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Ford E350 surging

I have a '99 E350 Ford van, V10, with 300K that runs like a charm until I slow down to stop. It begins to surge which will stop when I shift to neutral. I went to the local parts store and put it on their code reader. It only read ‘left bank lean’. Nothing else. Any suggestions?

Check for obvious vacuum leaks with the engine idling. You may have to take the doghouse off.
You may have an exhaust leak ahead of the upstream oxygen sensor on that bank. The sensor would interpret this excess oxygen as a lean condition.
You could have a dead or plugged injector on the left bank.
Any misfire on that bank would result in excess unburned oxygen, in other words a lean condition.

Surging is usually caused by an air/fuel ratio problem. Above comments are all good. I’d add to make sure all the routine engine maintenance is up to date. And if you (or anybody) has changed the setting on the idle speed adjustment, that can be a cause of surging. The idle speed adjustment shouldn’t be used to fix some other problem. If it has been changed, the first thing to do is set it back to the position it was when the car was new, and see if that helps.

Yup - usually if there’s a lean code showing, there’s a vacuum leak. Check for vacuum leaks.

Thanks for the comments guys. This is a great resource!

Have you pulled the doghouse yet?