2. 2 reasons I can think of:
    1: Radiator/head/hoses leak like sieves at 16psi
    2: Only thing they had at that time.

    3: that's all the pressure MVB can handle...and...nevermind...I got nothing else.

    ReplyDelete
  3. Head Gasket.

    If the gasket is blown and exhaust gas is getting into the coolant passages it will build up way too much pressure.
    Throwing a weak assed cap allows that pressure to vent.

    Obviously I can't diagnose something like that over the internet but I have seen this before. If this "goo" you speak of looks milky or has thick, mayonnaise consistency sludge around it then it is for sure the head gasket.

    Either way, you need to get it to a competent mechanic pronto.

    Of course you can try the OEM cap, just watch VERY carefully what happens when it comes up to operating temperature with the hood up at a safe distance.

    You could also start it cold with the cap off and as it warms up start looking for bubbles steadily coming into the radiator cap seat area.




    ReplyDelete
    Replies
    1. Thanks, Knuckles - my thought too. I ran with the new cap about 50 miles pretty hard (after getting the air pockets out), AC blasting, no hiccups. Tomorrow when everything is cool I'll pull the cap and see if we get bubbles. Oil looks good, no milky ring on the oil manifold cap - but I'm still keeping a wary eye.

      K

      Delete
    2. OK - started it up this morning cold, with radiator cap off. No bubbles or spurting at all, levels in overflow tank remained constant from last night, no sign of drips anywhere. I'm still reserving the possibility that the gasket has a flaw (versus a full blown failure) that will only show itself after a good distance on the highway. Unfortunately, all my big boy tools are many miles from here, and it's a holiday weekend - not a fun time to be crossing the vast open spaces of South Dakota.

      But, life is an adventure. ;)

      Roll the bones...

      K

      Delete
    3. Carry a few extra gallons of water for emergency radiator refills. I dont wanna see slammin sammy as a bleached skeleton in death valley...

      Delete
  4. Low information mechanic.

    Higher pressure cap raises boiling point of coolant, a good thing. Also check the thermostat is OEM. 195 deg is best operating point for modern engines. Vintage cars had 160, but computer controlled engines will have trouble with low temp and water in crankcase.

    ReplyDelete
  5. Bustednuckles........is right on as I have run into this being in the cab and auto repair business.

    ReplyDelete
  6. Regarding thermostats...DO NOT use one of those fail safe Motorad thingies. Complete and utter garbage. When they fail, they fail open.

    I put a brand new Motorad in my milk-wagon and it failed immediately...in the middle of last winter...stuck open...which means engine never gets up to temp...which means no heat...which means those 0 degree days sucked balls until it got warm enough outside in my driveway that I could put in an OEM Stant T-stat.

    After that, engine temp quickly got to a toasty 180-190 degrees, and all was right with the world.

    I can live with no AC in the summer. No heat in the winter blows. Hard.

    ReplyDelete
    Replies
    1. Ended up with a Stant from Walmart - every auto parts shop in the area was already closed for the weekend. The worst part about this era of Jeep is they don't have actual gauges - just warning lamps. By the time the lamp kicks on, you're already in trouble. This bad boy is going to take a little work and a few mods to get up to snuff.

      K

      Delete
    2. Busted is right on. These engines were susceptible to cooling system rusting when using ethylene glycol- the green antifreeze and required flushing every 2 years for proper maintenance- 5 yrs for extended-life stuff. If not flushed, the antifreeze turns acidic and starts to eat away gaskets and rusting the cast iron block/head. The rust particles will collect and start to clog the heater core and radiator- the radiator is not very big on the Cherokees and doesn't take much clogging to affect cooling.
      Using a lower than spec'd pressure radiator cap is not recommended. System should operate with no problem with the specified pressure cap. For every pound of pressure, the boiling point increases approx. 6 (if I remember correctly) degrees hence never take a cap off of a hot engine.
      Another issue, more especially high-mile units is the radiator fan clutch. A weak or failed fan clutch will cause overheating in traffic/idle scenarios but system works ok on the highway. The fan clutch stalls when road speed is attained thereby using ram air to passing through radiator and properly cool antifreeze.
      If I were you, I would also buy a spare crankshaft position sensor- high failure rate... just did one on Friday.
      I have been in the biz-ness for over 40 years and on my own for the past 33 years. I would be happy to work on the Jeep. I am only 12 miles north of Hagerstown... close to Miller too. Let me know if I can helpand will honor III to III anytime!

      Delete
    3. Wrench: Thank you! I will definitely be dropping this Jeep to you from time to time for maintenance and upgrades. I want to stay ahead of as many problems as possible, since we'll be doing so much traveling with the entire family aboard.

      We'll talk when I get back to the DC area after Brock's gig.

      K

      Delete

Please post anonymously and include your recognized online handle in the body of the comment.