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Discussion Starter #1
Ok, so I have an 08 LMM. Recently I have been getting a message on the dash every 4-5 days stating that engine coolant is low, please add fluid. I’ll wait for it to cool down then top off the reservoir tank. It takes maybe 1/2 of a quart to top it off each time. Never noticed a puddle under the truck and I am unable to trace any leaks.

The other thing is my heat isn’t working as good as normal. I think it is running under the normal temp. I have the CTS2 monitor in it and that is saying the coolant is at 165*, but the gauge in the cluster is reading about 200* so I’m not sure which one to trust. I had my plow on the truck over the weekend and did a little driving and the temp on the CTS2 read 220* and the heat was working awesome. Took the plow off and went back to 165*.

My thought is maybe a bad thermostat. Anyone else have this or similar issue or have any ideas what else to check?
 

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Bad thermostats won’t cause you to lose coolant, but they will cause it to run cooler. The CTS2 is the one to trust, the dash gauge is off.

All of the symptoms you’re describing point towards bad head gasket’s. Easiest way to check is to check your upper coolant hose in the morning after it has sat overnight. Bad sign if there is any pressure in it after the truck has totally cooled off.


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Discussion Starter #3
My first thought actually was head gaskets, but my mechanic said that’s not the issue. He did a quick check and ruled that out.
 

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He did a quick check and ruled that out.
There is no quick check on a Dmax.

1st, find the source of the coolant loss.
Wrap a rag around the overflow tube on the coolant surge tank, see if it's getting wet when driving.
 

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Like most all waterpumps, the Dmax water pump has a weep hole. I'ts veiwable with a flashlight thru the drivers side fenderwell.



My LLY had a s-l-o-w leak, and the coolant would evaporate off the hot engine block before it ever dripped on the ground.

Also, if this is the problem and you swap out the water pump, some have reported a gear noise for awhile ....like a "whirring" sound... while the new w/p gear mates with the other drive gears. While not normal, it does occur. Mine did this too.

Harbor Freight has a radiator pressurizer kit.... it's worth owning to find pesky leaks, like the weep hole.

https://www.harborfreight.com/radiator-pressure-tester-kit-63862.html

I've used mine, more than I thought I ever would..... it's pretty handy
 

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Two other possibilities.

1. The two plastic Quick Connects where your heater core nipples pass through the firewall and connect it to the coolant system. They are known to leak after time.

The fix for this is to remove them (takes a quick connect de-coupler) and put on new heater hoses with worm gear clamps

2. There is also a small 3/8" hose that connects the coolant system from your turbo to a valve on the aft side of the cross tube on top of the engine.

The fix is to replace the hose with Silicone Heater Hose so it will take the heat from the engine block which is in close proximity. Again you can use worm gear clamps instead of the factory spring loaded crappy ones.

Good Luck, let is know what you find.
 

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Two other possibilities.

1. The two plastic Quick Connects where your heater core nipples pass through the firewall and connect it to the coolant system. They are known to leak after time.
https://www.duramaxforum.com/forum/04-5-05-lly-duramax-powertrain/1004227-lly-headgasket-time.html

I have a set of plastic and a set of aluminum quick disconnect tools. I could not get them in there, so I used my Fein tool, split the connector and used a flat blade screwdriver in the split to open it up and slide it off....

 

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https://www.duramaxforum.com/forum/04-5-05-lly-duramax-powertrain/1004227-lly-headgasket-time.html

I have a set of plastic and a set of aluminum quick disconnect tools. I could not get them in there, so I used my Fein tool, split the connector and used a flat blade screwdriver in the split to open it up and slide it off....

When I did mine I had no problem with the inboard connector but it took some working the plastic tool in behind and into the outboard nipple. On each one I had to use a small water pump pliers to pull it into the connector though.

Just have to be VERY careful not to pierce the copper nipples from the Heater core with that vibrator cutter. I have one of those tools from HF, I LOVE it!!

I also like your pressure tester!
 
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