What is the Proper Idle RPM for the 4.0?

CrazyIvan888 · 02-27-2010, 01:27 AM

So, I'm sitting at a stop light, in gear, foot on the brake. The whole Jeep has a slight shudder, which is caused by engine vibration and of course it goes away when I accelerate.
This has been going on since I fixed an exhaust leak which was downstream of the muffler, if that has anything to do with it.

The question is, is my engine idling at too low of an RPM? If so, what would I replace or repair to fix it?

The harmonic balancer is new as of last Summer when I replaced the front main seal.

Thanks,
CI888

**ZJJay** · 02-27-2010, 02:44 AM

My '96 4.0 idles at about 700 RPM.

If it were mine, I'd first try erasing the memory in the PCM. Here's a link on how to do it:

http://www.jeepforum.com/forum/f13/question-about-how-reset-pcm-948495/

I was getting some error codes on mine, even after replacing some components. So I erased the PCM memory and the codes have not come back (yet), and the engine is running better. The capacitors in the PCMs get old, and the PCM starts to act up. At some point in time I'll probably have to get a rebuilt PCM.

An exhaust leak downstream of the muffler should have no effect.

Good luck.

Jay

befumo · 02-27-2010, 03:25 AM

Same idles at about 700. Mines a '95.

**comptiger5000** · 02-27-2010, 07:56 AM

Mom's 99 4.0 idles smooth as could be right on 600. I don't remember for sure, but I think her 95 4.0 idled at either 650 or 700.

melk · 02-27-2010, 12:33 PM

Have you cleaned the throttle body and IAC?

**jtsjeep99** · 02-27-2010, 02:44 PM

Around 700 rmp's for me also.

CrazyIvan888 · 02-27-2010, 10:11 PM

Ok, so I flashed the PCM like ZJJay said, that seemed to improve my fuel mileage a bit (sweet!) but it's still vibrating when stopped in gear. The tach needle sits just above the first slim line on the gauge, so I think it's a little low. The vibes go away if I touch the gas and bring the RPMs up a hair.

I'll do a search on the IAC cleaning, anything else that could be causing this?

CI888

**ZJJay** · 02-27-2010, 10:52 PM

Maybe the throttle position sensor, but I don't know of anything you can do with it but replace it. Clean the IAC first, because that is free. Here's how I did it: I used brake cleaner, but throttle body cleaner probably would be the better product. (I use brake cleaner for a lot of things besides brakes.) Rermove the IAC motor (plunger, valve), paying attention to the O-ring. Clean the plunger, then look to see if a groove is worn into it. If there is, the groove might be letting in an unmetered amount of air.

Then clean the housing that holds the IAC. Mine was really dirty and I didn't want to spray cleaner into it, only to have all of that gunk flow into somewhere else in the throttle body. Use a small mirror to look inside the housing and check your progress. I dipped a small rag (like the sleeve of an old T-shirt) into paint thinner, wrapped it over my finger, then wiped the inside of the housing. This took many tries. Then I used cotton swabs dipped in paint thinner to get the rest of the gunk.

Now you could spray the inside of the housing, just as a final rinse. Then clean the bore and butterfly of the throttle body.

PLease let us know how it turns out, and any change in performance.

Jay

Bowlerguy92 · 02-28-2010, 12:09 AM

Mine sits right on 600.

CrazyIvan888 · 02-28-2010, 01:22 PM

Before cleaning:

After cleaning:

I forgot to snap one of the clean butterfly.

I also re-reflashed the PCM to give it a fresh start on it's "learning". I'll be driving around quite a bit today, will update you guys tonight or tomorrow! Thanks ZJJay for the pocket write-up!
Thanks everyone else for the info!

CI888

CrazyIvan888 · 02-28-2010, 10:40 PM

Ok, so she still has the vibration. Here's where she sits at idle, stopped in gear:

Is this where the needle is supposed to be?

The vibes aren't really bad, I'm starting to wonder if I'm being paranoid.

Maverick_jco3 · 02-28-2010, 10:52 PM

My 98 was idiling @ 700 but when i seafoamed it on sat, rpms dropped about 200 rpm @ idle. Runnin smooth tho.

**comptiger5000** · 03-01-2010, 05:26 AM

That looks a bit low to me. It should be about 1/2 way between the marks, maybe a little more. It looks like it's running 450 - 500 now.

**ZJJay** · 03-01-2010, 05:33 AM

CI888 - That looks lower than mine, plus you say it's still idling rough. Was the shiny area on the IAC plunger smooth or an indentation?

If it were mine, I'd replace the throttle position sensor, about $60 and an easy install. I replaced mine recently. But I'm not much of a diagnostician. Other readers might have a better suggestion.

And no, you are not paranoid. Something new is going on...

Jay

**ZJJay** · 03-01-2010, 06:19 AM

CI888, I just thought of something else. Check for a vacuum leak. First do a visual, then check out this link: Engine Vacuum Leaks

I'm in the process of installing driving lights and will be mounting the relay on the fender wall next to the battery. I'll have to remove the battery to do this. While the battery is out I'm going to replace the vacuum line that goes from the vacuum reservoir (which is under the battery) to the hard plastic line at the fender. You might want to replace this hose if it hasn't been replaced already. But I'd guess if you have a leak it would probably be one of the rubber elbows near the intake manifold.

Your MAP sensor is mounted on the firewall, and it also has a vacuum line going to it. But if there was a leak in this line, I'd expect you to get a CEL for the MAP sensor not working properly.

Let us know what you find out.

Jay