Renault Megane 1600cc 1997 - engine temperature fluctuating

Honest John Back Room » Technical matters
View Threaded
Mon 15 Mar 2010 20:32
Renault Megane 1600cc 1997 - engine temperature fluctuating - njf

I replaced my thermostat when the temperature guage suddenly shot up above 3/4 one day. I checked the old thermostat and it was indeed faulty.

Then a new problem appeared. When warmed up, my engine temperature now fluctuates between 1/4 and 1/2 according to the guage. It never reaches a steady operating temperature, cycling between 1/4 and 1/2 continuously, every 30-40 seconds. Which is scary when driving!

I flushed the cooling system (it seems to flow fine), and bled it when refilling. The electric fan is working correctly. I think the guage itself is OK as it responds consistently.

I've tried it without a thermostat and it runs OK, but of course never warms up. If ticking over in traffic, it eventually reaches just over 1/2 and the fan kicks in as expected.

So it looks to me like a thermostat problem, and to make sure, I even bought another genuine Renault thermostat, but the problem persists.

I've never seen this before, and can't find reference to it in any forums. So any ideas gratefully received.

Cheers.

Reply | Report as offensive | Link
Mon 15 Mar 2010 21:21
Renault Megane 1600cc 1997 - engine temperature fluctuating - Fernando P

The temp gauge should read at about 1/4 hot in cold weather and just under 1/2 hot in hot summer weather and when running for some time at motorway speed. It sounds like you may not have fully purged the system of air, which is not as easy to do as I have experienced. There could be coolant sloshing about and partially circulating / cooling the system. There is an air "tap" at the top LHS of the rad (looking from the front of the rad) as well as an air bleed screw on the top hose supplying the heater. You then need to consider if the coolant temp gauge/warning light sender - (the upper of the two senders on the front of the thermostat housing) is functioning as it should. If the gauge needle remains at the "cold" end of the scale when the engine is hot, disconnect the sender wiring plug and earth the relevant wire to the cylinder head. If the needle then deflects when the ignition is switched on, the sender unit is proved faulty and should be renewed. But try purging the air / filling with more coolant first.

Reply | Report as offensive | Link
Mon 15 Mar 2010 22:40
Renault Megane 1600cc 1997 - engine temperature fluctuating - Altea Ego

The temp gauge should read at about 1/4 hot in cold weather and just under 1/2 hot in hot summer weather and when running for some time at motorway speed.

Nope shouldnt do that. A renault of that vintage and that engine should read normal at all speeds and all weathers.

I agree tho it sounds like you have an airlock.

Reply | Report as offensive | Link
Tue 16 Mar 2010 12:50
Renault Megane 1600cc 1997 - engine temperature fluctuating - Glaikit Wee Scunner {P}

Agreed as a simple starting point.

Not had recent direct experience of Renaults, but they always were very prone to airlocks when refilling the cooling system. Older Broomers will remember when changing the coolant and the radiator hoses was part of the routine servicing.

So get a Haynes manual or check one at the library, may be a sequence of clamping hoses and opening heater valves to purge the air.

Reply | Report as offensive | Link
Tue 16 Mar 2010 12:53
Renault Megane 1600cc 1997 - engine temperature fluctuating - njf

Thanks Fernando,

> The temp gauge should read at about 1/4 hot in cold weather and just under 1/2 hot in hot summer weather

Yes, that's what it used to do.

I've repeatedly tried purging the system of air, using the two bleed screws, but I'll try again!

What is strange is that the gauge moves like clockwork back and forth between 1/4 and 1/2, dropping quickly and rising more slowly.

I'll also test the sender, probably at the weekend.

Thanks again for the advice, and to Altea Ego.

Nigel

Reply | Report as offensive | Link