Transmission oil in vacuum lines?

JOE THE MARINE

MEMBER
2002 CHEVROLET BLAZER
4.3L
6 CYL
4WD
AUTOMATIC
180,000 MILES

I was having a problem with the vents inside blowing heat on the top vents, and not enough on the floor. Was told to replace the vacuum lines under the hood, and did it with a vacuum pump, and as I was doing this to find the leak before I replace all of them, I seen that as I took one by one off, rach had pink (transmission oil) coming out, which I figured this is the blockage of why I am not getting enough heat on the floor. Replaced all the lines with new silicone lines, still the same inside, no change. Now, when I put it in 4-wheel drive, I hear the transfer case kick in, but no 4-wheel drive. So far I haven't checked the line that goes down to the transmission, yet, and goes under the battery tray, which is a pain, will do this soon. Could this also be the problem with both of these issues I am having? This is driving me crazy, from one issue to another. I how you can solve this for me, please. Thanks for any info you may pass on. One thing, why or how did the transmission oil get in the lines?

Friday, January 5th, 2024 AT 4:51 PM

1 Reply

CARADIODOC

MECHANIC
34,330 POSTS

It looks like your transmission still uses a vacuum modulator valve in addition to electronic controls. If it does, you'll find it at the right rear just above the transmission pan, and it will have a vacuum hose connected to it. Transmission fluid can enter the vacuum lines when the modulator valve is leaking.

Many Fords used a similar valve that could develop the same leakage. The common symptom was different, however, because they commonly used a "vacuum tree" near the center / top of the firewall where all the other vacuum hoses connected to. Unused ports were capped off with rubber caps. Transmission fluid rots those rubber parts. The common symptom was high idle speed due to one of those caps falling off creating a vacuum leak.

GM didn't use that type of vacuum tree, but rubber hoses will still rot and deteriorate. You may have nothing more than a loose hose connection, but given the fluid you already found, it would be a good idea to replace as many as possible. That fluid will not have made its way into all of them. It's getting sucked out by intake manifold vacuum and is going into the engine where it is burned. It will tend to not flow into hoses where there's no vacuum pulling it in.

This article:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

goes into detail for finding a vacuum leak. Finding and repairing that will take care of most symptoms, but in this case you must also address the cause, which is the transmission fluid in the hoses.

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Friday, January 5th, 2024 AT 6:45 PM