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Discussion Starter #1
Hey all you clever people.
I have a 1.9 16V (old one with no swirl flaps, 2005 model).
It exhibits some strange behaviour in that when the engine is cold, it starts fairly easily (couple of cranks usually) but when the engine is hot, as in switched off, wait 5 minutes hot, then it often doesn't start properly.
By that I mean it seems to fire some of the cylinders, and in the wrong order. It's the most horrendous sound and vibration you can imagine (wish I has a recording). But open the bonnet, and let it cool for about 15 minutes (30 minuteswith closed bonnet), and it fires right up.
No fault codes, although there is a few and far between MCSF for the cam position sensor.
Any thoughts?

Thanks muchly
Leon
 

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Discussion Starter #3
I actually haven't, but given the amount of oil leaking past the turbo seals, due to axial play due to a stuffed thrust bearing due to a block cat (now removed, turbo to be repaired as soon as I have some downtime) I haven't seen much point...
So, two questions then:
1. Why would the MAFS cause hard starting?
2. How do you clean it without damaging it?
That said, I have ordered a new one from AliExpress. Will see how well that works.......

Thanks for the response

Leon
 

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I think one possibility is that an injector has developed an internal leak and is letting fuel spill back to the tank via the small hoses on the top of the injectors. This makes it hard for the HP pump to generate pressure when cranking and since hot fuel is thinner it will be worse during a hot start.



If the pump can only just generate the threshold pressure at which the ecu will deign to open the injectors the pressure will immediately fall under the threshold again so you get one cylinder firing and then a pause as the pressure builds up again, then another cylinder fires, and so on, making the engine shake.



I saw this recently on a friends Grand Voyager and found by chance that turning the key straight through from off to crank would often make it start. Americans are notoriously intolerant of cars which don’t start instantly so I suspect (but have no proof) that they programmed the ecu to open the injectors regardless of the pressure when the driver doesn’t wait for the heaters. I don’t know if it will work on an Italian car, but you could try.



To test for leakage: disconnect the return hose from the injectors and plug it. Crank the engine for at least ten seconds, or let it start. If fuel dribbles out from the injectors you have a problem. Google diesel leak-back test for details. You only need to test the injectors individually if you find a problem.



If not that then the crank or cam sensor may be failing when hot. Your codes say cam but I’m not sure whether the diagnostics can discriminate between them(?) Without an oscilloscope it’s not easy to test them, especially as you have limited time before the engine cools. After cleaning the connectors and checking the harness you might have to test by substitution.



As far as I know the maf sensor isn’t used during starting and only comes into play when the egr valve opens which on mine is a few seconds after start.
 

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I actually haven't, but given the amount of oil leaking past the turbo seals, due to axial play due to a stuffed thrust bearing due to a block cat (now removed, turbo to be repaired as soon as I have some downtime) I haven't seen much point...
So, two questions then:
1. Why would the MAFS cause hard starting?
2. How do you clean it without damaging it?
That said, I have ordered a new one from AliExpress. Will see how well that works.......

Thanks for the response

Leon
1. MAF determines the amount of air that is flowing into the cylinders. It is coupled with an IAT (intake air temperature) sensor which helps determine the temperature and therefore the ECU can calculate the air density (colder air more dense etc) and then give an optimal release of fuel from the injectors (i think the ratio was 14:1 ; 14 parts of air and 1 part of fuel for optimum combustion) ... if the MAF/IAT give off the wrong readings the whole calculation is wrong and you will get more parts of one or the other making it harder to start. When you let the engine cool down a little it is closer to the environmental temperature therefore closer to actual readings and starts easier. i'm assuming since its not running when hot then there is not enough fuel being thrown, to start the car. hope that makes some kind of sense. :)
Note: this is usually the first step of many things to check.

2. In the more developed countries you can get MAF cleaner spray. open it up and spray it, clean the IAT sensor which is usually visible with an ear cleaning buds. if you don't have access to MAF cleaner spray then use throttle or carburetor cleaner spray then some light compressed air from a distance to air it out and let the spray evaporate. MAF cleaner spray is not as harsh as throttle or carb cleaner but they are all quite similar in functionality (like different grades of fuel).

If that doesn't work then we can try other options.
 

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I think one possibility is that an injector has developed an internal leak and is letting fuel spill back to the tank via the small hoses on the top of the injectors. This makes it hard for the HP pump to generate pressure when cranking and since hot fuel is thinner it will be worse during a hot start.



If the pump can only just generate the threshold pressure at which the ecu will deign to open the injectors the pressure will immediately fall under the threshold again so you get one cylinder firing and then a pause as the pressure builds up again, then another cylinder fires, and so on, making the engine shake.



I saw this recently on a friends Grand Voyager and found by chance that turning the key straight through from off to crank would often make it start. Americans are notoriously intolerant of cars which don’t start instantly so I suspect (but have no proof) that they programmed the ecu to open the injectors regardless of the pressure when the driver doesn’t wait for the heaters. I don’t know if it will work on an Italian car, but you could try.



To test for leakage: disconnect the return hose from the injectors and plug it. Crank the engine for at least ten seconds, or let it start. If fuel dribbles out from the injectors you have a problem. Google diesel leak-back test for details. You only need to test the injectors individually if you find a problem.



If not that then the crank or cam sensor may be failing when hot. Your codes say cam but I’m not sure whether the diagnostics can discriminate between them(?) Without an oscilloscope it’s not easy to test them, especially as you have limited time before the engine cools. After cleaning the connectors and checking the harness you might have to test by substitution.



As far as I know the maf sensor isn’t used during starting and only comes into play when the egr valve opens which on mine is a few seconds after start.
definitely a possibility ... but i think it may be better to clear the easier/less labour intensive options first.
 

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Discussion Starter #7
Awesome, all thanks for the answers. I have thought about the injectors leaking, but my theory was that it leaks while still under pressure (I feel a Queen song coming on) and causes a localised cylinder to flood leading to bad starting. this of course will stop dripping when the pressure tapers off over a while and maybe leak past the rings slowly relieving the flooding. But the insufficient pressure thought also could be a cause, now that it's popped up. I try to avoid thinking about injectors (kind of like avoiding looking at a blacklight... Just hurts too much)
Re starting, I found that mostly, once it's had a bad start, if I immediately shut off and crank again it'll start. But not every time. Starts fine when cold first thing in the morning, but nowhere near as well as the 1.9 TDI Sharan That just catches straight away.
Also find myself wondering if the starter motor might need a refurb (bearings bushes etc) to spin it faster...

Thanks again. Got some new ideas.
 
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