Idle problem - Throttle position sensor? - Defender Source
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  #1  
Old December 20th, 2007, 08:33 PM
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Joe Barefoot
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Idle problem - Throttle position sensor?

I'm having an intermittent idle problem. At times, the beast starts right up and idles normally. Others, it starts then runs right past the idle point and shuts off. It dosen't seem to matter if the engine is hot or cold, Actually, it may happen more when the engine is warmed. I've noticed that if I just click the ignition on and off a few times before engaging the starter, it sometimes works and the engine idles normally. The engine idles smoothly when it idles properly. The engine runs great....good power, smooth, etc. I initially cleaned my stepper motor, then replaced it. I've also just replaced the coolant temperature sensor. I've reset the computer after each attempted fix. No unusual codes, just the 02 disconnect one. Does anyone think I may have a problem with my throttle position sensor? It is acting like a computer or sensor problem to me. Any other ideas? If it is the TPS, where is it and is it difficult to replace (beside the cost)? Miss my 110, but love this 90 even better, me thinks.
Thanks!,
Joe Barefoot
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  #2  
Old December 20th, 2007, 09:10 PM
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Charles Galpin
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Someone on the board recently had a similar problem - the truck wouldn't idle after initial start without revving it up a bit. It turned out to be a bad battery. Not sure it applies here, but might be worth checking.

charles
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  #3  
Old December 20th, 2007, 09:24 PM
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Joe Barefoot
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Thanks Charles...

I just repalced my battery with an Optima Yellow Top though. Intrestingly, my wife had a similar problem with a Rover that we owned previously (battery and bad idle that is).

Joe Barefoot
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  #4  
Old December 21st, 2007, 01:14 PM
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Chris Wherry
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Joe,

I had the same problem. I cleaned the stepper motor and it helped. check out the thread on nov. 17th.
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  #5  
Old December 21st, 2007, 07:15 PM
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Hey Chris!
I think I read that thread. I cleaned my original stepper...didn't work, so I replaced it with a new one...didn't work either....bummer! I thought that would be the problem for sure.
Thanks for the help,
Joe
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  #6  
Old December 21st, 2007, 08:04 PM
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If you just replaced the a battery, it does cause the learning algorithm in the computer to reset when it had the power disconnected. That can cause things such as high idle after coming to a stop and odd idle performance. You might want to try doing nothing for a week or so if you can, and see if things smooth out as the computer re-builds a new data table.
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  #7  
Old December 22nd, 2007, 04:47 PM
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Hans...that is very interesting. I have reset my computer several times...by unplugging it. I never thought about it "re-learning", but that makes good sense. I'll leave things alone for a bit and see. It is tolerable. It now seems to drop off of the first start attempt and cut off. The second attempt, literally seconds later, results in a normal start and idle. Kinda funny...that's why I think it is computer or sensor related. Any thought out there about the throttle position sensor. Logically, if it actually does what it sounds like it does, it seems it may cause similar symptoms.
Thanks again,
Joe Barefoot
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  #8  
Old December 22nd, 2007, 06:50 PM
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I had cold no-start conditions when I brought my truck out here. Didn't help that it was -18F, but changing plugs, wires, coil, etc. Didn't help. Finally, I just drove it, knowing that it sometimes would flood when cold, and it hasn't done it since.

As you said, just drive it.
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  #9  
Old December 23rd, 2007, 12:48 PM
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The TPS sensor should throw a computer fault code if there are problems with it, but you could also try possibly testing it with a multi-meter.... I just don't have the resistance values available right now.

Another possibility is that the fuel rails are losing pressure after you shut it down. Normally when you shut it off, the fuel system stays pressurized. However if an injector is dripping or the regulator isn't staying shut properly, then the fuel rail may have to build back the pressure. One way to see if that's the case is that after the truck has been sitting overnight, just turn the key to "ON" for about 10 seconds before you start it, in order to build up pressure again. Then start it, and it should be fine.
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  #10  
Old December 23rd, 2007, 06:08 PM
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Thank you again Hans...very insightfull. I don't mean to keep describing my problem, but recently the engine will start right up and idle well, then after running down the road and warming up, when I stop at a stop sign for instance, the engine will die. I restart and it will die again. Maybe the third or fourth time it will start and idle normally. After that, it runs strong and without hesitation. As I said, it is tolerable, but a bit of a PIA.
Joe B
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  #11  
Old December 23rd, 2007, 07:35 PM
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I know that the engine dying when coming to a stop is a common problem.

There are a couple possible causes for it, one is the stepper motor like you've been looking into already.

Another possibility is a bad vehicle speed sensor. It's only purpose that I know of is to tell the computer that the truck is moving, so that it can open up the stepper motor a bit to prevent stalling when coming to a stop.

The third is a dirty mass-air sensor, and I've never really done much looking into that particular piece, so I can't say what to look for in that.

But the odd thing is that most of them would normally throw a trouble code in the computer. But you don't have the check-engine light on, do you? I'd check the diagnostic box just in case to see if it's throwing a code, as sometimes a disreputable seller will just remove the bulb from the light, but leave you with the existing problems. It's a small brown box, about the size of a cigarette pack, right next to the computer. It will be labeled "Land Rover Diagnostic Display". Strangely enough, the LED is inside the box but there is no window to see it. But it's semi-transparent, so if it IS on, you should be able to read it through the side.
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  #12  
Old December 23rd, 2007, 09:29 PM
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could well be the ignition control module as well -- it would produce the same systems and not throw a code.
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  #13  
Old December 24th, 2007, 06:42 AM
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Hans and Tony...
Thanks again. I'm going to keep digging untill I get this thing figured out. I have checked the computer for codes, reset it...nothing unusual there that I see. I know the LED is working as I get a code 02 after reset which just means the power has been interrupted...normal. The vehicle speed sensor makes sense, but I've had the engine stall while coasting with the clutch in (getting ready to stop or just slow down) so I don't think thats it. In other words, if the speed sensor is supposed to tell the vehicle when it is stopped...I'm having the problem when the vehicle is still going also. So of the things mentioned I will research the mass air sensor and try to figure out about the ignition control module. I continue to think this is a sensor/computer problem because of the black/white, on/off, nature.
Joe Barefoot
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  #14  
Old December 24th, 2007, 10:04 AM
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Joe, do you hear a relay click when she dies? I had basically the symptoms you described and it turned out to be the fuel pump relay. It's cheaper to try than the VSS...

charles
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  #15  
Old December 24th, 2007, 10:18 AM
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I haven't noticed, but I'll certainly start listening closer. The problem is only intermittent and does not seem to be related to engine temperature. Thanks ...I'm all ears!
Joe B
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  #16  
Old December 25th, 2007, 12:35 AM
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Fuel relays are a good thought, cheap and easy enough that just replacing them is an option.

I'll go a bit more in detail on the speed sensor theory as well for you, just to make sure I have it right for you. It can get a bit confusing sometimes with so many interrelated systems and sensors. This is going to get wordy, but I figure that the more of how things work.... the better you can try and track down where the problem might be. I'm also going to go into detail for future readers too, just because I know the topic comes up from time to time. (Plus, I just feel like babbling today, I'm in a good mood).

Basically, what happens with it is that when you take your foot off the throttle a few things happen. One is that the throttle closes off air flow to the engine, and the throttle position sensor confirms this to the computer. Plus you also have the mass air sensor which shows reduced air flow. So the engine thinks "No air, no throttle, cut off the fuel too". However, the tricky part is that it still gets the ignition signal every revolution of the engine, which is spinning much faster than an engine with no fuel and air should be doing, which causes the computer to cut down on the fuel even further.

This can be especially pronounced when you are going at decent speeds, then suddenly take your foot off the gas and press the clutch. The computer will see a 3000rpm engine, 0% throttle and little air flow. Not your normal highway cruise situation. So it will really cut out the fuel supply fast. Sometimes it can cut it off so fast that the engine stumbles or dies before the computer notices that the RPM has dropped below idle settings and compensates for that. If you keep the truck in gear, the engine will still be getting rotated by the forward momentum, which will keep the RPM from dying too fast.

That is where the VSS sensor comes into play. It tells the computer that the truck is still at speed, not stopped. So the computer will open up the stepper motor a bit more to keep the air flowing a bit, and the idle speed a bit higher, to prevent that stalling at a sudden stop. Since air is also flowing, and the computer still sees the sparking ignition, it will also keep some fuel flowing to maintain a safe fuel/air ratio and prevent all the issues of an overly lean mixture.

Now, I don't know for sure if it really cares how fast you are going or not. Meaning that I don't know if it will do anything different at 60mph than at 5mph. What I do know though is that this is the specific area that the engine has a learning algorithm of some type built into it. You'll notice this right away in a truck that has been running fine for a few months, and you reset the computer. You will notice that after you come to a stop the idle speed will be really high for 5-10 seconds, and will come down in a series of steps. Over time this will go away, as the computer learns how the truck is performing. Kinda neat once you become aware of it actually.

Now, in your case something there isn't working as planned. Unfortunately, there are a lot of possibilities that we have thrown out there. For some reason, the engine isn't getting enough fuel or air in this situation. Realistically, it may also be one specific big problem. But it may be variety of small issues that would be meaningless individually.... but add up to enough. For instance, if the fuel system is dirty and you have reduced flow rates in your filter and injectors, with a tired pump. Maybe your regulator isn't keeping enough pressure in the rails. Add a dirty air filter and stepper motor, and a slightly wonky signal from a mass-air sensor that is known to have been commonly mis-manufactured.......

Normally, if a component failed it will throw a trouble light. But if it's still functioning, albeit marginally, you may not get that trouble code. So I'm thinking at this point that we may want to try and dig you out some values to get in there with a multi-meter and testing some sensors. I'd also poke around a bit to see if any more routine stuff such as filters need to be changed. Unfortunately, diagnosis is hard to do in situations like this. But I figured the more info I toss out there, it may spark you to have the urge to look at something you haven't seen before, and thus find the problem.
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  #17  
Old December 26th, 2007, 12:50 AM
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The engine engine should never die if there is spark and enough fuel and air. You need to reset the base idle adjustment.
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  #18  
Old December 26th, 2007, 08:00 AM
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Hi Jim,
My problem is intermittent. Yesterday, I cleaned my Mass Air Sensor contacts, as they were nearly corroded. On initial start after that, the idle was perfect. Initially around 1200 rpms, then after warming up, dropping to about 800 rpms. I took the trucjk for a spin and tried to get it to stall by letting my foot off of the accelerator and pushing in the clutch. I could not reproduce the problem. I was about ready to send Hans some flowers! I then cut the ignition, with the engine warm, and tried to restart. The vehicle starts but then promptly idles down to 0 rpms. I should add, the engine will idle properly again after several restart attempts. I have also turned the ignition on and off several times without engaging the starter, and that seems to work at times. I would agree that a base idle adjustment could be the culprit, but I wouldn't think that the problem would totally disappear at times if that were the problem.
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Joe Barefoot
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  #19  
Old December 26th, 2007, 05:07 PM
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Yeah something else is going on, but setting the base idle will at least prevent the truck from completely stalling.
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  #20  
Old January 6th, 2008, 11:09 AM
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Thumbs up Fixed!!!...I think

I spoke to a LR mechanic and ,like Jim, he suggested I try to adjust the base idle grossly (without a computer). One I figured out where the protective cap was, drilled a hole in it, I could insert a 3/16 allen wrench. 2 1/2 counter clockwise turns and I could hear the engine rev slightly. I then shut the ignition off, waited for the main relay click, and restarted. The idle was then quite high, around 2000 rpm, but returned to about 1200 in a few seconds. It was kinda cool to think about the computer relearning the new base idle and adjusting for it. I took the vehicle for a spin to warm it up more and noticed that it would idle around 1200 when moving. When I stopped, it idles at 800...perfect! I then tried to reproduce the dying idle after warm up and could not! Idles right at 800 rpm when warm. Time will tell but i think I'm good. Will report back if the problem returns. Thanks for all of the input guys!
Regards,
Joe Barefoot
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