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.
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.
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.
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
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.
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.
Joe
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