Airborne Janitor Posted August 16, 2022 Share Posted August 16, 2022 89 LWB MJ 4.0L. Block is from a 91 but has a renix head + components. All the old vacuum assist 4x4/CAD components have been removed. She's got a 97 XJ dana 30 front axle with AW4 + NP231. No AC. No heater control valve. 95 XJ brake booster. Using the smaller "vacuum football" with only one port. All EGR/airbox/vacuum lines are installed. Years back I did the axle/tranny/tcase swap, along with pulling the dash and replacing the heater core/seal and hoses. All the heater/vent/defrost options work fine, blend door works fine. But when I get down on the accelerator (going up a steep hill for example, or merging on the highway) air will begin to blow out the front defrost vents. If the heater or vent was on, air will stop blowing out those vents and be diverted to defrost. If the heater/vent was off then air will begin to come out the defrost vents regardless. Once you come off at accelerator everything returns to normal. Does not seem to affect motor/vehicle performance. Any ideas? Link to comment Share on other sites More sharing options...
Pete M Posted August 16, 2022 Share Posted August 16, 2022 well, I can't say for sure why it's happening, but it's definitely a weak vacuum strength if it's defaulting to defrost. Link to comment Share on other sites More sharing options...
howeitsdone Posted August 16, 2022 Share Posted August 16, 2022 My first thought is the vacuum actuator for defrost. You're obviously building a strong vacuum while the engine is working hard. Maybe the actuator isn't closing it off like it should? Not sure what those do when they fail. Link to comment Share on other sites More sharing options...
Pete M Posted August 16, 2022 Share Posted August 16, 2022 vacuum strength drops as engine load increases. but in theory it shouldn't drop far enough to fail at keeping the door open. it's the job of the vacuum reservoir to hold some reserve for just that occurrence, but I know guys that eliminated it and claim to have no side effects. the sphere isn't cracked is it? hose going to it is in good shape? Link to comment Share on other sites More sharing options...
howeitsdone Posted August 16, 2022 Share Posted August 16, 2022 Bernoulli is rolling in his grave. It's late... Link to comment Share on other sites More sharing options...
cruiser54 Posted August 16, 2022 Share Posted August 16, 2022 This is a very common issue and the fix never seems to be the actuator. Those vacuum lines in the engine bay are decades old. Here's a diagram. Scope this stuff out. Link to comment Share on other sites More sharing options...
Eagle Posted August 17, 2022 Share Posted August 17, 2022 The most common cause of this is deterioration of the vacuum line(s) to the football (or softball) where they pass beneath the battery tray. It is definitely caused by a vacuum leak. "All" you have to do is find it. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted August 18, 2022 Author Share Posted August 18, 2022 So here's my current setup. A little redneck... No cruise, no AC, no heater control valve. Using the smaller late model XJ "vacuum football" with the single port. Checked all the lines under the hood, no leaks. Thinking about replacing the vacuum football with the HO XJ model... Link to comment Share on other sites More sharing options...
cruiser54 Posted August 18, 2022 Share Posted August 18, 2022 Your vacuum ball only has one port? Link to comment Share on other sites More sharing options...
Airborne Janitor Posted August 18, 2022 Author Share Posted August 18, 2022 2 minutes ago, cruiser54 said: Your vacuum ball only has one port? Yes. Link to comment Share on other sites More sharing options...
cruiser54 Posted August 18, 2022 Share Posted August 18, 2022 A small one like you have is fine if it has 2 ports. https://www.oreillyauto.com/detail/c/dorman-help/engine-sensors---emissions/emissions---air-management/vacuum-reservoir/87c789837ca7/dorman-help-plastic-vacuum-tank-unit/mtm0/47076?pos=0 Link to comment Share on other sites More sharing options...
Airborne Janitor Posted August 18, 2022 Author Share Posted August 18, 2022 1 hour ago, cruiser54 said: A small one like you have is fine if it has 2 ports. https://www.oreillyauto.com/detail/c/dorman-help/engine-sensors---emissions/emissions---air-management/vacuum-reservoir/87c789837ca7/dorman-help-plastic-vacuum-tank-unit/mtm0/47076?pos=0 Whats the second port go to? Link to comment Share on other sites More sharing options...
cruiser54 Posted August 18, 2022 Share Posted August 18, 2022 One port goes to manifold vacuum and the other to the plastic tube going through the firewall to the heater control in the dash. Visit the diagram I posted above. The only difference in your case is you will be mounting the ball in the engine bay instead of up under the bumper. Link to comment Share on other sites More sharing options...
Airborne Janitor Posted August 18, 2022 Author Share Posted August 18, 2022 Solved! Seems my vacuum reservoir had a crack in it. I was able to actually pry the whole thing apart with my hands. Found another small vacuum canister with 2 ports, hooked it up and problem solved. I did end up just plugging the 2nd port on the new reservoir without issues. Thanks for the help all! Link to comment Share on other sites More sharing options...
cruiser54 Posted August 18, 2022 Share Posted August 18, 2022 wasn't that easy? Link to comment Share on other sites More sharing options...
Parei_doll_ia Posted August 21, 2022 Share Posted August 21, 2022 I would also check the vacuum check valve near the coolant pressure bottle. I had the same issue on my cross country trip because the engine vacuum drops under higher throttle. I took a few check valves from the JY and eventually found one that sealed enough to hold vacuum and keep the ac blowing Link to comment Share on other sites More sharing options...
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