Why my Maruti SX4 overheats intermittently, but only on highway drives

For example, it will happen if I'm driving at 90-100 km/h and slow down to 20-30 km/h and pick up speed again.

BHPian Kaumad recently shared this with other enthusiasts.

I have a 2008 Maruti SX4 ZXi which has clocked about 186k km.

Been facing a strange overheating problem over the past few months. I am listing out the behaviour based on my observation. Would love to hear inputs from members who can help me diagnose the problem.

Observations

• Overheating occurs only during highway drives where there is a “slow & go” situation involved. Say, cruising at 90-100 kmph and slow down to 20-30 kmph and pick up speed again. [Example Vellore – Chennai stretch]
• Overheating does not occur at city driving speeds. Also, does not occur when cruising at highway speed for extended periods of time. [Example Krishnagiri – Vellore stretch]
• During overheating, temperature shoots up to 115-120 DegC range, stays there for 2-3 minutes and drops back to 90-95 DegC range, as I pick up speeds back to highway cruising.
• Occasionally the temperature hits 125-128 DegC range. Not always, happens infrequently and stays there only for a few tenths of seconds.
• A/c cuts off when temperature crosses 105 DegC. Switches back on when the temperature drops under 100 DegC.
• During highway drives, quite frequently, the temperature drops to 45-50-55 DegC range. Stays here for 1-2 minutes and jumps back to the 90-95 DegC range. So, the temperature sensor is catching pockets of ‘relatively cooler fluid’, which indicates a ‘discontinuous’ coolant flow through the circuit.

Things to note

• Radiator has been changed recently to a new piece, so clogging of the radiator and external cooling circuit is ruled out. Fan was also replaced recently and comes on appropriately, so that's ruled out too.
• Coolant / water is all topped up and checked regularly, so fluid shortage is ruled out.
• Overheating almost exclusively occurs only during the “slow & go” highway drives, so essentially happens only during the ‘heat-soak’ period.

My gut feeling and hypothesis

• Cause 1 – there is a physical blockage / restriction in the engine block water jacket, which is causing the ‘discontinuous’ flow. This is my top concern as I really don’t want to dismantle the block jacket to diagnose the restriction.
• Cause 2 – the water pump is the culprit which is not pumping fluid continuously. If this is the case, then it would be an easy solution. Really hope this is it.

I'm keen on hearing perspectives from members – to see if I have missed anything. What do you think could be the cause of this intermittent but periodic and predictable overheating?

Please ask questions if I have missed mentioning anything important.

Here's what BHPian scorpian had to say about the matter:

Check the temperature sensor.

I had a similar issue where in the needle would jump to high and then low in a minute. Such drastic changes in coolant are not possible with a proper warmed-up engine. I replaced the temperature sensor and also cleaned all terminals with contact cleaner.

This resolved the issue.

Here's what BHPian Jeroen had to say about the matter:

I would swap out the thermostat. It appears it is not controlling the temperature sufficiently and/or with way too much hysteresis.

Swapping a thermostat is even simpler than swapping the cooling water pump.

As you need to, partly drain the cooling liquid system. I suggest you drain it completely, flush it and refill it with new cooling liquid as well.

Here's what BHPian NomadSK had to say about the matter:

Just being inquisitive, can you throw some light on whether the new radiator that was swapped was in line with the OEM part?

Because any change of radiator from the OEM (under sizing or different material of construction) will give an overheating indication during hard acceleration.

Also, you can go with 70:30 mix of coolant and distilled water.

Here's what BHPian Indian2003 had to say about the matter:

I don't think there is something wrong with the cooling system. I would place my bets on your clutch. It is not uncommon to overheat with a slipping clutch.

Place the front wheel against the curb, pull up the handbrake and try to drive over in second gear. The engine should spin but the car won't move if the clutch is worn.

Check out BHPian comments for more insights and information.