98 grand caravan radiator boils

My 1998 Grand Caravan just had a new tranny and radiator by AAMCO. Recently, after running the car for an extended period, we get home and shut it down. About a minute later, the reservoir starts boiling. The temp gage reads normal, I just replaced the thermostat thinking perhaps the flow was not good. The radiator fan comes on as expected for the temp gage indication which is every so often.

Any ideas? Again the Temp Gage reads normal yet it boils over. The only other thing I can think of is water pump but that doesn’t make sense either. None of this makes sense. Any help would be appreciated.

Thank you,


have you made sure the reservoir is actually ‘boiling’ vs. just flowing?

did this start since the visit to aamco?

it sounds like its possible the thermostat is faulty, but since they worked on it last, why not take it back there?

a water pump going bad sounds plausible too, but a competent mechanic could check that too.

have you let the car run, when shutting it off for a minute or two, (as opposed to just putting it in Park, and shutting it off) to let the coolant circulate some more?

Bubbling in the coolant resevoir could mean there is a headgasket problem. This happens when a cylinder under compression leaks the compressed air into the coolant from a bad spot in the headgasket.


  1. It is actually boiling, with water coming up and out. Big puddle on the ground.

  2. It started about a month ago, which is months from when it was serviced at AAMCO.

  3. I replaced the thermostat but the problem continues. It is a 195 degree thermostat.

  4. I hope it is not a water pump. I have no noise or leakage so I don’t think that is the problem. I replaced that about 5 years ago, so hopefully that is not it.

  5. I haven’t tried letting it run for awhile after stopping. That is a good idea. My co-worker suggested that the anti-freeze content is not where it should be and maybe it is boiling just do to the engine temp. I’ll check that too.

I appreciate the help.

Cougar - Man, I hope it is not the head gasket. I don’t think so because it goes on and on and on. I would think if it is pressure relieving, it would end. I think it has to do with temperature somehow. I really hope you are wrong. Thanks for the input.


I hope I am wrong also and from the sounds of it there may be another problem going on here. Since the fan is turning on it does seem like you are correct about the temperature being an issue. I like the idea about checking the coolant mixture ratio. Another thought may be the transmission fluid cooler built into the radiator. Perhaps the tranny fluid is getting too hot somehow.

Replace the radiator cap. And also replace the Dexron in the transmission, if that’s what they used (they probably did).

I second NyBo with the radiator cap. The rubber diaphragm in the pressure cap can deteriorate, and crack. causing the cap to no longer hold pressure.

you state two issues.

one, this stuff started after aamco did the transmission.

later you state this is months afterwards.

it may not seem important to you, but the time line IS important.

you need to have a check done on the antifreeze coolant to see if there is exhaust gas in it. that would eliminate (or confirm) a head gasket issue.

all else is a guess or a stab at possibles.

Thanks everyone for your advice. I’m thinking the transmission cooler in the radiator is not doing it’s job. I’m not sure what I’ll do about it. We got the job done in the winter and this boiling came up in the summer. We live in Seattle so the summer wasn’t that hot. I also noticed that if I run the car after stopping that the boiling isn’t as bad. So I think somehow the tranny is running hotter or the radiator isn’t cooling it as well.

I’ll report what I find.