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Believer
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Discussion Starter · #1 · (Edited)
So long story short, I bought another YJ about a year ago because I'm an idiot because I was itching to tinker on something. Ever since I bought it it has had what I can only describe as a "clatter" that never seemed to be that big of a deal and still doesn't. Seems to run and drive just fine, PO said it's been making that noise for as long as he can remember, it always starts up, etc. It is a 91 4.0 auto.

I have been working on the brakes lately and have been hunting down the source of my spongy pedal. Decided to replace the master cylinder and booster at the same time with one from Dave's Customs but wanted to check the vacuum prior to ordering. According to Dave's Customs the vacuum at idle on a warm engine should be around 18 inches.

I checked mine, 12 inches at idle. Needle is steady with no crazy fluctuations.

Things I have checked-
Sprayed all over the engine and vacuum hoses with brake cleaner trying to find a leak. Nothing.
Checked engine codes. Nothing. (12, 33, 55)
Idle RPM is 750ish when warm.
Fuel pressure is 34psi.
Timing at idle reads 13 degrees with a timing light.
Dry compression test showed all cylinders are between 132 and 140. Tested each cylinder twice, no variance larger than 4psi.
When I pulled the plugs they were dry but looked hot/lean. Very white.

Jeep seems to run and drive fine other than being slow, which I have always attributed to it just being a YJ with an auto and 31s.

Thoughts on where I should look next to find the reason my vacuum is so low? Think it may be related to the clatter at all?

My elevation is ~4500' if that matters.
 

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Premium Member
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Is the clatter from the catalytic converter? A plugged cat will cause vacuum issues. If the engine cannot push air out, it cannot suck it in either. It all works in perfect harmony. (Singing a 1970's coca cola commercial song to myself)
 

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An odd question, but is the brake cleaner you are using chlorinated or not? The older chlorinated cleaner is not flammable, whereas the non-chlorinated is. Carb cleaner is a good choice to find vacuum leaks because of it's flammability.

Just learned the difference between the two!
 

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Actually, some non-chlorinated brake cleaner is flammable. We have both kinds at work. If it is non-flammable it says on the can. At least ours does.
 

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Believer
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Discussion Starter · #5 ·
Is the clatter from the catalytic converter? A plugged cat will cause vacuum issues. If the engine cannot push air out, it cannot suck it in either. It all works in perfect harmony. (Singing a 1970's coca cola commercial song to myself)
Hmm good question. I've poked all around the engine with a stethoscope and it sounds loudest at the head. I'll check the cat this weekend. I did look at it before and confirmed it wasn't glowing red or something crazy while running.


An odd question, but is the brake cleaner you are using chlorinated or not? The older chlorinated cleaner is not flammable, whereas the non-chlorinated is. Carb cleaner is a good choice to find vacuum leaks because of it's flammability.

Just learned the difference between the two!
It's non-chlorinated but still the very flammable kind. I confirmed this with my torch and made a nice fireball lol.


Actually, some non-chlorinated brake cleaner is flammable. We have both kinds at work. If it is non-flammable it says on the can. At least ours does.
Yeah mine is non-chlorinated but still says extremely flammable on the can.
 
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