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GM-Frank, ASE Master Certified
Category: Chevy
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Experience:  37 Years Automotive Diagnosis and Repair. 25 Years of General Motors Dealer Experience
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I have a 1999 chevy 1500 4x4 and I noticed that gear oil is

Resolved Question:

I have a 1999 chevy 1500 4x4 and I noticed that gear oil is blowing back through the axel vent tube on the passenger front side. What causes this?
Submitted: 8 years ago.
Category: Chevy
Customer: replied 8 years ago.

1999 Chevrolet 1500 silverado 5.3

Already Tried:
Expert:  GM-Frank replied 8 years ago.




On your truck, there are a couple of possible causes for gear oil showing at the vent line either in the front or the rear differential.


The most common cause is over filling. Do not be afraid to have the level at the inspection/fill plug be down a bit. The next step is to be sure that the correct gear oil is used so that there is no possible foaming that would create pressure to push the fluid out the vent.


Another suggestion on the front differential is to replace the vent line adapting to a slightly larger hose at the upper end of the vent line, and relocate the vent slightly higher so that the differential must move the vented fluid farther up the vent line before it can escape.


But, number one is do not fill it right to the top and use the correct recommended fluid.


Below is a service bulletin on rear axle lubricant recommendations.


File in Section: 04 - Driveline Axle

Bulletin No.: 99-04-20-001

Date: June, 1999


Rear Axle Lubricant Compatibility and Usage




1999 Cadillac Escalade
1988-99 Chevrolet and GMC C/K, G Van and P1-3 Series Models
Built after the VIN Breakpoints (C/K) listed.


This bulletin is being revised to add the 1999 Model Year, VIN Breakpoint information, and additional text Please discard Corporate Bulletin Number 86-42-04 (Section 4 - Driveline/Axle).




This bulletin is being issued to inform dealers about the compatibility and usage of the new Fuel Efficient SAE 75W-90 Synthetic Axle Lubricant, GM P/N 12378261 (In Canada use P/N 10953455), with current axles, and with the non-synthetic axle lubricant, GM P/N(NNN) NNN-NNNN(In Canada use P/N 10950849), for the rear axles on the above light-duty trucks.

Testing has shown that the new synthetic lubricant will chemically attack the Room Temperature Vulcanizing (RTV) sealant. Use only the non-synthetic lubricant, P/N(NNN) NNN-NNNN in axles using RTV sealant (See 9.5 and 10.5 information below).

American Axle and Manufacturing (A.A.M.) has approved the use of the new Fuel Efficient SAE 75W-90 synthetic axle lubricant for rear axles with the following restrictions:

^All 8.6 inch axles are compatible as originally built.


^9.5 inch axles on vehicles built prior to mid-February 1998 are NOT compatible because RTV sealant is used on the cover pans.


On vehicles built after mid-February 1998 and equipped with 9.5 inch rear axles, synthetic lubricant, P/N 12378261, can be used because the RTV was replaced with a gasket. Other mid-February changes to the 9.5 inch rear axles include a new cover pan and cover pan fasteners common to other axles.

Important : The new cover pan is not retrofittable to the housing of 9.5 inch axles on vehicles built prior to mid-February 1998. A new retrofit kit is available that will enable the use of synthetic lubricant for vehicles built after mid-February, 1998.


^All 10.5 inch axles prior to March 3, 1999 are compatible if the axle shaft hub seal (RTV) is replaced with P/N 327739 gasket when changing to synthetic lubricant.


As a product improvement, this new lubricant has been released into 1999 current production C/K vehicles built after the above VIN breakpoints (Except HD3SOO models which use 75W-140 synthetic lubricant, P/N 12346140).

It is recommended that synthetic lubricant be used when changing rear axle lubricant for the following reasons:

^Potential fuel economy


^Lower operating temperature under severe usage conditions


Important :
^You do not need to wash/rinse any non-synthetic lubricant from the axle cavity when changing over to a synthetic lubricant. However, you should remove debris from the magnet and the bottom of the axle cavity whenever the axle fluid is changed.


^On light-duty trucks equipped with locking differentials, do NOT use a limited-slip additive.


Parts Information

Parts are currently available from GMSPO.


I hope this information is helpful.


Have a Great Day





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