Cooling System Mystery

[MilspecE6]

I purchased a '77 Dodge M882 (Comm truck) last week because I miss my old '74 Dodge W200 that I sold off years ago. Truck has great potential with almost no rust, but I have spent the last 3 days trying to get it road-ready and have run into a cooling system issue that doesn't even seem possible.

I first noticed something was off when I went to remove the radiator cap after it sat over-night and was met with a coolant bath...it was under pressure. It has a new aluminum radiator and over-flow tank, yet the system was under pressure. I also noticed that the lower radiator hose was leaking so I replaced that and refilled the radiator to about 1 inch below the neck. I left the cap off so I could look for any signs of combustion gas or oil mixing, plus I wanted to be ready to top it off after the t-stat opened.

Well, the truck idled for about 3 minutes, no bubbles or mixing present in the radiator. Then I started to see some coolant movement and the upper hose heat up so the t-stat was beginning to open. I walked over to the cab to grab a jug of coolant when the radiator turned into a volcano of scalding coolant all over the engine bay before I could shut down.

Now, I had already given the truck an oil change and tune-up and saw no signs of coolant issues in either one. There is no smoke or steam out of the exhaust either. Coolant was clean as well and yet my radiator blew up on me. I have never experienced anything like this and I admit to being fully stumped by it....and a little angry. I plan to pull the t-stat and run it again with constant flow to see what happens, but I don't know if that will really solve the mystery.

The truck runs and idles great, smooth and responsive, I have fixed up a dozen things already and ready to get it on the street to find out what I have, but this cooling system mystery has me at a loss.

 

[NDT]

Sounds to me that muck in the 50 year old block has plugged up the tubes in your brand new radiator.

 

[MilspecE6]

Sounds to me that muck in the 50 year old block has plugged up the tubes in your brand new radiator.

It is possible, can't rule it out. I am replacing pump and t-stat tomorrow to see if it is a matter of coolant staying in the block too long. Maybe by the time the t-stat opens it is really too little too late the whole thing over-heats / boils?

My gauge cluster stopped working (voltage limiter is working so I suspect a bad trace on the board) prior to all this so I have no idea what it happening internally temperature-wise. I may have to dig out my IR thermometer on the next start up. Really though, I am stumped by this whole thing, the pump is tight without any wobble or leakage and the t-stat opens up in about 3 minutes...yet volcano.

 

[87cr250r]

My CUCV with a 6.2 (I know it's a different engine) had a blown head gasket. It only leaked one way. The radiator would hold pressure for months if I wasn't driving it. Pop the cap, burst of coolant. Engine ran fine otherwise. As it got worse it started overflowing the overflow.

Anyways pulling the cap and getting s burst of coolant means there is are in the system. The sudden reduction in pressure causes that air bubble to get big. This may or may not indicate a head gasket issue. I say keep topping it up and if the problem goes away you are good. If it persists, you're probably going to be pulling heads.

 

[msgjd]

Having driven, maintained, and owned '69-79 LA-engines in cars and trucks incl M880's/890's, and big-blocks too since the mid-70s, I was initially going to suggest replacing the radiator cap or trying it without cap, but I see you had the cap off when it erupted.. My next thought was the water pump, and I see you are replacing the pump tomorrow.. You will get a small amount of convection flow seen in radiator via the thermo bypass , and of course as you noted, it will be more noticeable as the thermo opens.. I have seen waterpump impellers having corroded away to almost nothingness depending on what they were made of and the condition or PH of the coolant.. Some are steel, others are some sort of white metal, and some are cast aluminum..

As others have noted, the radiator core could be plugged with whatever was flushed out of the engine.. However I also have to wonder if there could be an obstruction within the heads or block that is creating an air pocket or "mud" pocket, and if coolant clears the obstruction and hits a hot spot, look out! .,. Best thing you could do before there's any damage is unhook the upper and lower hoses, manually flush the engine via the upper hose and see what pours out initially, then plug the lower hose and fill the engine, unplug the hose, and repeat a few times and observe what's coming out.. Using an old T-shirt as a filter can help see the fines that are coming out .. I would start there , and do same with radiator, even if your waterpump turns out to be bad when you remove it .. Make sure the arteries aren't clogged before you put that new heart in !

PS, also look to see if the expansion tank tube is clogged or pinched.. I have seen OEM tubes clogged with rusty "mud" .. One last thing, was the lower hose collapsing, especially at a higher than idle rpm? A restricted radiator combined with a lower hose without "slinky" insert will cause a similar eruptive scenario

 

[GopherHill]

I suggest removing a core plug in the block you can get to. It would be informative to see what has settled there.

 

[MilspecE6]

Having driven, maintained, and owned '69-79 LA-engines in cars and trucks incl M880's/890's, and big-blocks too since the mid-70s, I was initially going to suggest replacing the radiator cap or trying it without cap, but I see you had the cap off when it erupted.. My next thought was the water pump, and I see you are replacing the pump tomorrow.. You will get a small amount of convection flow seen in radiator via the thermo bypass , and of course as you noted, it will be more noticeable as the thermo opens.. I have seen waterpump impellers having corroded away to almost nothingness depending on what they were made of and the condition or PH of the coolant.. Some are steel, others are some sort of white metal, and some are cast aluminum..

As others have noted, the radiator core could be plugged with whatever was flushed out of the engine.. However I also have to wonder if there could be an obstruction within the heads or block that is creating an air pocket or "mud" pocket, and if coolant clears the obstruction and hits a hot spot, look out! .,. Best thing you could do before there's any damage is unhook the upper and lower hoses, manually flush the engine via the upper hose and see what pours out initially, then plug the lower hose and fill the engine, unplug the hose, and repeat a few times and observe what's coming out.. Using an old T-shirt as a filter can help see the fines that are coming out .. I would start there , and do same with radiator, even if your waterpump turns out to be bad when you remove it .. Make sure the arteries aren't clogged before you put that new heart in !

PS, also look to see if the expansion tank tube is clogged or pinched.. I have seen OEM tubes clogged with rusty "mud" .. One last thing, was the lower hose collapsing, especially at a higher than idle rpm? A restricted radiator combined with a lower hose without "slinky" insert will cause a similar eruptive scenario

All good suggestions...thanks. My first thought has been head gasket or cracked head from day one, but short of a gas sniffer or leakdown tester, I just don't know. I didn't see any signs of coolant on the plugs, none in the oil, no mixing in the coolant, and this truck doesn't smoke at all...yet I simply can't explain what is going on outside of a head gasket issue.

I had to work all day today and didn't get the chance to go forward on this. I have all the parts on hand and will be replacing the pump and t-stat tomorrow afternoon. The pump has been replaced before, but I have no idea when so it could be a flow problem where the hot coolant is hitting the radiator faster than the flow will accept. With nowhere to go, that coolant ejected out the neck. At least that is my hope at this point.

I have long enough list of tasks to complete on this truck, rebuilding heads wasn't on it, but my gut thinks it will end up there.

I have been wrenching since I was 7 in my Father's repair shop, yet I have never come across this situation. A bad head gasket was always pretty easy to confirm in the past, but I have never seen a radiator under pressure when cold like this.

 

[MilspecE6]

I suggest removing a core plug in the block you can get to. It would be informative to see what has settled there.

IF the water pump doesn't tell me anything, it is going to the shop. They have a better ability to pull core plugs and clean it out than I can...they can also do the leak down test. I just hope it doesn't come to that because I have never had an engine get new heads without losing the bottom end a year later. If it is head gasket or cracked head, it might be better to just drop a 360 in there to gain the 4 bbl and eliminate the Carter BBD issues at the same time.

Definitely going over-budget on this one, something I really wanted to avoid.

 

[GopherHill]

I guess that old 2 part cooling system flush isn't around any more. I cleaned a lot of rusty mud out of my M37 cooling system when I got it home.

 

[87cr250r]

Dishwasher detergent is part one and phosphoric acid is part two if you can't buy it locally. If you don't like using your own chemicals your local heavy duty diesel dealer has flushing kits.

 

[MilspecE6]

I guess I should include a picture of the project...previous owner might have been a member for all I know.

 

[msgjd]

I suggest removing a core plug in the block you can get to. It would be informative to see what has settled there.

good suggestion but need good luck when it comes to getting one to turn out.. I didn't mention it myself because I never was successful way back when, and never bothered to try since .. But yes, if the plug was pulled there would likely be casting sand and crud buildup in there, as I usually see on tractors and other engines.... With LA-blocks (273/318/340/360) , a plug can be found on each lower side next to the motor mount .. It's not easy to get to.. Good luck, milspec6 !

 

[msgjd]

A bad head gasket was always pretty easy to confirm in the past, but I have never seen a radiator under pressure when cold like this.

if head gasket issue with no coolant in oil nor vice-versa (compression leak only) , you would see bubbles in the coolant, no? .. And with a functional radiator cap, there should be abnormal blowback into the expansion tank while running, no? .. You are correct, you have a puzzling situation..

Not saying you don't have a head or head gasket issue, but my 70's CJ's and dodge's all have a small degree of pressure at times when "cold", especially during wider differentials of overnight/afternoon ambient air temps .. As the day heats up, so does the coolant thus expansion.. By evening, there can be some pressure under that cap, sometimes enough to blow out a little coolant when you release it,, if the coolant level is right up to the tippy top .. But certainly there should not be a geyser

 

[MilspecE6]

I spoke with a trusted mechanic on my mail route and described the situation. His response was that if this was a Chevy, he would suspect a cracked head immediately. Still thinks likely a cracked head, but also admitted that there was a slight chance it could be traced to the water pump not moving coolant. He must have felt sorry for me, because he gave me a coolant tester for detecting combustion gases in the radiator.

I just got home from work and after a quick lunch, will start on tearing things down. I plan to check for flow through the entire system, replace pump and t-stat, then if the problem persists...break out the combustion gas tester. Not real optimistic about what I will discover, but I need to solve this mystery either way.

 

[87cr250r]

If you open the t-stat housing, fill with water until it overflows, and run the engine with the belts off you may see bubbles indicating a leak.

 

[MilspecE6]

If you open the t-stat housing, fill with water until it overflows, and run the engine with the belts off you may see bubbles indicating a leak.

Not a bad idea, but it might be inconclusive. There may not be any leak until the heads / intake heats up and expands. If I wait for that to happen at the housing

 

[MilspecE6]

Well, that was fun. I haven't turn wrenches that much since I sold my Jeep Scrambler 12 years ago. I thought I wanted to get back into working on old rigs, but right now I am starting to wonder.

First, the stupid radiator drain didn't open...why are they always junk? So, I had to pull the lower radiator hose to drain the system. Normally, I cut the hose and let it drain cleanly into a bucket, but this was my last hose and only 2 days old, so I crawled under with a bucket and pulled the hose. I underestimated the situation as I was trying to catch the coolant from the radiator, I was getting drenched by the hose off the water pump. It was not a good start.

I first pulled the t-stat hoping to find a gummed up or wrong part, but it didn't look bad. Surprisingly, it was a 180 degree stat, so I am starting to think the PO knew he had an over-heating issue for awhile. A new radiator, cooler stat...could be a good indicator that it has been running hot. There was copious amounts of RTV under the housing along with the paper gasket which drives me insane. I really don't understand why people do that, but I spent plenty of time getting it all cleaned up. The intake looked pretty good inside, I took a bore scope in there and didn't see anything unusual. A little bit of rust, but really not bad.

When I pulled the water pump, same thing, looked okay, but did have a lot of RTV along with the paper gasket. I really wanted to find a bad water pump, but it was good and actually looked pretty new. Seals were good, shaft was tight, really nothing to indicate a circulation problem. That was very disappointing as it likely means that my motor is junk.

I spent hours putting it all back together, fixing some minor things that I discovered in the process along the way. It is all buttoned up minus the t-stat as the parts store gave me the wrong one (too small) but it really doesn't matter now. I will be the idiot with a lot of new parts on a junk engine...what an idiot.

BTW, I never use RTV with paper gaskets. I always use Permatex Aircraft sealant on one side of the gasket to help hold it in place for assembly and have never seen one leak in decades. I might have jinxed myself and see a leak tomorrow, but the history has been very good to date.

I will finish it up tomorrow and try the gas tester, but I think the answer is already clear. The only questions now are do I rebuild this motor or swap out for a 360? Actually, the 3rd question is do I cut my loses and bail out of this project as it already is going sideways?

 

[NDT]

You bail on 880s due to severe rust, not issues with a garden variety 318. Stay the course is my thoughts.

 

[MilspecE6]

You bail on 880s due to severe rust, not issues with a garden variety 318. Stay the course is my thoughts.

That is what got me to buy it in the first place, I have never seen one without rust, I can't say that I am not disappointed though. I really entered into this with a budget in mind and rebuilding the motor right out of the blocks wasn't in the planning.

 

[msgjd]

i bet a hundred good-running 70s-80s 318s are out there sitting in rust bucket cars and trucks all around the US .. I have kept all but two trucks working on a rotation of salvage engines for the last 45+ years .. One of the jeeps had its original engine machined but it never got reassembled nor bearings, pistons, etc... The bare block and crank have been encased in grease in a crate since 1982.. Three $100 to $200 junkyard replacements have kept it alive since.. As it did way back when, the cost of rebuilds far-surpass my time and cost of dumping a used motor into them every 10-15 years or more