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fstrfvo

RPM's hanging

18 posts in this topic

A lot of times I am driving I will come to a red light or stop sign and take my foot of the gas pedal and hit the brakes the rpm's will stick at around 1200 to 1800 rpms, i have to power brake the car to get it to stop, then after about 10 to 20 seconds the rpms drop and the car shuts off, almost like the tranny is shifting gears. It has also had a bad idle, the plugs, wires, Iac, pcv, coil have been replaced, Also the gasket/o ring at the intake manifold runner has been replaced as has the fuel filter. The throttle body has been removed and totally cleaned. Has been in 2 different shops and the Ford dealer once, nobody can find anything wrong. Not throwing any codes. It is a 06 Taurus with the Vulcan engine, has 92000 miles

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Possible vacuum leak? When parked, could start disconnecting vacuum lines and plugging them.

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Hi Ron!

I first-hand know how frustrating that is.
I assume you drive in overdrive most of the time like a normal person. Does it happen if you put it into just regular "D" as well?

Try spraying some brake cleaner around the EGR port on the back of the intake while at idle, the O-ring that seals that was in VERY bad condition on mine the last time I pulled my intake

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The hanging idle is very intermittent you never know when it is going to happen, that is what makes it dangerous.  I have not tried driving it in 2nd gear. But when I have to power brake it and not let off the brake it will hang for about 15 to 20 sec then the rpm's drop and the engine shuts off but if I am at a stop sign and am able to let off the brake the car will just take off without me stepping on the gas pedal then the rpm's will just drop to idle after a about 10 seconds then i start driving normal. No one can find any vacuum leaks anywhere and they would probably throw a code for a "lean" condition. The EGR valve was replaced.

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Throttle position sensor or transmission range sensor (neutral safety switch) perhaps?

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5 hours ago, Angrod said:

Throttle position sensor or transmission range sensor (neutral safety switch) perhaps?

Yeah, TRS was my guess. This is almost exactly what was happening with mine after the swap when my sensor was bad

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I was thinking about the tps as well but the 2 mechanic shops and the Ford dealer all said it would throw a code if it was going bad. What does the tranny range sensor do and how does it work?

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Never heard of the TPS throwing codes when it goes bad.

The TRS is connected to the gear shifter via the shift linkage. When you move the shifter the linkage moves a piece inside the TRS. It also moves a shaft that is connected to the valve body. The TRS communicates to the PCM what gear is selected so that the PCM can control the shifting of the tranny. If the sensor goes bad it will send erroneous signals to the PCM which can make the car act very weird including shifting issues all the way to general electrical problems.

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TPS throws a code of P1120, out of range, which 1 parameter it monitors  is closed throttle (idle and deceleration) which are what my problems are. i think I will just replace the TPS first as it is only about 40 dollars and pretty simple to replace. Does the TRS throw a code when going bad and is it easy to replace???

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A bad TRS could set codes or it might not. Replacement isn't too hard. Just have to be careful to not turn the shaft it is mounted on or turn the TRS or else the indicator on the dash will not line up correctly. If that happens you have to go through a process using a 3lb bag of sand hanging on the shifter and adjust the TRS so the indicator lines up. Unless you have floor shift.

If the throttle sensor is setting a code I would definitely start there.

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  No nothing is throwing codes. But I started using the a/c today and noticed the sticking/hanging was a lot worse with the A/C on. Almost every time i came to a stop it was sticking, then I turned the A/C off and it was back to normal, well normal for this car. Not sure what the connection is between the A/C and the sticking idle. Does that give you any ideas as where to look for the problem?

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is there a difference between the vehicle speed sensor and the TRS??

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17 hours ago, fstrfvo said:

is there a difference between the vehicle speed sensor and the TRS??

Yes. One tells the PCM the gear selector position and the other tells the vehicle speed.

There might be a continuous DTC that can help with the diagnosis. With the engine OFF, hold the reset button next to the instrument cluster, switch the key to ON and keep holding reset. After a few seconds the data center should read Engineering Test Mode and should go into a gauge test. Release the reset button. Now you can press and release the reset button and cycle through a bunch of information (it loops back to the start eventually.) One of the readouts will say if there are any continuous DTC's that are set.

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Cool thanks, I will try this before I go to work tomorrow. Will this show something other than what can be seen with a code scanner?

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I don't know. It might only show the same codes as a scanner or it might show other codes a scanner can't read.

The PCM is programmed to keep the engine RPM elevated when you lift off the throttle and gradually bring it down over a few seconds to provide a more comfortable ride. Something is feeding the PCM bad data and it sounds like the computer is keeping the RPM's elevated far longer than it should. Eventually it figures out the car is stopped and cuts back on the fuel and idle air but it still has bad input so it doesn't know how far to go and ends up stalling itself out. My first guess would be a bad throttle sensor. Bad inputs could cause the engine speed to stay elevated even with the throttle in idle position.

My next guess is the trans range sensor.These cars have a built in feature to prevent 'neutral bombing.' If you shift into neutral while moving the engine RPM will stay elevated in anticipation of the trans being re-engaged. I've tried this before and I have had the rpm stay elevated for up to 10 seconds after I came to a stop. The TRS might be telling the PCM that you have shifted into neutral while decelerating and the computer is reacting accordingly. I don't suspect this one as much though as you have not mentioned any issues with the transmission outside of the deceleration issue and it doesn't explain the stalling.

My last guess is the DPFE sensor. If it is malfunctioning it may be confusing the PCM and the EGR valve is getting opened when it shouldn't be. This doesn't exactly fit your symptoms though so it is less likely.

All in all it's kind of a crap shoot. The only way to really know for sure if it's a sensor or not would be to hook up a special diagnostic tool and watch the sensors in real time while driving. A mechanic might be able to do this. Unless you have a code for a specific sensor or circuit fault of course.

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It showed no DTS codes logged. It used to throw po300 (random misfire)  quite often, that is when i replaced the plugs, wires and IAC. Did not change a thing. The plugs had very little wear and were of the proper color, no fouling at all. I then replaced the coil, again no change. Both shops and the Ford dealer put the car on their big expensive scanners and the first shop did drive the car around with his scanner hooked up.  none of them could find anything wrong anywhere. The Ford dealer did replace the PCV valve (92000 on the original) and it seemed to help the idle quite a bit and has not thrown any codes since then.  But the hanging rpm,s are a total mystery to everyone. The tranny seems fine, I had it flushed out and refilled with Mercon V and a new filter, we have been to Vegas several times as well as Big Bear (lots of mountain driving) and it drives fine under power, no problems accelerating, no shuddering just the rpm,s sticking. i may just buy a TRS and just take it in and have them change both the TPS and the TRS at the same time. 

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It really seems like you have some kind of vacuum leak. Have you done a vacuum leak test at home? Plenty of ways you can create smoke and use to find.  Common on Vulcan - doesn't always throw a code, but causes random high idle. Egr o ring, iMrc gasket, plenum o rings, etc.

or you have a broken cruise control cable attachment on the throttle body stud causing it to jam the throttle open. Pop hood take the plastic throttle body cover off. Look at the cruise cable as well as the throttle cable. Make sure both snap on to the stud and stay when you manipulate the throttle.  Otherwise it'll jam the throttle.

 

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Yes we did smoke test as well as spray all gasket areas and vacuum lines with carb cleaner, I did it again at home with brake parts cleaner, no vacuum leaks found. Yes I did check the cruise control cable and it is in perfect condition.  i did drive the car in 2nd gear and it still did the same thing so what gear it is in does not seem to have any change, rpm's still hanging at around 1500 to 1800 rpm. Like i said before it seems to do it a lot more when the A/C is on, not sure what the connections is with that.

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