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Check my thinking, gang?

Just checking to be sure I’m thinking along the correct path. My 88 Mitsubishi pickup finally stranded me this morning. Been waiting for that to happen.

I replaced the lower radiator hose last weekend because it had a small leak in it. At the time I noticed that the reservoir tank cap had come off, but I failed to think much of it.

This morning I went over bumpy railroad tracks and instantly got big clouds of steam. It’s one of the heater hoses. Large leak because coolant is everywhere. Thing is, they’re only a couple of years old and still in great shape. It looks like the coolant leaked past the joint rather than from a hole in the hose.

And the reservoir cap came off with enough force to pull the dip tube completely out of the bottle. So clearly I have an overpressurization problem.

It could be a HG leak, but the coolant that drained out when I did the hose last weekend was perfect - absolutely no sign of anything in there that shouldn’t be, and I don’t have anything suspicious coming out of the exhaust.

I’m thinking replace the radiator cap and thermostat because one of them is probably sticking - most likely the cap, but the thermostat is cheap and easy - as well as the hoses just in case.

I’m pretty sure I’m on the right track but want validation! :wink:

Not the thermostat, unless you were big time over heating. Hope a new cap does the trick, but I’m skeptical.

Head gasket?

Check the plugs just to be sure. If any look bright white clean, you’ve either just changed them or they have been burning coolant. Rad cap is cheap and possible reason as well.

Plugs are brand new as of last fall, and the truck’s been driven maybe 200 miles since then so they won’t show much. The old plugs were definitely not clean.

I’m hoping it’s not the head gasket, and I’m not seeing any indication of it in the coolant or oil. I would think if it was a big enough leak to blow the reserve cap off that hard, the coolant should not be bright, clear green.

HG’s can and do fail in a one way check valve sorta way…they let cylinder compression leak into the cooling system…but not coolant into the engine… seen that many a time.

If you dont feel like doing the proper repair… You could check your cyl head bolt tension… or…just Blue Devil it… Blue Devil works like a mineral rich solution and seems to form a sort of “calcium” deposit…the stuff looks like wiper washer fluid…but it sure as hell works…and its permanent when it does work. Just a thought because the vehicle is so old…you might not be up for the proper repair…which you know full well what that is.


I’m thinking head gasket too. You need to have the coolant checked for combustion products first. Put a new radiator in and it’ll blow it out. On the one I had, there was no indication in the oil or the coolant except for the bubbles you’d see in the radiator.

I suppose I’ll pick up a test kit, then.

Yeah, I doubt if it is the HG I’ll bother replacing it. Probably more economical just to find another truck.

The Devil will help you… Muhu hahaha

Automatic trans, yes?
Here’s a little test that requires a helper:
With the engine cold take the radiator cap off.
Wear eye protection.
Have the helper put it in reverse, left foot firmly on the brake and press the gas for a second or two.
If a lot of bubbles and coolant spew out of the radiator you know the deal.
Put in reverse so if it slips the vehicle will move away from you while you’re standing in front of it.

Why you do that? @circuitsmith

I had a steady stream of bubbles just in idle. Made it home by loosening the radiator cap. Of course that was the famous diesel that always blew head gaskets.

Yeah…my dad ran a 65’ Plymouth Belvedere Slant 6 for over 10yrs with a blown head gasket like this… It would leak compression into the cooling system but nothing else. He put a razor slice in the rad cap gasket and motored on… Never a problem…but he knew about it so he checked fluid levels and such…kept an eye on it, but never had any major probs with it like that.

Hope the HG is good. One other guess before lower radiator typically came with a big spring inside to prevent collapse, If the radiator could not keep up with demand the lower hose would collapse preventing all flow, most of the time it was an overheat problem, but if the pump could build high enough pressure sure, why not, then add overheated coolant to the mix, maybe even getting to the boiling point,

Just a thought. But sure you plan of attack sounds fine.

You plan seems like the correct approach OP.

Blackbird, to raise the combustion pressure, which goes against the HG.

Oh… No need for the trans in any drive gear for that @circuitsmith …that only raises the pressure of danger. LOL Neutral with increased rpm’s should suffice dont you think?

Sure, try the milder test first, but sometimes the view is best at the edge of the cliff.


Well, that was interesting. I got a chemical test for hydrocarbons. For those that don’t know, it’s basically a large-bore syringe with a vacuum line. You pour a blue fluid in it, hook the line up to vehicle vacuum, and stick the bottom end over the radiator fill neck. The vacuum sucks a sample of what’s flowing through the radiator, and it’s theoretically weak enough that it will pull vapors without pulling coolant. The vapors bubble through the blue fluid, and if hydrocarbons are present the fluid turns green and then if it’s a really bad leak, yellow.

Well, the trouble is that sometimes the vacuum is a little stronger than the designers anticipated, and it sucks coolant in, which is green, and so your heart drops and you think the worst until you notice there’s more fluid in there than you started with. Oops.

The retest worked better and came back negative, so if I do have a HG leak, it’s miniscule and didn’t cause the issue.

However I did find a hose I didn’t know about (because this is the first carb’d vehicle I’ve fooled with since I started doing my own car work) which runs coolant into the recirculation barb on the carb. A guy on a vehicle-specific forum helped me identify it. You can’t see it until you get the whole air filter assembly off, which is how I missed it in my pre-summer inspection.

It crumbled in my hands when I went to take it off, so I think a lot of previous owners across 200,000 miles missed it too. :wink:

I also discovered that the overflow hose is clogged with some sort of crud, so I’ll replace that while I’m at it. And I’ll put a new radiator cap on as well, and I’m thinking that should solve the problem at minimal expense.