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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 54022
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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Normal idle, but engine dies if throttled hard. The engine

Customer Question

Normal idle, but engine dies if throttled hard. The engine will rev up if throttled slowly.
JA: Does the vehicle have a carburetor?
Customer: No. Fuel injected. 1988 560SL
JA: Are you fixing your 560SL yourself? What have you tried so far?
Customer: No, my mechanic is working on it, but the person knowledgeable in MBZ autos left, so he is puzzling through it. Replaced TPS & fuel pump relay.
JA: Anything else you want the mechanic to know before I connect you?
Customer: No
Submitted: 11 months ago.
Category: Mercedes
Customer: replied 11 months ago.
Got two error codes: one indicated bad TPS...replaced that. The other code says "Throttle valve switch, idle contact." I've tried finding a replacement "Throttle valve switch," but my search always turns up the TPS. Confounding.
Customer: replied 11 months ago.
Also replaced accumulator, as the original problem was stalling in traffic. It would always restart after a minute or so, but when the battery finally died, I had to have it towed to my mechanic's shop.
Customer: replied 11 months ago.
I have to leave home now. Please text response to my phone at +1(###) ###-#### Thanks. Paul
Expert:  Marty replied 11 months ago.

Hi, my name is***** the engine is stalling on acceleration I would first have the mechanic perform a fuel pressure test and a fuel volume test. If one of the fuel pumps is failing it could give you this symptom. If the fuel pressure and volume is good I would inspect the ignition system. A bad cap and rotor or a bad coil could cause this problem. The only TPS on this car is the throttle valve switch mounted to the side of the throttle body. This switch will not cause the problem you are having. There is also an air flow sensor potentiometer mounted to the side of the air flow meter. The potentiometer could cause the problem if it is bad.

Let me know if you have any questions.


Customer: replied 11 months ago.
Marty,Thanks. My mechanic did indeed perform a fuel pressure and volume test and advised that all is well there. The initial problem was that the car would stall in traffic. Typically, it would restart after a minute or so, and continue to run fine. However, after this occurred several times, it failed to restart and I drained the battery attempting to do so. I had the car towed to my mechanic, and he replaced the dead battery, but now the car will start and idle, but upon any swift movement of the throttle, it will die. He noted to me that when the throttle was pressed, the butterfly plate in the throttle body did not move. This led us to believe that the throttle position sensor might be bad. We got a code reader and two error codes came back. The first code was that the TPS was bad, so we replaced that, but the problem persisted. The second code indicated that the Throttle Valve Switch (idle contact) failed. Upon investigation, we were able to find little to no information on the Throttle Valve Switch, other than it is buried in the "V" between the heads, somewhere beneath the throttle body. When I searched the internet for a Throttle Valve Switch, the only thing that came up was the TPS, which we already replaced. Thus, we still don't know exactly what the Throttle Valve Switch is (if not the TPS), and how to source a new one and replace the old one. HELP!
Expert:  Marty replied 11 months ago.

The air flow sensor plate is strictly controlled by engine vacuum, there is no electronic control. If the plate did not deflect downwards on hard acceleration it is most likely due to the fact that the engine was stalling at the time. The throttle valve switch mounted to the side of the throttle body (down in the V of the motor) will not cause this problem. He needs to look elsewhere for the problem. I'd check for a bad coil, cap, rotor or ignition module. It sounds like you may be losing ignition high voltage on hard acceleration. Either that or there is a fuel pressure problem that he is missing. A restricted exhaust system would also cause the air flow sensor plate not to deflect downwards.