WOT throttle occasional stall, with excessive supper acceleration otherwise! (1 Viewer)

[2001LC]

WOT throttle occasional stall, with excessive supper acceleration otherwise!

So this 01LX w/360K miles will sometimes stall when “pedal to the metal ((PtTM) (sudden full depression of gas pedal))” as they say. When gas pedal slowly depressed, it never stalls. In fact once it has stalled during a PtTM, it doesn’t generally repeat.

I only need to let of the gas pedal after a PtTM stall, then depress at a normal easy pace again, and acceleration is normal. Then I can depress to WOT rapidly again, and it is fine most times. But the acceleration is more than any none VVT I’ve driven. In fact kick the my head back, may even have more acceleration than a well turned VVT 4.7L. More like a turbo charged engine.

Test by the book performed at 79F:

• Fuel pressure: Okay @ 44PSI.
• TPS, Okay @ 1.697 OMH.
• Throttle control motor, Okay @ 3.3 OMH.
• Clutch, Okay @ 5.3 OHM
• Accelerator pedal position sensor, Okay @ 1.872 OHM.

So anyone seen this or have some thoughts as to what’s going on.

Why does throttle do nothing at times (stall) and only during very rapid pedal depression?

Why, when working without stalls, is acceleration so strong?

 

[2020 Rocks]

I would inspect the fuel pump sock filter thing and probably replace the fuel filter. Could be rust in the gas tank, i've heard of people placing magnets on one side of the fuel tank to keep internal rust particles away from fuel pump side of tank, as a temp fix of course

 

[katit]

Feels like throttle position sensor? Can you read it's values?
And if it's good - then something with throttle body? For sure it's either lagging opening throttle or goes full 100%

 

[white_lx]

Why does throttle do nothing at times (stall) and only during very rapid pedal depression?

There is a possibility that the TB clutch is slipping under very quick throttle plate movements. Did you check to see if the throttle plate movements follow the pedal movements during the stall? I don't think that Techstream is fast enough to see APS and TPS during the stall. Maybe remove the air intake and see how the throttle plate moves to different throttle inputs (engine off).

Why, when working without stalls, is acceleration so strong?

The answer to this is the key the solving this puzzle. Erratic throttle plate movement?

 

[2001LC]

I would inspect the fuel pump sock filter thing and probably replace the fuel filter. Could be rust in the gas tank, i've heard of people placing magnets on one side of the fuel tank to keep internal rust particles away from fuel pump side of tank, as a temp fix of course

I'll keep on the list! THX.

But the fuel pressure test would not likely have passed. I suppose if some kind of gunk floating around in the boxed in area sock sit in, it could fool the test. But this would not explain all symptoms. But thanks for the thought.

Feels like throttle position sensor? Can you read it's values?
And if it's good - then something with throttle body? For sure it's either lagging opening throttle or goes full 100%

Than was first thought. But it does pass FSM test.

There is a possibility that the TB clutch is slipping under very quick throttle plate movements. Did you check to see if the throttle plate movements follow the pedal movements during the stall? I don't think that Techstream is fast enough to see APS and TPS during the stall. Maybe remove the air intake and see how the throttle plate moves to different throttle inputs (engine off).



The answer to this is the key the solving this puzzle. Erratic throttle plate movement?

I've not tried watching with air filter pipe off (out of the way) when engine running. Why? Well, in the past, when I've tried running a 4.7L 2UZ with air pipe off. They can barley run. I did look engine off, seems normal. But, engine off I can't stall, so have no way to tell if event is occurring.

"The answer to this is the key the solving this puzzle. Erratic throttle plate movement?"
This is my thought. That seems to be first WOT. Once it happens and I then accelerator slower (press pedal slower), it does seem to happen again. I'm thinking butterfly is either not opening "stall" or opens to wide to fast. But why would a weak clutch not open, than open to fast?

 

[MissouriLC]

I'll keep on the list! THX.

But the fuel pressure test would not likely have passed. I suppose if some kind of gunk floating around in the boxed in area sock sit in, it could fool the test. But this would not explain all symptoms. But thanks for the thought.

Than was first thought. But it does pass FSM test.

I've not tried watching with air filter pipe off (out of the way) when engine running. Why? Well, in the past, when I've tried running a 4.7L 2UZ with air pipe off. They can barley run. I did look engine off, seems normal. But, engine off I can't stall, so have no way to tell if event is occurring.

"The answer to this is the key the solving this puzzle. Erratic throttle plate movement?"
This is my thought. That seems to be first WOT. Once it happens and I then accelerator slower (press pedal slower), it does seem to happen again. I'm thinking butterfly is either not opening "stall" or opens to wide to fast. But why would a weak clutch not open, than open to fast?

I’m not familiar with the operation of the throttle plate clutch, but is it possible that there is some contamination—grit, a piece of fod—that is causing it to slip sometimes and lock up other times?

 

[white_lx]

I've not tried watching with air filter pipe off (out of the way) when engine running. Why? Well, in the past, when I've tried running a 4.7L 2UZ with air pipe off. They can barley run. I did look engine off, seems normal. But, engine off I can't stall, so have no way to tell if event is occurring.

Did you try to look at 'Throttle POS' in Techstream at the moment of the event? So with engine off it follows the accelerator instantly without stuttering? Any weird noises from the TPS? The other thing to check quickly first is that the throttle cable moves freely and does not get hung-up.

"The answer to this is the key the solving this puzzle. Erratic throttle plate movement?"
This is my thought. That seems to be first WOT. Once it happens and I then accelerator slower (press pedal slower), it does seem to happen again. I'm thinking butterfly is either not opening "stall" or opens to wide to fast. But why would a weak clutch not open, than open to fast?

In this generation there is no direct link between the throttle cable and the butterfly. The accelerator position sensor on the throttle body sets the target for the butterfly. The ECM then controls the butterfly to follow the accelerator target position as closely as possible This is done by moving the butterfly with the small motor in the throttle body. The small motor is coupled to the butterfly by the clutch. The clutch is there to have a backup throttle actuation with the cable in case of electronics failure. If the clutch is not powered the throttle cable can actuate the butterfly directly.

Now imagine you have go full WOT at a very fast rate. The ECM instructs the throttle motor to open the butterfly, but if there is some slippage in the clutch because of inertia, the plate will stay behind. The ECM notices this and instructs more power to the motor. Once the clutch starts to catch the butterfly will overshoot because the ECM is giving the motor the full beans. ECM drastically reduces the throttle motor power and undershoots the target etc.

Measuring the clutch's resistance verifies that the solenoid is working, but it does not guarantee that the clutch is not slipping. The same for the throttle motor. The resistance measures the windings of the throttle motor, but if it doesn't provide enough power under load (fast opening) it will not be able to move the butterfly.

In an ideal world, one would look at the APS and TPS sensors on a scope to see where the problem lies.

If you pull the connectors of the throttle body, it should default to the back-up control directly via the cable. It will need more throttle input, but it would allow you to quickly exclude all other suggested issues.

 

[2001LC]

I’m not familiar with the operation of the throttle plate clutch, but is it possible that there is some contamination—grit, a piece of fod—that is causing it to slip sometimes and lock up other times?

IDK, possible I suppose.

 

[2001LC]

Did you try to look at 'Throttle POS' in Techstream at the moment of the event? So with engine off it follows the accelerator instantly without stuttering? Any weird noises from the TPS? The other thing to check quickly first is that the throttle cable moves freely and does not get hung-up.



In this generation there is no direct link between the throttle cable and the butterfly. The accelerator position sensor on the throttle body sets the target for the butterfly. The ECM then controls the butterfly to follow the accelerator target position as closely as possible This is done by moving the butterfly with the small motor in the throttle body. The small motor is coupled to the butterfly by the clutch. The clutch is there to have a backup throttle actuation with the cable in case of electronics failure. If the clutch is not powered the throttle cable can actuate the butterfly directly.

Now imagine you have go full WOT at a very fast rate. The ECM instructs the throttle motor to open the butterfly, but if there is some slippage in the clutch because of inertia, the plate will stay behind. The ECM notices this and instructs more power to the motor. Once the clutch starts to catch the butterfly will overshoot because the ECM is giving the motor the full beans. ECM drastically reduces the throttle motor power and undershoots the target etc.

Measuring the clutch's resistance verifies that the solenoid is working, but it does not guarantee that the clutch is not slipping. The same for the throttle motor. The resistance measures the windings of the throttle motor, but if it doesn't provide enough power under load (fast opening) it will not be able to move the butterfly.

In an ideal world, one would look at the APS and TPS sensors on a scope to see where the problem lies.

If you pull the connectors of the throttle body, it should default to the back-up control directly via the cable. It will need more throttle input, but it would allow you to quickly exclude all other suggested issues.

Very good write-up. Thanks for taking the time.

I know of two 4.7L doing this stall. The 01LX I've experienced this in. The other, a 99LC, I've not been able to recreate.

Next time I've one of them in my shop. I'll see if I can get to happen with tech stream recording during a "stall". I'll also I try pulling the TB connector.