Here’s am unusual one, hopefully someone can help!

When the air temperature goes above 15 deg C my 140 TDi develops a really bad hiccup/surge on light throttle, like its running out of fuel. Idles perfectly, and pulls strongly once more throttle is applied. All through winter with lower temperatures it has run without problem, after showing the issue last summer.

The problem more or less goes after a long run ½ hr plus, though I can still feel a little lean-type response on light load.

Disconnected the MAF as an experiment, and it runs perfectly, really smooth and strong, and after a while the engine light comes on as expected. But otherwise runs faultlessly.

Recently replaced the MAF (Bosch) and exactly the same problem is still there!

So, something else linked to the MAF would appear to be the issue, question is, what? !

Thanks for any suggestions.

Steve