How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Matt Your Own Question
Matt
Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 21623
Experience:  BEng hons Mech engineering, in auto industry 22 years
12772297
Type Your Mercedes Question Here...
Matt is online now

Hi,I have a 2007 S550. Frequently, when I am driving, the

Customer Question

Hi, I have a 2007 S550. Frequently, when I am driving, the engine speed becomes restricted limiting the speed of the car. If I switch to neutral and back again to drive, the car will run normally until the next time the problem arises. It also arises when the car is in cruise control. No faults appear either on the dash or MB diagnostics (DAS). Thanks


I see a similar question: "2007 Mercedes S550: Intermittently..type of limp mode..cruising..rev" FYI. Thanks

Submitted: 2 years ago.
Category: Mercedes
Expert:  Matt replied 2 years ago.
Hello

I'd suspect a problem with a throttle body closing up when it shouldn't

I'd suggest cleaning both out with some brake cleaner and a clean rag and also check that the wire connections to both throttles are bright and clean with no damage to the wiring loom or chafing of the insulation
Customer: replied 2 years ago.
Hi,
What throttle body?
Thanks
Expert:  Matt replied 2 years ago.
Hello

there are 2 throttles 1 for each side / back of the engine

you have to strip off the inlet pipe work and the plastic engine cover to access them
Customer: replied 2 years ago.
Hi,
As I understand it, this is a common rail direct injection V8 with a single butterfly air intake / MAF. I am not aware of throttles. Can you comment please. Thanks
Expert:  Matt replied 2 years ago.
Hello

Ok

if this is the single air intake model then it'll still have a throttle or butterfly as you are referrring to it

remove the air intake pipe between the MAF and the engine to reveal the throttle butterfly
Customer: replied 2 years ago.
Hi, yes, done. Butterfly appears perfect. What is the next step please.
Expert:  Matt replied 2 years ago.
Hello

Next follow the throttle body wiring back into the main loom and check for damage / chafed insulation

also unplug the throttle and use some contact cleaner on the pins
Customer: replied 2 years ago.

I just pulled up WIS, so I know how to check the wiring back to the ECU. Plus I'll recheck the connections. I'll do this tomorrow and advise. Thanks

Expert:  Matt replied 2 years ago.
Hello

I should also mention that this issue can also come from the throttle pedal sensor

so its well worth doing the same for the pedal sensor, checking wiring and connector pins etc

as its not unheard of for the pedal sensor to be faulty
Customer: replied 2 years ago.

Hi Matt,

I checked all connections on the pedal sensor and the throttle. I checked over the looms. I can't see anything wrong. The butterfly moves freely on its spring as does the gas pedal. Everything is clean.

The issue remains, unchanged. Next steps? Best,

Expert:  Matt replied 2 years ago.
Hello Enda

Ok in the DAS can you see the throttle ( TPS) reading?

if you can then monitor the reading whilst your assistant pushes open the pedal and it should change smoothly with no sudden spikes in the reading

and spikes then it means its likely the pedal sensor or the position sensor on the throttle body is faulty

its more likley that the throttle body is the faulty item as its enviroment is far harsher than the throttle pedal
Customer: replied 2 years ago.
Hi Mat,
Yes, will run the check, thanks
Expert:  Matt replied 2 years ago.
Ok let me know what you find
Customer: replied 2 years ago.
Hi Mat, yes, will do. I've already checked everything in DAS, electrical signals and so forth. Looks fine / smooth and all signals are available to view. So the question I think relates to how the Butterfly is responding mechanically to the electrical signal. I'll check this today. Best
Expert:  Matt replied 2 years ago.
Understood
Customer: replied 2 years ago.
Hi Mat,
I removed both the throttle and accelerator assemblies, stripped, cleaned, lubricated and reassembled. Checked all connections again. Everything works perfectly when tested both engine running and stopped. Both units connect directly into the ECU.
Fault reappears once driving as before and resets when I shift into neutral and back to drive, only to reappear soon again.
Thoughts?
Expert:  Matt replied 2 years ago.
Hello Enda

I'd say its still most likley to be a faulty throttle

however it could also be a problem with the ECU putting the car into some sort of safety / limp home mode

if you disable the traction control does this help the issue at all?
Customer: replied 2 years ago.
Hi Matt,
The throttle is a Bosch unit, very simple assembly. Just a DC motor and dual pots. I couldn't see or create a problem with this unit. Also looks perfect on DAS. Same for the accelerator peddle. Symptoms suggest the issue only arises while driving and never in neutral.
I looked up Mercedes bulletins for the model and there are references to limp home issues plus a number of software updates in the years 2009 - 2010. Yet no fault is being recorded for the Transmission.
I see the fault also referenced on a number of sites including justanswer but none reach a definitive conclusion on corrective action. Traction control does not effect issue.
Thoughts?
Expert:  Matt replied 2 years ago.
Hello

in that case what might be worth trying is a software update by a dealership

I'd also suggest checking that the earth connections on the engine block are tight and free from corrosion

and there are a couple of small earth connections that exit from the loom and terminate in small ring terminals and are screwed to the engine
Customer: replied 2 years ago.

Hi Matt, we are setting up to update the software on the modules. Will do so and link back in a day on so, best,

Expert:  Matt replied 2 years ago.
Good luck

Related Mercedes Questions