Running hot: diagnosing high coolant temperatures

These are the symptoms owners reported and the things they tried. None of the threads ended with a confirmed cause, so treat the list as a set of things to check, in roughly the order that costs least.

What the symptoms looked like

• Hot in traffic. One owner saw 95 to 97°C, peaking over 100°C after a minute in traffic, and had to keep the revs up. Another saw normal running at about 92°C, then a rise of about 4°C within 20 seconds of stopping, which stayed there.
• Hot at constant speed. One owner hit 108°C on the motorway (mid to high revs, constant speed) but was fine at low revs and idle. After a hard run, the right-hand side of the radiator, the heater pipes and the right-hand hoses were very hot but the left-hand hoses were only warm. He took that to mean a restriction somewhere.
• Hot after a pump leak. One supercharged owner noticed his car running a couple of degrees hotter, rising to 95 to 96°C on hills, at the same time as the pump began to leak. He saw 104°C on track after five or six laps. He suspected the pump. See Water pump replacement.

Checks the owners made

1. Bleed it again. One owner thought the system "just needs another good bleeding". A Turbo owner found air still in the radiator was slowing the flow, and says to run the car up to temperature and loosen the radiator bleed nipple, letting plenty flow out because air keeps venting for a while. See Changing the coolant and bleeding the system.
2. Hoses and cap. One owner's top radiator hose bleed valve was leaking; after he replaced the hose and the coolant he saw 95 to 97°C and sometimes 91°C when pushing on, which was the only improvement he found. Another owner suggested an old header tank cap seal. That owner replied that he didn't think the cap would cause his symptoms.
3. Thermostat. One owner replaced his, and the symptoms stayed. Another tested both old and new thermostats in boiling water and watched them open and close together. A third took the thermostat out to check flow past the housing. See How the cooling circuit works for why an 82°C thermostat gave odd results (cold water dumped at higher revs).
4. Heater matrix flow. The heater circuit is the bypass, so reduced flow through it can raise the temperature. A quick test is to bypass the heater matrix and pipework with a 15 mm straight coolant connector. Results were mixed: one owner ran a bypass for 12 to 14 months and it cut the running temperature by only a degree or two; another saw no difference. A third found that taking too much heat out of the system with a new heater design made the thermostat close, which pushed the apparent temperature up by about 5°C.
5. Hoses and radiator. One owner pointed to a restricted radiator or pipe ("something got into the pipe"), and had suspicions about the pipe between the thermostat housing and the oil cooler. This was guesswork in the thread. See Radiator replacement and Cooling system flush.
6. The water pump. A garage told one owner the pump was the likely cause, from poor flow at idle and plastic blades going brittle. That owner's new GM pump had no plastic blades, and the thread couldn't confirm either. Checking the pump properly is a lot of work, so owners treated it as the last thing to rule out. One owner whose Turbo pump was four years and 11,000 miles old said he would replace it over the winter anyway.

Is the reading right?

• Gauge senders vary. One owner says different temperature sender units can read up to 7°C apart: one showed 92°C and its replacement showed 85°C in the same idling situation.
• Use the right sensor. Another owner would not fit a non-OEM sensor, because the coolant temperature sensor is used for fuel and ignition correction maps. His point is that the calibration may differ.
• Hot while moving is not the same as hot while stationary. One Turbo owner says not to worry about 105°C while moving, and that when static the engine isn't under much load. Another replied that shutting down at 105°C after a track session can cause a warped head or cracks, and that temperature shouldn't sit at 105°C unless the engine is under high load. Owners differ on this.

Related

Fitting a new pump is covered in Water pump replacement. For track use, see Turbo cooling circuit and upgrades.