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Unusual Sticking Throttle


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This one baffles me.

 

When (and only when) the motor is running, the throttle feels

like something is holding it closed. I have to step on it hard to

overcome the resistance. The pedal freely pivots, the cable

freely slides, and the throttle link and plate operate by hand

freely. When the motor is not running, the pedal is smooth

as silk. The throttle body is clean as new. I even disconnected

the cruise cable to make certain that wasn't a factor. This

just started a couple days ago on a road trip. Once the initial

resistance is overcome, the throttle is smooth throughout the

rest of the range of motion. It seems very improbable that

manifold vacuum could cause this, particularly since it just

started happening. Anyone have a clue what's going on?

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The IAC was a good suggestion, even though it was new last year and

my idle is perfectly smooth. To be certain, I pulled it out and found very

little contamination, which I cleaned anyways. Made no difference. Here's

one more clue. The sticking only happens when I'm moving. When I'm

sitting still, truck running, there is no throttle hesitation at all, not one

little bit. As soon as I'm moving, if I let off the throttle completely, I

have to tromp on it to get past the resistance. Most puzzling.

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Someone suggested the throttle cable fraying near the end. I've seen that before. Can you move the throttle under the hood easily or? If automatic, could the neutral shift cable from the column be causing an issue?

The throttle plate may be loose or needing adjusting.

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Nope. Let me recap:

truck off - throttle moves freely

ignition on (not running) - throttle moves freely

motor running (but parked) - throttle moves freely

driving (<10 mph) - throttle moves freely

driving (>20 mph) - throttle sticks badly

IAC and TB are very clean. Throttle plate is tight,

centered, smooth edges, and pristine. TB is nearly

new, and no marks on the bore that would suggest

any hangup or sticking due to fit. Cruise cable

is disconnected.

 

This all began less than 300 miles ago, midway

through a trip. Nothing was altered.

 

I cannot convince myself that manifold vacuum

is a factor, but nothing else makes sense either.

Vacuum reading is steady 20 in. at idle. Pop the

throttle and vacuum spikes low, spikes high, then

returns to steady 20 in., exactly as it should.

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Carnuck you're shooting from the hip, and not paying attention to what I write.

From most recent post:

 

Throttle plate is tight, centered, smooth edges, and pristine.

 

TB is nearly new, and no marks on the bore that would suggest

any hangup or sticking due to fit

 

Even if they were loose, which they are not, why do you think that

would only be an issue when moving?

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If tehre was play in the throttle plate screws and bushings, the suction could pull it enough to catch on an edge that it wouldn't without vacuum. Missed the other stuff :ack:

I'd try another throttlebody since nothing else seems to be causing it. 20 inches of vacuum is higher than most motors when not coasting though.

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To check that box, I tested fault codes and got only "55" (end of codes). Tomorrow I'm

going to install a NOS IAC I scored, just to rule out a sticking plunger, even though there's

no mechanical relationship between throttle and IAC. If money was no concern, my

solution might include one of these: http://www.4x4community.com/forum/showt ... hp?t=30945

 

Sometimes I really do miss carburetors.........

 

20 inches vacuum seems like an indicator of really good motor health, probably higher than many, but

not excessively so.

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Here's a thought. I'm way more up to speed on Renix but I'll throw this out there. Does your TB have a screw to stop the butterfly?

 

I'm wondering if the butterfly is closing too far into the bore of the TB and this is being exagerated when manifold vacuum is present. Perhaps backing the screw out, then tightening it in just until the butterfly begins to move open would be a good idea.

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Cruiser, you might get the prize! That is something that also came to mind as

a possible solution. My 62mm TB does have such a screw (at the linkage) and

I could easily try this, while watching the RPM so as to not raise the idle too much.

As you say, it would reduce the possibility of close-fitting butterfly compounded with

high manifold vacuum being the cause of this issue. I will post the results tonight.

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Crusier gets the prize! Crusier, I don't know what town you

live in, but if I'm ever there I will buy you a pitcher of the beverage

of your choosing. :cheers: :bowdown:

 

Installed a new IAC, no change. Pulled the cable and soaked it

with lubricant, no change. Pulled the TB and turned the setscrew

(cannot reach with TB installed) no more than 1/4 turn, and voila!!

No increase in idle speed, and throttle response EXACTLY as it should

be, regardless of speed. Yaa Hoo !!

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Don,

 

The screw is at the forward outboard corner of the TB mounting flange.

It rises vertically through the flange and contacts the throttle link. You

can see it in this image. It takes a 3/32" allen wrench from below. Probably

a special tool to reach this with TB installed, but quick to pull TB and

adjust.

 

 

Tom

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