There has been a super loud hissing sound in my 1990 xj on the drivers side, as well as the rpm's jumping like crazy when idling or at a stop. Might not be related, but I'm losing gas much more than normal. The engine also sounds awful and it keeps jumping on me. Initially I thought it was a vacuum hose leak, however, I couldn't find one, so more research was done and it might be a crack along the exhaust manifold. Any recommendations would be super helpful.

 

Sounds more like an intake manifold leak.

 

Check the front and rear nuts that secure the intake/exhaust manifold. They work loose periodically from all the expansion/contraction. I've had to tighten mine every few years. Next step would be to check/clean the butterfly valve for carbon/fuel build-up. Wouldn't hurt to remove/clean the IAT (Intake Air Sensor) ...they get gum'd up. Also wouldn't hurt to check all vacuum's, and finally the temp sensor (in case it's drifted).

The hissing might be an exhaust leak which could be affecting how your O2 sensor is operating (and possibly cause idle engine issues). The fix is to remove the exhaust manifold, inspect it, and re-weld any cracks. Also the flex bellows (if yours has them) do eventually crack. I've replaced exhaust manifolds ....and unless yours is unsalvageable, I'd try re-welding what you have. Cracked expansion bellows can cause a good exhaust manifold leak sound too. It's underneath the intake, you could probably see it with an inspection mirror. Personally, I'd remove the exhaust manifold and see what you're up against. They can crack all around. but like I said, easy to re-weld most of the time. The exhaust manifolds I've bought have been awful quality (even though they say, "High Quality"). I've had to re-work brand new ones that had gobs of built-up welds at the mounting plate (restricting the exhaust opening). Took 2 hours to grind out the awful looking and restricting welds with a carbide burr. The new manifolds I bought cracked within a year or two anyway ..so it was just money wasted. If yours has leaky expansion bellows just cut off and weld in a new one (below). But see what you have before you order a new one. That's my advice.

2" ID Bellow
https://www.ebay.com/itm/304712240157

1.75" ID:
https://www.ebay.com/itm/224325353405

 

How did you check for vacuum leaks?

 

Sounds like a vacuum leak

 

Thats what an engine vacuum gauge is made for

They are cheap, and easy to use, and tell you more about an engine than most any other tool

spraying engine start spray around a suspected vacuum leak will usually cause RPM to rise

 

I had the same situation, not my car runs like a dream with the new manifold ($90)

I also found an old cracked right angle vacuum hose connector (that goes into the side of the intake manifold. I probably would never have found it if I didn't try to replace my old 'tired, cracked, and full of holes exhaust manifold'. Check out my post:

https://www.cherokeeforum.com/f2/big...issues-273073/

ALSO this guy definitely made a perfect tutorial on how to do it. Anyone with basic tools can do it. TOTALLY worth every penny.


Highly recommend doing the exhaust even if you only suspect it is not right. I thought all I had was a hole, and found out the thing was FULLY cracked across....