Hello there,
Have a Ford E350 that sat for a while (~ 1 year); got it started and it ran fine, no noise, hesitation or otherwise any problem. Went out to it next day; hard to start, then ran really rough, large clouds of white smoke from the exhaust that got worse upon acceleration, and a tapping noise that seems most sever at idle. Oil gauge read really low. Shut off, restarted, same problem but no oil pressure on dash gauge at all.
Was stored with fuel that had no Sta-Bil or other fuel stabilizers added to it. Also noticed black oily residue around the joint where the head pipe bolts into the exhaust manifold. Would like to know what the likely problem is, and what it will take to fix the issue.

Yup. The engine is toast. Put butter on it. ;(

Ford E350 Motorhome

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1989 Ford Vanguard E350 Motorhome Manuals

1. 1989 LeisureCraft RV Ford E-350 Econoline. 1989 Ford E-350 Econoline Custom 7.5L V8 GAS Automatic 3 Speed Motor Home - Cutaway Added Sep 2018. 8 Fuel-ups. Property of LeisureCraft.
2. To determine the maximum trailer weight you can tow with your 1989 Ford E350 you will need to find both the Gross Combined Weight Rating (GCWR) and the Gross Vehicle Weight (GVW) of your van. This can all be found on the information tag located inside the driver door jamb.