SL500 around idle misfire - what next?

Nick-V

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Help! This is getting expensive and worrying for me…I recently asked a reputable local Mercedes “indi” to diagnose and resolve a misfire issue. However…we need some help as we are becoming quite baffled…

The Vehicle
• A 1995 UK-based SL500 R129-067, with the 119972 engine and LH control unit, with 87k miles (150k kilometres), running on 97 premium fuel, and owned since 2000.

Symptoms
For probably a year or two I have noticed that I always get:
• Slightly rough or shaky idle with an occasional misfire “pomph” in the pipes, not when cold, but when semi-warm and fully warm.
• Slight misfire when accelerating gently, or on a climb at very small throttle openings.
• Hesitation or cough at low revs when revving up from idle.

It might be a separate issue or a related issue but on very rare occasions, perhaps after a period of non-use, I also get:
• Several cylinders cutting out with significant drop power at small throttle openings when accelerating.

Probably separately, for some years since the LH Control Unit was replaced I get:
• The engine on acceleration sounding less deep and having a sort of fluttering sound. Sometime I wonder if it is pinking/pinging.

Background
There is some history I feel I should mention:
1. I bought the car from a proper Mercedes dealer who serviced the car for the first few years. I was eventually told they had manipulated things to pass the annual emission test (MOT) and I would have to address it properly somehow at some point. When I eventually went somewhere else for the test the car failed. This lead to Mercedes changing the O2 Sensor and then the LH Control Unit at great cost. Despite this being a relatively fast job I was made to wait a couple of hours more as apparently they had trouble “dialing things in”. I was never happy with the car after that. Although there was no misfire, the engine note on acceleration had changed from a low woomph to a slightly treble fluttering sounding “trrr-trrr-trrr-trrr-trrr”. Although the car seemed to run OK it reminded me of a lean condition. I have questioned this with many places over the years to no effect.
2. A Rotor Arm bolt sheared taking out the distributor cap etc. The car was run for a few miles on 4 cylinders to get home with a red hot exhaust from the burning of the un-ignited fuel.

Mechanical checks so far:
1. A compression test showed all 8 cylinders between 167 psi and 171 psi – no leak-down test has been performed.
2. A test for air leaks all around inlet manifold and all pipes found nothing.

Electrical checks so far:
1. Resistance of all 8 plug leads similar at ~1800 ohms .
2. Looking for sparks at night and spraying everything with water.
3. Alternative distributor caps and plug leads tried. Coils not tried.
4. Hermann diagnostics at idle showed occasional misfires on 1,2,3,5 (and perhaps others) so both banks and both distributor caps/coils are involved. Therefore, it is not an issue with one cylinder. A ‘scope variation also indicated a roughness at idle. However, Hermann indicated all electrics performing OK.
5. Number 3 plug sometimes has an uneven burn pattern. Unlike the other cylinders there is normal colour, but in an U shape on only one side of the insulator and not evenly all round.

Fuel System and Sensor checks so far:
1. Three lots of fuel cleaner.
2. Star error codes showed Crank Sensor and Camshaft Position Sensor. These were replaced and the codes have disappeared ‘so far’. We didn’t expect these to cause the misfire but there was also an occasional starting problem and we wanted to eliminate any confusion.
3. Star indicated injector timings etc. all correct. Star was used to reset everything – also used to clean the MAS hot wire.
4. MAS replaced temporarily – no effect.
5. Lambda sensor cleaned – not replaced. Diagnostics show this reading moving around a bit at idle – it should be more static – could be either the result OR the cause of the misfire?
6. Trim plug (in control unit bay) moved from position 1 (standard) to 5 (“Indi” thought this helped a bit) – no effect on misfire. This was in an effort to correct the fuel trim which was at 40% (from his memory) instead of the expected 50%. We may be masking the real cause here.
7. Fuel pressure checked – all OK.
8. Emissions tested and appear OK.
9. Pump the brake at idle and it nearly stalls

Potential next steps include?

1. LH Control Unit - swap with another car, fit a second hand unit, test it somehow or buy a new one. Problem…its expensive and coded to the vehicle.
2. Cam timing – could they be set up on drive wrongly? Could it have jumped when rotor arm and cap destructed?
3. Variable camshafts – does it have these…would cause a severe idling problem if set up wrong.
4. Leaky injectors or, bad spray pattern – surely this affect a specific cylinder(s) and not generally.
5. Incorrect timing – it does seem to pink/ping (weak or advanced timing).
6. Idle stabilization valve (ISV) is controlled by the LH unit. But surely this would cause a surging idle not a misfire.
7. Disconnect Lambda sensor so control unit allow rich running?
8. Temperature sensor, throttle position sensor?

Specific Questions

1. What should we try next?
2. How and who can test an LH Control Unit in the UK?
3. How can an LH Control Unit from a scrap or other vehicle be recoded to work on THIS vehicle?

Any help much appreciated….
 

television

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I still think dizzy cap or rotor arm
 
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Nick-V

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Thanks - that's on the already done list.
 

television

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Thanks - that's on the already done list.

Yes I read that,,but there are many that fail withing a very short time, only 1 make are supplied now
 
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Nick-V

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Yes its te 1st place to look normally. We swapped cap and leads off another car...fuel trim is not 50% - we suspect its lean and Hermann shows electrics ok. I rule out nothing but...
 

Alex Crow

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hello. so you have had this problem for a while? my first comment has to be - do not unerestimate the frailties of the HT system on this car! those ht leads may be ok on the resistance test, but you cannot test their insulation! you ought to replace them to be sure. also caps and arms can fail very quickly on these, i have had to replace a genuine arm after only 18 months on a 119.

also, please note that lambda values SHOULD fluctuate, usually between 100mv and 850mv, but at least by 500mv above and below 450mv.
 
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Nick-V

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Yep...

3. Alternative distributor caps and plug leads tried. Coils not tried.
4. Hermann diagnostics at idle showed occasional misfires on 1,2,3,5 (and perhaps others) so both banks and both distributor caps/coils are involved. Therefore, it is not an issue with one cylinder. A ‘scope variation also indicated a roughness at idle. However, Hermann indicated all electrics performing OK.
 

Alex Crow

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thanks for that recap nick, but i had read your post.

quote..."It might be a separate issue or a related issue but on very rare occasions, perhaps after a period of non-use, I also get:
• Several cylinders cutting out with significant drop power at small throttle openings when accelerating."

^^this is classic for condensation build up in the caps.


i have re-read your symptoms and the misfiring you describe is consistent with HT problems. sorry if you feel that HT has been ruled out, but i AM qualified to comment, and have seen so many HT problems on these cars. can you confirm that the rotor arms and caps are bosch, or possibly beru? throw any bremi or other items in the bin asap! also those coils can cause trouble, i know you feel the problem is with both HT system, but your hermann checks were done at idle which is not indicative of driveability issues.

what fault codes do you currently have? any in the GM?
 

television

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Alex has said my thoughts as well
 

Alex Crow

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nick, i would also be monitoring lambda values, live data and co, o2, hc etc at the exhaust, ignition missfires will show as greatly increased hc and o2.
 
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Nick-V

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Alexanda and television, many thanks for the inputs...

I agree about caps and rotors on this model but we did that bit - honest!...and I agree that damp is most likely the cause if the same bank goes down after a period of non-use...however, it is currently cleared up so I cannot chase this symptom at the moment. I'll certainly go directly to the caps when/if it returns...I mentioned it because it may be connected and may not. Standard Merc caps and rotor...Bosch I guess.

...I am currently chasing the idle and near-idle misfire and thinking about the LH control unit related change that I have lived sadly with for years.

I know that everyone always jumps on the ignition system when misfires are mentioned but I am told that the Hermann diagnotics equipment checks the spark and scopes things on each cylinder individually and APPARENTLY it shows the ignition is not at fault. I cannot ignore the test equipment can I?

This really does feel like a lean issue...actually both at idle and when accelerating (old LH Unit issue) - POSSIBLY two separate issue or the LH one has worsened?

I cannot ignore that the trim value is not 50% and I want to recheck this on STAR. Also I cannot ignore that idle gets a lot better when the MAS is disconnected...probably because its running open loop and ignoring Lambda corrections. The indi already tried a new MAS off another vehicle and reported no difference. I need to confirm he actually reset the LTFT setting after fitting it !

Exhaust readings are all OK (legal) indicating the LH Unit is correcting things?...so a guess is that Lambda is detecting rich (rightly or wrongly) and leaning off the injection incorrectly as the actual running appears lean - MAYBE. Actually the indi said the exhaust readings may look a little rich if anything...but perhaps due to the occasional pomph (remember its not a full blown missing).

I even sucked on the EGR valve pipe today to check it opens - yes it causes bad running so is PROBABLY closing at idle. There are many other factors - sadly !

When the air filter was off today the car even stalled a couple of times ! - Too lean? It also hesitates just on idle when I crack the throttle.

The indi mentioned the Lambda readings fluctuating...I have no idea what is normal but I take your point and will check again in more detail...I already suggested to him that the misfire/roughness means a bad burn which indeed would cause bad readings at Lambda and perhaps at the exhaust test. I need to check these readings again and get numbers.

Talking of numbers I seem to remember the injector timing was about 3.6 on all 8 cylinders according to another indi...sound ok?

Also...what does the trim plug on the front of the Control Box actually do? Does it allow tuning of mixture - additive or multiplicative ? What is standard position and which way is richer?

Appreciate the help guys...I feel very on my own in progressing this...was hoping a dealer would fix it easily. I'm even now looking at the costs of injector cleaning/checking and LH Control Unit checking...
 

television

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The trim plug is for the fueling to adjust from 92 octane and up,there is a place marked N this is for 95 but it is known to have little effect.

I will have a look at the info on this car in the morning on the fueling

If you take the wire off the air intake sensor or the water temp this will run the engine enriched you could try that
 

busby20

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9. Pump the brake at idle and it nearly stalls

I was intrigued by this comment, although I agree with all the other possible electronic faults diagnosed or suggested by others you have not mentioned why this happens? Given that the brakes utilise vacuum power to assist the Servo - which usually derives from the manifolds etc., is it not possible that you have a "leaky" manifold joint or vacuum connection which could cause the effects you describe - especially since the car "nearly stalls" when pumping the brakes? possibly worth checking:)
 

television

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9. Pump the brake at idle and it nearly stalls

I was intrigued by this comment, although I agree with all the other possible electronic faults diagnosed or suggested by others you have not mentioned why this happens? Given that the brakes utilise vacuum power to assist the Servo - which usually derives from the manifolds etc., is it not possible that you have a "leaky" manifold joint or vacuum connection which could cause the effects you describe - especially since the car "nearly stalls" when pumping the brakes? possibly worth checking:)

A leak in the vacuum system would normally increase the idle, as it can suck more air in, and the often hunt in that condition.

I do agree with you though that may be a link or clue, he can always pull the pipe off and blank it to test
 
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Nick-V

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television - thanks will try the wire-off trick. For clarity...I do not mean the ignition resistor plug to retard timing for bad fuel (mine cannot be altered). I mean a round wired up plug with numbers 1-7 (I think) sticking forwards out of the front of the control box case.
 
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Nick-V

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Busby - manifold and pipe leaks checked already by the indi but a leak is still a suspect...I looked at the EGR valvle for this reason. Still on the list...but where to look...

I've tested all pipes....however...how can I check (and find!) the idle air system I read about somewhere...
 

television

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I will see what I can find later, I am of to bed to rest my head;)
 
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Nick-V

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IMPORTANT CLUE TODAY PERHAPS: someone took one look inside the car today and said its running too cold...it never quite reaches 80c. I now THINK I remember that it used to be more like 90c before the LH Unit was replaced....

Can anyone tell me what the normal operating temperature should read...it may just help with my very long term running problem (not the misfire).
 

television

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You can blank off ½ the rad with some cardboard to test and get the temp higher, you have the gauge to watch. Also pulling off the wires that I told you,the engine will think that it is cold
 

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