[Kaumad]

I have a 2008 SX4 ZXi which has clocked about 186k kms.
Been facing a strange overheating problem over the past few months. I am listing out the behaviour based on 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 tens of seconds.
• A/c cuts off when temperature crosses 105 DegC. Switches back on when 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 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 radiator and external cooling circuit is ruled out. Fan was also replaced recently and comes on appropriately, so thats 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 feel 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 hoping this is it.

Am 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.

[scorpian]

Check 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.

[Kaumad]

I have installed an additional temperature sensor, with a digital readout inside the cabin. Hence the precise temperature readings.
The overheating and a/c cut off are corroborated by both sensors. When digital gauge crosses 120 DegC mark, the analogue stock needle creeps to the red zone.
So, unlikely that both sensors are kaput.

[Jeroen]

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 temperature.

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

Good luck
Br Jeroen

[NomadSK]

Originally Posted by Kaumad

Radiator has been changed recently to a new piece, so clogging of radiator and external cooling circuit is ruled out.

Just being inquisitive, Can you throw some light on this whether the new radiator which was swapped was in line with the OEM part.

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

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

[Indian2003]

Originally Posted by Kaumad

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.

I dont 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 handbrake and try to drive over in second gear. The engine should spin but car wont move if clutch is worn.

[Rahul Bhalgat]

I agree with Indian2003.

Since this is happening only when you accelerate to higher speeds after slowing down to 20 - 30 kmph, check if the clutch is slipping.

During this acceleration, ask someone to observe the speedo and tacho readings simultaneously. If the speedo needle movement lags the tacho during acceleration, the clutch is slipping.

[ssjr0498]

Did you bleed the cooling system?

[deep_bang]

Originally Posted by Jeroen

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 temperature.
Br Jeroen

This is the most likely and easiest to try first. You could even remove the thermostat valve just to check if this is the issue. This valve is present to not allow the coolant to circulate until the engine is warm enough. The idea is to let the engine warm up as fast as possible to reach its optimum temperature and then let the coolant start doing it's job. If this valve is kaput, then the behaviour you are seeing is possible.

[MinivanDriver]

As others have suggested, do replace the thermostat. This should be the cheapest option to try. Try to correlate when the cooling fan comes on with the temperature. Your vehicle probably has an electric cooling fan that is turned on based on temperature. Bad thermostat, bad fan, loose wiring etc can cause it to malfunction. Also, was the radiator replacement done with a genuine part? I had an experience where the neighborhood garage did not use genuine radiator and it was still overheating. The problem went away after we replaced with a genuine part. In that case also, it was difficult to reproduce overheating events .

[Indian2003]

Originally Posted by deep_bang

This is the most likely and easiest to try first. You could even remove the thermostat valve just to check if this is the issue. This valve is present to not allow the coolant to circulate until the engine is warm enough. The idea is to let the engine warm up as fast as possible to reach its optimum temperature and then let the coolant start doing it's job. If this valve is kaput, then the behaviour you are seeing is possible.

Thermostats that got stuck with the valve closed are a thing of the past. Todays thermostats always gets stuck in the open position.
I dont know how old your thermostat is but a faulty thermost should not cause overheating today. Your problem should be elsewhere.

[Rahul Bhalgat]

Originally Posted by Kaumad

I have a 2008 SX4 ZXi which has clocked about 186k kms.
Been facing a strange overheating problem over the past few months.

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]

One more possibility:
The water pump is driven by a separate belt in SX4. This belt is visible from outside. If this belt isn't fully tight, it may slip over the pulley during the acceleration. This will reduce the coolant flow rate for some time, causing the temperature needle to go up.

[svsantosh]

I would go in this order.

1) Electrically first check if the high speed fan and the low speed fan is working correctly. This is the easiest thing to check by an electrician. AC should turn on the high speed fan. Dont think if the high speed fan turns on it is working fine. (See my last comment)

2) For sake of checking, see if you can remove the thermostat and drive the car, ask your mechanic, he can remove/break the spring and keep the thermostat in open loop. Even without a thermostat if it heats then the issue is else where. Since the car has 186K high kms, a new thermostat is welcome anyways.

3) Why did you change the radiator? Was it muddy/rust colored? If yes, did you flush the engine water jacket, you would need 1 or 2 radiator flush cycles to clean the sludge around the engine block.

4) Remove the water pump and see the condition of the fins/blades. If it looks eroded/corroded then it will reduce the water flow. Spin it and see if the bearing is OK. Or just replace it since its a critical part and a Maruti should be cheap. Also the belt at the same time.

5) The sensor since you said is working fine because you checked it with a parallel unit. Remove the unit and see for youself.

If all the water cooling system check is done (not necessarily replace anything), then you can check for mechanical wear and tear causing spikes in temp. I dont suspect anything serious but if there any knock or misfire, starting trouble, excess fuel consumption? Anything abnormal you want to share?

PS - I once had a high speed fan issue in my Diesel Chevy Sail UVA and it was narrowed down to the relay inside the fan was weak, high speed did turn on but the relay was cooked partially after 6+ years and some 1.5Lac kms. A quick and cheap fix by the electrician on a new relay did the trick and sorted the overheating issue.

Edit
Check https://boodmo.com/catalog/part-moto...g_fan-6791474/ item. You need a competent electrician to see if the radiator fan motor is weak. He needs to see the current drawn when the HS fan comes on.

I would also advice to check the alternator health when the HS fan turn on- In tandem with the battery health.

[OffRoadFun]

My suspicion would be temperature sensor. In my earlier car, the sensor/valve was faulty which won't let the valve open fully leading to overheating when speed brought down or in stop/go traffic. That would mean there isn't sufficient flow of anti-freeze (radiator fluid) to withdraw the heat from the system and hence overheating.

[Sam800]

Originally Posted by svsantosh

I would go in this order.

Very valid points highlighted, the order of operation is good too.
However I would like to add one more point. Kindly check or replace the coolant-mix if required.
Usually 50-50 ratio of water and coolant works the best. But if this ratio is not maintained and it goes 25-75 or 75-25 then heat absorption property of coolant affects to a great measure. Also if you change the coolant, use the appropriate one as mentioned in manual, and not go for red or blue ones.

I once used MAHLE red coolant in my carb 800 and engine temp shot like anything.

Also, you can use Castrol radikool heavy duty coolant (green) , its trusted by many bhpians since years. If you want to keep the cooling system untouched for couple of years then you might use toyota long life coolant (pink).

I am using Amsoil Heavy duty long life coolant (pink) in my 800 currently and its the best coolant i have ever used.