Strange Idle Dip and High RPM's Even After Letting go of the Throttle - HELP!

[webby88]

So I have an '02 LS v6 with 73,xxx. I recently completed a top end overhaul with new:

• Valve Cover Gaskets
• Lower and Upper Intake Gaskets
• IMT Gasket
• All New PCV Components
• New Belt and Idler
• New IAC Valve
• New TPS
• New MAF Sensor
• New Plugs
• One New Coil

At first when I started the car after completing all of the above it was surging like crazy. At first I thought it needed to relearn all the parameters but it never settled down and then it threw a CEL with a MAF code.

I pulled out the new sensor and put the old one back in and the surging went away. I later discovered that the new MAF was damaged, one of the 'wire thingys' was torn.

What originally promoted the overhaul was a severe misfire due to a bad coil which ultimately lead to the car not starting. After determining that the fuel pumps and filter was fine, I found a suggestion on this forum which said to unplug the MAF and try starting it, and once unplugged it started up and idled like crap - which is why I bought a new MAF in the first place.

Anyways, now the car idles nice and drives fine only two issues - one of which can be serious. When idling, if I hit the throttle once the engine backs down it falls to lower idles - like 550-500rpm's - then recuperates and goes back to 750ish.

The other more serious issue is at highway speeds, when I back off the throttle it takes an extra few seconds to drop in RPM's and is still pulling even when I brake. Feels like a sensor or valve or something is staying stuck open or taking longer to retun to normal.

Any ideas where to start looking?

 

[1LoudLS]

throttle linkage...


you have old school throttle linkage from the pedal to the throttle body.

if you let off the pedal and it is still accelerating, then either the pedal is sticking or the throttle body is still staying open, I would think that even if a sensor was bad like the TPS and the computer was still throwing fuel into the motor, it wouldn't be accelerating because the throttle blade would be shut and not letting enough air into the motor for that fuel to burn.

 

[webby88]

throttle linkage...


you have old school throttle linkage from the pedal to the throttle body.

if you let off the pedal and it is still accelerating, then either the pedal is sticking or the throttle body is still staying open, I would think that even if a sensor was bad like the TPS and the computer was still throwing fuel into the motor, it wouldn't be accelerating because the throttle blade would be shut and not letting enough air into the motor for that fuel to burn.

That was the first thing I checked. When I let go of the pedal it immediately lets go of the TB.

The only thing I did was take off the TB and cleaned it up as well as changed its gasket, did do anything else besides the IAC.

 

[webby88]

Here are two video I took of my tachometer illustrating the problem:

[video=youtube;725Pbf86VaE]https://www.youtube.com/watch?v=725Pbf86VaE[/video]

[video=youtube;IthI1C9YNh4]https://www.youtube.com/watch?v=IthI1C9YNh4[/video]

Also, I noticed that there is a small adjustment screw on the throttle body. Is that something that I can/should try adjusting?

 

[LS4me]

The RPM will "hang" a little and not drop immediately to idle. That's normal.

 

[1LoudLS]

well since I have no idea what the gas pedal is having done to it at any part of those videos, I would say that it looks normal to me...

and no, I wouldn't mess with any adjustment screws unless you know what you are doing with them.

 

[webby88]

The RPM will "hang" a little and not drop immediately to idle. That's normal.

well since I have no idea what the gas pedal is having done to it at any part of those videos, I would say that it looks normal to me...

and no, I wouldn't mess with any adjustment screws unless you know what you are doing with them.

Allow me to clarify. In both videos I was braking pretty hard and completely off the throttle. When braking I was straining against a revving engine... as if I was still on the throttle.

If I let go of the throttle and idle at 20mph the car keeps revving and gaining speed.

The second video is more pronounced, I am going 3-4mph and holding the brake. Until I reach 1mph~ the engine does not let go and is still revving/accelerating.

I checked the throttle linkage and cable, they are both good and nothing is interfering or blocking. When I let go of the pedal it immediately lets go and the TB closes (as best I can tell).

I am beginning to suspect that the IAC may be behind all this, but I cannot be sure which is why I am looking for some more input. Thanks for all the help guys!

 

[joegr]

...I am beginning to suspect that the IAC may be behind all this, but I cannot be sure which is why I am looking for some more input. Thanks for all the help guys!

Okay, then here's one way to rule it out. Get the engine fully warmed up. Disconnect the electrical connector at the IAC. Hold the gas pedal down a little bit while starting it. You may have to stay on the pedal a little to keep it running. Carefully, in an isolated area, repeat your tests. Be aware that the engine may stall. If the unintended acceleration is gone, then maybe it is the IAC.

 

[Blueberryyum02]

Clean or replace your idle air valve. Mines was doing the same clean it and it stopped.

 

[LS4me]

Allow me to clarify. In both videos I was braking pretty hard and completely off the throttle. When braking I was straining against a revving engine... as if I was still on the throttle.

If I let go of the throttle and idle at 20mph the car keeps revving and gaining speed.

The second video is more pronounced, I am going 3-4mph and holding the brake. Until I reach 1mph~ the engine does not let go and is still revving/accelerating.

I checked the throttle linkage and cable, they are both good and nothing is interfering or blocking. When I let go of the pedal it immediately lets go and the TB closes (as best I can tell).

I am beginning to suspect that the IAC may be behind all this, but I cannot be sure which is why I am looking for some more input. Thanks for all the help guys!

Are you sure you don't really have a Toyota?????

 

[RigsLS]

... there is a small adjustment screw on the throttle body. Is that something that I can/should try adjusting?

DO NOT TOUCH !!!

Let the PCM make the decision, do NOT override idle control by means of forwarding that screw!

NOT A GOOD IDEA !!!

 

[joegr]

DO NOT TOUCH !!!

Let the PCM make the decision, do NOT override idle control by means of forwarding that screw!

NOT A GOOD IDEA !!!

Yes, the service manual does warn not to change the throttle hard-stop position adjustment. Certainly, it wouldn't help with his problem.

 

[webby88]

Okay, then here's one way to rule it out. Get the engine fully warmed up. Disconnect the electrical connector at the IAC. Hold the gas pedal down a little bit while starting it. You may have to stay on the pedal a little to keep it running. Carefully, in an isolated area, repeat your tests. Be aware that the engine may stall. If the unintended acceleration is gone, then maybe it is the IAC.

Great suggestion. I did it and positivly found the IAC to be the culprit! I already ordered a new motocraft branded one!

Are you sure you don't really have a Toyota?????

Why would you ask that?

 

[1LoudLS]

Why would you ask that?

because of that little thing where Toyotas couldn't be stopped, and they recalled a few of them...

 

[LS4me]

Great suggestion. I did it and positivly found the IAC to be the culprit! I already ordered a new motocraft branded one!


Why would you ask that?

because of that little thing where Toyotas couldn't be stopped, and they recalled a few of them...

Yea.... There was a little issue with Toyota's and unintended acceleration. A family here was killed when they couldn't stop their Lexus. It was a bit of a joke and poking fun at Toyota.