Four Wheel Drive vehicles are very much in vogue still, must say I Can’t understand why anyone should want to be a fashion victim. The particular vehicle in the limelight this week is a bit of a rarity in the off road world, the Mercedes 420 G. Powered by a 6 cylinder fuel injected Mercedes engine, automatic transmission, and with electric windows also it was rather better than most of it's kind. I couldn't help but recall the first time that this vehicle came to my attention was when an acquaintance of mine took a brand new one of these vehicles down onto a beach to demonstrate it's ability's to a customer. Unfortunately his ability as a salesman was not equalled by his ability as a mariner and the vehicle submerged along with his job. Anyway this particular vehicle had a fault of the engine cutting out intermittently. The problem here with diagnosis is that when an injected engine cuts out if it is due to ignition loss then the petrol pump is made to cut out automatically  a fraction of a second later as well - this is a safety feature to prevent the petrol pump from running in the event of accidents. When the engine is cutting out it is impossible to tell by looking at the petrol pressure and ignition signal together which cuts first since the two appear instantaneous. We found out which by datalogging the signals concerned onto a portable computer and then inspecting those signal levels afterwards for the time of the engine cutting out. It turned out to be the fuel pressure that was disappearing first, so the next step was to monitor the electical supply to the pump when the problem occurred. This cleared the pump of suspiscion since the pump was losing it's positive feed then. Tracing the wiring through to the pump revealed some tacky looking insulation, some completely bare and likely to be shorting out, so this was repaired and a relay that had probably sustained damage in consequence from the wiring renewed also. Subsequent testing could not make the fault re-occur, so the vehicle was returned to it's owner for reliable service once more.

The Mercedes was towed into us because it just wouldn't start at first inspection it appeared to have everything - plenty of sparks, petrol, good compression and correct timing and firing order.

     The fuel system was  Bosch K Jetronic - a mechanically and hydraulically controlled injection system which is normally very reliable -  never known it to wear badly enough as to cause total non starting though the fuel pressure from the pump and through the system are very important so we next checked those out. All checked out OK so we removed all the injectors and bench tested them. Their

   performance was not 100% but not so far out as to cause the problem. Testing the metering unit that regulates the petrol flow to the injectors revealed the problem though - petrol was bleeding through the metering unit to the injectors long after it should have stopped flowing and so flooding the engine to death. It was suspected that the unit had been taken apart at sometime causing the problem - it is impossible to get the unit backtogether satisfactorily without specialist equipment. The price of a new Mercedes metering unit would make your eyes water so a decent 2nd hand unit was found and once the fouled plugs cleared and a few tuning faults fixed the Mercedes was mobile once more.

Hessitation, power loss. This type of problem can be one of the most time consuming faults to find because the problem often is only slight and the fault only shows up when the car is being driven under load.

One of the most commen causes of this is fouled or sticking injectors that can be reconditioned at less than tenth the price of a new set. For more information on injector servicing click on injector.