intermittent engine power loss

[kfb118]

HELLO TO ALL, i am new to this forum and i would hope someone could help me with this problem.I have a 1999 s-280 w140 which has an intermittent fault. When i take off from a junction/roundabout etc. the engine has no power/like a miss, and i can hear a slight backfire for a second or two and then it takes off as normal. This seems to only happen when car reaches running temperature, and there are times when i dont experience this problem for days. Any help gratefully appreciated .

 

[television]

Hello and welcome with your nice 140.

The first thing to check is the MAF sensor, just unplug it and try the car when it plays up, and see how it goes

 

[kfb118]

Many thanks, i,ll give that a try and let you know how i get on.(knowing my luck it,ll probably run smooth for weeks now ! )

 

[television]

Many thanks, i,ll give that a try and let you know how i get on.(knowing my luck it,ll probably run smooth for weeks now ! )

This is one of the only things that can right itself on a restart, if it runs fine now,think of the money that you can spend on that something that you always wanted

 

[ash-uk]

Hi, My experience of a faulty MAF sensor on my W168 has been similar (I've got a 2001 A160)

Well it isn't actually a fault with the Mass Air Flow sensor itself , but contamination on the sensor which measure the air flow.

The way it works is the strips in the MAF sensor are thermal resistors (or thermistors). These thermistors limit the amount of current allowed to pass through them the hotter they get, so when the car is ticking over, the strips run at optimum temperature and as a result, only a little current passes, which the cars ECU measures the correct quantity of fuel for by doing a calculation from the measurment, but when you rev the car up, more air passes through which cools the thermistors and and allowing more current through.
The ECU measures ambient air temp, engine temp, engine revs, throttle opening position etc and all things being equal has been programmed to deliver the correct amount of fuel to the mix so the car runs correctly whatever the revs or engine temperature.

It is crucial that the air filter is changed regularly as even a slightest bit of contamination will affect the speed at which the thermistors cool (or warm) and measure the airflow.

ECUs are programmed to heat the thermistors up when the ignition is turned off which burns off any contamination acquired during the run, but when dust settles on them (due to infrequent filter changes) it attracts oil and this turns into a porridge on the thermistor which wicks away the heat and gives a faulty reading - less heat = it thinks there is more air flow = more fuel required which it supplies and then makes the car run rich at lower revs.

The thermistors are very small and very delicate so you should never attempt to handle them but they can be cleaned very carefully with a residue free solvent spray (chlorohexane or equivalent).

Mercedes code the ECU to the car and charge over £1000 for a replacement which has to be coded and then shipped from Germany.
There are other ECU repair companies who will replace these strips but charge £250 for the job. This may be the only option if the strips are damaged beyond repair, but cleaning them worked for me.

DON'T ATTEMPT TO MODIFY THE INDUCTION SYSTEM IN ANY WAY AND ALWAYS USE AN OEM AIR FILTER.

One other thing, the fault code will have to be reset after cleaning the MAF to put the dash warning light out.

 

[moosehead]

Malcolm - I mentioned in a recent thread that we had power loss problems on the SLK although the engine starts and runs fine. As you suggested I disconnected the MAF and although the engine started it almost immediately stopped running when I let it go back to tick over. Do MAF units pack up completely or can they lose their perfomance over a period of time bearing in mind this is the original unit fitted to a 10 year old vehicle?

 

[jibcl500]

The MAF sensor measure the air temp and air quantity, they can go out of spec or sometimes just fail, unplugging can stall the engine and this is common, sometimes you might think the MAF is fine and the car runs well but fitting a new one can show just how much performance you are missing out on.

Only ever buy a genuine MAF and never one off ebay, see atatched. The one on the right is a genuine Bosch the on on the left is a fake from China.

jib

 

[television]

Malcolm - I mentioned in a recent thread that we had power loss problems on the SLK although the engine starts and runs fine. As you suggested I disconnected the MAF and although the engine started it almost immediately stopped running when I let it go back to tick over. Do MAF units pack up completely or can they lose their perfomance over a period of time bearing in mind this is the original unit fitted to a 10 year old vehicle?

As Jib above, on some cars the unplugging can result in the car stalling at idle, better to try the car on an open road, to see if the power has come back.

Once they start going down they can get too far out of spec. 10 years or say 75K miles is a good life for one.

Cheapest from a Bosch Service Center or motor factor.

The MAF is one of the only things that can clear on a re start