2004 4.0 very low idle vacuum leak? - JeepForum.com
 
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post #1 of 12 Old 12-10-2019, 07:07 PM Thread Starter
j4bs4209333
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I have a 2004 4.0 with 315k that recently started having a really low idle once it's warmed up. Normally it idles around 900 but now when I get to a red light it'll go from 900 down to about 600 after a couple seconds. It doesn't feel like it's going to stall but you can feel it trembling. The idle is steady so I don't think it's the TPS. I cleaned out the throttle body plate and that didn't change anything. Putting it in park or neutral versus drive doesn't make any difference. No check engine light and no whistling coming from the engine. I replaced the crankshaft sensor a couple months back with an oem sensor. Fuel pump was replaced 2 years ago with one from a junkyard with less than 100k. I also have a 93 Mustang that did the exact same thing to me a few years back and it was a vacuum hose under the intake manifold. I've looked at the vacuum hoses but I can't find anything obvious. I'm wondering if anybody's had the same problem or if there's additional vacuum lines that are not in plain sight that I don't know about that would cause this issue. Any help would be appreciated.

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post #2 of 12 Old 12-10-2019, 07:18 PM
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My 4.7 always idles at 600 rpm. I thought that was normal. 900 sounds high
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post #3 of 12 Old 12-11-2019, 12:03 AM
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^ My 04 4.7 also idles around 600-650 rpm.

If you're looking for vacuum/emission hoses which may not be real visible try looking behind the driver's side rear tire plastic wheel well. You may find an octopus of hoses/fittings! Lol.

https://www.rockauto.com/en/moreinfo...sn=383&jsn=383

I used to think a LDP (leak detection pump) only came on California emissions Jeeps but my 04 4.7 with Federal emissions has one behind the left side wheel well on a metal bracket/shelf.

The filter for the LDP is in the middle of those hoses in the link which is for the 04 4.0 in addition to the 4.7. The charcoal canister is also back there and the hoses tend to rot/leak because of the location. Also it's a great place for rodents to chew hoses and I had a squirrel take a few bites out of a hose back there. I replaced it with fuel line which solved the vacuum leak. You may want to reach up in there and check your hoses while listening for leaks.

Below is a photo of my LDP with hoses with the rear wheel well cover and bumper removed. I was doing a rust restoration project and painted the bracket with some other parts. You don't have to remove the bumper to get to all these hoses. Good luck.
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #4 of 12 Old 12-11-2019, 07:55 AM Thread Starter
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Originally Posted by Uniblurb View Post
^ My 04 4.7 also idles around 600-650 rpm.

If you're looking for vacuum/emission hoses which may not be real visible try looking behind the driver's side rear tire plastic wheel well. You may find an octopus of hoses/fittings! Lol.

https://www.rockauto.com/en/moreinfo...sn=383&jsn=383

I used to think a LDP (leak detection pump) only came on California emissions Jeeps but my 04 4.7 with Federal emissions has one behind the left side wheel well on a metal bracket/shelf.

The filter for the LDP is in the middle of those hoses in the link which is for the 04 4.0 in addition to the 4.7. The charcoal canister is also back there and the hoses tend to rot/leak because of the location. Also it's a great place for rodents to chew hoses and I had a squirrel take a few bites out of a hose back there. I replaced it with fuel line which solved the vacuum leak. You may want to reach up in there and check your hoses while listening for leaks.

Below is a photo of my LDP with hoses with the rear wheel well cover and bumper removed. I was doing a rust restoration project and painted the bracket with some other parts. You don't have to remove the bumper to get to all these hoses. Good luck.


Thank you so much. I actually have oversized new Michelins I got for a steal on clearance and they're rubbing a hole in the plastic wheel well cover. If the vacuum octopus is back there that makes sense. I'll check it out today, Thanks so much.
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post #5 of 12 Old 12-11-2019, 11:46 AM
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Originally Posted by j4bs4209333 View Post
Thank you so much. I actually have oversized new Michelins I got for a steal on clearance and they're rubbing a hole in the plastic wheel well cover. If the vacuum octopus is back there that makes sense. I'll check it out today, Thanks so much.
Ahh, you're welcome, and wouldn't doubt that's where your vacuum leak is. Good luck and let us know what you find.

PS. the reason I used fuel hose instead of regular vacuum hose to replace one of those hoses back there is it's fuel resistant, regular vacuum hose often isn't, and bends easier w/o crimping than vacuum hose. Plus one of those hoses leads over to the fuel tank vent/roll-over-valve where there has to be quite a bit of fuel fumes/residue in the hose.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #6 of 12 Old 12-14-2019, 10:26 PM Thread Starter
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Originally Posted by Uniblurb View Post
Quote:
Originally Posted by j4bs4209333 View Post
Thank you so much. I actually have oversized new Michelins I got for a steal on clearance and they're rubbing a hole in the plastic wheel well cover. If the vacuum octopus is back there that makes sense. I'll check it out today, Thanks so much.
Ahh, you're welcome, and wouldn't doubt that's where your vacuum leak is. Good luck and let us know what you find.

PS. the reason I used fuel hose instead of regular vacuum hose to replace one of those hoses back there is it's fuel resistant, regular vacuum hose often isn't, and bends easier w/o crimping than vacuum hose. Plus one of those hoses leads over to the fuel tank vent/roll-over-valve where there has to be quite a bit of fuel fumes/residue in the hose.
I went through all those hoses and I'm not finding any issues. The only thing I noticed is a breather tube going from the valve cover to the intake manifold that pops off very easily. It seems like the fittings used to be rubber now they feel like brittle plastic. I don't really know what its function is and when I took it off the valve cover side is pretty oily. I know the picture is from a different Jeep but it's the same thing. Any idea if that would cause a low idle? Thanks
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post #7 of 12 Old 12-15-2019, 04:35 AM
WJ04JGC
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My 04 4.0 with 223,000mi seems to enjoy idling at around 800. I never had a low idle but I would probably check the IAC (idle air control valve) and maybe the spark plugs?
Hope you get it sorted out
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post #8 of 12 Old 12-15-2019, 01:25 PM
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Quote:
Originally Posted by j4bs4209333 View Post
I went through all those hoses and I'm not finding any issues. The only thing I noticed is a breather tube going from the valve cover to the intake manifold that pops off very easily. It seems like the fittings used to be rubber now they feel like brittle plastic. I don't really know what its function is and when I took it off the valve cover side is pretty oily. I know the picture is from a different Jeep but it's the same thing. Any idea if that would cause a low idle? Thanks
The 4.0 doesn't really have a PCV (positive crankcase ventilation) valve since that part of the inner ventilation system is built into the inside top of the valve cover. That 'fixed orifice fitting' in the diagrams in the top/rear of the valve cover is sometimes mistaken for a PCV valve but I've seen it also referred to as the CCV (crank case ventilation valve) which would be more correct.

Regardless, the way the system works is air is pulled into the front 'air inlet fitting' in the front/top of the valve cover then some of the inner engine fumes are pulled through the 'fixed orifice fitting' then pulled into the intake to be burnt by the engine.

If the hose from plastic line to the rear fixed orifice fitting is loose/hard you can use a spark plug boot if you have a spare one laying around to replace it. Believe the below $5 Dorman hose is also supposed to work. But that small hose says it's for the MAPS sensor which seems a little odd to me.

https://www.rockauto.com/en/moreinfo...420737&jsn=384

The rubber grommets in the valve cover are likely hard but supposedly the way you test that 'fixed orifice fitting' is to remove it with the line still attached, start the engine, and if you have vacuum pulling though the fitting making a hissing noise it's supposed to be good.

If replacing the IAC don't believe I'd use anything other than Mopar but that OE/Mopar (4874373AB) IAC motor is $100. Believe I'd try cleaning it as WJ04JGC suggested. You can use a toothbrush with some carb cleaner on the pintel (tip) but don't pull or twist on the pintel. Believe I'd also clean the housing where it fits in the throttle body.

I'd say check your fuel pressure since you installed a used fuel pump in your tank but unfortunately the 04 4.0 no longer came with a schrader valve/test port on the fuel rail. You can test the pressure if you had the proper fittings/hoses between the fuel line and rail or around the fuel filter/regulator.

I'd likely dump a bottle of "Heet" is your fuel tank first to help disperse any water and it also cleans fuel injectors. Good luck.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #9 of 12 Old 12-15-2019, 04:47 PM Thread Starter
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You guys are the best. I took the entire throttle body off took off the IAC and clean it all up and it's idling at a hair under 900 which is perfect. If you guys didn't suggest the IAC I probably wouldn't have bothered because usually they cause you to stall unless you throw it in neutral. Thank you so much for your help.
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post #10 of 12 Old 12-15-2019, 08:09 PM
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Good deal, it was WJ04JGC who originally suggested cleaning the IAC, and hopefully it's fixed for good!

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #11 of 12 Old 12-16-2019, 10:03 AM
WJ04JGC
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Glad everything worked out for ya!
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post #12 of 12 Old 12-16-2019, 12:16 PM
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Hello j4bs,


Agree with Spiderman and Uni...I have a 99' WJ 4.0 and 01' 4.7 Dodge Durango, both idle around the 600-700rpm range. So, I think you actually have a high Idle issue somewhere.


I know last year my Durango was suddenly idling around 1000rpm. I changed or cleaned the IAC, TPS, etc, to no evail, then I noticed that the PCV or Crank Ventilation Tube (think it's called) was cracked and disconnected itself. So I got it replaced, and was back to normal at 600-700rpm in a matter of couple seconds.


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