Extremely high idle after throttle body swap

[JimmyB]

OK, I'm a relative noob to the TJ, but I have a fairly good background in general automotive mechanics, so I'm not a complete moron...just a little incomplete
I'll start by apologizing for the length of the post, but in order to give you guys a full background of what's going on, it's necessary. 2003 TJ Rubicon with Auto Trans that I've only had for about a year now. 96K miles when I got it, about 104K now. Running just fine other than an occasional Check Engine light that was always intermittent and was caused by the TPS sensor voltage reading below acceptable range. This only happened very rarely and ALWAYS when I was idling with my foot on the break waiting to pull out into traffic. Since it's time to renew my registration, I figured that I'd simply adjust the idle a slight bit higher, reset the CEL, and move on down the road. Problem #1. Idle adjustment is from the underside of the throttle body. In the process of trying to remove set screw and replace from the top (where any sane engineer would have placed it), the tiny little set screw was cross threaded and refused to go back in. Phone call to 4 Wheel Parts and I have the upgraded BBK throttle body on the way. Upon receipt of said TB, I go to remove the remaining factory sensors from the stock TB and the TPS refuses to budge (previous owner/mechanic must have used loc-tite or something). Another trip to auto parts store and hardware store for new TPS and some machine screws to mount it. Finally get everything back together, start the TJ and the engine is basically bouncing off the rev limiter! WTF! I didn't have a high idle problem before, so what gives? I've had this thing in the warehouse at my office for nearly a week so any advise or ideas would be much appreciated.

JB

 

[CodaMan]

Sounds like a vacuum leak to me. Make sure a hose didn't fall off or that you are leaking under the throttle body.

 

[JimmyB]

Thanks. I'll check that. Another thought is that I may have just gotten a bad TPS. I'm going to give the original another shot at removing it from stock TB.

 

[CodaMan]

...Another thought is that I may have just gotten a bad TPS. I'm going to give the original another shot at removing it from stock TB.

Good idea

 

[Jerry Bransford]

High rpms are caused by a vacuum leak. Focus on fixing that and you'll fix the rpm. problem. I seriously doubt the TPS is causing the problem unless it wasn't installed properly and its leaking air into the throttle body.

There is no idle adjustment anywhere on the engine or throttle body. The idle is controlled strictly by the computer. The idle can be affected by a dirty Idle Air Control module or, less likely, the TPS, but make no mistake that the idle rpms are strictly under control by the computer.

The days of setting the idle rpms by a screw or linkage adjustment like on a carburetor are long over.

Perhaps the gasket between the intake manifold and throttle body is leaking. Make sure not to overtighten any of the four bolts that hold the throttle body to the intake manifold. The intake manifold bolt threads are easily stripped.

 

[Chris]

X2 to what Jerry said.

It's like either a vacuum leak (or maybe a vacuum hose you forgot to plug back in), the throttle body gasket, or maybe even a dirty IAC.

 

[tman66]

Thanks. I'll check that. Another thought is that I may have just gotten a bad TPS. I'm going to give the original another shot at removing it from stock TB.

Jimmy, what brand of TPS did you pick up at auto parts store? I was having high idle issues with a cheap one from the internet. Switched to Mopar and all is fine.

 

[JimmyB]

I got a BDS brand at O'Rielly (I think that's the brand), it was the only one in stock. And it turned out to be the TPS after all. Once I got the old one off the original TB and on to the new TB, it ran fine (for about 45 minutes). I checked all of the hoses and everything else for a vacuum leak and couldn't find anything. I literally didn't unhook any hoses (accidentally or on purpose). Switched the TPS and all of a sudden it was idling at 650 or so. Put away several tools and locked the warehouse door, drove around to the front of the office then went inside to shut off all of the remaining lights etc. Came back out and drove something like 50 feet and then it doesn't want to hold idle any longer. Get to the stop sign and it's fighting me to keep going. Made a few right turns until I circled the block and parked it in front of the office for the night(where it wants to idle around 4000 rpm again).
This morning, still at 4K rpm idle, I call AAA to tow it to the stealership because at this point, I'm so frustrated with it that I don't want to look at it. At the dealer, it starts at 4k idle but soon drops to about 1200 rpm with the service advisor standing there. Maybe the outside air temp has something to do with this, IDK. It's their problem now. I just have to pay for it.

 

[CodaMan]

That's too bad

 

[JimmyB]

High rpms are caused by a vacuum leak. Focus on fixing that and you'll fix the rpm. problem. I seriously doubt the TPS is causing the problem unless it wasn't installed properly and its leaking air into the throttle body.

There is no idle adjustment anywhere on the engine or throttle body. The idle is controlled strictly by the computer. The idle can be affected by a dirty Idle Air Control module or, less likely, the TPS, but make no mistake that the idle rpms are strictly under control by the computer.

The days of setting the idle rpms by a screw or linkage adjustment like on a carburetor are long over.

Perhaps the gasket between the intake manifold and throttle body is leaking. Make sure not to overtighten any of the four bolts that hold the throttle body to the intake manifold. The intake manifold bolt threads are easily stripped.

Jerry, there is an idle adjustment set screw on the throttle body. The problem is that in order to drum up more service dollars, Jeep installed the set screw with the hex head on the underside of the throttle body so you have to remove the throttle body from the intake manifold in order to adjust the idle set screw. Again, back to my original post, that is where I ran into the problem because that set screw didn't go in completely straight and ended up getting cross threaded. On the new BBK throttle body the set screw came installed the same way but I removed it and under better lighting conditions and with no dirt/grime on the new part, I was able to install the set screw from the TOP. Now I have the ability to adjust my idle by simply rotating the throttle lever out of the way and inserting the allen wrench (it helps to shut the engine off since you pretty much have to twist the throttle lever to full throttle in order to access the set screw). I realize that on a computer controlled vehicle that other factors such as those you mentioned also have an effect on the idle, but there is indeed a set screw in place on the throttle body.

 

[Jerry Bransford]

Jerry, there is an idle adjustment set screw on the throttle body.

I realize that, I knew that when I replied. But that screw is not there to be used after it has been initially set by the factory. When the engine is running properly, the engine computer sets the idle rpms as they should be. When something is wrong and causing the rpms to be incorrect, like a vacuum leak, bad TPS (throttle position sensor), IAC (idle air control), etc., you fix the specific problem causing the issue the engine computer isn't able to compensate for.

Like you don't apply a band-aid to fix a skin cancer, you don't use that set screw to try to correct the idle rpms when something else is making it impossible for the engine computer to set it. I grew up setting engine idle rpms on carbureted engines back in the 60's and 70's by adjusting the linkage and setting the carbs up, so it was odd when I figured out we don't set the engine idle like that once we got computerized engines, but that's how it is now and that's how it is on our TJs. Not even the engine timing is adjustable. That too is strictly controlled by the engine computer.

 

[CodaMan]

...Not even the engine timing is adjustable. That too is strictly controlled by the engine computer.

So the Sun timing light I used on my 68 GTO, 68 Z28, 70 Chevelle SS and 65 Impala SS is obsolete? BTW - If I still had those 4 vehicles I would have retired years ago

 

[JimmyB]

I realize that, I knew that when I replied. But that screw is not there to be used after it has been initially set by the factory. When the engine is running properly, the engine computer sets the idle rpms as they should be. When something is wrong and causing the rpms to be incorrect, like a vacuum leak, bad TPS (throttle position sensor), IAC (idle air control), etc., you fix the specific problem causing the issue the engine computer isn't able to compensate for.

Like you don't apply a band-aid to fix a skin cancer, you don't use that set screw to try to correct the idle rpms when something else is making it impossible for the engine computer to set it. I grew up setting engine idle rpms on carbureted engines back in the 60's and 70's by adjusting the linkage and setting the carbs up, so it was odd when I figured out we don't set the engine idle like that once we got computerized engines, but that's how it is now and that's how it is on our TJs. Not even the engine timing is adjustable. That too is strictly controlled by the engine computer.

Yeah. My idle wasn't off by enough to cause it to run poorly, it was merely causing an very intermittent CEL that was tripping a "TPS voltage below range" code. BY adjusting the idle a hundred rpm or so, I was hopefully going to eliminate that CEL code from coming back. That's what led to all of the trouble to begin with, that screw becoming cross-threaded...leading to the replacement TB and new, but faulty TPS.

 

[Jerry Bransford]

So the Sun timing light I used on my 68 GTO, 68 Z28, 70 Chevelle SS and 65 Impala SS is obsolete?

So far as TJs and JKs and pretty much all modern mainstream vehicles go, yep. Even if you were to rotate the distributor cap on an older TJ 4.0L engine, it wouldn't change the timing... the engine computer will keep the timing set to where it wants it to be.

 

[Jerry Bransford]

Dang computers taking over the world. We all know where that finally leads to.

Ha yep I agree but I for one don't miss these frequent needs at all that I used to have to perform regularly.... flat tires being fairly common, changing plugs every 5000 miles, changing the points, setting the dwell angle, setting the timing, carburetor rebuilds, adjusting the fuel mixture on the carburetor at sea level then the fuel mixture no longer being correct when I get to 6,000-8,000' in the mountains, etc.. I haven't had to worry about any of those on my Jeeps or even my cars in many years... and that's a good thing in my book.

My opinion is that the "good old days" weren't always so good in all respects.

 

[StG58]

Had a Z28 with two four barrels on it and a Mallory dual point distributor. Believe me when I say I don't miss tuning that thing up every couple of weeks.

 

[CodaMan]

Had a Z28 with two four barrels on it and a Mallory dual point distributor. Believe me when I say I don't miss tuning that thing up every couple of weeks.

Dual points were soooooooooo much fun

 

[WWII Jeeper]

Check the 90 degree vacuum elbow on the intake, just under the throttle body. My idle issues were due to that part being cracked and split, old age, crap rubber, and motor heat. Runs great now.