how to find vacuum leak?

'01 XJ, 70K miles.

about a month or so ago the engine light came on, so pulled the codes to find "small vacuum leak" and "large vacuum leak".

using the diagram on the firewall in the engine compartment i attempted to visually inspect all the hoses. typical wear & tear, but not too bad (certainly i wouldn't think for an '01 anyway, otherwise i suspect i'd hear of this problem more often from folks w/ older rigs?). anyway, i did find one section of hose (kinda near the CA mounts where the line goes up from the frame rails to the engine compartment) that was cracked a little more than the rest, so i gave it the ol' duct tape fix... reset the dummy light and thought i had fixed it as the light stayed off for a few days, but then came back on again.

any ideas? (and no gas cap is not loose).

thanks in advance.

Search for my posts on the subject, I've covered this before. Once you find a post with instructions, go get a can of carburettor cleaner and follow them.

It's a fairly simple test - I just know I've covered it several times, and honestly don't feel like doing so again (I've probably covered it a few times in the last six months...)

MMIXJ said:

that was cracked a little more than the rest, so i gave it the ol' duct tape fix... reset the dummy light and thought i had fixed it as the light stayed off for a few days, but then came back on again.

Click to expand...

Just a thought Terry. If you slapped some duct tape on there, and it worked for a few days, but now is pulling a code again; maybe you should do a more permanent fix than Alabama chrome. My first thought is that the duct tape isn't doing the job you want it to. I'd start there.

I also use a unlit propane torch, running it along the length of each line and around the top side of the intake. When the propane is sucked into the leak the engine will run differently.

Hey Terry... check all the lines and fittings associated with the carbon canister, leak detection pump and purge valve. Many leaks of this type are associated with a cracked hose over a fitting, usually at the carbon canister.

The codes don't have anything to do with engine vacuum leak but are actully triggered when the PCM cannot seal the fuel tank from the atmosphere.

I have been doing drive ability for about ten years now and have never seen a code that says vacuum leak. Sounds to me like an EVAP leak, common small and large leak. I suspect you might know this since you mentioned fuel cap. So if we are dealing with an EVAP leak (which is fuel vapors) we don't really want the propane torch around it. I suggest doing what "Digger87xj" says, then if you don't find a problem save yourself some money and send to a repair shop with a factory scanner and a smoke machine, if you can't find one you will have to take it to the dealer. Or just throw parts at it all day and spend more then you should. The time it takes for the code to set is dependent on many factors and the duct tape is probably not related (sealing then not). Try a Google search for TSB's you might find what your looking for. Hint hint.

thanks for the replies...

my bad-- i did search before posting, but now i know why i didn't find anything helpful... i was calling it a vacuum leak but you're right the code is for an EVAP leak. D'OH!

don, i did see some cracks on a few of the hose fittings, but like i said i assumed that that was normal. i'll take a second closer look focusing on the areas you recommended.

um, any hints on search terms?

5-90 said:

Search for my posts on the subject, I've covered this before.

Click to expand...

search posts by user "5-90"
"EVAP leak"
"evaporative"

=no results

Spray carb cleaner along all vacuum lines, intake gaskets, throttle body area, injectors, ect... when the engine wants to die or stumble, that is more and likley the place where the vacuum leak is.

Re: um, any hints on search terms?

MMIXJ said:

search posts by user "5-90"
"EVAP leak"
"evaporative"

=no results

Click to expand...

Bugger. Since you provided a few search terms, that should make this easier to find for the next guy. So...

Take your can of carburettor cleaner, and spray a "fog" around anything that is a vacuum seal - vacuum lines, throttle body base, intake mounting surface, ... You're after pretty much any leak downstream of the throttle valve. I'd not neglect the evapourative lines from the fuel tank, and make sure you also cover the EVAP cannister (about the size of a coffee can - just the lines on top should cover it.)

You will see the fog drawn into the engine, and hear the idle "run up" with the added combustibles. I'll use propane in a pinch, but I prefer getting more than one cue - so the carburettor cleaner.

Repair leak as indicated.

Search Terms

EVAP Leak
Evaporative leak
Evaporative containment
Vacuum leak
Vacuum leak diagnosis
Finding a vacuum leak
Locating a vacuum leak
How do I find a vacuum leak