Jerry rigged radiator cap cause of radiator leak?

Earlier this week, my car overheated while I was driving on the highway. I had it towed to a garage. They looked at it and found that there was air in the system from when the radiator had been installed (4 weeks earlier by a different mechanic). They bled the air and filled it up with fluid and sent me on my way.

As I was driving home, my car started to overheat again- only this time there was also steam coming from under the hood. I pulled over- there was antifreeze dripping from everywhere. I opened the hood and noticed that where my radiator cap should have been there was a plastic with a rubberband wrapped around it (see photo).

I had it towed back to the shop. They told me that- oh yeah- my cap had been missing and that they didn’t have one in stock (yes, this was a dealership). They then diagnosed the problem to be a radiator leak (which they admitted was not there when they had been working on it before). They claim a faulty radiator and think the mechanic who put it in in the first place is responsible.

I am wondering if the fact that they let me drive away with plastic bag and a rubber band in place of a radiator cap could have caused pressure to build in the radiator and thus blowing out a leak somewhere else.

That’s Not Jury Rigging. That’s Gross Incompetence To Let You Drive Away With A Plastic Bag “Radiator Cap”.

At this stage of the game the car needs to go to a competent mechanic and the cooling system needs to be pressure tested.

Car make, model, model-year ?


One of the purposes of a radiator cap is to allow pressure to build up in the radiator (typically about 16 lbs, IIRC), in order to raise the boiling point of the coolant. Using a piece of plastic and a rubber band would actually lead to lower pressure in the radiator than normal. However, this also means that the cooling system is more prone to overheating, because of the lack of a proper radiator cap. And, overheating on a modern engine is more than an inconvenience. It is a major cause of severe engine damage.

One thing that is unclear to me is whether you actually had a radiator cap in place prior to your first emergency visit to that garage. If it was not reinstalled by the mechanic who replaced the radiator 4 weeks prior, that is likely to be the start of your problems.

However, whether the radiator cap was missing prior to visiting this garage or not, for them to have allowed you to leave the shop without a radiator cap (and not informing you of same?) represents total incompetence.

Among the vital details that you omitted are the make, model, and odometer mileage of your mystery vehicle. If you are driving a '52 Dodge, I wouldn’t worry that much about two (or more) overheating incidents. On the other hand, if you have a modern vehicle with an aluminum engine (or even just an aluminum cylinder head), then your engine could well have sustained serious damage by now.

How about filling in these significant blanks for us before we go any further with this discussion?

I would call the national headquarters of the brand of auto this “dealership” represents and share your story including pictures with them. This is gross incompetence and they should loose their dealership if this is the kind of “service” they perform. That is insane.

With a proper cap the overall pressure in the system would not be very high, but when things overheat there are high pressures in areas of the system and the extra heat causes stress so lessor pressures can cause damage.

Basically due to hack job mechanics your cooling system is a mess. I hope you can find someone competent to put humpty dumpty back together again.

The car is a 2004 Honda Civic Hybrid.

I know the picture isn’t great- can somone confirm that it is indeed the radiator cap? (unfortunately I am not particularly mechanical). The cap that was missing was definitely connected to the radiator, but is there more than one cap?

I just want to make sure I have all of my information correct before persuing further action.


I question if this was really a bad decision by the dealer. I don’t believe it was.

Based on the picture, the plastic bag and rubber band were placed over the plastic overflow tank. That tank never has any pressure in it. That’s means it’s not meaningful to be concerned about it not having a pressure cap.

The pressure cap on the radiator is very different than the plastic cover that normally is used for a plastic overflow tank like this.

I can only fault the dealer for not notifying the customer of the temporary workaround.