SL/R129: sl320 inline 6 misfire under load
11-30-2017, 04:45 AM
#1
sl320 inline 6 misfire under load
hello everyone.
so here is my problem. the car is 1998 sl320 inline 6 with 70000 miles. the other day out of the blue it developed a misfire. took it to an indi garage. was told that one of the coils has packed up. so they changed the coil and all spark plugs. switched the car on and lovely. drove off and within 100 yards it started to misfire again. took it back to the garage and switched the car off. they switched it back on again to run tests and no misfire. so i drove off and the misfire came back. so if the misfire happens i turn the car off and on again and while its not moving there is no misfire. it only happens under load. after running more tests and moving things around the say its the ECU. so they send the ECU to be looked at and repaired or replaced. the ECU is water damaged so we get a new refurbished one. plug it in and the same misfire in the same two cylinders (3and4).
we checked all the leads and cables and they are in perfect condition. we dont think its the CPS because the fault goes away if you turn off then turn on the engine and it still happens regardless of engine temp. also there are no warning lights in the dash to tell me there is a problem.
please help.
so here is my problem. the car is 1998 sl320 inline 6 with 70000 miles. the other day out of the blue it developed a misfire. took it to an indi garage. was told that one of the coils has packed up. so they changed the coil and all spark plugs. switched the car on and lovely. drove off and within 100 yards it started to misfire again. took it back to the garage and switched the car off. they switched it back on again to run tests and no misfire. so i drove off and the misfire came back. so if the misfire happens i turn the car off and on again and while its not moving there is no misfire. it only happens under load. after running more tests and moving things around the say its the ECU. so they send the ECU to be looked at and repaired or replaced. the ECU is water damaged so we get a new refurbished one. plug it in and the same misfire in the same two cylinders (3and4).
we checked all the leads and cables and they are in perfect condition. we dont think its the CPS because the fault goes away if you turn off then turn on the engine and it still happens regardless of engine temp. also there are no warning lights in the dash to tell me there is a problem.
please help.
Last edited by reza_z; 11-30-2017 at 04:53 AM.
11-30-2017, 04:29 PM
#2
The engine harnesses and the throttle bodies are a widely known issue and the wiring degrading can cause stupid stuff like this. Confirm your engine harness build date by the tag in the top left compartment where the diag plug is in the engine bay.
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reza_z (12-01-2017)
12-01-2017, 12:47 PM
#3
thanks for getting in touch.
the car is at the mechanics at the moment. i have a suspicion that it is the wiring as well. I'll find out the date of production tomorrow however they opened part of the loom and the wires looked in very good shape. also as i mentioned once the car is misfiring if you turn the car off and turn it right back on the misfire goes away until you drive off. then it starts again.
I have another SL320 exactly the same so I am going to take the wiring harness off of that one and replace it with the misfiring car. If the misfire happens again then I think it will be time for a new ECU.
I'll keep you up to date.
12-10-2017, 12:03 PM
#4
Hi there.
so i changed over the harnesses from one SL to another and the problem is still there for same cylinders. all fingers are pointing to the ECU. I have located a new secondhand ECU and sent it off to a company that deals with these things all the time. as all these ECUs are vin locked the old ECU has to be cloned and all info sent into the new ECU. also the reason why i got a second hand ECU is that a new one is £1050 +vat and a refurbished one is around £300.
lets keep our fingers crossed.
I shall keep you poster.
so i changed over the harnesses from one SL to another and the problem is still there for same cylinders. all fingers are pointing to the ECU. I have located a new secondhand ECU and sent it off to a company that deals with these things all the time. as all these ECUs are vin locked the old ECU has to be cloned and all info sent into the new ECU. also the reason why i got a second hand ECU is that a new one is £1050 +vat and a refurbished one is around £300.
lets keep our fingers crossed.
I shall keep you poster.
12-11-2017, 08:47 PM
#5
Hi there.
so i changed over the harnesses from one SL to another and the problem is still there for same cylinders. all fingers are pointing to the ECU. I have located a new secondhand ECU and sent it off to a company that deals with these things all the time. as all these ECUs are vin locked the old ECU has to be cloned and all info sent into the new ECU. also the reason why i got a second hand ECU is that a new one is £1050 +vat and a refurbished one is around £300.
lets keep our fingers crossed.
I shall keep you poster.
so i changed over the harnesses from one SL to another and the problem is still there for same cylinders. all fingers are pointing to the ECU. I have located a new secondhand ECU and sent it off to a company that deals with these things all the time. as all these ECUs are vin locked the old ECU has to be cloned and all info sent into the new ECU. also the reason why i got a second hand ECU is that a new one is £1050 +vat and a refurbished one is around £300.
lets keep our fingers crossed.
I shall keep you poster.
05-03-2018, 07:07 AM
#6
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SL320 1997
Hello,
I have an SL320 inline 6 (R129 - 1997) and have solved my misfire problem - alternator/voltage regulator.
Problem first started following a week of parking and a mountain valley descent with engine near idle all the time. Engine started to seriously misfire. I stopped the engine and restarted - the problem cleared.
Days later it was misfiring on cylinder 1. I thought it might be the battery (4 years old) and fitted a new one. The engine went crazy and was worse than a failing lawnmower. The Mercedes garage changed the Mass Air Flow (MAF) sensor and the engine worked for a day or 2 before misfiring again.
I noticed that after charging the battery the engine would run for a few minutes without problem. Then cylinder 1 would gradually misfire as the battery voltage reduced from 14.0v. I checked the alternator output (13.7 to 14.0v).
I told the garage to change the voltage regulator in the alternator but it seemed unable to get the part! The alternator was changed and the misfire problem was solved.
The alternator output is now 13.9 to 14.0v.
Hope this helps.
I have an SL320 inline 6 (R129 - 1997) and have solved my misfire problem - alternator/voltage regulator.
Problem first started following a week of parking and a mountain valley descent with engine near idle all the time. Engine started to seriously misfire. I stopped the engine and restarted - the problem cleared.
Days later it was misfiring on cylinder 1. I thought it might be the battery (4 years old) and fitted a new one. The engine went crazy and was worse than a failing lawnmower. The Mercedes garage changed the Mass Air Flow (MAF) sensor and the engine worked for a day or 2 before misfiring again.
I noticed that after charging the battery the engine would run for a few minutes without problem. Then cylinder 1 would gradually misfire as the battery voltage reduced from 14.0v. I checked the alternator output (13.7 to 14.0v).
I told the garage to change the voltage regulator in the alternator but it seemed unable to get the part! The alternator was changed and the misfire problem was solved.
The alternator output is now 13.9 to 14.0v.
Hope this helps.
