2001 Dakota 3.9 automatic with A/C No Cruise. When first started the engine idles right at 750. After engaging the transmission the idle jumps up to 1000. If I take my foot off the brake and not touch the accelerator the truck will begin to move. Up to 15 MPH. If I'm driving say at 40mph and remove my foot from the accelerator the truck continues to go 40. If I brake to say 30 then the truck will continue to go 30, and that's without touching the accelerator. When I am stopped with the tranny engaged, the idle is 1000, If i place the tranny in neutral the idle jumps up to 1500>2000 and then settles down to 750. If I place it in drive it jumps back up to 1000. I've had a tune up and brought it in to the dealer but since there are no codes they don't seem to be able to diagnose the problem. Once I had the air horn off and with the e-brake applied I placed the tranny in drive. I immediately heard the IAC engage and they're was a hissing noise like a vacuum leak. But it was from within the throttle body. I've cleaned the IAC and Throttle body opening and checked all connections to the TBS and IAC
Any idea as to where the problem might be?
Thanks, Tom

 

My first guess on this would be a bad Throttle Position Sensor. If the IAC were bad the truck would idle rough and die and have a hard time staying running. You can test the TPS with a voltmeter. With the key in the run position but the engine is off you should have right about 5 volts when you measure across the 2 outside terminals on the TPS wire connector. With the throttle closed, on the center connector you should have from .35 to .90 volts, although some say .75 is what you want for the best throttle response. There's a mod for the TPS to set that voltage. Anyway when the throttle is wide open with the engine off the voltage on the center conductor should be right around 4.5 volts. If these are out of spec, replace the TPS.

The hissing sound you heard with the air hat off was probably normal. If your TPS is OK, you should check the Manifold Absolute Pressure sensor. Test is similar to that for the TPS. You should have 5 volts across the 2 outside terminals with the key on and engine not running. With the engine not running and ignition on, you should have 4 to 5 volts on the center wire. Start the engine and the voltage on the center wire should drop slowly to about 1.5 to 2 volts.

You can also check the adjustment on the throttle valve cable. There are probably posts here about that, but I don't know very much about it. My truck is a 5 speed manual. I don't know whether the 2001 even has a TV cable. Someone else on here will probably know. If you get the Haynes book for the 2001 it has directions on checking the adjustment on the cable, but the book is written for 2000 thru 2003's.

I do know that a faulty TPS will cause a lot of really strange problems. That is what I would start with in this case. Let us know what happens.

Jimmy

 

I'll check those readings... Thanks!
Tom

 

I replaced the TPS as the readings seemed to be slightly out of the range you suggested. (Outside terminal connections were at 3.7v) The sympton hasn't changed. (High idle when transmission is engaged)I also checked the voltage at the map sensor and those readings are similar to what you have suggested. I also checked the manifold vacumn and it makes good vacumn. At idle with the transmission in park the engine makes 23 lbs no fluctuation. So i don't think that there's a vacumn leak.

 

The TPS and MAP sensors get their supply voltage from the PCM. The voltage at each shold be 5 volts. Yours are low enough to make me think there is a problem with the PCM. The best way to check it would be at the Dodge dealer with the DRB scan tool, or a Snap-On scanner at an independent shop or electrical garage.

I am no mechanic, but this really makes me think the problem is voltage related somehow. What is the voltage across the battery terminals? You should have roughly 12.5 volts with the engine off and a high 13 to 14.5 volts with the engine running at idle. What kind of shape is the battery in? You can stop by AutoZone or Advance Auto Parts and they will load test the battery in the truck for free. Dakota's are really very sensitive to even the slightest low voltage situation, and a weak or marginal battery can cause the PCM to not receive the correct voltage, which would result in the MAP and TPS sensors not receiving or sending the correct voltage. Do you have any diagnostic trouble codes stored from the PCM? Usually when the PCM is bad it will display "NO BUS" in the odometer. A bad battery won't necessarily cause a trouble code though, unless it gets bad enough to cause a component (like the PCM) to fail. You can turn the ignition to run then back to off 3 times in a row and leave it in run after the third time and any trouble codes you have stored will show in the odometer. When it is done it will say "P Done". Check the codes, or maybe AutoZone or Advance will do it for free but I think the Check Engine Light has to be on for them to read the codes. I know they will load test the battery for you.

Your vacuum is a little high, if you ask me. At idle you should have exactly 20hG of vacuum if you have no leaks. Dakota's are also known for the evaporative canister lines under the hood and under the driver's side of the truck to crack and cause a small leak that is big enough to cause problems. You might take a look at that, but the cracked hoses will usually set a code and trip the Check Engine Light. I guess anything is possible but I can't see a slight vacuum leak causing the truck to idle so high in gear like that.

These are what I think may be causing the problem. Hopefully, others here will have more ideas. Check it out and let us know what you find.

Jimmy

 

I tried a different battery and new IAC. Additionally I checked for vacuum leaks but didn't discover any. Several months before this issue showed it self. the transmission was rebuilt. Is it possible that something in there is failing. No trouble codes are displayed either on the dash or the scan tools. What can cause excessive idle? Timing.

Thanks!
Tom

 

Maybe you could take the truck back to the shop that rebuilt the transmission and see if they can diagnose it. Hopefully they have a parts and labor warranty. I know nothing about automatics, but there has to be some way for the trans to know how fast the engine and torque convertor are moving, and vice versa. Maybe that's what the problem is.

I would start with the transmission shop.

Jimmy

 

I'm really not seeing the problem.
When you switch between gears (while not moving) like having your foot on the brake and switching from Park to Reverse, Reverse to Neutral, Neutral to Drive, your idle will jump for a little bit (a second or two).
It's normal operation to, when not having your foot on the brake, with the transmission in reverse or drive, the truck will move.

When you let your foot off the gas, you should start to coast down in speed, it might stay in the same relative area for several seconds, depending on the truck, the road, etc.

From what you said in your first post, nothing seems really out of line.

 

The behavior is really odd. The truck idles at 750, When I put it in gear and release the foot brake, the truck goes up to 20 mph without touching the gas. under normal circumstances I would expect it to creep up to 5, 6, mph a little but not up to 20. Now when I'm driving say 35 mph, and I remove my foot from the gas the truck continues to coast at 35 for a long time over 1/2 mile before it starts to coast down. Very slowly. So needless to say one issue is when I'm stopping for a traffic signal and I'm braking it feels like I'm trying to accelerate at the same time. As if I was stopping while stepping on the gas. And if I have to stop in a hurry, forget about it....I've owned this truck since it was new, and know exactly how it behaves when things are working correctly so I was able to recognize right away that something was wrong. I've had it back to the transmission shop where they put their scanner tool on it but found no issue either.

Thanks for your help and input though, it's appreciated.
Tom

 

OK this is really strange. The problems you described in that last post make me think the IAC is bad. That's exactly what the IAC does, it causes the idle to slowly decrease as the RPM decreases. It regulates idle speed, raising and lowering it as needed. It does this by opening and closing a "trim" valve to allow more or less airflow into the intake manifold when the throttle is closed. If there are no trouble codes stored and the transmission shop can't find anything, and you have already replaced the TPS, and you have acceptable output voltage from the MAP sensor, IAC is really the only other thing it could be, but that's just my opinion.

I'd try a new IAC valve and do a hard reset on the PCM and see how it does after that. You can reset the PCM by disconnecting the battery and holding the ignition key in the start position for about 30 seconds. Then connect the battery back up again.

Jimmy