Engine missfire when cold - restart engine fixes it

ChrisCL500

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CL500 Engine missfire when cold - restart engine fixes it

Hi

I appear to have a software problem on my 2001 CL500 in that *sometimes* the engine will misfire on one cylinder (bringing up the check engine light) and will continue to misfire like that for a few minutes. But if I turn off the engine and restart it - the misfire instantly disappears...

I paid to have a star analyser connected to it and it said number 1 cylinder was the issue but it didnt say what the actual problem was...

Its the restart clearing the issue that confuses me - any ideas?
 
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DSM10000

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I believe that if the ECU detects a misfire then it shuts down the injector on that cylinder to prevent unburnt fuel entering and possibly damaging the catalyst. Does your Check engine symbol stay lit?

If no misfire is detected at the next start then the car will run normally until the condition causing the misfire recurs. The STAR computer should have been able (as indeed any OBD II reader should) to look at the freeze frame data showing engine load, various temperature parameters, rpm etc at the time of the misfire. Does it happen at any particular engine speed?

Do you know when the spark plugs were last changed and by whom? The V6 and V8 plugs are a little tucked away and it is recommended that a special tool is used to remove the spark plug leads from them as they are tightly fitted and if you are not careful it is easy to break the internal carbon core.

A way of narrowing down the diagnosis is to swop the HT generator to another cylinder, if the misfire moves it is the HT generator or the leads assuming you moved them as well, if it stays it is the plugs (two per cylinder) or the injector. At least this way you can exclude some parts. You can investigate further by swopping remaining bits around, Not sure if STAR can interrogate and test injectors.
 
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ChrisCL500

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yes the check engine light stays lit.

Trouble is my Indi hasn't been able to repeat the problem to connect the star to it "at the time".

The engine speed at which it occurs is an interesting piece of data to capture though - i will keep an eye out for that.

Mind you the car is due for a service in 1500 miles so perhaps that will clear the issue anyway...
 

DSM10000

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I am unsure what "my Indi cant repeat the problem to connect the star to it "at the time" means.

As I said the ECU stores freeze frame data of such events so you can see those parameters , you do not need STAR nor an OBD reader connected at the time although you can get OBD readers that can be left plugged in to the port that record data over a number of trips and then the garage can download the data and analyze it.

If your service is to include spark plugs and it goes away you have your answer, if not you need a more detailed investigation.
 
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ChrisCL500

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ah sorry - what i meant was the car hasn't misfired when it was at the Indi... and he reset all the codes when the issue wasn't obvious. Perhaps I need to take it back to him.

Does a "B" service include plugs?
 

DSM10000

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.

Does a "B" service include plugs?

Depends on mileage / time since they were last changed.

Bit expensive on yours as it means 16 plugs!!:)
 
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ChrisCL500

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No... I see the misfire just driving off normally. I would never accelerate hard on a cold engine :)
 

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