ncbill

1998 SL500, around 85,000 miles.

Just had the front end repainted after a deer suicide last spring...looks new, want to get it running reliably again.

Starts right up when cold.

No problems while driving on surface streets or interstate...no hesitation on acceleration.

After warming up will stall in gear while stopped in gear, e.g. at a stoplight.

Starts right back up again...takes an extra few seconds to catch versus when cold.

Still the same problem after my independent mechanic replaced the original crankshaft position sensor with a Bosch OEM.

No codes after he plugged into the under-hood port...I assume he accessed the Star diagnostics?

If I'm reading forum posts correctly bad camshaft sensor(s) (I'm sure those are original as well) shouldn't cause the above?

Feedback on next steps for my mechanic:

1. Check distributor caps/rotors for cracks, carbon scoring.

2. Check/replace fuel filter, check fuel pump & pressure test fuel injectors (but wouldn't problems with those manifest in normal driving or while accelerating?)

3. Check over voltage protection relay (do I even have one of those on my 1998?)

4. Check/replace EGR purge valve?

5. Clean intake throttle body, MAF sensor? (these seem less likely to me)

Thanks for any feedback on what else to try...preferably the cheaper fixes first...

FATHERTIME

I had an air temp sensor do this to me on my 94 SL320. Can also be the MAF

ncbill

Thanks, this is the air temp sensor?

https://www.fcpeuro.com/products/mer...sor-0005428418

Where is it located?

FATHERTIME

Engine air temp its; in the air intake before the MAF.

bobterry99

Rather than play Parts Roulette I suggest purchasing a diagnostic tool capable of evaluating air temperature, crankshaft position, and other sensors. The cheapest ones cost around $15, plug into the vehicle OBD-II connector, and then output their data to a smart phone or tablet.

ncbill

Thanks, but mechanic noted there were no codes, not from the OBD-II connection or the connector under the hood (Star Diagnostics?)

I just read on one parts site that a reviewer also had a "stall when stuck in traffic problem" until they also replaced their camshaft sensors (as well as the crankshaft position sensor)..do those here concur?

bobterry99

A sensor could be faulty and not produce an OBD-II fault code. That's why you would want a tool which can evaluate sensors as well as read codes.

The camshaft sensor is used to synchronize the crankshaft sensor pulses with the fuel injection and ignition systems. This is necessary since the crankshaft makes two revolutions per 4-stroke operating cycle, and with only the crankshaft sensor the engine computer can not distinguish a compression stroke from an exhaust stroke. Once the synchronization is established at engine startup, I don't believe the camshaft signal is required. You can easily satisfy yourself that this is in fact the case by disconnecting the camshaft position sensor while the engine is running and observing that it continues to run.

ncbill

https://www.pelicanparts.com/techart...or_Removal.htm

Looks like the only way to test the sensors is to pull both & test resistance on them directly with an ohmmeter...going to that trouble why not just replace them (20+ years old) as well?

Here's an interesting comment from this SL500 M119 camshaft sensor replacement video:
"A LOT OF CAM SENSORS - it's the weak link in a Mercedes - it stops working when the car heats up, which causes the sensor to heat up and usually a broken wire in the winding of the sensor opens up and then the car doesnt run .. MBs will start and run well when they are cold also because the car is designed to dump more fuel into the vortex making it run better when it is cold, but as the car warms up the fuel system should lean out, but the cam sensor also sends a signal that is also used to control the fuel and air mixture -- so if it doesn't receive that signal the fuel mixture stays rich and the engine exhaust will smell rich the cat converters will heat up not being able to process the extra rich exhaust as well causing back pressure and popping when the exhaust valves open up, then the engine will stutter, stammer and bog down - and like my 92 500SL it will stop running all together when you get to a stop sign when the engine is supposed to idle down, because the air to duel mixture now is way too rich, then it will just die..."

bobterry99

I can think of three reasons. First, if you did the test I suggested, I believe you now have proof that the camshaft sensor is not the cause of your problem. Second, the sensor is installed with shims. It may be overkill, but you are supposed to remove the valve covers and set the gap between the sensor and the target it senses with shims. Third, I've owned a lot of 129s over the years (have four at this moment), and I have never experienced a failed camshaft sensor. Nor can I recall a thread on any forum where the camshaft sensor has failed.

Frankly, it's been a while since I read something so nonsensical. I hope the person who made the comment isn't passing themself off as anything more than a DIYer. What is interesting to me about the comment is how it demonstrates the power of the Placebo Effect, since the author of the video believes his engine revs better with the camshaft sensor connected.

Unlike with a 2001 car, on a '92 or earlier M119 the camshaft sensor plays no role at all in a properly-running engine. Since the fuel injectors spray continuously and distributors control which cylinder receives a spark, there is no need to synchronize the crankshaft with the injection and ignition pulses. In fact, the only function of the camshaft sensor on a '92 car is to identify which cylinder is misfiring in the event that a knock sensor detects detonation. The ignition for the misfiring cylinder is then retarded to hopefully correct the problem.

ncbill

So, what next, then, assuming the camshaft sensors are within spec when tested?

Clean the MAF sensor with CRC (or is there another recommended brand) MAF Sensor Cleaner?...if that solves it temporarily but the problem returns in a few weeks or months, replace the MAF sensor?

ncbill

Not that surprisingly, replacing the MAF sensor appears to have resolved the stalling issue.

If you're having the same problem, sure, replace the crankshaft position sensor first to see if that works.

But start saving your pennies for the OEM Bosch MAF sensor assembly.

Though there are pulls on eBay for 1/10th the price of a new one if you're a DIYer willing to take a chance.

Kebowers47

Your '98 R-129 is at the end of the defective wiring harness insulation. Check the insulation on the cable connecting to the MAF and the electric throttle actuator by carefully slitting open the plastic jacket and looking.

ncbill

Had it in for an oil change & asked the mechanic to check the above...he said it looked like it might have been replaced at some point.

So sure, do all the cheap fixes first...but again be prepared to have to replace the MAF sensor assembly...it's a common source of problems for many vehicles.