I have a Renault Scenic 1.9dCi on a 53 plate with 120,000 miles. Befoe Christmas it developed a problem where, under a certain rev/throttle opening combination, the engine momentarily cuts out, then cuts back in, sometimes once, sometimes repeatedly until I raise the revs by changing gear or slipping the clutch. It’s as if it’s not firing on one (or more) cylinders. The problem occurs under medium load at between 1,500- 2,000 revs, whether the engine is cold or warm.
Typically it’s when I’m pulling away, but it’s also really apparent on a cruise. Cruise at 51mph in 5th gear, accelerate and the car’s fine. Slow to 49mph (so the revs drop to just below 2,000rpm), fine, but when I accelerate the engine cuts out and cuts in repeatedly until I change down a gear. It’s the same at around 60mph in top.
There are no fault codes at all, and no warning lights are displayed. The engine starts fine hot or cold, there’s no noticeable loss of power, excessive smoking, or increased fuel consumption. My local mechanic replaced the crankshaft sensor, which didn’t help, and a diesel specialist tested all the injectors and reconditioned 2 of them, also to no avail. As a last resort I took it to a main dealer, who replaced the fuel filter. This helped a lot, curing most of the misfire when accelerating through the gears, but had no affect on the problem at a cruise.
Then, as it started getting warmer in the spring, the problem started to go away, and was only apparent on cold mornings. It's then that I worked out that the problem is related to a cold air temperature - typically below about 5-6 degrees. I've tried replacing the mass airflow meter, which didn't help. I would emphasise it's the outside air temperature that sets the problem off, it's not related to how warm the engine is. Just to add, the car had a new EGR valve in the autumn.
Wonder if anyone can give a diagnosis or suggest a cure, as otherwise it will have to be a replacement car in the autumn.