Warm start Starting... Problems..

[Simon mansell]

So I have a really frustrating fault

When I start the car in a morning it starts first time every time....

When the engine is up to temperature and I stop off at the shop for two minutes and get back in it won't start.... Cranks fine but won't start unless I pump the accelerator.....

Had it plugged in at the garage and it came back with the crankshaft sensor.... So got that replaced.... Also replaced the battery because had it tested and it was low.... But after having the crankshaft sensor replaced the fault is still there....

Any pointers or help would be really appreciated.... Scrolled hundreds of threads.... Rang several garages and Audi ''specialists" apparently it was common on the B7 model but it is really rare on the B8 model .... Seems I am very unlucky.....

Car runs perfectly when it is running no smoke....idle is perfect .... And has great pick up .... Just doesnt want to start when it is warm.... Have to turn the start stop off otherwise it gets really embarrassing at lights....

 

[jdp1962]

Good morning, Simon and welcome to Audi-Sport.net. We hope you’ll enjoy being here.

To help someone answer your question, some more information about the car would help: for example, age, mileage, which fuel type, which engine, whether manual or automatic etc.

 

[Retroman]

Only internet guessing, but as the fault is restricted to hot starts only, I am eliminating any sort of physical drive belt timing issues as this would be a problem on all starts. As it is hot starts only, I'm thinking it is a "heat soak" issue with a sensor plugged into the engine where it would get most exposure to heat. I am thinking camshaft position sensor which when faulty can often (in my experience) throw up erroneous crankshaft position sensor errors as well - in my experience crankshaft position sensors tend to be more reliable. I've also experienced damaged insulation in the wiring lead, leading to a camshaft position cause this sort of issue.

However, it's never good to speculatively throw sensors as a problem and ideally the car needs to be plugged into very specific VAG (VCDS) diagnostic software to try and pinpoint errors more specifically. A general garage might use something more generic like Bosch of Snap-on software diagnostics?

 

[Simon mansell]

Cheers guys for your responses

My car is

2012 Audi A4 Avant 2.0 TDi manual

86000 miles and had a cam belt change and water pump change 2 years ago

 

[Marky007]

Could it be a thermostat?

Thinking cars cold all time so starts fine when cold has same sort of issue on our old a3 1.9 tdi changed battery and thermostat it was fine.

 

[MarkWizard]

Are coolant temp sensors still problematic as they were on the 1.9s?

 

[Simon mansell]

If there was a problem with the coolant sensor surely I would have a gauge that would behave iraddically either showing it never getting warm or shooting straight up to hot on start up ... Mine seems to be operating ok

 

[MarkWizard]

If there was a problem with the coolant sensor surely I would have a gauge that would behave iraddically either showing it never getting warm or shooting straight up to hot on start up ... Mine seems to be operating ok

The green VW ones had 2 circuits on the sensor, 1 for the gauge and 1 for the ECU

 

[Simon mansell]

That's something to actually look into. Cheers for the info

 

[rum4mo]

Weird, pumping the accelerator in a TDI will/should do nothing, on an old carb'd petrol engined car it did pump fuel in to either assist or flood the engine.
As far as I know, a TDI simply loves extra fuel being dumped in while it is being cranked to start. okay the result will be some black smoke coming out the exhaust for a moment or two, but no "flooding" and failing to start, in fact the extra fuel in older engine even increases the compression ratio and as TDI are compression ignition it is win win.

Edit:- though, maybe the fuel delivery pressure in general is marginal or too low so when/if turned down when hot, there might not be enough pressure to initially start that engine, fuel filter getting changed now and again?

 

[Simon mansell]

Got an update for you

Turns out that it is the camshaft sensor that has gone/not operating correctly....

Hooked up an oscilloscope to it and found it wasn't operating correctly so there going to have to replace the cam shaft sensor..... Luckily car can still operate fine with it not working correctly but obviously will need it replacing.

Does anyone know if you have to take off the cam belt to change the cam shaft sensor..... Because that will determine whether or not it's over £500 or whether it's just over £100

 

[Joe Appleyard]

Got an update for you

Turns out that it is the camshaft sensor that has gone/not operating correctly....

Hooked up an oscilloscope to it and found it wasn't operating correctly so there going to have to replace the cam shaft sensor..... Luckily car can still operate fine with it not working correctly but obviously will need it replacing.

Does anyone know if you have to take off the cam belt to change the cam shaft sensor..... Because that will determine whether or not it's over £500 or whether it's just over £100

What was the outcome of this? Was it the crankshaft sensor? Is it easy enough to replace? I have the same issue as you described above, starts from cold with no issues but as soon as it’s up to temperature it won’t start unless I blip the throttle...
Once running is fine no problems.