If i go past 70mph the car slows down

[simon22]

I don't get it. Tomorrow is Monday. The garages will be open. get the codes read, find out the problem and get it fixed. Its not necessarily an expensive fix.

Just thought any of you Merc owners may have known, so i dont get fobbed of with something its not and the garage takes you to the cleaners.
Anybody else with any other idea's, nobody else had this problem

 

[renault12ts]

Simon, limp home mode is a common event with many reasons too numerous to mention. If we advised all sorts of things this could be worse than pinpointing the actual fault.

How long is a piece of string?

 

[simon22]

Simon, limp home mode is a common event with many reasons too numerous to mention. If we advised all sorts of things this could be worse than pinpointing the actual fault.

How long is a piece of string?

Thankyou for the reply, any surggestions will be taken lightly and not gospel.. just an idea, im not blaming anyone if its wrong, just nice to goto a garage with a few idea's, as you always get will need about 3hrs to look at it....

 

[renault12ts]

MAF, brake light switch, injector problem, wheel sensors,etc...

 

[BlackC55]

Right,

It wont be a fuel filter issue.

Some A170 don't have air mass sensors some do. It could be that if it has one fitted. Also it could be a sticky EGR valve, dodgy vacuum transducer, blocked CAT, split intercooler, split boost pipe, buggered turbo etc

I doubt many codes will come up (unless it has a faulty air mass sensor) as they rarely do.

 

[crankiejohn]

mine does a simular thing,but only when my wifes in pasenger seat.(sorry)

 

[Stratman]

The thing with 'limp home mode' is that it's the default response to a multitude of problems. The best, and cheapest, way to resolve it is to get the fault codes read.

Just some of the areas which can instigate limp home mode:-

Engine
Brakes
Gearbox
Fuel

Which of these would you like to throw money at first?

 

[simon22]

The thing with 'limp home mode' is that it's the default response to a multitude of problems. The best, and cheapest, way to resolve it is to get the fault codes read.

Just some of the areas which can instigate limp home mode:-

Engine
Brakes
Gearbox
Fuel

Which of these would you like to throw money at first?

I just might send it to the Car auction,s in manchester and noting it has a problem, and look for a another one, will get it on a Star manchine first.
the service did say it had only 200 miles togo so i got my mechanic to service it , this is why i said could it be the fuel filter as it was an after market one

 

[simon22]

The thing with 'limp home mode' is that it's the default response to a multitude of problems. The best, and cheapest, way to resolve it is to get the fault codes read.

Just some of the areas which can instigate limp home mode:-

Engine
Brakes
Gearbox
Fuel

Which of these would you like to throw money at first?

Brakes are a little soft to my own cars, something else to check tomorrow ,but do stop you lol

 

[simon22]

mine does a simular thing,but only when my wifes in pasenger seat.(sorry)

If thats a joke lol , if not sorry

 

[grober]

Does your car alternator have a freewheel pulley? Other possibility would be some wheel/tyre defect triggering the vehicle stability control system into a fault condition?

 

[BlackC55]

It will do. ^^

 

[grober]

Assuming the cars going into limp home mode was wondering if a faulty alternator clutch might cause voltage spiking enough to trigger it. Other possibility on the wheel front would be enough out of sync wheel signal to cause the stability control to throttle back the engine? That might be road speed related rather than engine speed. Just a couple of guesses. I'm sure there are lots of other more probable causes. If it was engine speed related you would think it would happen in the lower gears above 2,500rpm also.

 

[simon22]

Assuming the cars going into limp home mode was wondering if a faulty alternator clutch might cause voltage spiking enough to trigger it. Other possibility on the wheel front would be enough out of sync wheel signal to cause the stability control to throttle back the engine? That might be road speed related rather than engine speed. Just a couple of guesses. I'm sure there are lots of other more probable causes. If it was engine speed related you would think it would happen in the lower gears above 2,500rpm also.

good idea, but steering is smooth no shakes or rattles , if i let go of steering wheel at speed it goes straight.
im going to buy all new service items again this morning, but this time mercedes parts
e.g fuel filter, oil filter new oil again, cant bothered replacing air filter again that wont cause the problem , then ill get him to check all four brakes and if so bleed then all.
Any other views are very welcome, and you wont be held responsible, as they are only idea's

 

[Tony Russell]

I have one of these (as you well know by now from my constant questions ).

There are two actuator valve things which feed vacuum to the turbo.
One is below the wash bottle, and it is connected via a hose to the other which sits beneath the brake fluid reservoir.

Check the hose between these for kinks.
I let a "specialist" replace one of these only to find out later that the hose was kinked, thereby stopping the go faster juice to the turbo.

There is also a hose comes out the top of the intake manifold cover next to the oil filler which feeds a sensor located just below the drivers windscreen wiper.
Check that is in place.
Left it off once too and had the same effect.

There is a post of mine somewhere here with pictures describing it.

 

[simon22]

Well had another service ,oil and oil filter. air filter, fuel filter brakes bled and tested all pads / shoes ok, rad flushed and new coolant/antifreeze done, hand brake adjusted was 6 clicks now 3 , 4 is hard, just put some red-x injector cleaner for diesel in tank , thought it was all ok got 80mph ,i thought yes its done , slow down for traffic, then tried to put my foot down 2,500 revs 70mph again.
I forgot to mention the srs light is on, but not always,but it has done the 70mph even when srs is not lit, also had gearbox oil changed with new not that it needed it , came out clean, must have be done some time in the last 6 months.
But the problem is still there..

Star manchine is showing no faults, thought it may have.

Even the EPC light is not lit so not an engine fault i was told..

Anymore idea's please,
Thankyou for the picture will check that tomorrow as i dont know what bits are what, will ask mechanic.



Thanks for all the replies, more views welcome please

 

[NormanC]

With the symptoms as described, I find it difficult to believe that it is a "mechanical" problem. It seems to me that engine power is being reduced and an arbitrary rev limit of 2500rev/min is being imposed. Surely this is indicative of an electronic control problem?

Or am I wrong?

 

[simon22]

With the symptoms as described, I find it difficult to believe that it is a "mechanical" problem. It seems to me that engine power is being reduced and an arbitrary rev limit of 2500rev/min is being imposed. Surely this is indicative of an electronic control problem?

Or am I wrong?

Yep thats what i thought , this is now puzzling me also, there is no smoke from the exhaust the car runs very smooth, if you stay under 72mph its fine, but if you take your eyes off the speedo ,damit you have it 72mph.

Round town where you never hit more then 50mph its fine and very punchy and smooth.........

 

[charlie2]

:bannana:more than likley turbo overboosting and then going into limp mode,thats why when turning car off and waiting a bit and restarting car ok till it re over boosts.

 

[Tony Russell]

I am putting money on that vac pipe thingy.

50p says that's what it is.