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iDakota
Dodge Dakota
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6/13/2007
08:21:16
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 Subject: Rough Idle - when fuel tank is full
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I have been battling a rough idle and cylinder misfire codes for a few months now. And I have noticed a few things about the idle when the fuel tank is at different levels.
If I have a full tank of fuel the idle is terrible but at around half a tank the idle smooths out and sometimes by a quarter tank the check engine light goes off. I have been running my truck with half a tank seems to work at keeping the idle problem at a minimum. Truck runs best with less than eighth of a tank.
Recently we had a very hot humid day in the upper 90s and the rough idle came back even with less than a quarter of tank of gas. But as soon as the temp and humidity came down the rough idle disappeared and check engine light went out.
I am thinking the check valve in the fuel filter/fuel pressure regulater may be leaking. I do not have a fuel pressure gauge and have not check fuel pressure yet. I am hoping someone here may have a solution without me having to buy a pressure gauge.
Any thoughts?
I drive a 99 Dakota 4x4 3.9L with 160,000 miles. I replaced O2 sensors, plugs, wires, distributor cap, and roter. Tried fuel injector cleaners.
Thanks,
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daddio
Dodge Dakota
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6/13/2007
09:12:43
|  RE: Rough Idle - when fuel tank is full
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you need to find out what code is coming up when the CEL comes on. that should point you in the right direction.
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iDakota
Dodge Dakota
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6/13/2007
10:15:49
| RE: Rough Idle - when fuel tank is full
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They are multiple engine misfire code and cylinder 4 & 6 misfire codes.
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iDakota
Dodge Dakota
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6/19/2007
08:18:40
| RE: Rough Idle - when fuel tank is full
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Update:
I did a fuel pressure check. Fuel pressure while running right at 50 psi within specs. Turned engine off and monitored guage for 20 minutes. System only dropped to 48 psi after 5 minutes and only went to 46 psi after 20 minutes. My conclusion is fuel system operating normal(fuel pump, no leaks, no restrictions, fuel filter/pressure regulator good).
After the fuel pressure test I relized the level of the fuel tank was just a coincidence. So I decided to try a different fuel system cleaner (Seafoam). I have tried all kinds of fuel injector and fuel system treatment (none worked) but have heard Seafoam is just in a class by itself. I tried a can to half a tank of gas and noticed an improvment immediately. But I am not sure if it is the Seafoam or just another coincidence (idle seems to be intermittent). I decided to go a little further and poured it into the PCV intake line and used the throttle cable to keep it running (did not notice an improvement). I started focusing on how the vehicle runs during ambient air conditions and pulled the temperature sensor out of the intake maniford. Disgusting - it was black. Did a resistance test and it was off spec. So I took a can of carb cleaner and cleaned it up nice (now it is white and the sensor is blue). Tested it again and the resistence was within spec for the ambient air so I poured boiling water into a glass and tested the sensor and resistence reading looked good. Put it back in my truck and started it up - check engine light went off and noticed and improvment in the idle.
Truck is running good right now but still has a bit of a rough idle on hot humid days. Put another can of Seafoam in the tank with full tank of gas. Waiting to see if this was a temporary fix or not.
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iDakota
Dodge Dakota
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6/28/2007
06:48:19
| RE: Rough Idle - when fuel tank is full
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Truck still running good but the check engine light has come on and gone off and now is back on. It comes on at highway speeds, but the truck seems to be running good.
Is it likely, I have bad fuel injectors on cylinders 4 and 6?
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?
Dodge Dakota
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7/04/2007
08:42:54
| RE: Rough Idle - when fuel tank is full
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When you replaced plugs , did you check their gap ? Possible a little on the too much side
and possible - coil breaking down .
I don't think the injectors, sounds more like
water in gas, should be ok now after injector cleaner.
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iDakota
Dodge Dakota
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7/19/2007
09:29:54
| RE: Rough Idle - when fuel tank is full
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Well the rough idle came back with check engine light again. Put another can of SeaFoam in the fuel tank and the rough idle went away again after about 250 miles (I ran nearly the whole tank of treated gas). Plus the check engine light went off.
Yes, I checked gap when I install the plugs.
Should I try rotating the fuel injectors to see if when the rough idle and check engine light comes on that the misfire is in the same cyliners or has moved to different cylinders? When the check engine light comes on I get engine misfire codes for cylinders 4 and 6. In theory if the injectors are bad, and I move them to 1 and 3 the misfire codes should read 1 and 3. Any suggestions? Is there a reason why I can't rotate them? I have never removed injectors will it harm an O'ring or something? Can I attempt to clean them once removed?
Thanks for any responses.
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shadetree
Dodge Dakota
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7/19/2007
10:03:35
| RE: Rough Idle - when fuel tank is full
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iDakota
Since the problem is with cylinders 4 and 6 I would run a cylinder leak down test. If you dont have access to a gauge set I would try a compression test. The reason I would do this is because the cylinders are back to back which would make me beleive that the misfire would be caused by loss of compression because of a common leak between both cylinders like mabye a head gasket or other problem. We just got done replacing an engine in one of our fleet trucks (dodge 2500 with 5.9) at work that show the same symptoms as yours only the misfire codes were for cyl 1 and 3. However it was a ring problem. Now the new replacement engine has lost compression in the number 1 cyl do to a recessed exhaust valve. So we have to put a head on it. Anyway if you do decide to do a compresion test my general rule is you should not have less than 100 psi per cyl and the differecnce between the cylinders should not exceed 40 psi.
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iDakota
Dodge Dakota
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7/19/2007
11:10:09
| RE: Rough Idle - when fuel tank is full
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thanks shadetree
If I had a leak between cylinders or a ring problem would the rough idle be intermitent?
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daddio
Dodge Dakota
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7/19/2007
11:50:17
| RE: Rough Idle - when fuel tank is full
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since the problem is intermittent, i wouldn't bother with a cyl leakdown test.
check the plug wires to make sure they are not arcing on anything. best to do at night, while the truck is running open the hood and look for any sparking along those wires. if there is change them out.
if that checks ok, pull the electrical connectors from the injectors and clean them good with some contact cleaner and reconnect.
since your problem is intermittent and also seems to be affected by humidity leads me to believe it is an electrical problem.
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shadetree
Dodge Dakota
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7/19/2007
12:06:02
| RE: Rough Idle - when fuel tank is full
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It can be. Engine temp can have a lot to do with it also. The expansion and contraction will open and close leaks. Im not saying that this is your problem Im just suggesting it since you have pretty much covered all the other areas. You are right though If you still think its a fuel injector moving it to a different cylinder should cause the misfire code to change. Becareful removing the injector as the o rings will be brittle. If you notice a crack or flat spot in one you can get them in packs of 4 for like $8 and it takes 2 per injector. The truck we are having trouble with now ran great when we put the engine in, but then it would start to mis or even stall at random intervals. Then after that it would through a misfire code for the #1 cylinder. The way we caught this one was when I pulled the plug that I just install like 30 min before the test drive it was black. That lead us to a compression test which showed only 90# cold and 70# warm it actually would loose more compression when it was at operating temp. All other cylinders had 150# in them. After performing a leak down test it showed a 90% leak in the exhaust valve. But like I said it something to check if your fuel injector idea doesnt solve it. By the way this new engine only has 540 miles on it lol.
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Rich
Dodge Dakota
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7/24/2007
02:25:26
| RE: Rough Idle - when fuel tank is full
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I had this for over a month. It turned out to be a vacuum line off of the map sensor.
BTW. does anyone know what size diamater and where I can get a replacement hose for this. Right now I just taped up the cracked line and it solved the problem but this is only a temporary fix. I looked at silicone but would rather get a OEM part. Short of going to a junkyard or dealer any suggestions?
Thanks
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iDakota
Dodge Dakota
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7/26/2007
13:00:06
| RE: Rough Idle - when fuel tank is full
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Thanks for the tip I will check my MAP.
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iDakota
Dodge Dakota
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8/02/2007
12:09:44
| RE: Rough Idle - when fuel tank is full
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No problems with MAP vapor lines.
It has been nice enough to ride my motorcycle that I have not driven my truck in over a week and a half. I like the break from always troubleshoot problems with my truck.
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dakota49
Dodge Dakota
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8/05/2007
21:50:44
| RE: Rough Idle - when fuel tank is full
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Rich, just go to Auto Zone or some other parts store and they should have something that will work just fine. Take it off and show it to them.
Dakota4988 3.9 4x4 5 SPEED
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