W126 uneven throttle

[Scuff]

Hello Guys.
Hope someone can help me with this problem..
I have a W126 500sec coupe 1992. The problem I’m having is, she idles lovely, drives as it should but in traffic as soon as I get to approximately 1000rpm the engine revs up and down like I’m tapping the accelerator causing the car to serge, until I get to around 1300rpm? it clears and back to normal until I slow down and hit the 1000-1300rpm. It’s driving me crazy.

I’ve just replaced the idle throttle position censor, cleaned the small inline filter in main feed to fuel distributor, new spark plugs, new leads and new distributor cap and rotor arm.
Any advice or suggestions would be very much appreciated guys.
Thanks for your time.
Scuff..

 

[Dodipops]

Drawing in air somewhere??

 

[alabbasi]

I had a.560SEC that would fall flat on its **** at 1000-1300rpm. Turned out to the the fuel distributor.

 

[optimusprime]

Vacuum leak check all hoses and small bore pipes for splits along with the rubber joints .

 

[Scuff]

Hi Guys.
Think I’ve found it I cleaned the in-line filter in the main feed pipe for the fuel distributor and since then it’s been fine. Fingers crossed

Thank you for your reply’s.. they are very much appreciated..

 

[Pontoneer]

If it happens again , have a close look at the circuit boards inside the relays ; I had a 500SEL for seven years , same engine , and these were worth checking/resoldering ; besides the fuel pump relay , these cars have an idle control relay , but due to their age , just open up every relay , and if you aren't sure , find someone who knows electronics ( my degree was in Electrical and Electronic Engineering , so I was quite at home DIY-ing ) who can do this for you - new relays are expensive , but 30 or 40 years under a bonnet can take its toll on circuit boards , so re-soldering is an almost cost free thing that can reap dividends , and prevent future breakdowns .

Also don't ignore simple things like the fuses . The old style continental , or 'torpedo' , fuses used on these cars corrode at the ends and ( even if they don't blow ) can produce bad electrical contacts ; for the few quid it costs to go to Halfords and replace the whole lot , it's well worth doing ; don't forget to take a suitable sized drill bit and clean up the mating surfaces inside the fusebox ( disconnect the battery first to avoid risk of shorting anything while you do it ) . It will cost you around a tenner to buy enough fuses to replace every one in the car with brand new and is well worth it for ongoing reliability ( just check how many you need of each colour/rating : the fuses are colour coded according to how many amps they are rated at ) .

You might be pleasantly surprised how much better everything in the car works after doing that ( for an hour or so of your time and just a few quid spent ) ; I do it with every new ( old ) car I buy and it pays dividends . Just change each fuse one at a time with one of the same colour/rating ; or refer to the chart in the lid of the fusebox which details the correct rating ( amperage ) of each fuse in the box , so that you replace each fuse with the correct rating ( this is important ) . Go by the chart rather than what you find in the fusebox : someone before you may have replaced a fuse with one of the incorrect value .

It can never do any harm .