Late April in southern New Hampshire is when cooling systems get their first real test in months. The first 75-degree afternoon arrives, the A/C runs for the first time since the fall, and a cooling system that sat through a winter of salt spray and freeze-thaw suddenly has to work. Old coolant, a hose that hardened in the cold, a radiator caked with a season of road grime, or an electric fan that hasn't had to switch on since October all tend to show themselves right then, as car overheating. And the hardest test is still ahead: Memorial Day weekend, then a summer of stop-and-go on Route 101 toward the coast, I-93 north on Friday afternoons and the lights on Route 28 through Derry and Salem, often with a boat or a camper on the back.
If your temperature gauge is climbing right now, read the first section and stop there until you're safely parked. The rest can wait. At Vorenza Auto Repair in Derry, we'd much rather see an overheating car arrive on a tow truck than have it driven the last ten miles. Driving on an overheated engine is how a small cooling problem becomes a cylinder head or a head gasket.
Right now: A/C off, heater on full hot, then pull over as soon as it's safe and shut the engine off.
Don't: open the radiator cap or the coolant reservoir while the engine is hot. The system is under pressure.
Can you drive? Only far enough to get off the road safely. If the gauge is in the red, a warning light is on or there's steam, have it towed.
Most common causes: low coolant from a leak, a thermostat stuck shut, or a cooling fan that isn't running.
What to do the moment your temperature gauge starts climbing
Most cars show you a needle that normally sits around the middle and stays there. Some newer cars have only a warning light, which usually means it's already serious when you see it. Either way, the moment the temperature heads toward hot, work through these steps in order.
- Turn the A/C off and the heater on full hot, fan on high. It will be uncomfortable on a warm day, but the heater core is a small second radiator. Running it pulls some heat out of the engine, and switching off the A/C takes load off the engine and cooling fan. It buys you time to get somewhere safe. It doesn't fix anything.
- Ease off. Stop accelerating hard, leave a long gap in traffic, and don't race to the next exit.
- Get off the road and shut the engine off. Pull over as soon as you can do it safely. A breakdown lane, a parking lot or a wide shoulder all work. Put your hazards on, turn the engine off, and get yourself and your passengers somewhere safe, away from traffic.
- Leave the hood closed while it's steaming. Once the steam stops, opening the hood lets heat escape and tells other drivers you've broken down.
- Do not open the radiator cap or the coolant reservoir cap. A hot cooling system is pressurized, and the coolant inside can be well above boiling. Open it and the coolant can erupt out of the neck. Give it at least 20 to 30 minutes, longer if you can, until the top of the radiator and the hoses are cool to the touch.
- Check the coolant once it's cool. Look at the level in the overflow reservoir against its MIN and MAX marks. If it's low, top it up with the coolant your owner's manual calls for, ideally a 50/50 premix. Plain water will do in an emergency, as long as the engine has cooled first. Then get the system checked, because coolant doesn't disappear on its own.
- Look for the obvious. A puddle under the front of the car, a hose that's split or wet, a belt that's missing, or a fan that doesn't spin when the engine is restarted and warm are all clues worth reporting.
- Decide whether to tow. If the gauge went into the red, a warning light came on, you lost a lot of coolant, or you can see a burst hose or a broken belt, call for a tow.
Can you drive a car that's overheating?
The honest answer is: as little as possible. An engine is built to run within a narrow temperature range, and once it goes above it, the damage can come quickly. The aluminum cylinder head is usually the first casualty, because it can warp enough to break the seal of the head gasket underneath it. That's the difference between replacing a thermostat and spending a significant fraction of what the car is worth.
There is one grey area. If the needle crept a little above normal in traffic, then came back down once you turned the heater on and traffic started moving, the car may get you a short distance, gently, to a safe stop or a nearby shop. Treat that as a warning that something in the cooling system is marginal, not as a sign it's fine. If the needle keeps climbing, or comes back up as soon as traffic slows, stop.
Don't keep adding water and driving on. A cooling system that needs topping up every day has a leak, and that leak tends to get bigger at the worst possible moment.
The radiator cap holds the cooling system under pressure. When the engine is hot, the coolant under it can be well above boiling. Leave it closed until everything is cool to the touch.
Overheating in traffic, with the A/C on, or with a full reservoir
When it overheats is one of the best clues to why it overheats. Before you call a shop, notice which of these patterns matches yours. It'll make the diagnosis faster.
| When it overheats | Most likely causes | Why that pattern points there |
|---|---|---|
| At idle or in stop-and-go traffic, but fine on the highway | Cooling fan not running, a bad fan relay or fuse, or a radiator blocked with debris | At speed, air is forced through the radiator for free. Stopped, the fan has to do all the work. |
| Only with the A/C on | A weak cooling fan, or a clogged radiator and A/C condenser | The A/C condenser sits in front of the radiator and adds heat. A healthy system handles it; a marginal one doesn't. |
| On the highway or climbing hills | Thermostat stuck partly closed, a clogged radiator, a worn water pump, or low coolant | Plenty of airflow, so the problem is coolant flow or capacity, not the fan. |
| With the coolant reservoir full | Thermostat, water pump, an air pocket, an internally clogged radiator, or a head gasket | Coolant is there but not circulating, or the reservoir is full while the engine itself is low. |
| Needle jumps up and down, and the heater blows cold | Low coolant or a large air pocket | The temperature sensor and heater core need to be surrounded by coolant to work. Air gives erratic readings and no heat. |
| Right after a coolant service or repair | Air trapped in the system | Many engines need a specific bleeding procedure after the system is opened. |
What causes car overheating: the six usual suspects
Almost every overheating car we see comes down to one of these. Some are cheap and quick to fix, and a couple are serious. The only way to know which you have is to test for it rather than guess.
→Low coolant from a leak
The most common cause by far. Coolant escapes from a hose, the radiator, the water pump seal, a heater hose or a loose clamp, often slowly enough that nobody notices until a hot day finds the system a quart short. Clues: a sweet smell after driving, green, orange or pink drips under the front of the car, or crusty residue around hose ends.
→A thermostat stuck closed
The thermostat is a valve that keeps coolant in the engine until it warms up, then opens to send it to the radiator. If it sticks shut, the engine can overheat even with a full, healthy system. Clues: the gauge climbs steadily within 15 or 20 minutes of driving, and the upper radiator hose stays cooler than you'd expect.
→A cooling fan that isn't running
Most modern cars use an electric fan behind the radiator, switched on by the engine computer through a relay and a fuse. Any of those can fail, and so can the fan motor itself. Clues: the classic “fine on the highway, overheats in traffic” pattern. With the engine hot and the A/C on, you should normally hear the fan running.
→A worn water pump
The water pump keeps coolant moving. Its seal can leak, its bearing can wear, and on some engines its impeller can corrode or slip, so it spins without moving much coolant. On many engines the pump is driven by the serpentine belt, so a broken belt stops the pump too. Clues: a drip or crusty trail from a small weep hole near the front of the engine, or a grinding or whining noise from the pump area.
→A clogged radiator, a collapsed hose or a weak cap
Radiators clog from the outside with leaves, bugs and road grime, and from the inside with scale and old, broken-down coolant. A soft lower hose can collapse under suction at high engine speed. A radiator cap with a tired seal lets the system lose pressure, which lowers the boiling point. Why it matters here: years of salt and sand spray don't do the fins at the front of a New Hampshire radiator any favors.
→A head gasket, as a cause or a result
A failing head gasket can push hot combustion gas into the cooling system, which forces coolant out and creates air pockets. It can also be the damage left behind by an earlier overheat. Clues: white exhaust smoke with a sweet smell, coolant disappearing with no visible leak, bubbles in the reservoir with the engine running, or milky, tan oil on the dipstick.
What overheating does to an engine, and how to tell if it already has
A single mild overheat that was caught quickly often does no lasting harm. The trouble starts when the engine is driven while hot, or overheats repeatedly. In rough order of severity, this is what's at stake:
- Hoses, seals and the radiator. Extreme heat and pressure find the weakest point and open it up, so a slow seep becomes a real leak.
- The head gasket. The gasket between the cylinder head and the engine block is what usually gives when the head warps.
- The cylinder head. Aluminum heads warp and can crack. Fixing that means taking the top of the engine apart.
- Pistons, rings and bearings. In a severe overheat, metal parts expand past their clearances and scuff, and the engine can seize.
If the car has overheated recently, watch for the signs that it left damage behind: white, sweet-smelling exhaust that doesn't clear once the engine is warm, coolant that keeps going down with no leak you can find, a rough idle or misfire, or oil that looks milky on the dipstick. Any of those means a proper test before you rely on the car again. If you're buying a used car, those same signs are worth knowing about, and an inspection before you buy is the place to find them.
An overheat will often set off the check engine light too, sometimes with a code that points straight at the thermostat or a coolant temperature sensor. A code is a starting point rather than an answer, which is why our diagnostics go on to test the part the code points at.
Towing in warm weather is the hardest job a cooling system does. Pulled over, hood up and waiting for it to cool is the right call, not the embarrassing one.
How we track down the cause
Overheating is one of the easiest problems to throw parts at and one of the most expensive to guess wrong on. A new thermostat won't help a car with a dead fan, and a new radiator won't help one with a failing head gasket. Here's how we narrow it down at our Derry shop.
- Your description. When it overheats, how fast, whether the heater blows hot, and whether you've been adding coolant. The patterns in the table above save time.
- A visual inspection and pressure test. We pressurize the cooling system while it's cold to make slow leaks show themselves, and test the radiator cap.
- Fan and thermostat checks. We confirm the fan switches on when it should and that the thermostat opens at the right point, using live temperature data from the engine computer.
- A scan for stored codes. Temperature sensor and fan circuit faults often leave a code behind.
- Head gasket testing, only when the signs point there. Tests for combustion gas in the coolant tell us whether the head gasket is involved, before anyone talks about taking the engine apart.
That's the same approach our diagnostics service takes to any problem: find the cause first, then quote the repair.
What fixing an overheating car involves, and what it costs
Cooling system repairs range from a part you could hold in one hand to a job that takes the engine apart. We don't publish dollar figures for these, because they depend heavily on the vehicle. This is roughly how they compare.
| Repair | What it involves | Relative size |
|---|---|---|
| Radiator cap, hose or clamp | Replacing the part, refilling and bleeding the system | Small |
| Thermostat | Easy to reach on some engines, buried on others; refill and bleed | Small to moderate |
| Cooling fan, relay or fuse | From a fuse to a complete fan assembly, depending on what failed | Small to moderate |
| Radiator | Removing and replacing the radiator, refilling and bleeding | Moderate |
| Water pump | Simple on some engines; on others it sits behind the timing belt and is replaced at the same time | Moderate to major |
| Head gasket | Removing the cylinder head, checking it for flatness and cracks, and rebuilding the top of the engine | Major |
| Service at Vorenza | Price |
|---|---|
| Quick code read A scan for stored codes, including temperature sensor and fan circuit faults. | typically free |
| Full overheating diagnosis Pressure test, fan and thermostat checks, live data and further testing as needed. A fee may apply, and we tell you before we start. | quoted up front |
| Cooling system repairs Caps, hoses, thermostats, fans, radiators, water pumps and head gaskets, with parts and labor broken out. | written estimate |
| Oil change with multi-point courtesy inspection Covers up to 5 quarts; additional oil is $8.95 per quart. Includes a fluid level check, which is where low coolant gets caught early. | starting at $89.95 |
Nothing gets repaired until you've seen the estimate and approved it, and every repair we perform is backed by a 12-month / 12,000-mile warranty on parts and labor. You can see the full range of engine and auto repairs we handle on our services page.
Coolant wears out. It loses its ability to protect against corrosion long before it stops looking like coolant, and many manufacturers recommend coolant flushes every 30,000–60,000 miles. Your owner's manual has the exact interval. A coolant level check is part of the fluid check that comes with every oil change. If you've spotted a sweet-smelling puddle under the front of the car since the snow melted, our guide to what fluid is leaking from your car will help you confirm it's coolant. A spring check now costs far less than finding the problem in July traffic.
Overheating FAQs — from drivers in Derry, Manchester & Southern NH
What should I do if my car is overheating?
Turn the A/C off and the heater on full hot with the fan on high, which pulls some heat out of the engine. Then get off the road as soon as it's safe, put your hazards on and shut the engine off. Don't open the radiator cap or the coolant reservoir while the engine is hot. Wait at least 20 to 30 minutes, check the coolant level once everything is cool, and call for help if it's low, leaking or the gauge went into the red.
Can I drive my car if it's overheating?
Only far enough to get safely off the road. If the gauge is in the red, a temperature warning light is on, or you see steam, stop and shut the engine off. Driving on an engine that is overheating can warp the cylinder head or blow the head gasket, which turns a small repair into a very large one. A tow almost always costs less than that.
Why does my car overheat when it's idling or in traffic?
At highway speed, air is forced through the radiator for free. Stopped in traffic, the electric cooling fan has to do all of that work. A car that runs fine on the highway but overheats at idle or in stop-and-go traffic usually has a cooling fan that isn't running, a bad fan relay or fuse, or a radiator blocked with debris on the outside.
Why is my car overheating when the coolant is full?
Coolant has to circulate as well as be there. A thermostat stuck closed, a worn water pump, a radiator clogged inside, a collapsed hose or an air pocket can all stop it moving. A full reservoir can also hide a low radiator or engine, and a head gasket leak can push combustion gas into the cooling system. Those causes need testing to tell apart.
Why does my car overheat with the A/C on?
The A/C condenser sits in front of the radiator and adds its own heat to the air flowing through it, and the A/C also makes the engine work harder. A healthy cooling system handles that easily. If the temperature only climbs with the A/C on, the system is usually already marginal, most often because of a weak or failed cooling fan or a radiator and condenser clogged with debris.
How much does it cost to fix a car that's overheating?
It depends entirely on the cause, which is why we find it before we quote it. A radiator cap or a hose is a small job; a water pump or radiator is a moderate one; a head gasket is a major one. Quick code reads at our Derry shop are typically free, full diagnostics may carry a fee that we tell you about before we start, and every repair comes with a written estimate you approve first. Call (603) 825-3815; we work by appointment, open daily 8am–6pm.
Bottom line: car overheating is a stop sign, not a suggestion
Car overheating is almost always a cooling-system problem that started small: a slow leak, a sticking thermostat, a fan that quit. It turns expensive when the car is driven anyway. Heater on, pull over, engine off, and let it cool before you touch anything. Then find the cause properly instead of topping it up and hoping. A tow and a diagnosis cost far less than an engine.
Questions first? Call (603) 825-3815 or send a message through our contact form. We serve Derry, Manchester, Londonderry, Salem, Windham and the rest of southern New Hampshire.