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MB Paul
MB Paul, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4317
Experience:  Mercedes Benz Qualified Diagnostic Technician
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hi i have a merc ml 270 cdi 2000 ive not had it long only done

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hi i have a merc ml 270 cdi 2000 ive not had it long only done abont a few hundred miles it was going good until i was climbing up an incline it started making a noise like leaking compression and immediatly lost power it lasted about 6 or 7 seconds then cleared and was ok for about 6 miles then it repeated,i had a look in my shed took the engine top cover off the injectors and all looked fine i gave it a few good revs and the noise retured it sounded like it was agine losing compression and down maybe 1 or 2 cylinders for about 6 or 7 seconds the noise seemed to coming from around the near side of the engine but cant find were from, it done it a few times as i reved it then stopted can anyone help ?

It is difficult to say for sure without actually hearing the noise in person but I will try to assist.

The injector seals leaking is a common problem but if you have checked under the top engine cover & these are ok then we can rule this out.

The inlet ports in the intake manifold is another common problem - they are quite hard to see and are small butterfly flap located in the intake manifold near where it bolts to the cylinder head - the flaps can break up & leak air through the manifold. Look for excess oil leaking down the engine block under the manifold - a tell tale sign that these are leaking.
A test with a proper intake pressure test kit may be required to confirm this.

However from what you have described with the problem happening at full loads up hill another strong possibility is with blocked catalytic converters - under high load the exhaust gas backs up & causes issues like you have described. Lood for excess soot around the turbo & cat joints. The only real way of confirming this however is to remove both cats to inspect.

We do see problems with blocked cats & both should be replaced if this is the case. Also check for codes & injector values & all sensor readings on a diagnsotic tool such as Mercedes STAR to make sure there are no running problems that have caused this.

There are of course many other possibilities but I would look into these issues first.

Best regards, Paul
Customer: replied 3 years ago.
thanks for the reply i did notice oil on the block but it looked old could the problems with the inlet come and go today the noise of like leaking compreshion lasted a good 60 seconds but i still couldent find it when it clears it goes great i can get it to make the noice by just lightly reving it without moving should i remove the inlet?

If you can hear a hissing noise when revving from under the manifold then I would say the manifold will need replacing. A pressure test or a smoke test is a better idea first as it is quite a big job to remove the intake manifold!

I would still also check the cats at this stage for blockage also.

Best regards, Paul
Customer: replied 3 years ago.
hi i had look tonight and it looks like the inlet has already had the flaps removed and bypassed, no leaks or marks around cat joints,i pulled of the big pipe to the egr whitch look dirty,i ran the engine with the pipe off and when the noise returned it sounded and looked like it was exausting through the inlet but agine only for about 6 or 7 seconds and the engine was at idle the hole time then it runs good with loads or power what could cause this?

Well to be honest I have never tried to bypass the inlet port valve & not sure what implications that may have! The ECU usually relies on feedback from the motor that opens/closes tha valves!

If the EGR valve is very dirty then it could be the valve is not always shutting off when it should & allowing exhaust gas into the intake. I would give it a good cleanout & make sure the valve opens & closes smoothly when vacuum is applied.

Best regards, Paul
Customer: replied 3 years ago.
hi i checked and cleaned the egr valve i worked it back and forth no problem with a vacume pump i blocked the vacume hose and ran the engine with egr shut it still made the noise like a train interintermitantly but this time it wont go over 3000 rpm no full boost i reconected egr and sill no full boost switchted engine on and off lots of times but still the same i then checked the turbo waste gate it was moving fine i then thought about the cats blockted so i tryed holding a bit of wood over tail pipe to simulate a blockted cat to see if the noise would return but it never {very heath robertson i know} straight away i got full boost i then drove it a good ten miles no problem,did the presure in the exaust move dirt? should i remove cats? no eml light on at any time

I would probably be looking in the cats at this point - it is very symptomatic of a blocked cat.

A check for fault codes would be a good idea first - low boost pressure codes may be stored which could also be an indication.

A bad MAF sensor can also give a 3000rpm limit - but not usually with the noise you describe.

Aother real possibility is an air leak in the intake system - a hose or split intercooler opening up at pressure. As mentioned previously a correct intake pressure test kit is the only way to reliably test this to check for leaks.

A sticking turbo actuator may also be a cause & also check all vacuum lines.

My gut feeling from your description is still blocked cats!

It is very difficult to say for sure without further testing & seeing the car to experiance the problem first hand.

Best regards, Paul
Customer: replied 3 years ago.
hi i drove th best part of 30 miles with no problem today,i did plug it in and got load of codes p1630 imobiliser com error, p0703 brake light switch error,p0100 mass or volume air flow circuit voltage low+circuit not plausible, p0105 boost presure sencor voltage too low thay maybe old codes before i got it.i cleared all codes and went for a run and none returned, do you think a sticking valve in the head could be a cause given its so intermitant and such a nasty sound like exausting through the inlet but for such a short time, i would of thought blockted cats would make it sluggish at all times? but if you think its likely thats my next job thanks iain
Hi Iain,

ok so the 2 codes that may cause the issue are the MAF & the low boost pressure voltage.

A bad MAF sensor may be the cause but as said it is unlikely to cause a noise like you have described. They are quite cheap & easy to replace and are very prone to fail so it may be worth trying a MAF sensor first but think in all honesty that the fault may be induced by another issue.

Both of these faults can be caused by either a leak in the intake and/or a blocked exhaust.

If you are sure that the intake is ok then I would be looking at removing the cats to inspect next..

Best regards, Paul
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