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500 SEC lack of power - bad acceleration

H

helgi33

Active Member
Joined
Dec 14, 2012
Messages
62
Location
Crawley
Car
500 SEC & CL600
Have problem with my 500SEC. it seriusly lacks power and acclereration. It starts perfectly, idles perfectly, responsive and almost normal on low rpm and partial throttle but when the throttle is opened up more then a quarter then it really acclerates slowly and is really hard to get over 4000rpm.. there is no misfiring and it is the same cold or hot...

the car was tested a a dealer and star system didnt see anything wrong...

have now replaced plugs, leads, distributor cap and rotor... the car runs smoother and is much more responsive on low rpm partial throttle but it is seriusly lacking power and acceleration above 1500rpm and quarter throttle.. almost like pulling a big truck....

would be greatful if anybody can lead me in the right direction....
 
Possibly a loose accelerator linkage or an air leak at idle and the mixture has been adjusted to cover that, but is too rich when the throttle opens.
 
I changed the plugs tonight and they werent black .. more like medium brown and had a slightly damp... but then I had just driven it quarter of a mile all cold to a friend garage so that running cold might have made the plugs damp... they werent really wet..

the symptoms are a bit like the ignition is way retarded on higher rpm... above 1500 rpm.. the car idles nromally when in D around 5-600 but idles higher when in P or N or maybe 800rpm...
 
Check the timing and also make sure, if the car has cats, that they're not blocked

AFAIK there is nothing on an SEC for Star to talk to. But models in your country may differ from UK ones

Nick Froome
 
from what I understand from all I have studied the ignition.. then it is totally mapped in the ECU and there is no way to advance or retatard it... The car doesnt have Cats... my car is an UK car.. keep it in Crawley as have to spend lot of time there..
 
Is there maybe a mercedes specialsit in the Crawley/Gatwick area.. ? i would be grateful if somebody can give me name and number... I could end up spending huge hours on fixing this .....
 
Try Alan Robertson the Car Surgeon in Hove. He's a good from-first-principles engineer who won't be worried by the age of the car

Nick Froome
 
Hi Nick..
Thanks for the input.. would you have any number for him ?
 
I am still fighting this problem with really slow accleration and really hard to go over 3500rpm. I have now done the following

changed both fuel pumps and filters
changed Distributor cap, Rotor, Leads, Coil and Plugs

It runs a bit better now... starts right away.. very smooth idle acclrates fine from idle up to around 2500 rpm if throttle is not opened more than 1/4 but ig given full throttle then it acclerates slowly and very slowly above 3000 rpm.. on Motorway then it is really slow accleration above 70 mph and can hardly go over 90-95mph.. it used to go quite easily to 140mph when everything was normal..

I have been all over different mercedes forum and found various threads with similar description and different things that cuased it.. like

Clogged fuel filters
worn fuel pumps csausing low fuel pressure
ignition leads
ignition coil

I have now replaced all of those and then there are few I havent checked yet...

overload protection relay
vacuum leak (supposed to throw the mapping of ECU off)
wire harness on engine
MAS - Mass Air Flow Sensor (think though that is really more applicable to newer models)

I would be so grateful if somebody can give me input on this...
 
You seemed to have covered the obvious things already, so these are long-shots:

Not sure what age your car is, but later ones would appear to have a sensor linked between throttle/transmission, as this link (under drivetrain technologies):
Mercedes-Benz W126 - Wikipedia, the free encyclopedia

It's unlikely, but what is the air filter like (I don't think you mentioned that)

If your car has a throttle position sensor, I believe that they can sometimes be 'reset' - ensure battery fully charged(idle for a while if necessary), turn off, disconnect battery negative lead for 5-10mins to reset the ecu, reconnect, turn ignition key to position 2, but don't start car, then gradually depress throttle to fully open, then gradually release. However, I don't know whether this would be relevant to your car.

Are there any exhaust restrictions - I have known one car whereby someone had jammed things in the exhaust, although in that case it also didn't start as well as usual.

Any fuel tank venting problems? I did wonder whether the car is parked up for long periods, but if the fuel had 'gone off', I suspect that it wouldn't start properly.
 
Unbolt the exhaust from the manifold and allow the gases to escape free of the exhaust system and see if the problem clears.
 
Hi tonysmb

to answer your questions.. then my car is 1988 500SEC and yes have replaced the air filter and it doesn not have throttle position sensor and there arent any exhaust restrictions.. mine doesnt have cats which are the usual suspects if there are exhaust restrictions..

yes that had been sitting for very long but have been through few tanks already and added fuel injector cleaner with one of the tank filling and yes I did have ventilation problem in the beginning but have replaced the cap while ago..

I think my next step will be to borrow from a friend a Snap on emission test equipment which also measures AFR (Air Fuel ratio) and then read what it is doing when it is struggling to accelerate... if the mixture is too rich then I have ignition problem as the engine is not burning all the fuel... if it is too lean then the engine is not getting enough fuel and the problem is with the Fuel Injection system (could also be faulty MAS)

Even though it is tempting to give up and take the car to a W126 specialist... then am getting a bit stubborn Now.. :)

E270 Owner: I am not really getting your advice.. ?
 
helgi33 said:
Hi tonysmb
to answer your questions.. then my car is 1988 500SEC and yes have replaced the air filter and it doesn not have throttle position sensor and there arent any exhaust restrictions.. mine doesnt have cats which are the usual suspects if there are exhaust restrictions..

yes that had been sitting for very long but have been through few tanks already and added fuel injector cleaner with one of the tank filling and yes I did have ventilation problem in the beginning but have replaced the cap while ago..
l
Click to expand...

Hi helgi

right! coupe of things, 1st your car does have a throttle position sensor but i can guarantee that your problem wont be that. The tps on the ke jetronic only provides 2 signals on/off, its not progressive.

as suggested on a previous reply, check the camshaft timing, if its out it would cause those symptoms that you describe.

if the timing is correct, then check the air flow sensor plate on the fuel distributor, if its sticking it would cause fueling issues and therefor slow acceleration.while you are there, check if the throttle body is moving freely
open/shut, you should just about see throttle body it if you press the air flow sensor plate downwards, the TB sits right below sensor plate.

i would put £10 that the problem is been caused by the fuel pump relay
maybe you can borrow one from someone and try it on your car

rgds
mazza
 
mazza said:
Hi helgi

i would put £10 that the problem is been caused by the fuel pump relay
maybe you can borrow one from someone and try it on your car

rgds
mazza
Click to expand...

I did think about this, but i thought its either on or off?
Or do you think its the RPM sensor causing it to cut out too early?

If it could be the culprit could he just jumper the pins to see if it is the problem or not.
 
Last edited:
ivan1983 said:
I did think about this, but i thought its either on or off?
Or do you think its the RPM sensor causing it to cut out too early?

If it could be the culprit could he just jumper the pins to see if it is the problem or not.
Click to expand...

Hi

mate, the fuel pump relay is a complex box, full of surprises..hehehe
it could be rpm related, or some other issue.

best way would be to use another (working) relay and run the engine under load

mazza
 
Tank ventilation problem? That will be the vent valve, its only about £7 and just pops off to be changed. It's near the rear axle area.
Otherwise your tank pressures will be off. I lost two tanks to this, both times they got vacuumed smaller. Those fuel pumps are strong! My vent line was also blocked so eventually it was disconnected.
Did you buy the proper MB fuel cap?

My car ran fine through that though, oddly.
 
My 1984 1w126 500sec

Have you cured your sec ,my sec have the same problem.

helgi33 said:
Have problem with my 500SEC. it seriusly lacks power and acclereration. It starts perfectly, idles perfectly, responsive and almost normal on low rpm and partial throttle but when the throttle is opened up more then a quarter then it really acclerates slowly and is really hard to get over 4000rpm.. there is no misfiring and it is the same cold or hot...

the car was tested a a dealer and star system didnt see anything wrong...

have now replaced plugs, leads, distributor cap and rotor... the car runs smoother and is much more responsive on low rpm partial throttle but it is seriusly lacking power and acceleration above 1500rpm and quarter throttle.. almost like pulling a big truck....

would be greatful if anybody can lead me in the right direction....
Click to expand...
 
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