Have a 97 4x4 f-150. The front differential will sometimes spew diff fluid from the vent. The truck does not go off-road. All driving is highway miles. I drained the diff to make sure there was no water. Fluid was not milky. Replaced with the proper gear oil. It does it on 60 degree days, does it on 110 degree days. It may do it today and won't do it again for 3 months. I'm lost. Any ideas?

 

when driving is it possible the wind passing by the vent is creating a negative pressure and sucking gear oil out of the diff?

 

No way, the vent is ran under the hood. Any other suggestions?

 

Have you verified that the vent is working? If it isn't working properly it may be causing pressure to build before venting and therefore spewing oil out the vent. Also too much oil in diff can cause this.