Nick-V
New Member
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. Mass AirFlow Sensor 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….
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. Mass AirFlow Sensor 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….
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