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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 20384
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have recently bought a 1919 R129 SL500 while driving

Customer Question

Hi I have recently bought a 1919 R129 SL500
while driving it today it has developed a problem?
the car will not accelerate properly as when depressed the maximum speed reached is 45-50 MPH.
The car idles very smooth and will rev without any problems.
The main problem is the lack of power
any ideas?
Cheers Mike
Submitted: 1 year ago.
Category: Mercedes
Expert:  Shantal-Mod replied 1 year ago.
Hello Mike,
I'm Shantal and I'm a moderator for this topic.
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I was checking to see if you had already found your answer or if you still need assistance from one of the Professionals.
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Thank you,
Expert:  Matt replied 1 year ago.


this sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,

Ideally measure the pressure at the injectors and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

Expert:  Matt replied 1 year ago.

do you still need help ?
Customer: replied 1 year ago.

Hi Matt, yes it could be a fuel problem but I am also getting a warning sign on the instrument dash board which lights up when I try to gain more power.

I had the battery disconected just before the problem started could it need a fault code clearing?

The car will accelerate when pushed but when the revs drop thats when the lack of power and the warning sign shows.

Could it be a vacuum problem.

I will change the fuel filter to see if it is a starvation issue.


Expert:  Matt replied 1 year ago.

thanks for the extra info
On any car if you get a check engine light or other fault light this typically means one or more of the sensors has failed or has a faulty connection.

The fault can be something quite simple and nothing that’s going to stop the car to something quite major that will reduce engine power and possibly stop the engine

The ECU senses this and may put the car into 'limp home' mode which typically limits the revs and the amount of power available. Usually the car is safe to drive for a short distance as the mode is intended is intended to get you home without incurring any engine damage

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.You can also try resetting the car by removing both battery leads and clamping them together for 10 mins as this will reset everything so if the light re-appears then the fault is still there.
Disconnect the battery cables and clamp them together for 10 mins. Remove both battery leads from the battery then use a nut and bolt or G-clamp to hold both battery lead terminals together.

This discharges any capacitors / memory within the ECU and around the car so ensures all memory settings are wiped clear - back to the factory default.

This will reset all the systems on the car and you may need the radio security code to get it going again, and ensure that the keys are in your hand when you re-connect the battery as the car may lock and turn on the alarm

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