Coolant boils over when engine shut off?

I took my truck on a trip from California to Montana and started noticing the truck was running hotter than usual somewhere in Idaho. I had hit a several massive swarms of locusts out in eastern Oregon so I thought maybe the condenser was plugged up with dead locusts so I pulled into a rest area and pulled the grill off and cleaned it out as best as I could. since I was stopped and I had a spare thermostat I decided to replace it once it had cooled down enough. the new thermostat was a 180 degree unit as opposed to the stock temperature 190 one.

it seemed to run a little cooler for maybe 30 minutes but then the temperature climbed back to hotter than usual. I pulled over again and took a look at the fan, I noticed that it would spin freely by hand. I figured that the fan clutch was shot and no one in the area I was in had a spare so i just kept moving but with the AC off and keeping an eye on the temp gauge, pulling over as needed to let it cool down.

when I got to Montana I ordered a brand new Aisin fan clutch and a good used fan blade to replace my slightly warped one that had some damaged blades. I also flushed out the coolant since it has been pretty rusty and now its nice and clean. I also inspected the radiator cap that I had replaced about a year ago with a good quality Sankei brand made in japan one since the Napa Balkamp ones would go bad after a couple months, it looked good too.

sometime between the trip and now it has developed a weird symptom where it will boil over when I stop and shut off the engine after a drive. It does it so much that the overflow bottle vents out the vent tube onto the ground. If I start the truck and let it idle a bit it will take all the coolant back in and stop boiling after just a minute of idling.

any ideas? I suppose the one thing that hasn't been checked in a while would be the water pump, I sure hope the impeller isn't eroded away!

The thermostat may be bad. I’ve had new ones fail, especially the fail-safe variety. Try running it without a thermostat and see if it boils over after shut off. If it won’t get hot enough, restrict air flow through the radiator with a piece of cardboard. Also, how old is the radiator? It may be plugged with the rust and crud. Insert a long, stout wire into the passages accessible via the cap. If it won’t reach the bottom tank, the radiator needs cleaning and rodding out.

This is why newer vehicles have electric fans that run on their own regardless of whether or not you are driving.
When overheating and you have to drive it also helps to turn the heat all the way up and the blower motor on high.
I wish I could help but I have a similar problem with mine not being able to stay cool in warm weather. I lost a head 40 miles up the interstate the only time I tried driving it on the highway out of town.

When you shut an engine off the coolant temperature increases for a short while. If it's already running hot, it can boil over. I have had several causes of slow heat buildup with my 2.6. First was a cracked head, drove the truck for 10k miles on that head and only changed it out because I was starting a very long commute. Second was the stock fan and clutch could not keep up in heavy traffic on a very hot day. Third was that about 1/3 of the fins rotted off the core of my radiator.
Given the G54B's penchant for cracking jet valve heads, I would do a combustion gas test in cooling system first.
Available at most auto parts stores, speed shops, or Amazon
https://www.summitracing.com/parts/lil-75500

Thanks for the replies! I may try putting the old thermostat back in since the new one may be faulty as suggested, Im starting to really distrust most non OEM parts. The radiator is original equipment and before i ran some flush solvent I did notice one of the passages did look a bit plugged, I will have to see if anyone in this area cleans radiators. I have another stock radiator back home that is in very nice shape, although it appears to have had some of that stop leak junk run through it so it will need a good cleaning too. maybe ill look into a new radiator until I can get my stock one cleaned? I forgot to mention that the truck does run cooler for a while now before it finally climbs to right in the middle of operating range. the truck seems to be boiling over even in cooler weather too and the temp gauge is right in the middle when it does this. I should also note that it does climb higher than the middle of operating range when pulling hills so I'm thinking that the temp gauge is working properly. Ill try pulling the thermostat first and seeing what what that does. Ill check for water pump function too while i do this.

Hows your radiator cap, if its not holding pressure it will boil over lower.

I'm pretty sure its good. its a Sankei brand one that's made in Japan and has good rubber. All the cheapo, made in mexico, store brand ones would fail after about 2 months of use so I got what I thought would be a good quality one and the rubber looks like new. Maybe ill see if i can get a NipponDenso one?

Check your overflow bottle diaphragm. If it has perished, you'll slowly lose coolant and then you'll have a overheat issue from lack of coolant return. Next is a chemical flush. I would remove the radiator and fill it with a batch of citric acid powder dissolved in hot water (only have to leave the cap on and have it lying flat with the inlet/outlet facing upward) You will probably need to drain, flush and repeat the process a few times (if you have pressure cleaner, this will help will freeing scale and rust in the cores and bottom tank). Give the radiator a decent coat of black epoxy paint too. It does help with heat dissipation.

Oh? I didn't know that it had a diaphragm? I thought it was just an inlet tube with an overflow port at the top. I will have to give that a good close inspection. I did start the truck earlier and ran it without a thermostat and covered the radiator after a while since it was taking a long time to hit operating temp. I did notice once it cooled down that it wouldn't pull coolant back from the bottle into the cooling system, of course it didn't cool completely but it normally seems to do it when it cools just a bit. I'm starting to think it may be pushing coolant out but not taking it in, then boiling due to air in the system. I'm going to take a look at it very closely and see.

just checked it out and the bottle has a seal on the edge of the cap but no diaphragm. I disconnected the hose and sucked on it slightly and then blew into it to see what passed vacuum (i really need to quit being a tight wad and buy a proper vacuum pump/gauge:grin:). it let air into the radiator but also let some out. next I cleaned the radiator cap for the 10th time and put it back on, now it lets air in but not out ( so coolant can go back in the radiator but has to be forced out at the pressure rating of the cap). I'm thinking that the radiator spout has some wear and isn't quite sealing up properly sometimes.

I just found this free shipping. https://www.ebay.com/itm/Coolant-Overflow-Bottle-Tank-For-Mitsubishi-L200-Mighty-Max-Pickup-1986-1996/164264908879?fits=Year%3A1987%7CModel%3AMighty+Max

I grabbed one just in case that is my issue too. The top on mine keeps popping off and I can't find the top I grabbed at the JY the last time I was out there.

Interesting. it looks like it is a different shape than the stock one for my 87' but it appears to have the same mounting slide. my coolant bottle did crack on me but I have one back home so I had my parents ship it to me, in the mean time I used a soldering iron to melt mine back together for the time being.

I did just get back from a fairly long drive of about 30-45 minutes and the needle stayed right in the middle with the old 190 degree thermostat. it did not boil over when I parked it and the hoses all were pressurized nicely. I did notice that they were a bit more pressurized than usual and my original upper radiator hose has swolen up and has a tiny pinhole leak when you bend it, not bad for 33 years of service though! I would order one from the original supplier if I knew who that was and if they even are still available. for now I have a good barley used one from my parts truck that I brought with me for if I needed it. It is a different shape than the original but will work for now.

Keep an eye on your cooling system. They can develop leaks in places you can't see. 2 places that are the usual suspects are the external coolant pipe that runs around the back of the block and (mostly relates to G63B's) is the coolant chamber in the base of the inlet manifold. They are known to corrode out and form small pin holes from all of the crap from the block slowly collecting in it. And don't quote me on the diaphragm. I've seen overflows with and without, but if coolant isn't returning to the system and you're getting air you must be losing coolant somewhere.

I took a good close look at the radiator fill port earlier after swapping out the bad radiator hose and noticed the top lip was deformed ever so slightly. I tried blowing into the overflow hose and heard a leak. I then removed the cap and rotated the inner seal assembly a bit to see what would happen if it seated in a different orientation and blew into it again. I did this several times and it would leak a little about 2 out of three times. I then got a scotch bright pad and carefully polished both the lower and upper seating surfaces for the cap. once this was done I could see the low spot on the upper seating surface so I very very carefully raised it with some pliers and then re polished it. I then repeated the cap removal and re seating checks and it now holds at about 4 out of 5 times. I started thinking some more about it and released that most likely my problems must have been caused by the lower seal not seating since it does most of the pressure holding and the upper simply does the job of directing the coolant to the overflow and holding vacuum to pull it back in. it seems to be holding vacuum well now too so it probably is good to go now hopefully.

Ill definitely keep a good eye on everything else now too since it seems to have some more pressure in the system now that it doesn't leak.

I peeked in the old upper hose that had swolen and the inner rubber lining had several splits in it running along the bottom and it appeared the inner webbing was starting to fail as well. its good I replaced it when I did since there was not much keeping it from totally blowing out.

The cap on my overflow has been popping off for a while. And it seems to lose below the fill to line. I haven't noticed any leaks. Once I get the new bottle and make it fit I will try some safe long distance rides to see how it does cooling wise. I also have a new since I blew the head fan clutch and an OE cowl.

Quote:

---

Originally Posted by StarquestMan

Thanks for the replies! I may try putting the old thermostat back in since the new one may be faulty as suggested, Im starting to really distrust most non OEM parts. The radiator is original equipment and before i ran some flush solvent I did notice one of the passages did look a bit plugged, I will have to see if anyone in this area cleans radiators. I have another stock radiator back home that is in very nice shape, although it appears to have had some of that stop leak junk run through it so it will need a good cleaning too. maybe ill look into a new radiator until I can get my stock one cleaned? I forgot to mention that the truck does run cooler for a while now before it finally climbs to right in the middle of operating range. the truck seems to be boiling over even in cooler weather too and the temp gauge is right in the middle when it does this. I should also note that it does climb higher than the middle of operating range when pulling hills so I'm thinking that the temp gauge is working properly. Ill try pulling the thermostat first and seeing what what that does. Ill check for water pump function too while i do this.

---

Email GARY: parts@mitsubishipartscentral.com

He sent me a genuine OEM Mitsubishi thermostat ~ works good so far :thumbup:

Overflow came. It fits fine. I like the tight snap of the top. Seems to be made to clear the wiper washer tank.
Attachment 25478Attachment 25479

85Ram50 ~ good catch on ebay ~ it fits & it holds more than the original tank... Do you plan to put a length of tygon tube for drain to run down through the tank mount..?

Yes George, that is what I had on the other. The outlet nipple is a lot larger on this top so I couldn't use the old hose. I expect its because people were hooking up the hoses wrong. If you blow through them the one goes out the tube that hangs inside the tank the other goes out right inside the top.