SL500 around idle misfire - what next?

[ben12345]

From my experience of the LH jetronic system on my Porsche, the engine temperature sensor and air flow meter are the biggest culprits with poor running.

I'm not sure about on the M119, but there may be two sensors. One for the temperature gauge on the dash, and one for the LH. These can normally be troubleshooted with a simple ohm meter - the specification should be in the service manual.

The air flow meter, being a hot wire type, won't last forever. They also don't just stop working, they gradually fail. If the sensor is anything like on my Porsche, they are mega money to replace, but I do know of a nice man in Cambridge that offers a testing and refurbishment service.

 

[Nick-V]

Yeah the temp thing is worth chasing and there are indeed two sensors. I clamped the top rad hose to get the engine very warm indeed and pulled off cables...no difference...but I want to see what the ECU thinks the temp is next time it goes on the STAR diags.

Merc's MAS is some £544 - eek. I believe there is a black (plastic) Siemens equivilent and I would not touch ebay or China! However, we APPARENTLY tested this by putting on a new one off another car in service there. HOWEVER, I need to confirm that the LTFT reading was reset after fitting. The Indi reported no difference in the idle misfire.

I take on board the gradual misfire point...an option is to wait until it gradually gets worse...?

I'd love to know the number of your nice man! Thanks

 

[ben12345]

There's no need to wait for STAR to find out the temp... like I said, a simple ohm meter (multimeter) will do the trick. Measure once at cold, again at temperature, and compare the results with the manual.

The man to speak to about the MAF is called John Speake, well known in the Porsche 928 world... Hopefully your air flow meter is not too different to the one they used in the 928, and he can give it the once over. http://www.jdsporsche.com/main.html

Yes... avoid the chinese ones. A member of my family bought one for their Alfa. It's lasted about 10k so far and is already causing a rough idle and hesitation.

 

[Alex Crow]

nick, where are you? i know a very good garage in the stowmarket area in suffolk. star machine and all plenty of experience of these cars/engines.

 

[television]

I will confirm that he is very good

 

[Nick-V]

I am in Wimbledon very near to a well regarded Indi with Star and Hermann etc...Of course I can travel to Suffolk but should I really be moving around and starting again...

...I don't know much at all about vehicles especially with all the unreliable complex junk they have on them these days...

...but I am reasonable at asking questions and interpreting answers...at present I am taking on a lot of opinions about causes...and conclude there must be engineering methods of testing each rather than throwing thousands at parts that are fine!

I'm currently listing all the suggestions, trying some, and plan to get more specific test results and a good discussion with the Indi very shortly. There seem to be a lot of potential causes and I have not written off the ones that we believe have already been tested (been there!).

There are loads of potential causes - I believe I now need to prioritise and test then spend or sell...they include:

MAS, ETA, Throttle position sensor, Lambda Sensor, cats, LH Control Unit, several temperature sensors, wiring looms, thermostats, injectors, knock sensors, adjusting the fuel trim plug, rotor arms, distributor caps, plug leads, coil packs, valves and compression, egr valve etc, Idle Stablisation Valve, brake/evap and other air leaks, fuel pressure regulator, cam and spark timing.

One or two of them might fix it !

Trying to keep it simple...STAR shows a long term fuel trim value - not to be ignored. Pulling the MAS plug (into open loop) improves idle - interesting. The dashboard displays less than 80c but used to display more (before LH). We believe it displays lean running symptoms (slight miss at idle and hesitation on revving). It was always different after the LH was replaced years ago but the misfire at idle and sometime at low revs when accelerating is relatively new.

 

[Number_Cruncher]

Thankfully, most items on your list can be checked easily, and without too much special equipment.

>>Throttle position sensor

A check of the output using a moving coil voltmenter, or even better an oscilloscope checks this part out completely.

>>cats

Very difficult to see how a cat could cause this problem, but, to check, simply measure the temperature of the exhaust pipe upstream and just downstream of the cat - the gas leaving the cat, and hence the pipe should be hotter, at least 50C

>>several temperature sensors

These are easy to check. Simply measure the voltage output from the connected sensor as the engine warms up - the voltage should be high when cold, dropping as the engine warms up. This method of checking is better than simply probing the resistance of the sensor, because it also checks the wiring from the ECU and to earth.

>>rotor arms, distributor caps, plug leads, coil packs,

All these can be checked with a good scope. You mention a Herman tester - can this report what the burn times and firing voltages are?

>>wiring looms

This is a case of a simple visual inspection

>>spark timing

An old fashioned stroboscope can be used to check this out.

>>cam (timing)

The only way to check this is to take the covers off to enable the marks on the cams to be seen.

>>thermostats

A simple check with a thermocouple on the thermostat output should tell you what the opening temperature is with the thermostat in-situ. Failing that, take it out, and heat it in a pan of water, and measure the temperature of the water with a thermometer, watching for the valve opening.

>>Idle Stablisation Valve

Difficult to see how this could cause a misfire at idle. But, if this is a rotary idle actuator, with the engine warm, pinch off the hose (either upstream or down from the valve it doesn't matter), and artificially slow the idle speed down for a minute or two. Let go of the pipe - the idle speed should shoot up, and possibly oscilate for a while before smoothly settling.

>>valves and compression

Easy enough to check, but, worth doing both cold and hot.

>>brake/evap and other air leaks

Block off all non essential vacuum take offs, and see if it omproves running. Spray something flammable around the intake system - if the running suddenly improves, you've found the leak.

>>egr valve etc

For a temporary check, fit a blanking gasket made from thin sheet metal.

>>fuel pressure regulator

Usually, a fuel pressure regulator on systems like this is holding the pressure at its minimum during idling. If it had failed, I would expect to see either a very rich idle, or extreme weakness at high engine loads - your description of the fault doesn't sound like this to me? I've always found this type of fuel pressure regulator to be very reliable and trouble free.

It's always good value to check that there's a healthy return flow of fuel back to the tank.

>>knock sensors

Temporarily, remove them from the engine block but keep them electrically connected. Remember that if you do find they are retarding the ignition too much they can also be "hearing" noises from elsewhere in the engine.

>>adjusting the fuel trim plug

I would return this to its standard setting and leave it there until the underlying fault is cleared.

>>Lambda Sensor

Check to see if it switching properly, and that there is a reasonable current flowing in the heater coils.

>>MAS

I think you've already checked this by substitution - which is a good way to eliminate it.

>>injectors

Not easy to check without removing and checking flow properly.

>>LH Control Unit

Very unlikely to be the fault, but, it may be worth asking BBA Reman if they can check it.

>>ETA

Sorry, that a TLA too far for me. Is that Mercedes speak for the carbon canister purge valve? If so, simply plug the pipe for a temporary check.

 

[Nick-V]

Number cruncher....fantastic response thanks...I much prefer this approach...this should really help...I will include these recommended tests in my discussion with the Indi...

Unfortunately I don't have a manual, the spec values on things or test instructions or I'd get the multimeter on things myself.

Sorry about the TLAs ! ETA is apparently the Electronic Throttle Actuator...in my terms the "butterfly" under the MAS.

 

[Number_Cruncher]

>>ETA is apparently the Electronic Throttle Actuator,....

Ah!, this page may be helpful;

http://v12uberalles.com/throttle_actuator_rewire.htm

The piccies also show the general form of insulation degradation on the wiring of cars from the early to mid 90's.

If your car has a ETA, then I don't see why it would also have an idle stabilisation valve - does it? - the ETA would naturally perform that function.

 

[Alex Crow]

yes NC, the LH is very similar to the HFM in that it uses the throttle valve to control idle speed.

nick, you can get all the test data you could ever want in cd format from a certain internet auction site. all for less than £10 including postage. it will need a PC with XP SP2 or above btw. here you can find very specific data for all engine systems over various load conditions.

 

[rayhennig]

Wimbledon MB

I am in Wimbledon very near to a well regarded Indi with Star and Hermann etc...Of course I can travel to Suffolk but should I really be moving around and starting again...

If you are in SW19, have you tried Steve Redfearn? Maybe he's your "well regarded Indi"?

I've used him over the years and my 300CE really needs a few days with him when funds permit. He looked after my 280CE, 450SLC, 500SL, 500SEL and 280TE and, with few exceptions, the results were better than I've had so far with the 300CE.

Anyway, good luck and remember me to Steve if you visit him.

Bon courage.

RayH

 

[Nick-V]

I'll say hi from Ray Hennig when I see Steve!

 

[Nick-V]

number cruncher - thanks, I read somewhere else about the wiring and potentiometer issues - its an expensive part - lets hope its something else. I believe the car maintains idle ok...

...I heard about air leaks from the idle circuit so mentioned the ISV...I can't get to http://www.detali.ru/ at the moment to check and point to it but I think it has one...although alexander may be saying otherwise...?

Both the ETA and ISV should be on the list I suspect...?

Alexander - many thanks - I'm having a look right away for that CD ! Will try a few searches in the hope there is only 1 brand of CD...wonder if EMULE has it ?

 

[Nick-V]

Alexander - can you give me any more clues on the CD test data...I have looked at Autodata and Tolerance Data and they only really provide the resistance of temp sensors and manifold pressure...did you have something more in mind?

 

[Alex Crow]

i have sent you a PM nick, but i guess as a newbie you cannot read it yet. email me and i will get back to you.

 

[Nick-V]

alexander - Thanks I got the PM - I'm checking now !

number cruncher - thanks for the knock sensor test to check it is not retarding due to other noise...but how can I check it IS retarding when there is a knock...which is the way it might be...do they throw codes ???

 

[Nick-V]

A quick update as I am still waiting for a discussion with the "Indi" I use...

Fixed misfire on pulling away - I found a hairline crack inside the circle on the back of one of the rotor arms.

Improved general running slightly - I found the intake air temperature sensor was from a later model (resistance values different) causing the engine to think the incoming air temp was lower - replaced. I also found the car running at about 65c instead of 80c due to the thermostat opening at too low a temperature - replaced and coolant system flushed.

The occasional misfire (pomph) at idle and fast idle remains...

...thanks for the help so far guys...working thru things...Happy Xmas.

 

[Alex Crow]

...... I found a hairline crack inside the circle on the back of one of the rotor arms........


...thanks for the help so far guys...working thru things...Happy Xmas.

happy christmas nick, a rotor arm problem! who would have thought it

on a serious note :neutral:, thanks for updating the thread, i look forward to the final post with your car running perfectly and we will all be smiles. there are too many threads that are missing the concluding posts, for various reasons. this forum stands as a massive and constantly updated reference resource for thousands of folk, past present and future, so keep up the good work!

 

[television]

That was what I said it would be in post 2 it just had all of the symptoms,,Thanks for posting back

 

[Nick-V]

I too am a little disappointed that the test I paid for on the Hermann machine failed to conclusively exclude electrics as I was told. Consequently, the misfire at idle could still be coils etc.

Clearly there are 2 or more issues...very possibly the rotor issue was masked by the remaining idle issue (in defence of the Indi).

Luckily, I try to not be mislead by tests and try to think for myself (shame, I wanted to pay someone else to fix it because I'm not a mechanic). Actually, lots of causes have been suggested to me and I'm confident that if I was to try them all one or two would fix it (and my bank account!).

Its great that I found the Rotor issue...its sad the replacement sensors and other expensive work did nothing...and its a relief to have the support of everyone at this forum.