Elusive oil leak

[Matthew Jeschke]

I have a buddy with a 2005 Z71 truck that is absolutely cherry, despite it's been his daily driver ever since he bought it. There's one issue though, it leaks oil. I looked at it for him and immediately though it was the oil pan gasket. The oil comes from the rear parting lines (by the rear cover) and down to the drain bolt on the pan so I replaced the oil pan gasket. This did nothing. It still leaks in the same fashion. We've done all the following as well chasing this leak...

Crank sensor - new one installed with a new seal

Sending unit - new one installed with a new crush seal.

Rear Main seal - inspected, inside bell housing is dry as a bone, even used a endoscope to look at the seal and it's dry.

Valve covers - completely dry, no leaks, also along the bellhousing where they'd have to be leaking for oil to end up where they leak is dry.

New oil pan gasket - I'd hope not but maybe I didn't get enough RTV on parting lines between covers? The front is fine, I did the rear in the same fashion per instructions... That said, can I have a leak above in the rear cover that would drain down leading one to believe it's the oil pan gasket when it's the rear cover?

Not looked at...

Cam sensor - I think his is on the front of the motor so I don't believe that's the issue? I couldn't spot it though as the water pump is in the way. Didn't they move the cam sensor to the timing cover after the split model year?

Main bolts - there's the bolts that hold the side of the mains... I put my finger in one of those holes, and couldn't tell if it was wet or not (oily). I'm curious are those side main bolts known to leak? I know when I built my engine I sealed them and or they had an oring or something. Anybody had an issue with that before?

Any other ideas?

 

[Fless]

Cam sensor on a Gen III engine is next to the oil pressure sensor, aft of the intake manifold.

The valley cover could be leaking back there, too.

Looking forward from the rear of the engine, the red arrow points to the oil pressure sensor, green points to the cam sensor, and yellow is the valley cover.

 

[S33k3r]

Is it actually motor oil? Could it be transmission fluid, power steering fluid, etc? Could it be a different leaking stirring up old oil from a previous leak?

 

[mikez71]

Not the rear cover that holds the rear main seal? It sounds like a common leak on the GMT900's anyway..

 

[Matthew Jeschke]

Cam sensor on a Gen III engine is next to the oil pressure sensor, aft of the intake manifold.

The valley cover could be leaking back there, too.

Looking forward from the rear of the engine, the red arrow points to the oil pressure sensor, green points to the cam sensor, and yellow is the valley cover.

View attachment 483841

Darn for some reason I assumed they moved the cam sensor to the front of the engine after the split model year. Nice to see if with the cover rear cover off. I assume, if that sucker leaks it's going to look an awful lot like the oil pan seal. My plan was to just pull it and put a different o-ring on it.

Is it actually motor oil? Could it be transmission fluid, power steering fluid, etc? Could it be a different leaking stirring up old oil from a previous leak?

Good thinking... I think it's motor oil but will double check the transmission isn't leaking as well as the power steering and brake reservoirs.

Not the rear cover that holds the rear main seal? It sounds like a common leak on the GMT900's anyway..

The leak is on the sides of the rear cover. I imagine the GMT900 would be same as GMT800... Right where the oil pan mates with the rear rear cover.




Looking at that cam sensor photo, I think if the o-ring is shot it'll leak in the same spot. That's my next thing. Thanks again for the photo @Fless !!

 

[C1E2]

Any update on this? I have an oil leak I'm looking for and I'm curious if any of this helped. I never see oil on the driveway and it seems that it only leaks when the trick ('16 Tahoe LTZ) is running/driving. It's only been an issue for the past few months but I've gotten a "Low Oil" warning twice so far. Yesterday's warning comes with only 1000 miles since my last oil change.

 

[6speedblazer]

95% of the time it's the rear cover that leaks on these.

 

[C1E2]

95% of the time it's the rear cover that leaks on these.

Sorry, I don't know what you mean by 'rear cover' - are you referring to the rear side of the valley cover? Just want to make sure I'm understanding this correctly. Thanks!

 

[6speedblazer]

LS engines have what's commonly referred to as the "rear cover" it's essentially the rear main seal housing but covers quite a bit more than the older SBC style rear main seal housing. the design was updated at least one time over the years.