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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22902
Experience:  BEng hons Mech engineering, in auto industry 22 years
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The ML300 blue efficiency is hard to start especially when

The ML300 blue efficiency is... Show More

The ML300 blue efficiency is hard to start especially when warm, it takes 3-4 goes before it fires, when you put your foot down nothing happens, the heater plug light stays on for 2-3 minutes after the engine has started. Now the ABS lights are on as well The car has only done 29,000 kms and has not been driven hard

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this sounds like a couple of faults

however if there's really no compression then the engine is badly damaged and will need rebuilding or replacing

so thats the 1st thing to confirm and measurements should be taken on all cylinders as they should all be within 10-15 psi of each other

the ABS fault could be a number of things,

The quickest route to repair is to have the car plugged into a diagnostic machine which will read off the fault code stored in the ECU (providing the light is still on at this point) and indicate which sensor has failed or whether the fault is more serious.

However with ABS systems 99% of the time its a problem with one of the wheel speed sensors and I'd first check the connections at all four corners first for any signs of corrosion or chafed wiring.

The sensor is located in the wheel hub so you'll have to remove each wheel in turn to inspect them all.

Also check that the toothed wheel that the sensor points to is clean and free of any debris.

you can check each sensor's function by plugging the two output wires into an AC voltmeter and spinning the wheel quite quickly, you should get a voltage signal that increases with rev's.

Its possible that the fault is only a bad connection to one of the wheel sensors.

This may be remedied by removing each wheel in turn and unplugging each sensor and cleaning the contacts with a squirt of electrical contact cleaner. Then refitting.

Customer reply replied 3 years ago.

Matt thats great a good explanation,however its the hard starting that concerns me and the complete lack of power when you put your foot down, it used to jump up and go but now will only pick up very gradually.

If it is stationary and in park it revs's freely and no black smoke.

when you turn the key in the morning it will start before it cuts out, if you do the same when its warm it will only start on the 3d attempt


Hello Les

if there's really no compression then the engine will be very down on power as its relying on the turbo to make any power at all

measure the compression pressures 1st and if low the 1st thing to check is that the cam timing is correct as this can be out and lower the compressions on two cylinders, so ask if this has been done 1st, also the next step is if the cam timing is right then is to squirt a little engine oil into the bores and re-do the compression measurement, if it shoots up then the issue is piston wear if it makes no difference then its a valve that needs replacing.

As you can imagine if its the latter this reduces the repair cost greatly as the engine can stay in the car and the head be removed for valve replacement and possible valve seat machining.

If however it bore wear then the engine can be re-paired by stripping it down , machining the piston bore out and fitting bigger pistons but its often more cost effective to source a good 2nd hand engine for less than this, I'd imagine a 2nd hand engine costs for this year and model would be around 800-£1000