Erratic hesitation

I have a 2004 BMW E60 525d that has done 185,000 miles. The oil is changed every 10,000 miles and it returns 43mpg on a long run. Since I got it at 80,000 miles, it hasn’t missed a beat until recently. It still has its original turbo, diesel particulate filter and diesel injectors (which may be the problem).

In the last month or so, it has developed an annoying fault: hesitant periods that feel just like a misfire. I know you can’t have a misfire with a diesel, but it feels as if a cylinder or two are misfiring. It will run perfectly for an hour, then after 30 minutes of hesitancy it will be back to normal. It’s not a complete cut-out, just a brief stutter, but it is erratic and lasts for lengthy periods before going back to normal for an hour, then there’s another hesitant period. The economy gauge fluctuates quite a bit when this is happening, as if you are blipping the accelerator up and down when it’s actually rock steady. Putting my foot to the floor still makes the car accelerate quickly, but even then you can feel it stuttering.

I put in a new fuel filter, even though it wasn’t due a change, but it made no difference. There are no fault codes when I connect my LNPA lead. Could one or two of the injectors be misfuelling? Can I use a diagnostic lead with the engine running to check out the injectors? Jim Harrower My first comment would be that you CAN have a misfire on a diesel engine. The common rail system on your BMW means that the diesel pressure is constantly applied to the injectors, which are then fired when triggered by the electronic control system. In the same way that a spark plug or HT circuit may fail in operation, causing an engine to misfire, so can the injection system.

As you suggest, the problem may be one or more of the injectors beginning to fail. The first test I would carry out would be to check the electrical connections to the injectors to ensure that these are secure. Diagnostic equipment will be able to detect if the operating signals are correct.

One of the more common problems on your engine is caused by the swirl flaps sticking or failing. If this is the cause of your problem then action is required quite urgently to prevent total failure. When the swirl flaps fail it is possible for the metal parts to be ingested by the engine, causing serious damage.

Given that your vehicle has a DPF, one other possibility is that the regeneration system is causing the problem. To regenerate the DPF, the fuel system injects extra fuel, which is then burnt in the DPF to clean the unit. The problem you are feeling may be a symptom of this action. Connecting a diagnostic tester would be a good way to retrieve any information that may indicate the cause of the problem.