Low rpms on a 2004 tj

I have a 2004 TJ, 4.0 sport 5 speed manual. 30 inch tires, no lift. Just replaced TPS and IAC after check engine like error code p1294 I think. (wasted money. Did nothing :BangHead cleaned out throttle body, still get a rough idle. Doesn't stall or anything.

There was a lift in it that was removed right before I bought it, and the speedo is consistently 10mph off. I assume this is the speedo gear, however I will bring it up since my tachometer and speedometer is off.

When I start my jeep the rpms are about 1000, then they drop to about 650-700 for a few seconds or less then it drops to zero or about 100. The idle rpm "thinks" it is between zero and about 100. Obviously it would stall out at that rpm. I have plugged in an OBD scanner and it is about 550 rpms off. It seems to stay consistent throughout gears. Going 65 in 5th gear it's about 1500 Rpms. Which, according to others with the same gears and similar tire size, that seems about 500 low still.

It has 3.07 gears :facepalm: and Dana 30 front Dana 35 rear.

This issue has been purely an annoyance. Haven't had any issues wheeling and mudding or daily driving with it. every time I start it it sits at about 600-750 and I'm always thinking "finally it's normal!" (yes I do this probably 5 times a day every time I start it. )

My oil pressure battery and gas gauge seem to be fine. First jeep, had 'er a little more than a year.

Looking for ideas on how to fix it and what is causing it the needle to sit almost at 0 rpms.

Thanks in advance.

My apologies for this being long and rambly its been a long day. :dance1:

Chris said:

It sounds to me like you might have a vacuum leak somewhere in your system.

You can check for a vacuum leak by having the engine running and spraying carb cleaner at various spots on/around engine, especially intake and vacuum hoses etc. and listening for changes in engine idle. If you spray and the engine revs up a bit you've found where the vacuum leak is at it suck in the carb cleaner.

It could also be your intake manifold gasket as well, I've heard of this causing the P1294 code as well as vacuum leaks.

The speedo being off is because you need a new speedo gear. You can read more about that here: Speedo Gear Change

3.07 gears are quite possibly the worst gearing setup you could have, no kidding. Even with only 30" tires, that gear ratio just flat out sucks. It would be far worse with larger tires, but I'm just putting it out there anyways.

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Really? That must be the same vacuum leak then that is causing my ac to not work. It only blows out the dash. I see the line, it is what comes out of the right side of engine, small tube goes splits off into T. One side goes In to firewall the other is broken and just hangs there. I've also asked many mechanics and all can't figure it out. Unbelievable. How can they not check a schematic and figure it out (I say this as I cannot figure it out myself haha) I could post some pictures tomorrow.

Could that be the same one? Im not aware of any others however I will try to find them tomorrow. I assume I take off the plastic coating around the lines and spray them?


Thanks

Chris said:

Yep, that could very well be the case. Those blend doors behind the dash that control where the air blows out are controlled by vacuum lines, so that stands to reason.

Post some photos and one of us can tell you where the line goes. It's hard to say without photos or my TJ in front of me, but once we see what we're looking at, we should know.

Here's some tips on how to easily figure out if you have a vacuum leak, and how to identify where it's located:

1. Start the engine and allow it to idle, spray the cleaner near and around the throttle bore gasket, if the gasket has failed the engine will react to the spray by increasing the engine idle speed as the cleaner is burned. If the engine is idling rough it will temporarily smooth out as the cleaner is consumed.
2. Continue testing by spraying around the intake manifold plenum and base gaskets while observing engine speed, if the gasket has failed the engine speed will increase.
3. Moving on to external components such as EVAP control solenoids, valves and connections while still observing engine speed.
4. A large amount of vacuum is used to work the brake booster and can allow major leaks, spray around the booster while observing engine speed. Also, an audible hissing noise can sometimes be heard coming from around the brake pedal when the brakes are applied which also indicates the booster has failed. An alternative method of a vacuum leak inspection is to pinch a supply line to the accessory and observe engine speed, an increase indicates a leak.
5. Check small vacuum tubes for cracks and broken pieces, these tubes provide vacuum to many accessories such as the heater/air conditioner, leaks under the dashboard near the heater plenum can also occur.

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Hopefully these uploaded. First is where the vacuum line starts, second is the line before the T section, third is the t section where it Goes in to the firewall, next is where found the line (sticking behind the battery connected to nothing pointing down) and I laslty the jeep running, in neutral, rpms dropped to basically 0. My dash is in the process of being taken apart, as I'm hoping I can find part of a severed line in there somewhere.

I feel like this is too many pictures but hopefully it is helpful! Hopefully I can spray the engine tomorrow to see if I can find any other leaks.

Thanks

Jeepaholic said:

There is a vacuum reservoir under the battery. That line you have in your hands needs to be attached to it. That should fix your air vent problem.

However--am I the only one that noticed that tach at 0 rpm and there is oil pressure? This means the jeep is running, possibly the tachometer itself or whatever it gets the reading from is malfunctioning.

Fix that vacuum leak and go from there.

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Great I'll check there tomorrow for the reservoir.

And the tps and iac were replaced with Mopar parts the other day. Oil sending unit replaced a few months ago after the "oil pressure drops to 0 at idle" no mechanic has had any useful input on rpms at zero. Two dealership mechanics and proably 5 local ones.

Thanks guys

Brantley said:

A vacuum leak will cause a high idle, not a low idle. My guess would be one of your sensors causing the low idle, probably the IAC needs cleaning or TPS needs replacement.

Side note: do yourself a favor and quit driving your Jeep in 5th gear. 1500 rpm is doing no good for you whatsoever. Try 4th and 3rd at that speed. Neither of those will be perfect, 3rd will have you somewhere in the 3000's and 4th will have you doing about 2100, which isn't much better power-wise than the 1500 in 5th, but at least it's better on the engine.

This is why 3.07 gears suck so bad and why many people including myself recommend regearing so strongly.

Edit: your tachometer may be off but with 30" tires, 3.07 will still only have you going about 17-1800 rpm in 5th at 65...still not good enough. I can't say I know why your tach is off.

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It's about 2200 or so rpms in 5th gear highway driving. According to most people I've talked to that's good and normal. Especially for 3.07s. I'm not comfortable pushing it to 3000-4000 rpms highway driving. first of all gas milage will be in the toilet. Getting like 17 is bad enough, I can't afford to fill entire tank just on way to work. Second the engine clearly doesn't like being in 4th gear going 70. I do downshift to pass and stuff. Yes I will regear. Honestly not looking for input on how shitty 3.07s are. They suck. I get it. Hence the face palm emoji thing. I still daily drive my jeep 120 miles to and from work and wheel with my dad with his rubicon and I've been able to keep up. Sometimes get stuck but it's fine. I don't live near Moab so it's local OHV parks and anywhere I can find. I do not have $4000 to regear and throw a Dana 44 on, so please no more about the 3.07s haha. Someday I'll regear. I want to fix rpms now.

Well, I installed vacuum reservoir under battery, ac still blowing only out dash but I'm sure there's another leak I can fix soon.

I fixed the rpms, I pulled the needle off and wiped off some of the silicon grease, put it back and it's fine. I'm surprised I didn't try that sooner.

Thanks everyone for your advice. I definelty feel dumb for not trying this sooner, I just figured it couldn't be that simple. :smash2: Rpms hang at a consistent 600-800 range, which seems normal. :thumbup:

Problem solved. Now to fix the AC leaks.

Jerry Bransford said:

The rpms that work best vary by the tire size, the bigger the tire the higher the rpms need to be. WIth 35's, I'm real happy at 2650-2700 on the highway. I'd like to regear my axles to 5.38 (the lowest ratio I could go and still keep my present Rubicon lockers) given my 42RLE transmission to get closer to those RPMs like I used to see on my previous TJ.

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Thanks for the input Jerry, I always look for your response for parts or anything TJ related. I know you get this a lot but you really are the expert!

I know this is beating a dead horse, but I wasn't able to find the 3.07 rpms threads I thought I had boookmarked.

So, I plan to regear next summer hopefully (4.11s with 32s?), but with the 3.07s, where is my power band with 30s? Is there a chart for this I can save? I stayed in 4th and have stopped using 5th entirely after reading a lot of this on this thread.

I'm getting around 15mpg, was getting closer to 17 or 18 in 5th, which is worse than my dads JKU rubicon (which seems to get 17). Getting about 2600 rpms in 4th gear going 60. And I did see your post about "worrying about gas milage in a jeep is like worrying about your sisters virginity," which was funny and I get your point, however I would like to get better than 15, especially being a college student, money is tight.

I don't have one of those superchips (want one but feel it's a waste of money for a TJ) so I don't know exactly how accurate my rpms are, will use the OBD scanner when I stop at my parents before heading home, to check idle.

First manual so I do apologize for any of this being obvious common knowledge. Being a gunsmith student, I know guns and cars are somewhat of a foreign language, but I prefer to do all I can myself. Learning more every day about TJs though.

And sorry to steer the post topic for one question, I went to get tires aligned the other day. They are unable to do it and say I need adjustable lower control arms and a left upper adjustable Ball joint for 1 degree. I hydroplaned and hit a curd hard and the jeep has pulled since, and I'm getting pretty bad shaking. Most people on forums say that means my axel is bent, so
Of course I asked the mechanic if it was bent and he said nope not at all, and both are fine.

Should I try replacing the left Dana 30 axel shaft (where the adjustable ball joint is needed), or so what the mechanic says? I read "adjustable ball
Joints should never be needed unless the axel is bent," cannot remember who said it.

Thanks everyone!