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High Engine RPM on Startup

19K views 20 replies 4 participants last post by  Frango100 
#1 ·
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
 
#2 · (Edited)
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 don´t 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).
 
#4 · (Edited)
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-bluetooth-odb2-v1-5-car-diagnostic-interface-tool-blue-232018
 
#6 ·
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.
 
#7 ·
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.
 
#8 ·
First clean the IAC stepper motor plunjer and air canal and check again. If still high on idle, then its probably the TPS.
 
#9 ·
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.
 
#10 ·
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.
 
#11 ·
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
 

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#12 ·
Did you have problems with the cruise control? The hose is going to the vacuum reservoir, which is used for the cruise control.
 
#13 ·
I wrote a bit too early. Today on start up, high idle RPM. I took off the IAC stepper motor and it was very dirty, looked like carbon build up. Cleaned it up, started engine and sprayed cleaner in the throttle body and replaced. Engine ran very poorly for about 10 minutes, would not idle at all, then smoothed out and ran like normal. We'll see how it starts tomorrow when cold. I did have to reset the trouble code for IAC. To be expected since it ran with it off and disconnected.

Thanks for the reply Frank. I did not have problems with Cruise Control, worked perfectly. I appreciate your responses. I'll keep working on this until I get it right with your help and others.
 
#15 ·
I cleaned the IAC and the air channel very well. It improved the fast idle but it is still very erratic. Starts at 1500 rpm and then never really settles down. I see 600 to 1100 rpm just sitting at idle in neutral after driving for 30 minutes. Suggest TPS or IAC replacement?
 
#18 · (Edited)
Erratic idle like you have is the TPS. A faulty IAC would give a constant idle (high or low) and a fault code for the IAC.
Erratic is normally the TPS. If you have a scanner which shows live data, you can see the TPS % going up and down a bit. Its normally around 12 % at idle.
 
#19 ·
Thanks, no fault codes. I took the TPS off but no obvious faults. Desert weather here with temps well into the 100's during summer so plastic and rubber parts just cook. Jeep is over 11 years old with only 78k miles. I'll get a replacement and see if that fixes the issue.
 
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