How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Matt Your Own Question
Matt
Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22166
Experience:  BEng hons Mech engineering, in auto industry 22 years
12772297
Type Your Mercedes Question Here...
Matt is online now

I have a ml270 w163 on a 53 plate can you tell me why it

Customer Question

i have a ml270 w163 on a 53 plate can you tell me why it keeps going into limp mode or will not go over 2500/3000 rpm , i keep changing the mass air sensor and before it worked but this time it didnt ?
i had it in a local garage thats how i learnt about the mas sensor ,this guy used to work for mercedes Aberdeen and claimed if it happend again it will come down to the engine management ?
he did some wiring work charged me £130 and sent me on my way !!!!!!!!!!
its happend 3 times since then but this time no joy , can you help please ?
mark hollick in aberdeenshire
Submitted: 1 year ago.
Category: Mercedes
Expert:  Matt replied 1 year ago.

Hello

if you have a check engine light on then the best route is to plug a scan tool in to read off the stored fault code as this will point to the fault

but if there's no fault code then this could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes 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 /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

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.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine