Question QUESTION: My 156 has a rough idle after it is fully warmed up. It also has hesitation every now and again anywhere between idle and 2000 rpm. I have had new spark plugs fitted because the diagnostics could not give a answer. Now I believe I have two options: MAF or Lambda sensor. Which one should I go for?
ANSWER: Neither. If you do not have access to an oscilloscope then you must do a "plug cut" to start eliminating possibilities. Drive at 4000 rpm for one mile then cut the ignition and coast to a stop in neutral. Remove all spark plugs and look at the pattern. An ignition fault should show as an even pattern on one or more spark plugs. An uneven pattern will suggest faulty injectors. An uneven pattern on the outer ring of the plug will indicate coolant leaking into the cylinder. By the way, clean the battery terminals (do it anyway) and unplug and replug the MAF sensor connector several times.
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QUESTION: So you think it is an ignition or injector fault? Why then is it only at low revs? The car runs fine above 2000rpm. And mostly below as well. If it was a coolant leak, would I not have seen a drop in the coolant level?
ANSWER: You are being subjective. These items must be checked to see if they really are ok. By the way, it is typical of a spark plug to first fail at either high or low rpm. But, just replacing items is very expensive and that is why checks or tests must be made. Each step will indicate the next step to the well trained mechanic.
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QUESTION: Sorry to bother again. I am a newly grad mech eng, but not very hands-on. So I am trying to figure this out as best I could so I could guide my mechanic in a direction.
The spark plugs were replaced less than 5000 km ago. Can I cancel them of the list? The car had the same symptoms before I replaced them. What you are implying I check is the ignition system (coils etc.??), the injectors, and the MAF connection. Is this correct?
The reason I had MAF on my list is because I have read that these units did fail quite a lot. But I do not know the exact symptoms of MAF failure.
Should the modern diagnostic equipment not show up ignition system failure? Or injector failure?
Thanks a lot already for the help.
Answer The modern scanner is not capable of finding these faults. It only shows a data stream in not very fast time and any codes the brain notices. Only a DSO is fast enough to find some problems and it requires an experienced hand to use it. So down to reality. I suggested checking the spark plugs because the picture on the insulator will tell the experienced mechanic a lot of information quicker than hooking up all the electronic mumbo jumbo (which I do on a regular basis--hook up the mumbo jumbo). When one is dealing with an elusive and intermittant problem step by step is used to eliminate the obvious--each step must be taken because humans make mistakes all the time--so step by step by step helps to come to the answer. By the way, one of the first steps is what I told you about cleaning grounds and the battery terminals and unplugging the MAF sensor connection. If it is narrowed to the MAF sensor then before it is replaced the interior should be cleaned to clean the sensor (not the temp sensor in the big opening). Too many of these units are replaced when it is just corrosion on the plug or dirt on the sensor causing the problem. I could go on and on but enough for now.
