Low vacuum under load

97 XJ Country Auto 4x4 3.5" RE Lift on 33x12.50 KM2s

Engine has basic bolt on mods, K&N cold air intake, 99+ Mopar intake manifold, Banks header, 703 injectors, Magnaflow high flow cat, Flowmaster 50 series muffler.

My 97 has had this issue for years but now I feel like it happens much more easily than before. In the beginning days of the issue it only happened at highway speeds but now its happens as slow as 35 MPH under load.

Whenever I am under the slightest load and deep into the accelerator, my HVAC controls default to defrost. This happens even with the system off and just the force of the wind outside hitting the front of the Jeep and forcing through the HVAC system.

I have done a visual check over most of the vacuum system finding no visual leaks. I had the HVAC box out recently to change my heater core and did a bench test with a vacuum pump and it held pressure, though the defrost vent flap for the side window defrost vents didn't like to close which makes sense since they always seem to blow air no matter the setting.

I looked over my vacuum canister and it seems good. I am scratching my head here and its getting annoying to have this issue.

Anyone got any suggestions?

check the vacuum hoses around the battery going to the canister. That caused that exact issue on my '99.

N8N_99xj said:

check the vacuum hoses around the battery going to the canister. That caused that exact issue on my '99.

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So you had the same issue where you had enough vacuum to run the HVAC controls normally but would lose them at times? I figured if there was a leak really of any kind that there would not be enough vacuum to run the HVAC controls and it would just default to defrost.

Ben824 said:

So you had the same issue where you had enough vacuum to run the HVAC controls normally but would lose them at times? I figured if there was a leak really of any kind that there would not be enough vacuum to run the HVAC controls and it would just default to defrost.

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I had that issue where it would work until I started on the gas. Then go to defrost only. I found a leak in the line on the firewall going from the manifold to the T right at the heater core.
So yeah, you'll get normal heat/ac even with a leak as long as there isn't another system needing the vacuum.

Ben824 said:

So you had the same issue where you had enough vacuum to run the HVAC controls normally but would lose them at times? I figured if there was a leak really of any kind that there would not be enough vacuum to run the HVAC controls and it would just default to defrost.

Click to expand...

The leak was small enough that the vacuum from the engine would over power it. But when engine vacuum dropped, the canister would just fill with outside air immediately and the HVAC system would go into default (defrost) mode because the hose going to the canister that stores vacuum for vacuum controlled accessories is right by the battery where battery acid can drip on it and corrode it.

The other possible thing is that the one way valve in the canister has gone bad, but more likely you have a leak.

Sound like a classic vacuum check valve failure.
As mentioned before, the vacuum reservoir has a built in check valve that can go bad.
Adding an inline check valve can fix this without having to replace the reservoir.

The carb cleaner / ether method can quickly find a vacuum leak if you have one, but you have to be careful.

Wow thank for all the responses!!!! I have been annoyed by this issue for far too long! I have had my XJ for 15 years this August as my daily so when I say its been a while, it has probably been a developing issue for longer than some others keep a vehicle in general.

So I will swing in a parts store and snag a check valve to rule that out. Would a smoke leak detection machine be what I would need to check for other leaks? I know I could do the carb cleaner thing but for under the battery tray for example I wouldn't be able to get to that with the carb cleaner.

if you can get a smoke detector or similar i would go for it.

Smoke machine is how I found my leak. My buddy has a shop so fortunately it only cost me a short piece of hose and a bit of my time.