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-   -   High Engine RPM on Startup (http://www.jeepgarage.org/f196/high-engine-rpm-on-startup-70325.html)

dons2346 01-13-2014 12:10 AM

High Engine RPM on Startup
 
On startup, the RPM will climb up to 2,000 and then idle back down to around 6oo. It doesn't matter if the engine is hot or cold. Engine is the 4.7 in a '99

Could this be a function of the throttle position sensor and if not, what could cause this? Thanks

Frango100 01-13-2014 09:22 AM

Re: High Engine RPM on Startup
 
This could have to do with the TPS, but also with the idle air control (IAC) stepper motor. A dirty air passage from the IAC can give idle problems.
Remove the IAC stepper motor and clean the plunjer and air passage very well.
You will have to remove the resonator box for this, because the passage starts at the front of the throttle body. You could run some throttle body cleaner through the air passage with the engine running.
If you have a scanner which shows live data, you could see the TPS sensor % and check if there is any change to the figure while starting.
If you donīt have such scanner, but you do have an Android phone, then you could download the free app "Torque" and just buy a cheap bluetooth dongle and there you go. Besides many live data figures, it will also show you fault codes from the PCM and give you the possibility to errase the PCM fault data. Very handy and cheap scanner (at least if you have the Android phone already).

dons2346 01-14-2014 12:06 AM

Re: High Engine RPM on Startup
 
Thanks for the info Frank. I do have a Droid phone and tablet so I will get the app.

Now, one more question, where will I find the usb port to plug the dongle into?

Don

Frango100 01-14-2014 01:42 AM

Re: High Engine RPM on Startup
 
Hi Don. The dongle is not an USB device, but it just fits into the Scanner connector left under the steering wheel. I bought mine directly from China at www.dealextreme.com. I think i payed around 20 USD including shipping.
Quite a nice site with a lot of other things as well. You can maybe even buy it locally, don't know about that.
Here the link to the site, eventhough its not the exact same dongle as i have: http://dx.com/p/super-mini-elm327-bl...ol-blue-232018

dons2346 01-14-2014 11:37 PM

Re: High Engine RPM on Startup
 
Ok, I got it now. Looked at the site and there are quite a few different dongles to sort through. I did look at the Droid apps and there are a few of those also.

Thanks for your help.

Don

Frango100 01-15-2014 06:07 AM

Re: High Engine RPM on Startup
 
I see that they now also sell wifi dongles besides the bluetooth, but they are more or less twice the price.
There are indeed several apps available, but " Torque" is at least a very good one.

intlaero 09-15-2014 10:24 PM

Re: High Engine RPM on Startup
 
Quote:

Originally Posted by dons2346 (Post 958935)
On startup, the RPM will climb up to 2,000 and then idle back down to around 6oo. It doesn't matter if the engine is hot or cold. Engine is the 4.7 in a '99

Could this be a function of the throttle position sensor and if not, what could cause this? Thanks


Recently started to have this problem also. 2004 4.7 HO. Fast idle on cold start up and takes 5-10 minutes to go back to normal. Outside temps are 100F so engine is not really cold.

Frango100 09-15-2014 10:27 PM

Re: High Engine RPM on Startup
 
First clean the IAC stepper motor plunjer and air canal and check again. If still high on idle, then its probably the TPS.

intlaero 09-15-2014 10:45 PM

Re: High Engine RPM on Startup
 
Thanks, I've recently changed to a K & N cold air intake so maybe the dust and dirt got into the idle air control. I would hope not. Desert conditions here.

I do have an apple iPad with a wifi scanner. It gives real time data on the engine. App is OBD Fusion. Scanner is ELM 327 from Amazon. Make sure you get the wifi version to work with iPad.

Frango100 09-16-2014 06:38 PM

Re: High Engine RPM on Startup
 
The IAC plunjer and air canal normally gets dirty with oil from the head mounted breather lines. I would not expect the K&N filter have to do with it, or it would be filtering very badly. There normally forms quite some oily residue on the IAC plunjer , which interferes with the fine regulation of the idle speed.

intlaero 09-20-2014 04:43 PM

Re: High Engine RPM on Startup
 
1 Attachment(s)
Spent some time examining the air intake throttle body and found a cracked and broken vacuum hose. It is the one from the throttle body to a check valve to a "U" shaped hose coming off the rear of the driver side. I replaced the hose for $1.39 and engine works perfectly again. I checked all of the rest of the vacuum hoses and they are all good. Only this small 2" piece had perished, cracked, and turned brittle and hard. Photo of the "U" bend hose is attached.

2004 4.7HO

Frango100 09-20-2014 06:02 PM

Re: High Engine RPM on Startup
 
Did you have problems with the cruise control? The hose is going to the vacuum reservoir, which is used for the cruise control.


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