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-   -   4.0 tdi hot start problem (http://forum.a8parts.co.uk/showthread.php?t=12968)

erubus 4th May 2017 06:54 PM

4.0 tdi hot start problem
Hi guys.

I thought I had this one sorted. Car never started when it was hot. would just spin the engine round for ten seconds or so and would do it till the vattery was dead. Always started with a jump start though when it was hot. started fine once it had cooled down for an hour or so though.

I had it scanned a while back and the only thing the (audi trained) guy could find was an egr fault that he reckoned could be stopping the thing start, he reckoned the valve was sticking open. When I thought about it a bit though I couldnt see how a jump start would get it going no problem every time.

I fitted a new (intermotor) crank sensor a couple of weeks ago. since then it has started first time every time no matter how hot it was but over the last couple of days I noticed it cranking for longer when hot and tonight it was back to spinning round endlessly without catching.

I really thought I had it licked with the crank sensor, is it possible that it has to be an OEM sensor? I'd still expect an aftermarket one to last more than 10 days or so.

The other possibility is the power management module as I understand from researching the problem, especially as I know it has been jumped straight to the battery terminals but howcould a new sensor make it work for as long as it did?

any help gratefully received, gill

ainarssems 5th May 2017 10:41 AM

On 1.9TDI's there is a common hot start issue that there is a map in ECU that has minimum rpm before start of fuel injection depending on temperature, I do not know exact figures but it could be something like 200rpm for cold and 300rpm for warm. If the battery or starter is a bit weak and engine does not turn fast enough ECU does not inject any fuel and engine does not start. The resolution is either new battery or starter or ECU remap.

I am not saying that it is definitely your problem as I have never heard it on 4.0TDI but then again it is not very common engine either. And the fact that you say it will start when jump starting could mean that starter is cranking faster when jump starting so worth investigating or trying new battery.

Mechcanico Lee 6th May 2017 03:08 PM

I think you are already on the right track with the crank sensor , the symptoms sound correct ....ok at cold start and trouble at warm start .

You say fitting the new sensor made it behave great for a period of time , the trouble we have at work with after market sensors I have lost count of the number of times..... crank , cam , air mass meters , egrs , temp sensors in the after market have caused issues and confused the diagnostic pathway testing .

I would be tempted to fit a genuine crank sensor and try again , the vehicle parts market is flooded with Chinese copy ting tong parts ......I would also say Intermotor are not the quality company that they used to be back in the day ....it's all about profit now and not quality sadly .

Here's how a ac generation crank sensor looks on a scope


Draw a horizontal line at the 0 volts through the trace , you have plus volts and minus volts because it's an ac generation sensor ....as a rule whilst cranking the ecm wants to see around 3 volts peak to peak signal amplitude , the change in the trace so far through is the missing tooth on the flywheel reluctor ring or front pulley toothed ring whatever the application , that missing tooth is how the ecm picks up the top dead reference .
So when crank sensor signal and cam sensor signal are seen to be in synchronisation when cranking this is when ecm triggers injection begin pulse.

What happens with the hot start is the heat soak affects signal amplitude from the sensor , in some cases jump starting it just gives the starter extra speed so a larger amplitude of signal is generated and away it goes ....or just let it cool off and it re starts also .

Of course it may be something else ......lol !

erubus 10th May 2017 05:38 PM

Excellent information, thanks. Now, I'm primarily a mechanical engineer so I'm having to dig deep to the depths of my memory of my training for electrcal principals haha. I think I'm right to say that the faster the engine turns the greater the signal amplitude will be from the sensor, assuming the signal isn't conditioned in some way, such as through a voltage regulator.

If I want to test the sensor are there any checks I can do without having to set it up with an exciter ring (reluctor ring?) and scope? I should be able to get a shot of a picoscope that I can rig up with the sensor still on the car so I can get a trace with the engine cranking hot and cold. Is this something that can be checked using VCDS?

I'm reluctant to spend money on another sensor on the off chance, although the evidence does point towards the intermotor one failing very quickly!

What manufacturer is the OEM sensor and does anyone know the manufacturer's part number? I should be able to get a Bosch one fairly cheap through my father's work if that's OEM.

As for the cranking speed, I did check it with my OBDII reader on live data and I didnt think it was all that fast but I cant remember off the top of my head. I should check it again.

Sorry for all the questions. Thanks in advance, gill

Mechcanico Lee 10th May 2017 10:57 PM

Yes , ac generation sensor ....faster speed means more signal amplitude , I have not seen any voltage clipping with ac sensors but not to say that it does not excist .You can test across pins of crank sensor for ohmage and test to earth shielding but these test donot prove what ac voltage it will generate .

You can use a multi meter set on ac volts and test but I use scope as it gives a much clearer picture and shows missing tooth segment and shows any interference and signal anomalies much better .....but obviously you need a scope to do this .

You say car starts well from cold ......you could summise this will be the slowest cranking speed of any start ups , you would think that warm start ups would be faster cranking speed but get this is were your fault is ..... very typical issue for crank sensors . With vcds you could look at what cranking speed is and you could see if there is a data block for crank / cam synchronisation to see if it shows yes /no whilst cranking .

The orginal sensor most likely will be a Bosch one , even though you can buy a Bosch sensor in the aftermarket as I said before there is to many copy parts and Chinese copy's floating around ..... I would only use genuine and pay the extra cost for the quality .....it allows you to move on in the diagnostic pathway testing without questioning ...." Is that sensor definatley good "

I have been caught out in the past fitting OE after market sensors only after hours and hours further testing and going over everything again only to find the genuine sensor fixes it first time

erubus 12th May 2017 04:25 PM

I stuck the OBDII reader on while starting it from warm yesterday evening. It gave a reading of 165 RPM which is what it said last time I tried I'm fairly sure. I don't know just how accurate that is though as when it actually starts up it went up in two or three increments quite slowly up to idle speed so it could well have been fasyter. I need to check it again when the car is having one of it's moods and not starting, that way i'll have ten seconds or so cranking for the reader to get a proper speed reading.

If I can get the proper bosch part number I should be able to get a genuine one cost price from my dad, so won't have to worry about whether its a chinese fake or not. I woner if the intermotor one wasactually a fake... I usually find intermotor parts fairly reliable but this one was off of ebay and suspiciously cheap so who knows.


erubus 4th June 2017 07:22 PM

Well here's an update.

I got a genuine bosch crank sensor. As in it came from bosch direct from germany, so definitely genuine. I didnt even get the six days of happy hot starting. God only knows why I got 6 days from the intermotor one. I really can't explain that and i'm really really p*ssed off now. Got stuck 30 miles from home and had to ask several people for a jump styart before anyone actually said yes.

It is vaguely possible that the sensor was the wrong one as when i ordered it with the part number off the original I was told it had been superceded and the last digit was different. The cable was maybe 6 inches shorter but otherwise it was identical.

exact same symptoms as before. My next line of enquiry is going to be the starter motor. When it's cranking over and not starting it is turning between 150 - 160 rpm. When I jump it, it's definitely turning over faster, although my generic cheapo obd reader doesnt react fast enough to tell me the speed before it started.

Has anyone tried removing a started from a 4.0 tdi before? is it an engine out job? If i can get it out and on the bench then its an easy job to see if it is in need of a refurb, and I can easily clean up the commutator on my lathe, fit new brushes etc and hopefully that will be it........ If thats the problem!!

ainarssems 5th June 2017 06:45 AM

150-160rpm sound too low for hot start. On 1.9TDI which experience similar problems it needs to be 250-280rpm when engine is hot. Have you had your battery tested or replaced? Considering that it turns faster and starts when jumping it tells me that battery is too weak as that's the only thing that changes.

Another thing you could try is to unplug coolant temperature sensor when it's hot and refusing to start that way ECU will not know temperature and may allow to start injection at lower rpm.

Joe2.0E 5th June 2017 05:31 PM

Starter motor would be worth checking if it's the original, I recently replaced one on a 2006 3.0TDI Q7 that was difficult to start. Long cranking time almost flattening the battery but very random, happened mostly when car sat idle for more than a few hours but then sometimes started perfectly. No real pattern as when car was hot or cold though.

erubus 6th June 2017 05:25 PM

Thanks guys. Starter motor is the chief suspect now for me anyway. The battery has been replaced several times over the past year by the PO and by myself, going through the paperwork the PPO changed it as well about two years ago.

I will try unplugging the coolant sensor if i can get to it. I believe on this engine it is underneath the AC pump a d requires the pump to be removed and the AC evacuated and re-gassed before turnign the engine on due to there not being a clutch on the pump. Not sure what the truth of that is though.

I suppose i shouldn't overlook obvious things like the earth strap connections from the engine to the body. Corrosion between the steel of the strap and the aluminium of the bodywork could just be impeding the current enough to not let the engine turn over fast enough. It seems to spin over at much the same speed hot or cold, by ear anyway. As I mentioned i my last post, my code reader doesnt reacy fast enough to get a reliable rpm figure unless its cranking for ages and not starting.

Has anyone had any experience of pulling a starter on one of these things? When I fitted the crank sensor it was quite late and I hadn't put the axle stands anywhere near high enough, so I was really squeezing myself underneath and as i was convinced that the sensor was the culprit I didnt really look any further to look at starters or anything.

Its still a huge mystery as to why it worked perfectly with the last sensor for that 6 days.


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