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Uneven throttle response in W126


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Hello everyone.
I am seeking assistance with this difficulty and I hope someone can provide it.
I own a 1992 W126 500sec coupe. The issue I am experiencing is that although the vehicle idles smoothly and runs normally, when I am in traffic and the engine reaches about 1000rpm, it starts fluctuating in revs as if I am repeatedly pressing the accelerator. This causes the car to surge until the engine reaches around 1300rpm. The issue resolves itself and returns to its usual state, but only temporarily, till I decelerate and reach the 1000-1300rpm range. It is causing me extreme frustration.

I have recently changed the idle throttle position sensor, cleaned the little inline filter in the main fuel flow to the fuel distributor, installed new spark plugs, new leads, and a new distributor cap and rotor arm.
We would much welcome any ideas or recommendations from you all.
Thank you for your time.
The sound of a scuffing noise.

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If the issue reoccurs, carefully inspect the circuit boards within the relays. During the seven years I owned a 500SEL with the same engine, I found it beneficial to examine and potentially repair these circuit boards. In addition to the fuel pump relay, these vehicles also have an idle control relay. However, due to their age, it is advisable to open up all the relays and seek assistance from someone knowledgeable in electronics, such as myself with a degree in Electrical and Electronic Engineering, to perform this task if you are uncertain. Although new relays may be costly, the prolonged exposure of circuit boards to the conditions under a car's hood for 30 or 40 years can cause damage. Re-soldering, on the other hand, is a nearly cost-free solution that can provide significant benefits and avoid future malfunctions.

Furthermore, it is important not to overlook basic components such as the fuses. The outdated continental-style fuses, also known as 'torpedo' fuses, commonly used in these cars tend to corrode at the ends. This corrosion can lead to poor electrical connections, even if the fuses themselves do not blow. It is highly recommended to replace all of these fuses, which can be done inexpensively at Halfords. Additionally, it is important to clean the mating surfaces inside the fusebox using a drill bit of suitable size. To avoid any risk of short-circuiting, remember to disconnect the battery before performing this task. Replacing all the fuses in your vehicle with new ones will cost you around ten pounds. This investment is highly recommended for long-term dependability. Make sure to determine the quantity of fuses needed for each colour and rating, since the fuses are color-coded based on their amp rating.

You may see a significant improvement in the functionality of various automotive components after investing around an hour of your time and a little amount of money. I personally do this practice with every newly acquired (previously owned) vehicle, and it yields substantial benefits. Replace each fuse individually with one of the same colour and rating, or consult the table included in the fusebox lid for the exact amperage rating of each fuse. It is crucial to ensure that each fuse is replaced with the appropriate rating. Refer to the chart instead than relying on the contents of the fusebox, since a previous individual may have replaced a fuse with one that has an inappropriate value.

It is incapable of causing any damage.

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