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Matt, Engineer
Category: Mercedes
Satisfied Customers: 21973
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Vapour lock on 1999 MB cdi. Injectors seem to be ok. What

Customer Question

Vapour lock on 1999 MB cdi. Injectors seem to be ok. What next???
It is comonon rail direct injection with bosh high preasure pump. Mercedes 1999 (w202) C220 cdi.
I am getting vapour lock. Every time. 100% times i turn a hot engine off.
I did a leak test of injectors. All fore leaked exactly same amount. Can they still be the couse of the (internal) leak?
I dont see any external leak in the common rail area.
Do I go now on the pump? How do I test the pump?
There is a slight hesitation on acceleration but otherwise the pump (engine) seems to be working well. There is that chaffing noice at lower rpm. But i dont hear it at higher rpm and the engine seems to have a good power. But there seems to be a drop in power once the turbo turns on. which might relate to the injection? Dont know.
PS: i am nearly 100% sure the leak of the high preasure is happening back through the high preasure pump. Timewise it is quite rapid as allready after a minute after turning off the vapour lock manifests itself. I could also see the vapour pushing the fuel back trough the fuel lines through the low preasure pump whichone thenafter ends up without contact with fuel as the fuel line between it and fuel filter is getting filled with the vapour. Which further more inhibited the cranking of the engine.
I had to crank extremely long time i reckon mainly cause of this "secondary" issue. I say i "had to" becouse i did put additional inline fuel pump before the fuel filter. Which now refills or at least preasurise fuel in the line making it readily available for the low preasure pump and so high presure pump. So now i am just overcomming the vapour in the rail when cranking still hot enginy. After engine has cooled down i wouldnt even noticed anything. But when the temperature is very low freezing i have cranking issue as well. So? Please i dont want to hear about changing 1000 eur high preasure pump. I saw some set of new rubber seals for these bosh pumps. On ebay. Is that anything which would help? Is it the pump which responsible for holding the preasure after turn off? Or some other component? There is something before the pump, maybe this component is the problem. I dont know what is this component. No documentation for this vehicle available.
Submitted: 1 year ago.
Category: Mercedes
Expert:  Matt replied 1 year ago.


This could be an injector leaking fuel back through the overspill pipe, so not getting enough fuel pressure to tell ECU to fire the injectors, this is trickier to find and you’ll need the garage to do a ‘run back’ test to identify which one is the issue

and as you say if the high pressure pump is leaking back then this too can create this issue

if this was the case I'd expect that the fuel pressure would be down on what its supposed to be

this can be checked by a fuel injection specialist or if you have a garage with the MB STAR system that can plug into the diagnostic port to read off the engine pressure sensor

Customer: replied 1 year ago.
With no insult its just too general. In fact what i expected. A general answer not containing anything which i would not know allready and therefore not helping me forward in anydirection. To not to be pathetic and refusing your answer on same general level... 1) I did specificaly ask if the injectors could be still a problem despite the leak down test came out negative as i wrote. You ignored these lines of my enquiry. 2) Also Matt, its not the computer not giving command in the situation of 'vapour lock' . It does bloody try hard to crank it by several means. It is just the fysical difference between compressed gass and compressed fluid. The first one will just not open the injector as a result of how they (constructed). At least this particular Bosh from 1999. 3) What about the people refurbishing the high preasure pump with those replacement rubber seals. You dont say anythink about it neither. Sorry as i said no insult meant but just a useless answer for me not bringing me any new info-info-direction or idea. Even for a risk it would be a wrong direction idea. Your answer is lime copied from internet forum which i went through many. All good man. Regards. PS: do not insult further customers with advising them to bring their vehicle to a service with 'star' diagnostic. Its realy an insult on ones inteligents as i can hardly imagine anybody not knowing about this possible solution (bring his/her car to a garage)
Customer: replied 1 year ago.
Realy no insult meant Matt. Take care.
Expert:  Matt replied 1 year ago.

no offence taken

also when I 1st looked at this post all the data in your PS part wasn't there for some reason?

still, moving on

I'd try and get the fuel pressure measurement off the car as this will show if the main pump is healthy this can be done either with a mechanical gauge or via the diagnostic tool I mentioned earlier

if the leak back test is Ok then I doubt the injector are the problem as that tests their sealing

regarding rebuilding the pump with some Ebay seals I'd be wary of this as generally Bosch don't refurbish the the pumps as a lot of the sealing comes from the fit of the mechanical metal parts