Author Topic: Coolant leak at high rpm  (Read 4084 times)

richard230sl

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Coolant leak at high rpm
« on: May 23, 2021, 16:26:53 »
I am struggling to find the most effective way of finding the cause of the following problem.

My engine runs great. And when I am cruising off highway everything is functioning fine.
However:
If I am driving at higher rpm s  (lets say 4000rpm )for a longer period of time for example on the highway, there is a pressure build up in the cooling system and i am loosing coolant.


I am thinking that the head (gasket) might have a small leak but I am not sure because of the following:
1 My oil stays clean, no mayonnaise in there.
2 The coolant stays clean: no oil there
3 no bubbles can be seen in the expansion tank when revving the engine: no smell of combustion gasses in it
4 The normal coolant pressure stays in the system after driving for a couple of hours. (When I open the expansion tank cap after the vehicle is at rest for 5 hours, there is still pressure releasing which makes me unsure what to diagnose: the head seems not to be leaking from that point of view)
5 radiator has been re cored and the temperature reading stays perfect: well below the red, of course it rises a bit on the highway.
6 No combustion smell can be detected when smelling at the reservoir.
7 The waterflow seems to be good and the system has been flushed; coolant is clean
8 I followed the procedure of bleeding air from the coolant.
9 The cap is releasing perfectly at the 1 bar pressure that is written on top of it.

Still I sometimes have an empty expansion reservoir and coolant is pressed out through several connections due to the pressure (small leaks at hoses, overflow from the tank)
The upper radiator hose is under pressure


Should I be thinking of a small crack in the head (gasket) or about air trapped in the coolant?
Any tips or thoughts about similar cases or tips on how to find the problem are welcome:
I would hate it to draw a wrong conclusion when simple things are overseen.
I will make sure to replace the thermostat and drill a hole in it for the air bleeding but am not sure if that will help. Also I will see if i can do a leakdown test.


Thanks for helping!



« Last Edit: May 23, 2021, 17:41:03 by richard230sl »

Tyler S

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Re: Coolant leak at high rpm
« Reply #1 on: May 24, 2021, 01:43:00 »
Even though you can’t smell combustion gasses in the coolant it still may be present. Sometimes head gasket leaks are so small that they only manifest under high combustion pressures and temperatures.
If a local shop has a gas analyzer (emissions testing machine), they can use the sniffer probe to check for small amounts of combustion gasses in the expansion tank. A fellow member and I used this technique to verify a head gasket issue on a 280sl.
Even better would be to have the car on a rolling dyno to duplicate the load at 4000 rpm.
One other thing would be to do a cooling system pressure check when cold. Sometimes when they are done hot the leaks are missed as the coolant evaporates before it has a chance to pool and be noticeable.
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Cees Klumper

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Re: Coolant leak at high rpm
« Reply #2 on: May 24, 2021, 04:01:51 »
My '90 Bronco was losing coolant steadily. Like your SL, there was no coolant in the oil or vice versa and no apparent leaks. However, when I took off the valve cover, the 'mayonaise' was sitting there, a lot of it. Replaced the heads (V6) and no more disappearing coolant.
Cees Klumper
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Mike Hughes

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Re: Coolant leak at high rpm
« Reply #3 on: May 24, 2021, 14:00:55 »
Like Cees, what I am about to relate is not about my Pagoda, but germane to the conversation.

I thought I had some kind of coolant leak in my '56 Dodge V8 pickup with only 13K miles, as I was constantly topping up the radiator.  I would notice the gauge would read higher when going up the mountain but settle down a bit going down and stay down on the flatter roads.  However it was a pain to restart when fully warmed up unless I raised the hood when I parked it.  Even though I had drained and replaced the brown coolant last spring it was brown again. Last fall it overheated on a longer run on fairly level ground.  Once it cooled down I pulled the thermostat, refilled with water supplied by a passing neighbor and started to drive the remaining five miles home.  It overheated again barely half way home!  Another neighbor passing by on his way to the dump came back with more water.  I let it cool down again, refilled it and got it the rest of the way home before it got too hot, drained the remaining (brownish) water once it cooled off and parked it for the winter.

After talking with a couple of locals who knew the truck before I bought it, the consensus was the culprit was a silted up radiator.  Sixty-five year old Dodge pickups do not have quite the same broad parts availability as Ford or Chevy pickups of that era.  Going on-line, I found a place down in Alabama that makes custom and reproduction radiators, and had a new one made (made in the USA!) and UPS delivered it 10 days after the order.  Last weekend Lucy and I pulled the original honeycomb radiator and installed the new one, which was a perfect fit, and a new thermostat.  The gauge reads rock steady up and down the mountain, it starts up right away after coming out of the Post office or the store without having to raise the hood when parking it, and the coolant level has not dropped or changed color after a week of constant use.

Low mileage vehicles that have spent most of their decades sitting in wait of the next occasional drive can have problems, too!
- Mike Hughes  -ô¿ô-
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richard230sl

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Re: Coolant leak at high rpm
« Reply #5 on: June 01, 2021, 18:24:32 »
Thanks everyone for the advise!

I took my car to a garage with a gas analyzer (thanks tyler s for the suggestion!) and we could not sense any combustion gasses in the coolant at first.
After I took the car for another spin and had the car work a little harder .16% co was measured in the expansion tank.

As it turns out I have a small leak in the head (gasket) which only appears when driven hard.






teahead

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Garry

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Re: Coolant leak at high rpm
« Reply #7 on: June 03, 2021, 21:48:46 »
 Stop leak is a band-aid fix for some other problem and i would never ever put it in my Pagoda unless i was planning a radiator flush and an engine rebuild in the near future.
« Last Edit: June 06, 2021, 09:22:23 by Garry »
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stickandrudderman

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Re: Coolant leak at high rpm
« Reply #8 on: June 06, 2021, 08:31:20 »
Stop leak is a band-aid fix for some other problem and i would never put it in my Pagoda unless i was planning a radiator flush and/or an engine rebuild in the near future.
For emphasis, I would never use it period.

Jordan

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Re: Coolant leak at high rpm
« Reply #9 on: June 06, 2021, 11:05:26 »

I thought I had some kind of coolant leak in my '56 Dodge V8 pickup with only 13K miles, as I was constantly topping up the radiator.  I would notice the gauge would read higher when going up the mountain but settle down a bit going down and stay down on the flatter roads.  However it was a pain to restart when fully warmed up unless I raised the hood when I parked it.  Even though I had drained and replaced the brown coolant last spring it was brown again. Last fall it overheated on a longer run on fairly level ground.  Once it cooled down I pulled the thermostat, refilled with water supplied by a passing neighbor and started to drive the remaining five miles home.  It overheated again barely half way home!  Another neighbor passing by on his way to the dump came back with more water.  I let it cool down again, refilled it and got it the rest of the way home before it got too hot, drained the remaining (brownish) water once it cooled off and parked it for the winter.


Mike, is your Dodge a 2WD or a Power Wagon?
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Mike Hughes

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Re: Coolant leak at high rpm
« Reply #10 on: June 08, 2021, 18:57:47 »
Its a 2WD.  You can see a photo in the first entry of this link:  https://www.sl113.org/forums/index.php?topic=28006.50

Power Wagons are another kettle of fish altogether.  They have a strong following and parts are far more available for them as Chrysler produced military and civilian models with very few changes for over thirty years.
- Mike Hughes  -ô¿ô-
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  Havana Brown (408)
  Light Beige (181)
  Cream M-B Tex (121)