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Locating possible vacuum leak affecting heater controls?

Two years ago I purchased this vehicle used (~150k), head gasket had just been replaced, otherwise in good repair, and it runs well. (It has the 2.2 liter flex fuel engine.) I noticed gradually since then air is reluctant to flow from floor registers and now only comes out of dash board even when set to defrost or floor (any fan setting). A/C works fine, and system gets hot and cold just fine. I took apart the center console and glove box and performed visual inspection of control and vacuum hoses/vacuum connectors and even removed sleeve around hose bundles to check for unseen kinks- all looked fine. I disconnected and reconnected various actuators and noticed all functioned but were sluggish (such as opening of recirculating door during MAX A/C operation. I saw a vacuum hose that disappeared into the firewall above the accelerator but could not locate corresponding exit from firewall @ engine side… Hose connections at the engine intake are secure and appear intact. Any particular trouble spots or known/likely culprits that anyone could recommend pursuing would be welcome! It sounds like a vacuum leak to me, but I am having trouble finding the dang thing: could it be something else?

You probably have not gotten any responses because no one is sure what year vehicle you are working with. Flex fuel motors are generally newer so I will take a swag here.

The newer vehicles may have went to electrical actuators that I have seen in my mother-in-laws Envoy, I’m not sure. Hers would only blow to the floor. One of the actuators went out of calibration and could not get to one of the stops so it stuck in that position.

The older S-10s had the vacuum lines from the dash to the engine. Those would crack easily. The line was routed along the driver’s side and entered under the brake booster if I remember correctly.

I started to notice rot in the vacuum lines about 2 years ago with my 2000 S-10 Blazer. There is a vacuum reservoir inside the left front fender behind the ABS controller that I was going to have to access to replace some of the lines (I traded the Blazer in a week ago). Check under the hood on the firewall where the vacuum lines originate. I was using 5/32" and 7/32" ID replacement hose.

If all else fails, I used a coat hanger threaded through the floor outlet and poked around until the defrost and heater outlets were about 50/50 for flow. The heater controls were still able to switch between the dash vents and the defrost/heater vents. I believe this was a problem with a blend door and not vacuum.

Ed B.

I know I entered the vehicle make, model, and year in one of the required fields when I posted… where the hell is the information displayed for the forum members? ANYWAY, it is a 2000 Chevrolet S-10 with a 2.2 liter flex fuel engine, and thank you for those that responded; I will be checking the brake booster reservoir this weekend (and the lines entering/leaving) for the line rot mentioned in the responses… I think we are on the right track here since the lines in the passenger compartment looked pristine. Thank you again, next time I will post make/model etc. in the header and/or body of my message to avoid confusion… I’ll post again after additional diagnosis. THANKS AGAIN!

With that many miles, you need to put a vacuum gauge on it to see if you have normal vacuum. If the vacuum is good at the manifold, see if this truck has a vacuum accumulator.They sometimes crack with age.
If the vacuum is not good, check the compression. If it is good, check the timing and the egr system, they sometimes leak air into the intake manifold.

OK, I checked on the engine side, located the vacuum line (it goes straight to the intake and the other end feeds into the black line going through the firewall to the climate control) and I assume the idiot who took the engine out to replace the head gasket before I bought the vehicle (an unnecessary step, by the way) left the vacuum line divided with a “T” connector near the master cylinder where a straight or 90 degree only should have been. The extra six inches of vacuum line dead-ended with NO CAP, open to atmosphere! I could not locate anything that it should have been connected to within the radius/reach of that hose. So, I shoved a golf tee in the hose to plug it and everything works great now! By the way, the “mechanic” who worked on the engine before me got the engine running OK but did a half-ass job putting everything back: The wipers were put on the shafts incorrectly clocked, there were screws missing on the intake box (and many places elsewhere), half the straps that tether and support the vacuum lines & wire harness weren’t re-used at all, cowling behind the hood was FUBAR, etc., etc. I felt like contacting the seller and telling him to stay the hell away from whatever idiot worked on this truck, but I’ve lost track of him… Thanks to all for your suggestions!

Glad this inspection worked out for you and at least the dash stuff is operating like it should.