Radiator cap leaking
 

I just returned from a 3000 mile road trip. Midway through the trip, the top coolant cap(near the thermostat) started to blow off through the cap. I replaced the cap with a 13 psi cap, but the problem continues. I would write this off to the coolant finding it's own level, but when this is happening, no coolant is going into the overflow bottle. This continues until the low coolant buzzer starts.

I have checked the engine temperature with a point-and-shoot thermometer, and the engine temperature never goes above 160 degrees, while the radiator temperature hovers just under 160 degrees, so the car is not overheating.

The engine exhibits no sign of distress, and purrs like a kitten at all speeds and idle.

Anyone got any idea what the heck is going on here? I am at my wit's end with this.:scratch:

sounds like a coolant seal blew. top off your system, and let it idle for awhile (half hour or more), leave the thermostat cap off during this time and watch what the coolant does. I have a feeling it will start bubbling nonstop

I just did what you suggested, and there were, in fact, a few bubbles coming out, but not any kind of a steady stream and after a short while they stopped.

My question is, would there not be some other indication of this problem? ie, smoke from the exhaust or rough running?

Also, why does the coolant escape from this point rather than into the over flow tank?

Not being stupid here, just a bit confused. Thanks.

Is the passage to the overflow clear?

the engine should be over 160F when warm, how are you monitoring temps?

Yes, the overflow passage is clear.

I am monitoring with an infrared thermometer. Since it only measures the external surface temp, I expect it is a bit warmer than the readings show.

is that as hot as it gets or just until it starts spitting coolant out? has the gauge ever gotten to half or higher?

That's as hot as it gets. The inside gauge never rises past 1/4. there is no indication that the car is running hot at all, except for the steam rising off of the engine where the water hits. What is coming out of the cap is water, not steam.

Earlier this evening I ran the thing without the cap on for about 45 minutes. There was some bubbling at the beginning, but after a while it just sat there with no problem. You could see the water flowing past through the passage. Needle never got above 1/4, a bit lower than that, actually. Smooth idle, etc.

I have had this car and been running this car for over a year and a half, and it has just started doing this. New water pump, hoses and thermostat. I am also located in Florida, near Tampa, and have run this car in some pretty hot weather with no problem.

well im only skeptical because it could only run that cold with a broken or missing thermostat, since you see coolant flowing then one of those has to be true. however that still doesn't explain the purging coolant, it could be a damaged filler neck or debris not allowing the cap to sit flush.

i'm probably a bit too far for you to want to take a trip to have it looked at, about 2 hours from you.

when did you replace the water pump and all the other bits and pieces? did it start doing this right after replacing those? How/did you burp the system afterwords?

Probably coolant seals, but you could check your radiator. May be partially clogged, but i wouldn't put money on it.

I replaced the thermostat soon after I bought the car, about 1 1/2 years ago. The water pump, about a month before I left for my trip and the hoses sometime last Summer. I have never really had any trouble with "burping" before and am familiar with the process.

What temperature should the engine run?

Rotary Evolution: I am not adverse to a two hour drive. Where are you located?

if you did all that, you might as well test or replace the radiator considering its the last variable besides coolant seals.

Yes, that thought has occurred to me.

Or you could go down to speedy or jiffy lube, and they should be able to pressure test your coolant system. its like 150 bucks.

a shop might be able to tell you if there is a leak but probably not internally definitively unless they know rotary engines. a speedy lube place is the last place you would want to go for anything besides an oil change, most of the workers have no experience actually working on cars. they could cause more harm than good, a reputable regular repair shop or rotary specialist are the 2 best options.

there's actually 3 ways the engine itself can leak internally:

combustion gases only goes into the cooling system
coolant only leaks into the engine
coolant and combustion gases flow in either direction

this depends on the type of coolant seal failure and the location where it is in the engine.

it may be a simple issue, it might not.

I am gonna go down to Autozone and "borrow" their pressure tester and see what I can see. I will also get a new thermostat while I am there.

My biggest problem with all of this is I don't understand why it is leaking out of the top cap instead of into the overflow. You would think that that is the path of least resistance.

for some reason i missed that it is the top cap on the engine, that filler housing tends to crack and leak, clean it off thoroughly and inspect it for warping and for cracks in the plastic. an aluminum counterpart can be found on the automatic FD3S thermostat housing from mazda.

you should only use factory thermostats or at the very least a premium aftermarket with a jiggle pin.

pictures of where it is leaking would help a bit for verification.

Thats if its a s5, i haven't seen any mention what series of engine

That and i thought only s4's had the rad cap on the thermo neck?

my post refers to the S4 actually, which also has a plastic filler cap adapter on top of the aluminum thermostat housing.

Ooohhh ya, i forgot about that, i thought you were referring to the whole neck being plastic, which is a s5 thing

Its a simple test which doesnt involve much. They hook the the machine to the inlet, and to the outlet. pressurize and watch the gauge. even an idiot could do it. you just need the machine.

I thought I better get back to everyone and thank you all for your input. Last night I was pretty bumbed out because when I finally found a pressure tester that would work I put it on the top opening and the damn thing just would not hold pressure. So, all night I worried about a tear down to replace the seals, which I really don't have time to do with some other projects underway. (A TR8 engine rebuild and an MGB.)

Anyway, this morning I got a bee in my bonnet and attached the tester to the actual radiator cap and capped off the top filler with a new cap. I pumped the thing up and there was the problem. The neck of the filler was cracked and leaking. So, I removed it and made a block off plate and installed it and pumped it up again, and she held just like she was supposed to.

While I was at it, I replaced the thermostat with a 180 degree one. I swear she idles better and runs better now at the higher temperature. (Up to about 178 degrees according to the infrared tester at the thermostat housing.)

So, Rotary Evolution, you nailed it in one of your posts. I can't thank you guys enough for jumping in and helping out with this.

no problem, here is the aluminum counterpart i was referring to:

Cart Price Check Part:15-17Y0-N323

I had similar issue the plastic piece where the cap hooks to was just plain worn out