Idle surging and dead gas pedal - Mercedes Benz SLK Forum

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#1 Old 11-10-2017
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Idle surging and dead gas pedal

Started this week on my wife’s 99 SLK230. Idle cycling from 800 to 1400. Go to D, *clunk*! Need heavy brake to keep car at bay. Gas pedal has no effect.

Limped home and hit the interwebs. Showing BAS/ and code P1580 (throttle motor). Studied this forum and others and removed and cleaned the throttle body and mating connectors. All looked fine. Light carbon and good electrical (including dual potentiometers). Reassembled, cleared code, reset pedal position.

Car started fine. Idled well. Waited for the startup speed to go to normal, backed out for a test drive. Went one block, touched the brake at a corner and BAM! Back into the surging.

Removed the throttle body connector and proved the connections. About 500-1400 ohms, depending on position on both pots. About 3-5 ohms on the motor contacts. Cleaned the connectors again. Awaiting thorough drying to reconnect.

Ideas?? Some say replace the brake pedal switch. Some say replace the throttle body and/or pedal sensor. Some say while wiring harness has to go.

Just rebuilt the car cosmetically 28 months ago: new paint, interior, seats, mirrors, rebuilt top hydraulics, new engine mounts and torque bushings. Battery is 4 years old but seems to crank fine. Past repairs include AC system and MAF around 70,000 miles. Otherwise no significant engine problems. About 110,000 miles.

Heard you gents might have ideas: @savcom, @stu32, @efair.

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Last edited by jbanks15; 11-16-2017 at 02:37 PM.
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#2 Old 11-10-2017
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The BAS/ESP fault can be caused by brake switch and no response to throttle can be brake switch. Not sure about the surging of the idle speed. The fault code is throttle motor and that would cause 2 of the 3 problems also.
You went for a drive after cleaning things and code re-set, then first touch of the brake and fault returns, I would change the brake switch and clear code then try again. Brake switch not expensive and prone to failure so quick and easy to eliminate. Poss clean the MAF too but only do one thing at a time and quick test after each potential fix.
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#3 Old 11-10-2017
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@stu32's logic is good and it's probably simpler to keep to one train of diagnosis or we'd all get confused.
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#4 Old 11-10-2017
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Originally Posted by stu32 View Post
The BAS/ESP fault can be caused by brake switch and no response to throttle can be brake switch. Not sure about the surging of the idle speed. The fault code is throttle motor and that would cause 2 of the 3 problems also.
You went for a drive after cleaning things and code re-set, then first touch of the brake and fault returns, I would change the brake switch and clear code then try again. Brake switch not expensive and prone to failure so quick and easy to eliminate. Poss clean the MAF too but only do one thing at a time and quick test after each potential fix.


Thanks. Already ordered the brake switch. We’ll see...
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#5 Old 11-10-2017
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Update this thread when you replace the switch please, Thanks
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#6 Old 11-10-2017
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I'd use a scanner program like Torque Pro to monitor the throttle position, brake switch, rpm, and MAF flow. Something is not registering.
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#7 Old 11-13-2017
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Update: bought and installed a new brake switch today. PITA with all the screws to pull the lower dash panel without breaking the (rebuilt) brittle plastic.

New switch installed. Gas pedal reset routine done. *Same* symptom: surging idle. Bummer.

Before I did this I cleaned the throttle body connector and plug with alcohol and let them dry for a day.

Next step (pending advice from you folks) is to buy some carb cleaner (and maybe electrical cleaner) and pull / clean the MAF. Yes, I know they’re fragile - used to work on them as a vendor to Delco.

If that gets nowhere, guess I look for a rebuilt one. No other code fails still.

Ideas??
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#8 Old 11-14-2017
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Do you have any malfunction lights on now such as engine check or BAS/ESP ?
Agree at this stage to clean MAF next, then if that doesn't work it's probably be down to the throttle motor.
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#9 Old 11-14-2017
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May be an ESP module problem. I did a quick search and came up with one tread with surging issues related to the ESP.

Everything is fine until Phil hits the brake. When that happens, some logic somewhere, calls for throttle cut to either the gas pedal or/and/via the throttle body. What does that? ESP? When back on the gas, and no response the ECU throws a TB motor code. (I'm speculating here)

On a separate line of thinking (but does not account for the dead gas pedal) check your hoses that connect to the TB and just ahead of the TB. They get old and split. I traced surge on my car to one of those and replaced it. Also the vacuum hose that goes to the brake booster has a Very brittle connection at the intake manifold.
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#10 Old 11-14-2017
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MAF will often generate a code (P0170 and P0173 is common). If no codes (set or pending) I would not replace the MAF ($100+) but would clean it (CRC MAF cleaner).
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#11 Old 11-14-2017
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With BAS/ESP and throttle response it could still be low battery voltage even though your battery is only 4 years old. Check the voltage with and without the engine running and expect 12.6 to 12.8 volts at rest then 13.6 to 14.6 volts when engine running for a good battery and charging system.
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#12 Old 11-14-2017
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Thanks for the comments, folks. Tomorrow I dive in again. Let you know...
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#13 Old 11-15-2017
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Battery voltage at rest this AM is 12.3. Will replace today when I go get some MAF cleaner. Four years in Florida maybe is slowly draining it!!

More interesting, read 12.16 on my X5 battery and it’s only one year old!! Have to have a chat with the dealer.... since they replaced it....
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#14 Old 11-16-2017
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Have just checked our cars C32 and SLK32 batteries this morning. Both read 12.35 volts after standing in the winter cold all night. If it's warmer where you are and I guess it will be you would expect better readings than that. However, our cars work ok at that voltage from startup. I started the c class and it was running 13.9 volts on idle.
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#15 Old 11-16-2017
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OK: all fixed! Might be one or two things -- here's the sequence in order:

Symptoms: Surging at idle, *slamming* when entering any gear, no throttle response at all. P1580 Throttle body code, BAS/ASR light, check engine light

1. Cleared codes with OBDII reader and iPhone program
Throttle body: dismantled, cleaned with alcohol even though looked fine - symptoms remained
Reset throttle position: all OK! Problem returned 300' later on test drive.

2. Ordered and changed brake light switch: symptoms remained.

3. Battery: read low, four years old, replaced - symptoms remained

4. Mass airflow sensor: cleaned with CRC cleaner, seemed pretty clean (no oil) anyway.
Throttle body: dismantled, cleaned rotary pot traces with CRC and scrubbed lightly with cotton.
Also added a bit more scrubbing tension on brushes.
Checked motor for function with a 9V battery - spins freely either direction
CRC cleaned both MAF and TB plugs and sockets
Reset throttle position.

SYMPTOMS GONE! No warning lights, smoother speed idle, no surging.
5 mile test ride - mixed speed / city driving and 75 MPH highway - no bad symptoms / no warning lights.

Thanks for the help, guys! (Sorry I wasn't 'pure' in checking after the MAF cleaning but the wife was hounding me to get her car back on the road!)

Was it the MAF cleaning? Was it the extra attention to the throttle body pots???
Will keep an eye on this and see if it comes back.
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#16 Old 11-16-2017
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P.S. Used an inexpensive KOBRA Wireless OBD2 Car Code Reader Scan Tool ($17.99) and 'DashCMD' for iPhone. Interesting readouts, tons of data, lots of fun to make graphs of RMP, Speed, MAF intake pressure..... But ultimately just fun. Trying one thing at a time in order or probability (thanks again, folks) and increasing $$ did the trick!
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#17 Old 11-16-2017
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Arghhh!!! Came back!!!
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#18 Old 11-16-2017
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Well at least you didn't have to wait long... Maybe it's heat related? Can your scanner report throttle body position and throttle pedal position?
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#19 Old 11-17-2017
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Well at least you didn't have to wait long... Maybe it's heat related? Can your scanner report throttle body position and throttle pedal position?


Checked through the 1,000 columns of data on the recorded file - no throttle position or pedal position. Multiple MAF readings, timing calc, CO2, power and others. Maybe the ‘power’ one is calculated? Curiosity makes me want to know but practicality wants the car fixed!

See what I can find today...
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#20 Old 11-17-2017
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Noooooo!! surely you must have a fault code showing.
Is it coincidence that each time you clean and re-connect the throttle body it works breifly?
Does @Anders S SLK02 have any idea's? he has a lot of knowlege on this model.
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