throttle being held abnormally high - strange...

[MattM]

in town or highway speeds, if I take my foot off of the throttle I would expect the car to slow down at a consistent rate, downshift at some tapered rate, and if I had enough time/room the car would eventually downshift until it hits the creep-speed, maybe a few mph.

however with some frequency (not with every outing) I'm finding that the car wants to continue at an elevated rpm after I take my foot off of the accellerator. this is especially pronounced if I have the a/c running.

for instance, If I'm driving in-town at 45 mph and take my foot off of the accellerator, the car will frequently continue to run strongly around 40-43. when this happens, it seems like I have to force the car to downshift by braking. I can actually get to a stoplight and feel the car finally downshift 2 gears after I've stopped - thump thump. not good, whatever the cause is.

there are other times where the car does not "misbehave" and the car slows when I take my foot off the pedal, etc. Might be my imagination, but it seems that when the car is driven in-town all day and is hot, the symptoms are more prevalent.

so like i said, it's strange.

I've been reading the forums and wondering if this may simply be a IAC or EGR component misbehaving.

other considerations... the transmission seems to perform fine except for the disagreement he may be having with the throttle. no slipping, grinding or whining.

the car has 100k miles on it. transmission fluid was flushed at 65 I think.

thoughts?

 

[BattelWagun]

Your EGR wouldn't cause these problems, it vents exhaust gasses into your intake at cruising speeds and won't affect your idle/throttle. Only thing its known to cause is rough idling and issues of that sort. Is your idle higher/lower than normal? I've never seen this issue before, but if your truck isn't acting up while in park, or just sitting at a stop light it most likely isn't that. Does your gas pedal seem like it's sticking? And also do you have any trouble codes either?

 

[gpracer1]

When it happens while driving, put it in neutral and see if the rpms shoot up. I would guess there may be a problem with the IAC motor on the throttle body. I would take it off and clean it real good with carb cleaner, the throttle body also.

 

[MattM]

idle is smooth and sits around 600 rpm. really nice actually.

accellerator pedal seems to be smooth with no hanging.

with my limited background, I was presuming something on the intake manifold was opening up to increase airflow and rpm, and then not closing-off.

man, when the ac compressor kicks in, I really have to fight the brake to slow her down sometimes.

oh, and no check engine light. I haven't put a reader on it to see if it is throwing a code.

I guess one more superstition/symptom/clue...
this behavior came with the car at 70k. pretty consistently showing up as I described above.
Then I replaced the intake manifold gaskets a few months back. stopped the coolant leak etc.
afterwards, this whole symptom disappeared for a couple of months. I thought I must have reseated something because the car was driving like a dream. then this problem began to reappear.
I'm feeling it's on the intake side of life rather than a transmission problem or freaky ecu thing.
grrrr....

 

[retorq]

When you put the intake back on perhaps you bound up the throttle cable on something ... when I did my top end swap the brake booster hose was interfering with it, I had to reroute it to get them to place nicely.

 

[Phone Man]

I have had this exact same problem along with surging idle. I did so many changes to correct the problem, i can suggest a few. First take out the IAC and see if it works by leaving it connected and just turn the key forward with out starting the engine and see if the pindle moves. Each time the key is turned forward that automatically resets each time to a start position.

Next clean the throttle body really well. Also check the throttle position sensor by putting a volt meter on it and see if you get a smooth voltage change from 0-5 volts. Clean the MAF with MAF cleaner.

And finally this didn't make any sense to me, but i changed the air intake temp sensor and and unplugged the computer for an hour, took it for a drive on various roads for highway and stop and go traffic and it evened out.

Oh, also check the temp sensor on the thermo housing. That is the one the computer uses, not the one in the block.

Hope this helps.

If i am inaccurate, there are a lot of good people in here that know a lot Sunlite Comet knows his stuff

 

[SunlitComet]

Thank you, Thank you - I will be here all week.

Disconnect your cruise control cable to isolate it as being an issue. if installed. and if so are you on the correct TB crank guide. And you say tranny seems fine but do you notice any firmness change or shift timing off then before? I would have to plug in yo your matrix to be sure has any body or thing been plugged into you OBD2 connector lately that might have change the PCM programming?

 

[gpracer1]

idle is smooth and sits around 600 rpm. really nice actually.

accellerator pedal seems to be smooth with no hanging.

with my limited background, I was presuming something on the intake manifold was opening up to increase airflow and rpm, and then not closing-off.

man, when the ac compressor kicks in, I really have to fight the brake to slow her down sometimes.oh, and no check engine light. I haven't put a reader on it to see if it is throwing a code.

I guess one more superstition/symptom/clue...
this behavior came with the car at 70k. pretty consistently showing up as I described above.
Then I replaced the intake manifold gaskets a few months back. stopped the coolant leak etc.
afterwards, this whole symptom disappeared for a couple of months. I thought I must have reseated something because the car was driving like a dream. then this problem began to reappear.
I'm feeling it's on the intake side of life rather than a transmission problem or freaky ecu thing.
grrrr....

Totally a symptom of IAC motor.
When you turn on the AC, it bumps up the idle with the IAC.

 

[SunlitComet]

Sure sound like that is the most definite of any of the ideas. But I wanted to add something. Weather or not you end up replacing the IAC motor when you are a done and the same or new motor is installed you will have to reset it again while installed or it is going to act up. and the reset procedure actually has a required time to follow.

1. Turn the ignition switch to the ON position for 5 seconds.
2. Turn the ignition switch to the OFF position for 10 seconds.
3. Start the engine.
4. Check for the proper idle operation.

 

[MattM]

awesome. thanks for the ideas.
I'll go through the list of things one at a time and post back how it goes.