'97 Lesabre AC to Defrost: Vacuum Leak?

[Smith1000]

Just returned from a road trip to Colorado. Lesabre ran great; however, the ac would switch from the vents to defrost on long grades, particularly when accelerating. It had been doing this some during my regular commute, but not all that often. During this trip though, it did it continually. I found if I ran it on bi-level it seemed to hold better and would not switch to defrost as much and if it did when on bi-level, it would return to the vents on its own on the downhill side. I found that if I shut the ac off when cruising when it occurred, it would not always return to the vents (when climbing) when the ac was turned back on.

I also found with the ac on, it got about 21.5 MPG cruising. One morning when cool out, I shut the ac/vent off all together. While crusing that morning, it got about 27.6 MPG.

This morning, I pulled out the controller behind the glovebox and inspected the nipples and tubing. It all looks good and the nipples coming out of the controller are not flexible and soft. I really think that part looks okay. There is a vacuum where the violet vac. tube meets the black tube.

I am tending to think that it is losing sufficient vacuum due to a leak somewhere and that the problem lies somewhere in the main vacuum line that comes into the cab. The leak may be partially responsible for the decreased gas mileage I have been experiencing over about the past 1.5 to 2 years. It has gone from 26 combined to around 21 or less.

I cannot seem to find where the black vacuum line enters the cab (to trace it back). Does anyone know where the vacuum line enters the cab? Is it on the drivers side? My Hanes manual doesn't seem to show any vacuum line diagrams. I followed the line off the engine (near injectors) that connects near where the brake boost vacuum connects. It appears to go directly to the canister under the right front fender. I kind of lose it there. It appears only one vacuum line connects to the canister, but I can't tell for sure what the top of the canister looks like. Is there only one line to the vacuum canister? I do see a line that I believe is vacuum that runs near the bottom of the radiator over under the ECU and loops to the transmission and engine vaccums. Not sure where the one is going that ends up inside for the ac/defrost controls. I am about to go out and take another look. Thanks for any help. I would like to run a new line and connect somewhere in a good spot in the engine compartment.

[grcore]

I went through the same issue recently with my '96. On my car, the vacuum for the vent controls starts where the brake booster fitting is. From there it is fed into the wiring harness via a plastic tube. Then near the passenger side it comes out (may need to lift the fuse cover) and there is a three way (with a check valve) with one end feeding the canister and the other going through the firewall. The rubber transition pieces were rotting, but it did not look like they were failing, collapsing or leaking (but I replaced them anyway.

Where I found my problem was that translucent rubber tubing connctor that hooks up to the programmer. It was leaking at the top pair of fittings. I ended up dabing a bit of silicone on each fitting and plugging it on (leaving time for the silicone to set). If I ever have problems with it again, its going to be a pain to remove, but that was the only way I could think of to fix it.

[Smith1000]

I found where the vacuum line goes through the firewall into the cab. It tees off by the firewall (was wrapped in tape). One line goes to the canister. The other goes to the controller. I took the line loose at the engine and used a vacuum tester on it. I put it up to 10 lbs. It held fairly well. Over the course of about 30 minutes or more, it decreased only about 1 lb. The line seems fairly good and the canister looks good.

The line I was looking at under the radiator goes to the fuel tank and relates to the venting into the charcoal canister under the ECU. Guess I will drive it another week and see how it does.

[Smith1000]

The '96 set up sounds exactly the same as the '97. I had the rubbery piece that is connected to the controller off. I looked it over and put it back on. I may have corrected the leak when messing with it. I took the controller out too and put it back in. I also found that some of the transition segments of hose were weak, but not failing. When I drove it around yesterday, it never did it again, but I didn't get it out on the hi-way. Today, I'll run it down the hi-way and see what it does.

[grcore]

On mine, all I had to do was wiggle that connector and you would hear the vacumm leaking. If I pushed it down, it would stop.

Over time, it appears that connector lost some of its elasticity causing the poor seal and eventual leak.

[Smith1000]

I bet I have the same thing going on. I thought I could hear a slight hiss when I had the controller exposed, engine running. Thought it may have been my imagination. It wasn't much though because it was hard to hear over the engine running. It didn't act up at all today, so I may have it pressed on there tight enough for the time being. Regular commute today and much cooler. I like the silicone solution. I think I'll give that a try next time it acts up. Shouldn't take much to seal it. I appreciate your help.

[Flatrater]

When you are climbing a hill at full throttle you lose vaccum and when the vaccum tank gets empty the mode door defaults to defrost position. Vaccum is produced by the throttle blade and when you open the throttle all the way you will decrease the amount of vaccum made by the engine.

[Smith1000]

It hasn't done it since I have been back from the road trip, so maybe it is okay for now. It will return in the future and at least I'll know what to focus on.

I am getting code P1870 now. The check engine light came on during the trip. The last couple of days it has had a little "chuckle" at times, so my torque converter lock-up is formally going out. It has been going out for the past 75,000 miles, so it probably has a ways to go before it completely goes out. I put some Lucas transmission additive in it today. Might help some for awhile.