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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22908
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Mersaded Vito 56 plate, engine light come on when I go over

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Mersaded Vito 56 plate, engine light come on when I go over 75mph and the engine goes in to limp mode?

if you've got an engine light coming on then by far the quickest route to repair is to plug the van into a diagnostic port and read off the stored fault code

from your description I'd consider changing the fuel filter as if this partially blocked this too will reduce pressure at the main pump

If OK then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.

If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues and its best to have the boost pressure measured actually measured with a boost gauge to check that the turbo is healthy and that the sensor is reading correctly

If its a variable vane turbo (they’ll be an actuator on the turbo body - but not a wastegate) then check its vacuum pipework as above and check the connections to the diaphragm / solenoid valve

Also worth checking that the glow-plug relay is switching off as they can stick on and leave the glow plugs also on - on some cars this can force it into limp home

Also worth considering a bottle of injector cleaner into the tank as a clogged injector nozzle will reduce power and give poor combustion - the next stage on from this is to remove all the injectors and have them ultrasonically cleaned and flow checked

Also check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.

If the above are all OK then try checking for airleaks after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and leans the engine out causing rough running. As its a mechanical fault it tends not to turn on the fault light and you can sometime hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas around each joint in turn. If the engine rev's up you've found your leak.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty
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Customer: replied 4 years ago.
I've double checked all the pipe/hoses for crack etc can't find anything checked all the wiring and all ok, I suspect it's something to do with the dual intake system on top of the engine, the light comes on when I maintain a speed of 80 mph. Disconnected the plug from injection lead on top of the engine Ada trial and this has the same affect-engine light come on an goes into limp mode, this is why I suspect it's a pressure fault in this area, any other suggestions
Hello Kevin

I'd really reccommend getting the van plugged into a diagnostic device as the next step - I know its about £50 - £80 but you'll save so much time its money well spent as you don't have to have the garage that does the diagnostic do the repair
Customer: replied 4 years ago.
Yes I've done that and it comes up as the engine booster pressure switch which is situated behind on the left by water screen wash, I've replaced this to no affect what do you think next.