engine sucking up transmission oil - The 1947 - Present Chevrolet & GMC Truck Message Board Network

marvxyz1 · 08-18-2006, 07:17 PM

I have a 1980 gmc with a six automatic. I've been losing transmission oil. I checked under the hood and on the left side of the engine as I am facing it. There is a plug on the side of the engine.with two vac. lines going to it. With the motor running and the vac lines connected, I get transmission oil leaking from it. I also seem to be getting transmission oil sucked into the engine, as I get gray smoke coming from the exhaust, when I start the engine. what is this plug called and what would be the fix for this problem?? I would appreciate any info you may have on this problem, Thanks for your interest..

f.monroe · 08-18-2006, 10:33 PM

Sounds like a bad vacuum modulator valve on the automatic trans . My 87 did the same thing and the valve was bad . Good thing is the valve takes about 10 minutes to change and is a pretty cheap fix. The valve should be mounted on the passenger side of the trans towards the back if it is a TH 350 . Frank

marvxyz1 · 08-19-2006, 02:47 AM

I started to think about it and I believe you may be right!! With transmission oil going through your cylinders when your engine is running, will this cause engine damage?? Thanks for the info f.monroe.

krue · 08-19-2006, 07:07 AM

No, it may actually clean it up some.

It's not very good for the tranny tho.

piecesparts · 08-20-2006, 12:26 PM

The tranny will get to a point where it willl not shift anymore, if the modulator fails. You may even gbe noticing a sharp, hard shift even now, that is caused by a loss of vacuum effect from the modulator. Changing it out is not that hard.

marvxyz1 · 08-20-2006, 08:55 PM

I was concerned about possible engine damage, with trans. oil goiing through the engine!!! Thanks for the info. guys.

Longhorn Man · 08-20-2006, 11:00 PM

Well, you CAN damage the engine. I had one that had a huge carb fire, melted the carb, everything on top of the carb, plug wires, and paint on the hood.

krue · 08-21-2006, 12:46 AM

That's an extreme case.

If you haven't noticed anything catastrophic by now, you're probably ok.

piecesparts · 08-21-2006, 09:13 PM

The tranny fluid would gum up the intake some, but actually will help to clean some of the carbon off of the top of the pistons. As for a fire in the engine bay, unless the fluid is spreding all over the top of the motor and onto the exhaust manifolds, highly unlikely. There have been cars driving around the United States, gobbling up oil for as long as there have been highways and that is a remote thing. A fire would come from a defective carb float and flooding of the gasoline all over the intake and exhaust manifolds. The real loss to your car will be the wear and tear on the tranny from running it low on fluid and not shifting properly.

08-18-2006, 07:17 PM	#1
marvxyz1 Registered User Join Date: Oct 2005 Location: Pittsburgh, Pa Posts: 106	engine sucking up transmission oil I have a 1980 gmc with a six automatic. I've been losing transmission oil. I checked under the hood and on the left side of the engine as I am facing it. There is a plug on the side of the engine.with two vac. lines going to it. With the motor running and the vac lines connected, I get transmission oil leaking from it. I also seem to be getting transmission oil sucked into the engine, as I get gray smoke coming from the exhaust, when I start the engine. what is this plug called and what would be the fix for this problem?? I would appreciate any info you may have on this problem, Thanks for your interest..

08-18-2006, 10:33 PM	#2
f.monroe Senior Member Join Date: May 2004 Location: Caldwell,Tx. Posts: 3,648	Re: engine sucking up transmission oil Sounds like a bad vacuum modulator valve on the automatic trans . My 87 did the same thing and the valve was bad . Good thing is the valve takes about 10 minutes to change and is a pretty cheap fix. The valve should be mounted on the passenger side of the trans towards the back if it is a TH 350 . Frank __________________ Born and proudly residing in a Red State !

08-19-2006, 02:47 AM	#3
marvxyz1 Registered User Join Date: Oct 2005 Location: Pittsburgh, Pa Posts: 106	Re: engine sucking up transmission oil I started to think about it and I believe you may be right!! With transmission oil going through your cylinders when your engine is running, will this cause engine damage?? Thanks for the info f.monroe.

08-19-2006, 07:07 AM	#4
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Re: engine sucking up transmission oil No, it may actually clean it up some. It's not very good for the tranny tho. __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

08-20-2006, 12:26 PM	#5
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: engine sucking up transmission oil The tranny will get to a point where it willl not shift anymore, if the modulator fails. You may even gbe noticing a sharp, hard shift even now, that is caused by a loss of vacuum effect from the modulator. Changing it out is not that hard.

08-20-2006, 08:55 PM	#6
marvxyz1 Registered User Join Date: Oct 2005 Location: Pittsburgh, Pa Posts: 106	Re: engine sucking up transmission oil I was concerned about possible engine damage, with trans. oil goiing through the engine!!! Thanks for the info. guys.

08-20-2006, 11:00 PM	#7
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: engine sucking up transmission oil Well, you CAN damage the engine. I had one that had a huge carb fire, melted the carb, everything on top of the carb, plug wires, and paint on the hood.

08-21-2006, 12:46 AM	#8
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Re: engine sucking up transmission oil That's an extreme case. If you haven't noticed anything catastrophic by now, you're probably ok. __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

08-21-2006, 09:13 PM	#9
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: engine sucking up transmission oil The tranny fluid would gum up the intake some, but actually will help to clean some of the carbon off of the top of the pistons. As for a fire in the engine bay, unless the fluid is spreding all over the top of the motor and onto the exhaust manifolds, highly unlikely. There have been cars driving around the United States, gobbling up oil for as long as there have been highways and that is a remote thing. A fire would come from a defective carb float and flooding of the gasoline all over the intake and exhaust manifolds. The real loss to your car will be the wear and tear on the tranny from running it low on fluid and not shifting properly.



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