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Vacuum Problem

2K views 4 replies 4 participants last post by  soccerpapa 
#1 ·
OK guys, need to pick your brains here.

MY CJ7 has an AMC 360 V* with about 4000 miles on it. It's driven 95% off road. It's always ben a strong puller even with 35" tires. Has 3.56 gears and LockRites in both axles. Dana 300 and TF999.

About 10 years ago, I installed an aftermarket vacuum gauge. Vacuum would read 16-18 inches when idling, depending on the choke setting. Vacuum was 20-21 inches when engine braking down hill.

Went trail riding this weekend. From the git-go, vacuum at idle was only 5 inches at idle and didn't rise appreciably when engine braking. During mild acceleration, it dropped to 2 inches.

During the ride, I noted that my power brakes were not as good as normal. But I was in low range, lots of gear, so didn't use the brakes that much. Since the brake booster (from a Chevy pickup) is vacuum operated, I figured the low vacuum was also the cause of reduced power braking.

The Jeep did OK on the trail, until we came to a fairly steep nill with very loose, almost sandy soil. Other guys had some problems with it, but eventually got p it. I was aired down to 14 psi, low range, TF999 in 2nd gear. Started up the hill, was doing great. It was obvious that I was going to clear the hill without any problems. Don't remember looking at the tach, But I was probably between 3000 and 3500 rpm. Two-thirds of the way up the hill, the engine stumbled - rpms went to idle, forward movement stopped. In less than one second, rps jumped back up, I continued on for another 20-30 feet, and then the engine died totally. Had to back down the hill with no power brfakes. No power steering with a front locker made it even more exciting. After I got down the hill, I started the engine and it continued to run well the rest of the day, except for low vacuum reading, although I didn't try any more sandy hills requiring high rpms. I did get up some steep hills that were hard and I was able to crawl up without high rpms.

I checked the back of the vacuum gauge, and the vacuum line is still hooked up. The engine compatment was too hot (engine temp was OK) -- outside temp was about 95 degrees -- to do any trouble shooting on the trail.

So something is causing the thing to lose vaccuum. Since there is still some vacuum on he gauge, I'm guessing that it's not the case that the vacuum hose from the engine to the gauge came off. So here's some questions:

1. Is my guess that low vacuum also resulted in the poor power braking correct?

2. Is there a likely (common occurance) cause for the low vacuum?

3. Could whatever is causing the low vacuum also cause the engine dying at high rpm?

Thanks in advance for any thoughts you may have.
 
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#2 ·
I'm no expert, but here are my thoughts...
Question #1: Yes
Question #2: Check for vacuum leaks with all plumbing, check your brake booster for internal or external vacuum leaks, check intake manifold and carburator mounting hardware for torque.
Question #3: Maybe??? (remember, I'm no expert)
 
#3 ·
It's also possible that your poor power braking may be causing the vacuum loss. What I mean is: if your power booster diaphragm is leaking, that would be considered a significant vacuum leak. To check, put moderate pressure on the brake pedal with the engine off, then start the engine. You should feel the pedal drop a little farther once the engine starts. If not, your booster is shot. Good luck:thumbsup:
 
#4 ·
Thanks for the hints. If the problem is a leaky brake booster, I should be able to determine that pretty quickly. I will disconnect the vacuum hose going to the booster. If the hose looks bad, I'll replace it. If the hose looks good, I'll disconnect it from the booster and plug it. If a leaking booster was the problem, then I should immediately see the vacuum gauge go back up to 16-18 inches.
 
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