Temp gauge/coolant temperature sensor query

[RoyE]

Hi all, I have a 1.8T FR Ibiza (2004) with increasingly bad hot start problems.

After researching the forums I suspect coolant temperature gauge may be faulty (leading to flooding the engine) - can anyone confirm if the following is a good sign of this:

After engine fully warmed up gauge has gone up to middle (just under 90 degrees) which it always has done. But if I stop the car for a minute then turn the ignition back on the gauge only goes up very slightly (to 50 degrees) and car is hard to start - it takes a couple of miles to climb back to 90 again even though coolant must still be boiling hot?

I can't recall how it used to be but shouldn't it be close to 90 as soon as you turn the ignition back on if you've only just that minute stopped after a long hot run?

Any help is much appreciated.

Other info - no fault codes or warning lights, battery 18 months old, cold start is fine, good mpg and runs well. Can still get hot air through blowers.
Hot start issue is that engine/starter 'skitters' or else sort of fires up a bit (like very low revs) whilst key is still turned - after a couple of seconds of this it kicks in/revs up normally.
Can take 2 or 3 attempts to get it going but doesn't sound at all like a dying battery does.

Many thanks

 

[RUM4MO]

Just one comment, all VAG cars with a temperature gauge have a dual sensor engine coolant temperature sensor, so, if this is not working as expected, either the gauge could show wrong temperature or the engine ECU could be getting wrong temperature signal, one sensor feeds the gauge and one sensor feeds the engine ECU. Also "bad" but in range temperature values will not throw a fault code. If buying a new dual sensor, buy a new O-ring seal and retainer clip.

 

[RoyE]

Thanks for your reply.

I had thought that only the 4 pin sensors (like the golf mk4 ones) were dual and that the Ibiza 2 pin sensors weren't - which is why I thought the sensor misreading hot coolant as cold could lead to the ecu flooding the engine?

Either way, are you able to confirm if the gauge should show a hot engine as 90 degrees-ish when ignition turned back on immediately after stopping - or is starting at 50 degrees again for a while normal? Just seems odd to me as the coolant must still be almost boiling...

Many thanks

 

[RUM4MO]

I'd agree that a fully warmed up engine showing a gauge reading of 90C - then switched off and then switched back on a short time later should still show 90C or nearly 90C.

In my experience, so far, a VAG car with temperature gauge will always have a 4-pin dual sensor. My daughter's late 2009 Ibiza does not have a temperature gauge - so only has a 2-pin sensor - but I'd still expect your car to have a 4-pin. Though there is no reason to suggest that VAG or any other marque should need to do things that way.

One comment, though the temperature gauge looks and acts like an analogue instrument, it will be getting fed with digital data, so it will be driven up the scale in steps, would will mean that it will show "90C" probably from something like 75C to 95C - these values are just a guess to give you an idea of how it works. These gauges will never operate in a linear way, more in temperature jumps - but we will never notice that!

 

[RoyE]

Ok, I've now replaced the sensor with a genuine new one (it was 2 pin, seems that all the mk4 Ibizas have 2 pin regardless of a temp gauge).

However, there's no change - could anyone with a Mk4 Ibiza with a temp gauge confirm for certain if it goes back up to near 90 degrees on ignition when engine is very hot or not?

Mine is still only going up slightly to c 50 degrees on ignition after a long journey - if that is in fact normal then I guess I now need to look into other suspects e.g. battery or starter motor.

Many thanks

 

[Badger]

Not thought about the thermostat?

Also how old is your battery?

 

[RoyE]

Battery is 16 months old (Varta Silver).

I hadn't thought about thermostat as temp goes up to slightly under 90 degrees after a few miles and stays there on a good run, doesn't overheat, doesn't drop when fan/heater is blowing hot or cold - same as always. Blower works fine - both hot and cold ok.

Double checked today and, immediately after stopping after a long run (gauge was on 90), on ignition the temp gauge goes up momentarily to about 80ish then drops immediately to 50 - after starting it climbed back up to 90 again after a mile or so.

 

[R3k1355]

The temp gauge isn't very accurate, it does generally tend to read 90 whether the engine is running at 70 or 90.

 

[RoyE]

I'll take it to garage next week to check things out, especially the battery/earths and starter motor.

Came across a few octavia vrs posts that reckoned a duff starter motor could cause hot start issues like this even though it does turn over as "when warm engine must meet minimum rpm on starter before fuel is injected" so perhaps the same might apply...

Thanks for the help - I was able to change the sensor myself thanks to some great earlier posts in this forum

 

[RoyE]

Temp Gauge issue SOLVED

This is now solved

For anyone who finds this thread and has a similar issue, this is what my garage did after checking for causes:

Cleaned throttle housing and butterfly valve (well dirty, sticking a bit, car has done 80K miles)
Reset ecu defaults, relearnt idle speed and angle
Checked for air leaks and checked hoses/vac pipes etc, all ok

I guess the ecu reset was needed to adapt fully to the new coolant sensor - now the temp gauge does go back up to 80+ on ignition after a long run and it doesn't overfuel on hot starts.

Idles nicely now as soon as starts up when engine warm and one bonus is that the car seems quite a bit more responsive - was on revo stage 2 anyway but I suspect the ecu reset has helped.

 

[RUM4MO]

One thing that I can't see as being true is the "ECU relearning the new temperature sensor" - the problem I have with that is, none of the car's system can cross check actual engine coolant temperatures with what the temperature sensor is seeing, it can only accept that the temperature readings being logged are correct.

The bottom line though, if that garage sorted your car then they did something right!

 

[Mike Hodgkins]

Temp Gauge issue SOLVED

This is now solved

For anyone who finds this thread and has a similar issue, this is what my garage did after checking for causes:

Cleaned throttle housing and butterfly valve (well dirty, sticking a bit, car has done 80K miles)
Reset ecu defaults, relearnt idle speed and angle
Checked for air leaks and checked hoses/vac pipes etc, all ok

I guess the ecu reset was needed to adapt fully to the new coolant sensor - now the temp gauge does go back up to 80+ on ignition after a long run and it doesn't overfuel on hot starts.

Idles nicely now as soon as starts up when engine warm and one bonus is that the car seems quite a bit more responsive - was on revo stage 2 anyway but I suspect the ecu reset has helped.

I know I’m 3 years late, I have the same issue.
Although I have cts code and eml light. Runs perfect for hours and if I turn the car off and run into a shop or fill up with petrol (basically if I turn the car off and back on) coolant temp gauge doesn’t move when I turn the car back on, runs very rich, sputters when I touch the accelerator, rough/high idle.

Battery replaced - no difference although it did need doing was becoming slow to start
Cts replaced with non genuine replacement - no difference
Genuine cts in today and still no difference.

it is 2 pin on the bjx engine which is in the fr and polo gti by the way

So do you think if I went to my garage and asked them to reset the ecu this would fix the issue? I have had the codes reset a few times and Obviously resets things when I replaced the battery but not sure to what extent (eml light went off but back on the next day).

Feeling very stuck, also read the thread relating to possible starter motor issues but mine cranks over fast and strong hot or cold since replacing the battery.

 

[RUM4MO]

I would guess that an ECU reset fixing this is illogical, but if so, what fixed the OP's problem!

The OP's car sounds like it had the TB removed from the car and cleaned, after doing that you should perform Basic Setting using VCDS or the VW Group workshop diagnostic tool - which sounds like what that garage would have done.

Again, doing the TB cleaning and readapting it is a good thing to do - but your issue, like the OP's sounds like being a temperature sensor failure - but no work was carried out on the OP's car in relation to any temperature sensor, and now that car then worked okay!!

 

[Mike Hodgkins]

I would guess that an ECU reset fixing this is illogical, but if so, what fixed the OP's problem!

The OP's car sounds like it had the TB removed from the car and cleaned, after doing that you should perform Basic Setting using VCDS or the VW Group workshop diagnostic tool - which sounds like what that garage would have done.

Again, doing the TB cleaning and readapting it is a good thing to do - but your issue, like the OP's sounds like being a temperature sensor failure - but no work was carried out on the OP's car in relation to any temperature sensor, and now that car then worked okay!!

Very difficult one. I think I will go and have the reset done as well as a check on the health of the starter motor as another thread said that could be the issue (can’t see how). If that doesn’t work then check all wiring and contacts and coolant flush just for good measure. Failing that.. well I have no idea

 

[R3k1355]

Have you checked the starter solenoid wire?

Mine did the same and this wire was worn through, fixed the wire and the problems were gone.

 

[Mike Hodgkins]

Will give that one a go too, thanks mate.