Fiesta Mk7 highway overheating

[gt097]

Hello! I bought a 2010 mk7 fiesta earlier this year. It has 115k kilometers on it (72k mi), 1.2 82hp petrol engine. First thing I did was NEW timing belt + water pump + thermostatic valve + radiator + new coolant since the timing belt was 14 yrs old and the radiator was pretty dirt and corroded.

Radiator fan is coming on correctly, when I turn on AC the fan instantly kicks in and turns off within a couple of minutes when AC is off. Similarly the fan turns on when the water temp hits 105C (I don't have temperature gauge so I read this data from the test menu). If I drive around the city, also in slow traffic, also in extreme hot temps (average of 35 degrees slow driving for 40 minutes) nothing happens, AC works correctly and everything is fine.

However, last time I drove it on the highway (100-140kmh or 80-90mph) after less than 30 minutes the water temp reaches almost 120C, and I think that the AC goes in protection mode (past 112C the compressor turns off and the fan pushes outside air in the vehicle). Warning temp light did not show up because I went easy on the gas and I used the AC fan set on heat to redirect engine heat in the cockpit to get to my destination.

I really dont know why the car is overheating only on the highway, where the airflow is much stronger and the radiator should cooldown easily. Any ideas?

It's like if the engine's cooling system can handle the slow-paced-in-hot-weather city drive but can't handle high speed/rpm highway driving.

Thanks in advance

 

[Handy Andy]

Check your coolant levels during this overheat, loosen but do not remove the cap on that tank to release pressure and observe for foam or bubbling air rising up and foaming in the tank - this is a sign that you may have a gasket fail ready to erupt.

Take some time to make sure your car rolls easily - if a brake caliper hangs up or it needs an alignment due to a bad bearing or hub worn out, it can place a load on the engine at highway speeds and generate a shudder like an unbalanced wheel - this can cause the drag and overheat by forcing the engine to work harder. More-so than it is designed for at such speeds and level of load to torque being needed to keep that speed.

You may also have a model that uses a "shutter" that uses a sensor to determine the air temperature and the shutter closes and opens due to the air temp - but is overridden by the A/C signal that forces the shutter to open because of the AC demand so air will flow - the fan kicks on - and the engine cools down. It may be the sensor has failed, jarred loose or has become un-clipped and just dangles so it doesn't see any airflow thru the spot it needs to be placed in. So what happens? The shutter stays closed until its either too hot or you asked for AC to be on and the shutter operates thru a redundant system

 

[gt097]

Hey! Thanks a lot for your reply! Coolant seems clean, I have no smoke from the exhaust and no coolant drops. It is slightly above the max and I've done almost a thousand kilometers (630mi circa) since the flush and re-fill and it is still there. Oil il fine as well, no foamy oil cap and no level drops. It can't be a gasket leak .
However I've noticed a couple of weeks ago a whistling sound coming from my front wheels, maybe some bad bearing? idk.

Another couple of things I've noticed during the last overheating episode were:
1. the AC compressor cutting out at 112C (233F) but I think that is to lift some load off the engine;
2. when I was having the foot off the gas and the road was just enough downhill my temperature dropped quite fast from 100+ C (212+ F) to 90-95C (194-202F). Is this temperature drop normal? I don't think so.
3. with very cold engine and while it is not running, if I squeeze the bottom radiator hose the coolant in the tank bubbles. Is it normal?

 

[Handy Andy]

I'll do my best to give you answers...

Another couple of things I've noticed during the last overheating episode were:
1. the AC compressor cutting out at 112C (233F) but I think that is to lift some load off the engine;
2. when I was having the foot off the gas and the road was just enough downhill my temperature dropped quite fast from 100+ C (212+ F) to 90-95C (194-202F). Is this temperature drop normal? I don't think so.
3. with very cold engine and while it is not running, if I squeeze the bottom radiator hose the coolant in the tank bubbles. Is it normal?

When the AC cuts out, is this from load, or are you seeing this as a "temperature" issue?

So yes, it would cut out to reduce load on the engine, but only during moments of high engine torque load and or acceleration. The AC can be used by the system as an Engine Brake by looking for low-torque (coasting) opportunities to engage while the engine spins down as - per-se; coming to a light to stop - however it does also look at high temp situations and will cut out to reduce load and results being a lower engine temperature (hopefully) while driving - meaning its seeing itself moving.

When you time the cooling portion of this in question 2 - it's not hard to imagine the thermostat being fully open but if the water pump and it's pulley are having issues - this may mean a replacement of the pump is in order. You just replaced it so I'd like to think of it as just the system needs to be burped. So the system may only be partially filled.

The rapid decline of the temp usually indicates the efficiency of the new radiator versus the older one. I am not sure of how many seconds this takes for you imply that the "drop not normal?". But I too have observed the temperature thru the HEC mode in the Instrument cluster (23rd step I believe) shows easily how quickly the standing in traffic (110C) - to open road above 40MPH (>- 82C) can cool the motor down in less than two minutes - so not sure of the time frame your using...

But the last question is what throws me because the issue of the squeezing the lower hose - yes - you're displacing fluid, but why is it not still under pressure? So something is releasing pressure to help you squeeze and move enough fluid to see bubbling which has me wondering if the reservoir is cracked and can't hold pressure. This can generate air pockets inside even when the engines running that can prevent gasses from the hot spots that cause a boil condition - to escape.

 

[gt097]

I'll do my best to give you answers...

When the AC cuts out, is this from load, or are you seeing this as a "temperature" issue?

Yes, definitely a temperature issue. The compressor cuts out at 112C coolant temp and goes back on at 111. I think it's to reduce load.

The rapid decline of the temp usually indicates the efficiency of the new radiator versus the older one. I am not sure of how many seconds this takes for you imply that the "drop not normal?".

The time frame is nearly the same as yours. A couple of minutes are enough to cool down from 100+ to 85/90 so I guess that's a normal thing.

But the last question is what throws me because the issue of the squeezing the lower hose - yes - you're displacing fluid, but why is it not still under pressure? So something is releasing pressure to help you squeeze and move enough fluid to see bubbling which has me wondering if the reservoir is cracked and can't hold pressure. This can generate air pockets inside even when the engines running that can prevent gasses from the hot spots that cause a boil condition - to escape.

Exactly what I was thinking. When the engine is hot, like after a normal ride, If I try to squeeze the rad hoses I can feel the pressure. Once the engine cools down I can easily (really, two fingers are enough) squeeze the hose and see bubbles in the tank.
According to this are you saying that the cooling system leaks pressure from somewhere or the tank cannot hold pressure leading to air pockets being formed? In a normal situation even with a stone cold engine and parked car the cooling system should be under (little) pressure from the previous ride?

 

[tabijan]

I just tried now on cold engine. Squeezing the hoses causes coolant in the reservoir to move but there are no bubbles.

Andy might have suggested this already. The pressure cap is tightened until it clicks? It takes a lot of turns to tighten the cap. You replaced most of the cooling system. Have the pressure cap tested or just replace it.

What was the condition of the coolant when you got the car? If it was rusty, maybe you should have the cooling system descaled with citric acid.

One hypothesis is the water pump has an inefficient impeller that works sufficiently at low engine rpm but not so well at continuous high engine rpm on the highway.

Sixto
2014 SE 1.6 hatchback 13K miles

 

[gt097]

The problem was there even with the old water pump, I don't think that's the culprit. The coolant was pretty dirty when I first bought the car, so I had it changed.

I'm going to buy a new tank cap and do the test but I'm 100% sure that the actual cap is tightened properly. When you squeezed your hose was it hard or soft?

 

[tabijan]

Hose is soft. No perceptible pressure release when I remove the pressure cap. Hose isn’t softer after I remove the cap.

New coolant is good but a block with corrosion is another matter. Was the system flushed before adding new coolant?

Sixto
2014 SE 1.6 hatchback 136K miles

 

[gt097]

New coolant is good but a block with corrosion is another matter. Was the system flushed before adding new coolant?

Nope, don't think so. I'll do that as well at this point, after the tank cap replacement.

Thank you btw, I'll post updates as soon as I get the job done.