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jolind
Dodge Dakota
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3/24/2003
17:53:59
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 Subject: 3.9 L stuck running lean
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I have a 2000 Dakota with a 3.9L engine. That is the extent of my knowledge of the problem with this thing.
The problem is this. The engine is running lean constantly. It used to only do it for a couple minutes or less on startup and then straighten itself out, and it didn't happen every time. Now it's stuck that way.
It sounds like I have a shop vac under the hood at 2000 rpm. I have a loss of power running up through the gears because it's sucking so much air. I'm constantly shifting too soon because it sounds over revved when it's not - if I revved it to around 3k during a shift, I'm fairly sure it would just implode and cause a black hole. Other than that it runs fine - no missing, no other probs.
Replaced the IAC, cleaned the throttle body, checked for vacuum leaks (17.5 and steady @ idle), and took it to the dealer. Dealer told me no codes had been set and it was because my fuel injectors were clogged that my truck was running lean. Despite my doubts I let them flush the system. The truck runs better, but still runs lean...go figure.
The theory that I'm currently working on is this - and I'd appreciate any others. Is it possible that some sensor (most likely an oxygen sensor) is telling the PCM that there is a rich condition when there really isn't causing the PCM to dump extra air into this beast? More importantly, is it possible for that condition to happen and not set a trouble code?
Please help because the dealer isn't and I'm at a loss...
Thanks
Dan
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Jon
Dodge Dakota
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3/24/2003
18:40:32
|  RE: 3.9 L stuck running lean
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Yes it is possible for the o2 sensors to fool the computer into running lean, but usually after a little while of driving the computer catches on and sets a code. The dealer should have been able to check this by looking at the o2 sensor voltages on the scantool anyways. Here's one idea. The computer does not measure fuel pressure. It assumes that the fuel is at a certain pressure. If the pressure is too low it could run lean without setting a code. Sometimes the computer will catch on that there is a problem because it is having to lenghten injector pulses to get good readings from the o2 sensors, but sometimes it's not bad enough for the computer to see the problem. Are you sure that it is running lean? How can you tell?
-Jon
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jolind
Dodge Dakota
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3/24/2003
20:22:18
| RE: 3.9 L stuck running lean
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Ok, to be honest - I'm just assuming that the engine is running lean because of the sound it's making. It has that overwhelming roar of an engine that's getting too much air with respect to fuel. It doesn't drop off between shifts when you let off of the gas like it should. It does litteraly sound like a shop vac at 2k rpm. At 70mph it sounds like I'm in 4th gear even though the rpms are only slightly higher than normal at that speed.
I haven't had any definitive diagnostics done on it yet. The dealer should have, could have, would have...but didn't. All they checked for was trouble codes - but they must have isolated it to a fuel pressure problem, that's why they cleaned the injectors. So they may have been on the right track, but they didn't correct the problem.
Your advice of there being too little fuel instead of too much air makes sense. I was chasing my tail in the wrong direction. Is it correct to say that there really is no way for too much air to enter the engine (without manifold leaks that is)? Isn't that governed solely by the throttle plate opening?
So if I understand you correctly, either the O2 sensors might be telling the PCM that the mixture is too rich and dropping the fuel injection OR the fuel pump is running at a reduced level. Since, as you said, the PCM SHOULD catch a faulty O2, I'm leaning toward the fuel pump. Am I on the right track now? Hope so - I'm getting dizzy...
Thanks
Dan
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Simpleman96
Dodge Dakota
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3/25/2003
12:07:43
| RE: 3.9 L stuck running lean
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I would look at the 02 sensors, especially the first before the cat. I just corrected a similar problem. The heating element on the sensor went bad, but it never threw a code. after nine months and a couple of grand it throws a code for that sensor. turns out that the heating element wasn't keeping the sensor hot enough so the PCM would tell it to run lean, but since it was still a good circuit, no mil light. I had a significant loss of power, and a stumble after start up. Since Ive replaced the sensor the power has returned and no stumble. I eat up more gas but that is because the PCM has to relearn the fuel curve again.
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NYSubDak
Dodge Dakota
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3/25/2003
12:45:09
|  RE: 3.9 L stuck running lean
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I'm gonna go out on a limb with this one. I think your heading down the wrong path all together. I don't believe you are running lean at all. I don't even think the sound you hear is your engine. I think it is your fan. Your thermostatic clutch (or what ever it is called) is permanently locked up. Because of this, your fan is always turning at engine speed, Which at 2000 rpms is a huge load on your engine. This is actually a common problem. My truck (2000 3.9)does it every once in a while the first thing on a cold morning. It is usually gone by the time I get to the end of my street. From what I understand, the way the fan is supposed to work is this: When you first start the truck, the fan should turn at roughly idle speed. But as engine speed picks up, the fan should slip more and more because the friction of the air exceeds the friction of the clutch. As the fan clutch heats up due to the radiant heat from the engine, the clutch should slowly engage, causing less and less slip, and thereby more and more airflow. This could be called self-regulating. Shoot me if I am wrong, but I'll bet this is it. To fix it you could do two things. You could have the clutch replaced, or remove the fan and install an electric fan. A lot of guys on the forum have switched to electric fans because they say that it gives a performance boost since the stock fan is big and heavy and always turning at some speed.
Wade
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Jon
Dodge Dakota
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3/25/2003
19:43:04
| RE: 3.9 L stuck running lean
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It could be the fan, but hopefully the dealer could have cought that but you never know. It is possible that since you complained about a fuel problem they sold you the injector service anyways. The fan clutch as well as the fuel pressure can be tested pretty easily. The fan clutch thing does explain the shop vac noise, usually when tit is cold out they spin at engine speed and really roar for a few seconds, when it warms up they slip more and run quiet. See how easily you can turn the fan with your fingers when the engine is off. There should be resistance but you should beable to turn it with a couple fingers.
-Jon
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Jon
Dodge Dakota
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3/25/2003
19:43:10
| RE: 3.9 L stuck running lean
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It could be the fan, but hopefully the dealer could have cought that but you never know. It is possible that since you complained about a fuel problem they sold you the injector service anyways. The fan clutch as well as the fuel pressure can be tested pretty easily. The fan clutch thing does explain the shop vac noise, usually when tit is cold out they spin at engine speed and really roar for a few seconds, when it warms up they slip more and run quiet. See how easily you can turn the fan with your fingers when the engine is off. There should be resistance but you should beable to turn it with a couple fingers.
-Jon
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jolind
Dodge Dakota
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3/25/2003
20:10:43
| RE: 3.9 L stuck running lean
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Thanks for the suggestions - will check into everything and post my findings. The dealer did mention that my fan clutch was noisy and suggested I get that repaired too. He may have been on to something from the sound of all of this.
Again thanks,
Dan
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jolind
Dodge Dakota
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3/26/2003
22:20:28
| RE: 3.9 L stuck running lean
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The winner is...the fan clutch. Tried turning it when cold and it was so tight I could've turned the engine over with it. Wade's discription of what his truck is doing is exactly what my did before it locked up all together.
Any sources for the electric fan would be appreciated.
Thanks to all
Dan
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Lattimer
Dodge Dakota
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3/26/2003
22:45:03
| RE: 3.9 L stuck running lean
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Summitracing.com is where I got my Black magic 150 fan.
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