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Reader Scott pointed us to his blog where he asks the following:

What are these little blue and grey caps called? What do they do? Are they part of the cooling system? What happens if they are not there?

Ellen’s 2001 Eclipse is overheating now and then. Her coolant reservoir is empty, we filled it. The radiator seemed to still be full of coolant. Near the coolant reservoir there is a metal pipe/line that looks like it has one of these little caps missing. There is also a short hose coming out of the reservoir that is not attached to anything but we assume this is an overflow hose, are we right?

Before I launch into some possible explanations, I’d like to point out that I’m not going to make fun of Scott. And I’d like to ask that you don’t either. Some of us were lucky enough to have parents or friends (or just plain situation) that gave us an understanding of basic automotive troubleshooting before we were old enough to drink legally. But some didn’t.

Our job as Toolmongers isn’t to deride them but to pass on that knowledge — to remind our less-experienced friends that they can figure it out by using the same reasoning they apply to other problems in life.

As I point out in my recent jackass project post, having a manual for your car — even a cheap-ass crappy manual — makes troubleshooting a lot easier. The manual will show, for example, what parts comprise the Eclipse’s cooling system and how they work together. From that point you can make some reasonable guesses as to the problem based on the symptoms with a little imagination and logic.

Personally, I know nothing at all about the Eclipse. And I’m too poor to shell out for a manual (even a subscription one) just to take a stab at this question. But anyone who’s messed with cars much will know the basics: There’s an upper and lower connection on the engine which connects to the radiator via hoses, usually soft hoses but sometimes a combination of soft and metal. Somewhere in the system you’ll also find a thermostat which closes off coolant flow from the radiator until the coolant reaches a specific temperature. This helps get the coolant to its proper operating temperature more quickly and reduces overall system wear.

Let’s start with a little imagination. What if the thermostat sticks closed for a bit sometimes before letting go and doing it’s job? That’d cause Scott’s reported symptoms — the Eclipse only overheating “every now and then” and not appearing to leak a lot — wouldn’t it?

Now let’s apply a little logic: Automotive thermostats contain a cylinder with a bit of wax inside that melts at a given temperature, allowing a spring-loaded plunger to depress and open a valve. (If you’d like to know more, HowStuffWorks.com has a great, detailed explanation of the process.)

In fact, thermostats often become corroded and fail in the closed position either temporarily or permanently. Thankfully they’re also relatively easy to remove (at least on non-Porsches), and you can test them by placing them in a pot of water over a stove burner set on high. Place a thermometer in the water and note the temperature at which the thermostat opens. If it’s above the specified temperature — or it doesn’t open at all — it’s boned.

Or better yet, if you’ve gone to the trouble of removing a thermostat, just replace it. They usually cost less than $20, so why bother?

Other issues can reduce cooling efficiency without leaking coolant as well. The radiator depends on pushing coolant through lots of tiny tubes to effectively sap heat from the coolant and radiate it safely away. If gunk or corrosion build up within the radiator, some of these tubes can get sealed off reducing the radiator’s capacity or effectiveness. Rocks and road debris can also damage radiators, closing them off without puncturing them. Sometimes a simple radiator flush can clear out the mess and get you going again, but if the radiator is too far gone, you’ll have to replace it.

But my point is this: If you’re going to try and troubleshoot car problems on your own, this is the procedure you’ll follow.

  1. Read and understand (at least in simple terms) the vehicle’s systems.
  2. Simulate in your mind what would happen if a given system failed.
  3. When you find one that matches your symptoms, read up on the system and how it can fail.
  4. Follow the manual’s procedures for testing the system and replacing it if necessary.

Or if you’ve dumped the dosh for a dealer’s maintenance manual you can just follow the step-by-step troubleshooting and decision charts within. Either way, you’ll get there, though probably not without getting some grease under your nails first.

And Scott, regarding the little blue cap: Remember that I don’t know squat about the Eclipse. But I’ll bet you a quarter that your missing cap covers a Schraeder valve on a freon line for the car’s air conditioning system. I’m certain that the gray cap does. I’d also bet that removing the cap won’t affect the system at all — except for allowing the valve to corrode over time or collect crap from the engine compartment.

PS: Even if you’re not willing to dive in, you can always roll the dice with Google. For kicks I plugged in “2001 Eclipse overheating” and got the following advice:

  • Bleed the cooling system (first result)
  • Blown head gasket (second result, though not applicable to you because you don’t have coolant in your oil and the car runs)
  • Thermostat, radiator flush (fourth result, though spelled atrociously)
  • Cooling fan/relay (seventh result, from a Diamond-Star Motors forum with lots of helpful responses)

Just keep in mind that car enthusiasts will probably jump right in to help you solve your problem — as long as you’re willing to help yourself.

Scott’s Blog Post [ApertureQuiet]


10 Responses to Basic Automotive Troubleshooting, Explained

  1. Justin says:


    The blue and grey caps look like they are ports for charging the air conditioning system with refrigerant, they are not part of the engine cooling system. The other little hose is a vent that allows for water to drain out of the top of the radiator if it is over-filled. When the water gets hot, it expands. If the caps aren’t their, nothing will happen except that the inside of the ports may get dirty. This isn’t a problem if they are cleaned before servicing the AC. Most cars use the radiator as a condenser for the AC. A condenser is the part of the system that rejects the heat you don’t want in the cooled space, in this case the inside of the car.

  2. Chris W says:

    A bad thermostat can let enough coolant flow when driving at highway speed to bring the temp gauge back into the normal range. It is the first thing I would replace. Also remember to turn the heat/fan on maximum if your car starts to overheat, but pull over if the gauge goes into the red. I learned the hard way years ago. I agree the caps are for the high/low test/service ports for the AC, but I doubt they are Schraeder valves on a 2001.

  3. If the reservoir is empty, the coolant is going somewhere. With modern coolants, corrosion isn’t a super likely problem, so I wouldn’t jump to conclusions regarding the thermostat and radiator being crusty. I’d check the thermostat, but figure out where the coolant is off to as well.

    You’re going to find one of the following:
    1) A crusty deposit all over something near a coolant hose (probably whatever color the coolant is..yellow, pink, green, blue, etc)
    2) A slippery slick of coolant all over something near a hose (though coolant can spurt quite a way when it wants to)
    3) A sweet smell from the tailpipe when the car is warmed up.

    #3 is a head gasket. A bad HG does not necessarily lead to mixing oil and water, so this is definitely a possibility. You can often spot it by watching inside the coolant reservoir while the car is running…if there are air bubbles, get ready to shell out cash.

    When is it overheating? If on the highway or on cool days, it probably isn’t the cooling fan/relay. That would tend to have you overheating in traffic on hot days.

    My money is on a simple leak causing a big air bubble in the cooling system. Fix the leak, fill the system and bleed it however the manufacturer suggests. But your best bet is probably to post this question in whatever user forums exist for this car.

  4. Toolhearty says:

    On the subject of simple questions to forums:

    One car site that I used to hang out at had a post in the technical section from a guy who wanted to change the oil in his car with his son as a sort of father/son bonding activity. Problem was he had never done it and, being from a single-parent household (mom, of course) really didn’t have much experience with mechanical things and had a lot of questions. His first question was: How do you get the old oil out?

    I cringed as I imagined everyone jumping all over the guy as he could have at least searched the web for instructions (I’m sure they’re out there), but instead everyone was pretty helpful. One guy even offered his phone number and told him to give a call and he’d walk him through it as, he too, was raised by his mom and and to figure out a lot of things for himself.

  5. Scott says:

    Thanks for the post and all the answers. As it turns out, there was a loose radiator hose. We might have figured this out yesterday if it wasn’t pitch black darkness and freezing cold (I’m trapped in a cubicle during daylight hours). I am usually pretty good at upstanding how car related things work in theory, but when its cold and dark, when I’m unfamiliar with the rats nest of parts under the hood, and when my wife is saying ‘lets just drive it over to the gas station’, I am prone to give up. I did search the internet for a cooling system diagram, but was unable to find one. In any event, thanks again, and hopefully all the info in this post will help other people when diagnosing their overheating problems.

    I forgot to mention, I had also noticed that her radiator fan wasn’t running, but I have since learned that in a lot of cars it doesn’t run until the engine heats up.

    And, the thermostat explanation was very interesting. Thanks again.

  6. Bill P. says:

    I’ve been working on cars for years, over twenty years I guess, and made my living as a mechanic into my twenties. I must have replaced a hundred thermostats, and I never knew that it was a slug of wax that did the work. I always figured it was a bimetallic something-or-other. Learn something every day. I want to say that I’m very impressed, and frankly touched by the considerate, helpful way in which Scott’s question was addressed. I like Toolmonger very much, and read it almost daily, but now I feel that I have your measure as men. Decent people are few and far between. Kudos to you and your readers, and if you’re ever near NE PA, I’d like to buy you all a beverage.

  7. Mitch says:

    I agree, Bill. Toolmonger posters are pretty much reasonable people exhibiting proper adult behavior. My guess is probably because there aren’t teenagers posting. (and I immediately thought I’m probably insulting all the teenagers posting on this blog – sorry!)

    I like the very wide variety of tinkering info I get here. From machinist tips to asphalt. Like the link to the asphalt and concrete repair company that was posted. I never knew about it and now I’m probably going to order some of their products next year.

  8. ShopMonger says:

    Knowledge is Power, and you cannot get knowledge from sarcasm. Well done gentlemen. I once again bow my head in honor of the Collective mind that is TOOLMONGER>


  9. kyle says:

    Mitch I have to disagree with your comment about teenagers because i am turning 14 on this thanksgiving. However i am quite mature for my age.

  10. Mitch says:

    heheh – the moment I said that I knew there would be some youngster reading my comment. My sincere apologies to you and other mature teenagers. You may in fact know more about cars and tools and could teach me stuff. You know what I mean though. I’ve read a lot of other forums where there’s a lot of derogatory comments and attacks and foul language instead of discussion and helpfulness. Maybe I’m wrong but my feeling was that the amount of cursing increased as the proportion of younger commenters on the forum increased.

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