Mysterious Vacuum Problem in 300 TDT

This is easy! It is that yellow vacuum checkvalve that is shot... it has one in and two out... usually located by the firewall and fusebox area... Change it and all is well again. I also recall you also have a vacuum bellow reservoir? Not sure about this last part... I believe is located in trunk area?

Just for your information... Remember that even though your locks work after sitting, that doesn't mean that you have vacuum in the entire system. The locks are branched off the rest of the system with a check valve and have a huge reservoir located under the package shelf. It looks like a plastic version of grapefruit packaging. You'll know what I mean once you find it. It might be behind the fuel tank wall, but I don't think so. Can't remember right now. There is no reason to access it other than curiosity, since you obviously don't have a problem with that particular part.

Thanks. I was wondering if it could be a check valve.

The wagons do have a big vacuum reservoir. It's located under the car behind the fuel tank.

Out of curiousity, how does a bad vacuum check valve result in the climate system working correctly at idle, but failing at highway speed?

You should also take a good look at the rubber coupling elbows that attach to the check valve. Any leaks there will cause the symptoms you describe. Peter

Test that tank to see if it holds vacuum... chances are after all these years... has deteriorated.

Climate control system has alot of diaphragms that are activated by vacuum force... For example... the defrost vent are normally open when engine is off... it is closed by vacuum when engine is running unless you selected defrost or bi-level...

When you accelerate, there is no vacuum generated... that's where the checkvalve comes in... prevent loss of vacuum... so if the checkvalve is bad, vacuum in the climate control system is removed and thus the defrost vent opens up again.

Yes, that I know.

Doesn't this car generate the vacuum from a pump, rather than the intake manifold? And the OP said the symptom was loss of climate control vacuum during highway speed, not during acceleration. That's why I can't see how a check valve has anything to do with it.

Yes.

At highway speed, the engine is not constant speed so vacuum is not always present. To prevent loss of vacuum, the checkvalve 'locks' the vacuum in the climate control system.

You are right on turbo diesel engine or diesel engine where we have vacuum pump. In this case, we have that reservoir that helps maintain this vacuum.

Vacuum pump may be weak and need to be rebuilt.

What I described is for gas engine... normally apirated... turbo or supercharged system should also have vacuum pump as there is no vacuum generated in those engine.

If not the check valve, what do you think it could be? It does seem related to the amount of vacuum the engine is contributing to the system, which could be accentuated in its effect by a check valve that has failed.

The engine does contribute some vacuum with a turbo diesel, just not enough so it is supplemented by the vacuum pump. I rebuilt mine not too far back, maybe 50,000 miles.

Use vacuum gauge/pump to test your system. That is the only way to tell which part is bad.

Possible part failure...

1. Check valve.
2. Vacuum canister
3. Defrost vent actuator (they do go bad)
4. Vacuum pump
5. Vacuum hose connectors.
6. Vacuum hose cracked.

I'm finding it a little difficult to imagine how a turbo pressurizing the intake manifoild results in a vacuum there.

Does the turbo pressurize the intake manifold at idle?

Thanks Tiger for the checklist. I have to take the dash out to inspect the AC evaporator for leaks soon and when I do that I will do a comprehensive series of tests on the vacuum system. I remembered yesterday, though, when I was driving the car, that several years ago I had a similar problem because one of the vacuum lines became loose when I was working on the instrument panel. I recently replaced the bulb on my alternator light so I was wrestling with the kick panel on the driver's side, so I may have loosened that vacuum line.

Thanks again.

Paul

There are a pair of vacuum valves that are controlled via a cam that is part of the throttle linkage. They are under a black plastic cover, and a fitting comes out the front of the case that has 5 female openings with three of them used. One tube goes to the vaccum source line that runs from the vacuum pump to the power brake reservoir. One goes to the shutoff actuator on the injection pump and one goes to the vacuum controller on the EGR valve.

There are little plastic riders that ride on the big black plastic cam that is part of the throttle linkage and depress the vaccum valves at certain throttle settings, controlling the vaccum system. The plastic riders on my vaccum valves were worn and so the vacuum valves were not operating at the right times. This caused the climate control vacuum system to malfunction when the throttle was above idle speed, but operated properly at idle, hence the mystery of the irregular operation.

The valves are a commonly replaced part and the dealer had two in stock, $55 each. This solved the mysterious problem.