Tyray3P

Hello! I'm new here, please be gentle. I'll bold important stuff in case you'd just like to skim through :)

I've got an old 1994 Grand Cherokee (Straight 6) that's got just around 385,000 miles on her.

She runs beautifully, never had an issue with the engine. Transmission rebuilt once, and the radiator's been redone a couple times. She's only had one owner, but I'm driving it now and I do plan to buy it off the owner.

Anyhow, to the point: It's basically the title. Air blows fine through the vents unless I'm giving her more gas, like accelerating to highway speeds, or going uphill. At which point, the air stops blowing out of the front vents properly and instead switches to the defrost vents without the actual switch being touched. When it's hot out, which is *very* often (Florida) it'll be stuck in defrost the same way. And sometimes, when the A/C is turned off and back on, it'll do it as well.

I'm thinking it's a solenoid issue, possibly a vacuum leak or something. I'm not entirely sure. The mechanic that the car owner takes it to refuses to do anything about it because they don't know what's causing the issue and refuses to touch anything.

I really just wanted to know if anyone else had has a similar issue, or just happens to know what's the cause.


On a second note: There is another slightly concerning thing. She generally runs pretty cold, the temp gauge rarely goes over 150° F. When she's just sitting there running though, she does build up some heat. No air blowing over after all.

Anyway, once the dial starts to get around 200°, about the halfway point on the gauge, when accelerating she'll make a kinda quiet grinding noise. Until she cools back down at least.

It's concerning but I know what causes it, so I simply refuse to let her get that hot, and I haven't heard the noise since (Been about half a year since discovery) I'm sure if I did let her get that hot it would happen again.

Anyway, thanks for your time! Please let me know if you know anything :)

wingless

Welcome to the forum.

That '94 ZJ was available w/ either Automatic Temperature Control (ATC) w/ a digital temperature setpoint display or manual control.

Assuming manual control, the dash ***** rotate to route engine vacuum to the appropriate HVAC vacuum motors.

It sounds like there are vacuum leaks that must be corrected. Assuming the vacuum hose from the engine to the firewall is intact, there are several ways to detect vacuum leaks posted on-line. Select the method that comports to your mechanical skills and comfort level.

With the engine cold, remove the radiator cap. There should be 50/50 water / antifreeze mixture to the top to the radiator, if not fill, then start engine and keep filling, then replace cap while running. Also fill bottle halfway. 100% speculation is that the thermostat has been yanked from the engine.

My assumption is the oil level (engine off) is correct and the oil is fresh.

Sorry, I don't know anything, but others on the forum have smarts...

Tyray3P

Thanks for the feedback! Sorry I didn't specify, I didn't know there were alternatives to the dash setup.

It is manual control, and I think you're right about the vacuum leaks. I had already plugged a rather big one that was hurting the idle revs and stalling the car every now and then. My guess is that they've never been maintained, and there's probably more that have likely cracked in some way or another.

On the note of the overheating grinding noise, I know the oil is changed every 2,500 miles, and the radiator is checked regularly with a correct mixture (I've never had an issue with overheating), and the Thermostat is working fine to my knowledge, I've never actually checked myself but the temperature has never looked out of the ordinary to me, aside from a few times the radiator seized (Years ago, has been replaced since)

I do actually have audio of the noise in a quick video I took. It's only during the first three seconds that it's noticeable.. I'll link the video. I took it as the noise was starting to fade as the engine cooled back down, probably wont help at all but ehe, why not. Anyway, thank you very much for the advice, I'll look into any vacuum leaks next chance I get.

Noah911

I am just a little worried that maybe your engine temperature gauge is not reading accurate engine temperatures. The lower temperatures you are accustomed to ussually seeing on the gauge may actually be much higher than what you're thinking they are... When a vehicle begins to overheat going upwards in temperature to around the 260* range. The engine sometimes will begin to shudder, and it will often make a grinding type of sound noise when they are that hot! Do you know how and if the mechanic, or anybody has tested to see if the engine temperature gauge is reading accurately the actual engine temperature?

Noah911

A nearly half million mile Jeep.. that is some very impressive mileage! The 500,000 mile mark is right around the corner for you after when you take full ownership of it. I would have to make getting to the 1/2 million miles status mark a goal for this Jeep if it were mine.

I always thought the 150,000 mile mark on these older Jeeps was when they were just beginning to start getting broken-in good.

Someone took good care of this Jeep huh? Outside of the normal wear items, I bet they really just needed to change the oil on time mostly.. These Jeeps when they are a one-owner vehicle you can almost always count on them ussually being great. When I purchased mine - My WJ was a one-owner Jeep going on 15 years old. My ZJ was a one-owner too. It was going on 23 years old when I picked it up earlier this year. Whoever previously owned my ZJ, they might have kept it even longer? Except, there were a few small problems happening with it that were seemingly much worse than they actually were.

I think it sounds like a vacuum leak for the other issue mentioned with the heat/air, vents, and switching itself to defrost.

97grand4.0

My zj is the manual control and I lost everything but defrost last week, and it turned out I had a break in the 3/32 line to the dash from under the hood, right by where the AC lines go inside. It was 2" from where it taps into the vacuum line going to the cruise control and the vacuum reservoir.
Advance auto had a pack of rubber caps for a couple bucks, just cut the end off one and it was the perfect splice. Now it easily changes vents and have all kinds of heat again.
The factory service manual does a great job of explaining the system.

Noah911

Same thing happened here for on my ZJ too..