S3 overheating when driving. Normal when idling

Hey folks. Before anyone says it a tried using the search button but didn't find anything with similar issues.

As title says. My S3 diesel overheats when I'm driving but as soon as I stop at some lights the needle comes back down.

Thought to try swapping the stat for a new one. Checked it opened normally before fitting and I'm still having the issue.

It has a new waterpump last year when I had the engine rebuilt and all was well and fine back then.

Any ideas?

check rad gets hot all over and out put of alt with engine running at idle and revving

Rad gets hot everywhere I can reach including top hose from stat. Alternator I presume is doing fine as I have a dual charge system for my secondary and it only activates at 13.8v? I have an led for when it's active and it does light up

Weirdly if I rev engine when stationary the needle will drop a little but not all the way back to normal

Stormintrooper said:

Rad gets hot everywhere I can reach including top hose from stat. Alternator I presume is doing fine as I have a dual charge system for my secondary and it only activates at 13.8v? I have an led for when it's active and it does light up

Weirdly if I rev engine when stationary the needle will drop a little but not all the way back to normal

Click to expand...

it was to test to see if alt output rose with revs

Goes from between 12.4 up to 13.8 depending on revs

I think you need to separate whether its a sender / gauge issue or and engine temp issue. If it changes quickly it sounds more like a sender / gauge issue (which includes voltage and electricals), if it overheats and takes time to come back down its more likely actually overheating. Unfortunatly the only time I've had a similar symptom was when the head gasket had gone so I'd do some more checks before committing to anything. The first question is "does the engine and or rad feel hot?" If its overheating it will feel very hot. The other thing is how load affects it, is is revs dependent or load dependent?

Think I may have found the problem. Didn't think to grab the bottom hose for a feel

Stone cold! And very bottom of the rad isnt as hot as the top. So either a block in the rad. Faulty new britpart pump or hoses?

I'd rather it not the rad but to check the rest iv gotta take it out anyway so a flush thru will tell.

Don't I feel stupid!

On a side note. Shouldn't my alt be getting up to about 14.4v?

If the battery is staying charged then its OK. Is your overheating a stuck thermostat?

Already tried the stat. Guess I could check by running it without a stat to see if there's flow but iv already swapped it for a new one

does your temp sender circuit have the voltage stabiliser fitted , reduces voltage to 10v so stat reading does not change with revs.

The fuel gauge might be doing the same if it’s the reg.

Fuel gauge is stable

Hey guys

So I swapped the pump
All new hoses
Tested the new thermostat to see if it opens
Ran water through the rad which happily went through

I still have an over heating problem

I did try pouring water directly through the thermostat opening to make sure water went through the block which it did

But I have a rad that's piping hot at the top and cold at the bottom. And the bottom hose feels tight like it's got pressure behind it

Any other ideas before I buy a new rad?

I had a problem with my 200di series today whilst driving back home whilst doing some roadside checks put my hand on top of radiator and it was that hot had to take it off straight away 4" down rad was cool

Blackburn said:

I had a problem with my 200di series today whilst driving back home whilst doing some roadside checks put my hand on top of radiator and it was that hot had to take it off straight away 4" down rad was cool

Click to expand...

replace the rad

jamesmartin said:

replace the rad

Click to expand...

Same goes for me? Even though when I stuck the hose in the top it ran freely out the bottom?

Some thoughts on this
1) Are you getting an air lock? I had a similar situation where it (109 2.25 petrol) ran at a decent temp then as soon as I started to climb a gradient the temp shot up. It was too quick a reaction to be the actual engine temp increase due to loading. Later when I pulled the cooling system apart to replace the thermostat due to other issues it disappeared. No reason why the thermostat would be affected by gradient
2) It's not the valve timing or what ever you have on diesels that make it go bang is it?
3) Head gasket.

You could try some agressive heating descaler, I think its phosophoric acide, (I bleive you can use brick cleaner too) , take the rad off, block the bottom and fill it up. It will either work or make the decision on a new rad for you. If it works be sure to flush and neutralise it before you refit.

When you say overheating...can you say how you know...does it boil over...or is it just that the temp gauge reads high...not the same.

Infra red thermometer cheap on ebay...tells you all you need to know.

The engine and top of rad can easy reach 75c in normal use...thats the opening temp of the stat.

Stormintrooper said:

Same goes for me? Even though when I stuck the hose in the top it ran freely out the bottom?

Click to expand...

no, his problem is hot top of rad cold 4 inches down,rad should gradually cool but be warm all over