I’ve chased vacuum leaks for more than 50 years. First thing you always look for is something you just touched whether it was intentional or unintentional. And if there is a sudden change, you just inch along the different vacuum lines and you can usually spot a split or a cracked connector or something like that. But on older vehicles, that is rarely the problem. The hoses get hard, joints get loose, and they do it very subtly and slow slowly.
More than half the time it’s not something sudden, especially on an older vehicle. Harbor Freight sells a pretty good smoke machine for very little money. I bought one a couple years ago, and I’ve been kicking myself in the head ever since on why I didn’t get it 50 years ago. It clips to the battery for power, you put a little special oil in it, and it blows smoke. So if you tie it into the vacuum line anywhere, and you just wait a minute, you start seeing smoke coming from here and there and everywhere.
When I redid the AC on my 97 Ranger, I had about a half a dozen leaks, nothing big, but they added up. I was still astounded with how well it worked. I used it on my other 70s and 80s vintage vehicles, and I found dozens of leaks.
On my 88 Towncar, I had the same symptoms: when you stepped hard on the gas, the vacuum would drop, and the default position for the air-conditioning starts is to blow hot air up onto the windshield for safety. I worked on that off and on for probably two months. I finally took it to the shop and learned something I would’ve never thought of.
That 88 Towncar has one of the big “soup can” vacuum reservoirs. But it’s connected to the system with a little skinny hose. That connecting hose was 90% blocked. So when you checked the system with a handheld vacuum pump, you could draw vacuum and it wouldn’t release. The problem was, you had all that vacuum sitting in the vacuum reservoir, but it couldn’t get out because the line was restricted when it was under load. When my Mechanic showed me that, that’s when I bought my own machine.
I’m not sure what engine set up you have, but on my 97 4.0, there’s a Little Red vacuum line that starts on the driver side of the engine plugged into the intake manifold. It runs around the back of the top of the engine to the passenger side, where there is a check valve. There’s a few rubber connectors on that red plastic line, check and make sure the line is in them very tightly, it shouldn’t rotate with a little bit of pressure.
That check valve can be the problem. It’s as simple as taking it out of the circuit and sucking on one side and then sucking on the other side and making sure that it will hold. They are pennies if you buy a half a dozen at a time or more on eBay.
On the passenger side, the line splits in two or three different directions, but it doesn’t go far. There should be a vacuum reservoir (about the size of a softball, sphere) sitting on top of the frame up near the radiator. Same thing, you can disconnect the line and just suck on it, and see if it will hold the vacuum. On the other lines, you just have to run them down and make sure they’re not rock hard from age and that they’re sitting tightly on whatever fittings and connectors. You shouldn’t be able to rotate the line in or on a connector very easily, it should be snug.
One line runs to the blend door behind the glove box, and the other line runs to the back of the heat and air controls in the middle of the dashboard. It’s hard to check them with the vacuum pump because the controls are notorious for having little leaks, but usually not enough to undermine the air-conditioning under load. But if you connect the smoke machine to it, you could spot even the tiniest imperfections, and of course you can see the big leaks like a billboard.
I couldn’t find it on the Harbor Freight website, but this is the kind of pump I’m talking about. I think mine was about $50 but that was a few years ago.
I hope it helps.
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