tmgrant

I've done quite a bit of modifications over the past year to my 96 Cherokee 4.0L that could impact this. So background info first:

New Exhaust (Down pipe, cat, muffler, exhaust pipe, [headers in a couple weeks])
Neon Fuel Injectors (better spray pattern)
Cold Air intake
New Fuel pump and filter

Running 31" tires ~4.5" of lift, and stock 3.55 gearing (AW4 Trans)

Today I just added a 63mm bored throttle body, took it for a drive and noticed the engine is a lot quieter in general and the RPMs are down by about 300-500 rpm. I noticed it gradually dropped from the other mods, but nothing like it did with the bored throttle body. Could this just be that the fuel/air mixture has finally been able to catch up and equalize?

I'm not saying any of this is a bad thing, it's just surprising to me. Parked and running I am showing just barely above 0 RPMs. The engine doesn't sound like it's struggling and RPMs are stable at idle. On the interstate going 75 I'm slightly over 2k. In town I'm a little under 1.5k. The engine also seems noticeably quieter than before, oil pressure is lower both at idle and driving. Throttle response is still quick and responsive (got that from the fuel injectors).

Thoughts?

Rockhead

With 31 inch tires and 3:55 gears, according to the charts, your rpm at 65mph (I think the charts are based on 65mph) should be 2500. Maybe your tachometer is going bad. I have not done any of those mods to my engine but have not read anything on here about rpm's dropping with the larger throttle body.

edit: http://www.jeep4x4center.com/jeep-ti...atio-chart.htm

Yup, 65mph highway speed.

XJmember

Pretty sure your tac is shot, you can take the cluster out and then the tac itself and find a small adjustment screw and try to make more accurate for the time being. Also that rpm chart doesn't seem very accurate either.

None of those "upgrades" will cause lower oil pressure or lower idle rpms .

Rockhead

Hey XJmember. If you know of a chart that is more accurate please pass it on, most of CF members would appreciate it.

Crazy 8s

Perhaps your old TB had some hidden issues and the remanned TB you just put in just works as it should?

tmgrant

XJMember, I was wondering the same thing. I just hooked up a good quality wireless OBDII connector that can log all of the information from the computer. The tach isn't off by much according to it, only about 100-150 rpms. My idle when the engine is warmed up is 700-750rpms. Starting from cold it's about 800-900.

I need to double check again at 65mph, but pretty sure it was about 1.6k 2k rpm, at 80mph it was 2.4k.

As for the oil, it still stays about the same spectrum the range has just shifted to the lower end a bit. Driving it doesn't get as high as what it did before, at idle it goes a little bit lower than it did before.

Everything seems to be reading properly... As I mentioned before, this was unexpected to me and was wondering if anyone could explain what would cause this. So the only question now is could there be an underlying problem or could it lead to future problems? Another thought I had was could this be the PCM trying to figure out what the hell is going on? I reset the PCM after the fuel injector swap which was maybe a month ago.

XJmember

I personally dont know of a chart that is other than the grimm jeeper gear calculator. Only downside is you have to plug in all the info to get the rpm. I believe the chart you posted would be more accurate for a manual transmission.

I have had a faulty tac before, it would go from being accurate then to showing almost zero at idle. As for the oil pressure, none of those mods would affect it. Id go find another tac and swap it in and see if that makes a difference.

tmgrant

So the readings from the OBDII are off by about 100rpm. Doesn't look like I can get oil pressure readings from it. But I have a log that is pretty consistent with what I have been seeing on the tach. Not sure what is causing it.

extrashaky

700 RPM is about normal for hot idle. Cold idle looks all right also. Looking back at your original post:

If your hot idle was 300-500 higher than where it is now, it sounds like you may have been running lean and fixed the issue when you replaced the throttle body. It should not idle at 1000-1200 RPM when hot. It sounds as though you were running wrong for so long that you became alarmed when it actually started running the way it should!

Also, when you start talking about RPM at cruising speed, you're introducing new variables. None of us can know if your RPM at 65MPH are correct, because we don't know if your speedometer is reading the correct speed. You have changed tire diameter from stock. Are you certain you have corrected for that appropriately?

Something you might consider is to install an engine monitoring app on your phone (Torque if you have Android), set up a virtual dashboard with all the regular gauges and monitor them for a while so that you can see how close your OEM gauges are tracking. That will allow you to not only track the accuracy of your tach, but you can also get a good idea of the accuracy of your speedometer against GPS speed. Then you'll get accustomed to where the gauges normally ride without some of the guesswork. You can also use that to log your engine activity so that you can go back and look if you have any question.

I have a mount for my phone in the dead space just above the dash to the left of the steering wheel where I can easily glance over and monitor the computer's output without it obstructing my view. Eventually I'll end up running a cable behind the dash to power it.

yarom

Did you rest the computer? First thing to do when modifying fuel/air mixture..

tmgrant

Great feedback and you make a great point, the old throttle body was stock and I know it was beat on pretty hard. It looked pretty worn out and was pretty dirty. I have an OBDLink bluetooth connector that I used to run Torque. I did change out my speedo gear and torque confirms that the accuracy is pretty spot on. And at 65mph I am roughly at 2k rpms. The one thing that I have noticed is that the gear/throttle ratio charts typically reference an AX5 (or being in 4th gear with a ratio of 1:1) and not the AW4 (which 4th gear would be a ratio of 0.75). To get a better idea on the ratio, should I run in 3rd and see what my RPMs are at 60 (giving me the 1:1 ratio the charts use)?