TQS Hesitant To Start

MaverickTDi · May 26, 2017

Since becoming a TQS owner all has been good until today.

I went to use the car having not been used for 5 days and it it cranked over for what seemed ages. I switched the ignition off then back on, let everything settle and then tried again and after several cranks it reluctantly started. Once going it's fine - engine seems responsive etc.

Got to work and when I came back to it some 6hrs later it still was hesitating. It's at this point I realised that on both occasions whereby it struggled to start I had the air con on. I switched it off and tried starting and it seemed to start better.

Could the air con on before start up somehow drag it down? Or could there be other reasons why it's struggling to start?

All suggestions welcome, cheers.

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Samuel c5 Q · May 26, 2017

Check battery voltage level. Ignition or fuelling, I.e. Evap system/check valve/vacuum tube is possible.

Samuel c5 Q · May 26, 2017

Fuel system or check valve maybe.

Chriscooke · May 26, 2017

Mine does similar especially when it's hot out. I've had fuel pressure checked, fuel filter changed, plugs changed, temp sensor swapped and it still does it.

It's never failed to start but it's odd.

aragorn · May 27, 2017

Check CTS readings with VCDS and make sure its reading correctly.

MaverickTDi · May 27, 2017

aragorn said:
Check CTS readings with VCDS and make sure its reading correctly.

CTS? What is this?

Is there a VCDS set up you'd recommend?

Cheers.

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aragorn · May 27, 2017

coolant temperature sensor

I just use a cheap ebay copy lead.

MaverickTDi · May 27, 2017

So a knackered CTS would cause the engine to take a while to turn over?

Cheers - I'll have a look on eBay.

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aragorn · May 27, 2017

yeh, the ECU uses the coolant temperature to calculate the equivalent of how much "choke" is required when starting the car. If its faulty it can report the wrong temperature, causing starting to become difficult.

MaverickTDi · May 27, 2017

Makes sense. Where is the CTS located?

Cheers bud

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aragorn · May 28, 2017

in the plastic elbow thing on the back of the cylinder head.

PaulJC84 · May 28, 2017

Yeah I have had this twice in my 13 years of b5 ownership and I needed a new coolant temp sensor.

MaverickTDi · May 28, 2017

Cheers guys

Are CTS's expensive? I assume the usual rule applies whereby it's better to buy OEM once than cheap 2-3 times?

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aragorn · May 28, 2017

about 25 quid from the dealer, dont buy a fake one.

MaverickTDi · May 29, 2017

aragorn said:
about 25 quid from the dealer, dont buy a fake one.

Cheers mate

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MaverickTDi · Jun 17, 2017

Just after thoughts/opinions please;

So the missus went to use the car today and after being sat for a week it started fine. Drove approx 12 miles parked up, came back and it just cranked over but didn't start. Left it a few mins and it started no probs.

Is the general consensus a intermittent CTS or could it be something else?

Cheers chaps.

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aragorn · Jun 17, 2017

how long had it sat after the 12 mile drive?

MaverickTDi · Jun 17, 2017

About 1hr I'd say.

Cheers.

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aragorn · Jun 17, 2017

well unless you get some diagnostics on it, its all just guess work.

MaverickTDi · Jun 17, 2017

Yeah I appreciate it could be a multitude of things but just wondered what component or components may cause these symptoms so I can gauge the possible expense.

The brakes need replacing soon so I'll ask my local garage to plug their diagnostic computer in and see what it spits out.

Would the car log a 'fail to start' condition?

Let's say we looked at the CTS, what would a scan tell me if it's intermittent?

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aragorn · Jun 17, 2017

a scan alone may or may not show anything. It depends exactly whats wrong.

A faulty sensor can fail in different ways. If it goes open circuit or short circuit or something, then the ECU can clearly see thats an issue. It'll log a fault code and will almost certainly run perfectly fine, as it ignores the faulty sensor. If however it simply reports "30" instead of "90" or whatever, then the ECU wont realise its faulty, as the values reported are believable, but simply wrong. Thats when you get issues, as the ECU believes the incorrect value, and then does the wrong thing.

VCDS allows you to view live data from the sensors as well as showing the fault codes etc, and thats where you really start to figure out whats going wrong. A garage will just throw parts at it. Someone with a brain will look at whats going on and make decisions from there. If you plug in VCDS and the temp readings are all perfect, then clearly the CTS is fine!

As an example, a faulty thermostat will often cause an "implausible signal" fault code for the coolant sensor. Thats because the ECU expects the tempertaure to rise up to 90c within x minutes of starting the car, and then stay there. If it never reaches 90c or gets there but falls off again, the ECU realises something isnt right and flags a code saying the signal doesnt make sense. It ofcourse has no way of knowing if the temperature sensor is faulty, or if its measuring correctly and theres something else going on. Some people will see that code and replace the sensor, but ofcourse that doesnt fix it!

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