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01 Dakota SLT 3.9 4x4
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01 Dakota 3.9 SLT 4x4. If I unhook the battery & reset the ecm it will run basically perfect until it gets to operating temp. Then it slowly starts to misfire and the vacuum gauge goes crazy. Needle goes from top to bottom of a round gague constantly very fast. It will misfire so bad u can barely move.
U can be riding 30 mph while the engine is misfiring badly and all of a sudden it will kick in like nitrous and run perfect for 10 seconds. Because the pedal is pushed halfway down. Then it will start misfiring from some to basically all cylinders. Then kick in again. Only code that comes up is a o2 sensor. The sensor ripped from the exhaust while trying to remove it using mapp gas and a wrench. Now I have to replace the y pipe. Fun times.
Could a 02 sensor cause the computer to adjust the fuel so much that it causes the insane misfires and vacuum readings? I've replaced fuel pump, injectors, filler neck & gas cap. All the ignition items other then distributer shaft is new. New valve cover gaskets. Crank sensor. Bench tested throttle sensor. The fuel pressure gauge bounces when its misfiring. It keeps steady pressure while driving. Could the evap system cause the vacuum and misfires.? If I pull any vacuum hose off of the evap system there isn't any changes on the vacuum gauge. The gauge is hooked onto the intake where the evap system leads to. It idels a little low on vacuum but when u drive it the vacuum reads in the green on the gauge like it should. When it misfires the vacuum readings are bouncing all over the place. With the vacuum issues while its misfiring and reseting the ecm making it run fine until it gets hot makes me think its more then a 02 sensor. I'm replacing the y pipe next along with the rest of the exhaust. Any info would be great. I need this truck to run.
 

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Mine is a 03 3.9 V6 RWD, and suffers from the same type running issues. My list of replaced parts are about the same. Three inch from Y pipe back new up and down O2s, high flow cat, turbo muffler std location 3 inch exit. Crank and cam position sensors, MAS, TPS, IAC, PCV, plugs, wires, air filter. I also attached a vacuum gauge to the intake. When it runs fine the vacuum is in the correct range. As it begins running poorly the vacuum falls off to nearly nothing. My question is how much of a vacuum is required to kill vacuum. My last trip was 1000 miles all same day with this condition. I figure gas MPG for every tank. One tank gave 21 MPG the next 6.5 MPG. It would seem to me once the vacuum is gone the PCM thinks full throttle and starts dumping fuel. If you play with the throttle and TPS agrees with fuel delivery it will take off like a it should. Mine will accelerate to about 75 MPH lifting will go back to missing and coast down to around 55 MPH and I can catch it again and accelerate again. I'm currently removing the engine that has given me so many issues. It will be replaced with the engine I have been building the last six months. The crank, mains std, rods reground .010, block bored .020, intake with VRP mods, timing cover, and valve covers are only parts reused. Every thing else is new. Attempting to purchase all sensors MOPAR/ Dodge. As the old engine is coming out I feel a cooling issue as well as a Keg plate leak are at the bottom of my issues. As soon as the old engine is out I'll clear and detail the engine compartment checking wires and connections. Cleaning grounds and replacing hoses as I go. I kind of lost count but feel like I have replaced 11 vacuum lines some twice and found another under the battery tray split on both ends. I also need my truck to run and plan on it doing so. Steve
 

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Could the evap system cause the vacuum and misfires.? If I pull any vacuum hose off of the evap system there isn't any changes on the vacuum gauge.
I could be wrong, but I am thinking not.
When I got mine, those lines were so dry & brittle & cracked & broken they might as well have not been there. I got a check engine light for the evap system, but it ran fine.
 

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When it misfires the vacuum readings are bouncing all over the place. With the vacuum issues while its misfiring and reseting the ecm making it run fine until it gets hot makes me think its more then a 02 sensor
I don't have personal experience with this, but related to a thread posted on this forum, Ralph P. once gave me a pretty good explanation of the 2 piece intake manifold for the 3.9/5.2/5.9 engines. (& then I did some reading on my own.) That being that the body of the manifold is aluminum but the bottom is a removeable bolt on steel pan, & that due to the dis-similar materials & rates of expansion, over time these engines invariably (invariably, according to Ralph & the sites I went to to read about it) start leaking vacuum from there.

So a possible hypothesis might be that the intake vacuum is able to, at least somewhat, self-seal itself off to begin with, but as it gets hot it just gets to be too much (rates of expansion?) & gets out of control.

The advice Ralph gave was: with the engine off, hold the plate on the TB open & look down there with a light for oil at the bottom of the intake manifold. Oil being there meaning it's leaking.

But as I prefaced my reply, I have never had to work on this part of my Dak yet, & this is just a wild guess I am throwing out there.

Good luck & I hope you get her running right.
 

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I have been taking my time getting the old engine out. The last bolt holding the Y pipe to the right side exhaust manifold tucked behind the oil filter had a rusted head which no wrench fit. Small Vise grips would not hold and large ones not easy to attach. You can find my efforts rebuilding under 3.9 engine problems. In spite of having four customer engines to build and each receive the same level of attention as my "Not for the faint of heart 3.9 build" I'm looking forward to taking a look inside the old V6. I took pictures as I stripped the core engine but had no history of it's issues. The core did have both heads cracked and oil in the Keg. Bearings and cylinders looked really good for a high mileage engine. A quick look shows the Keg on my old engine has had a leak for some time. The valve covers are leaking, the timing cover where it meets the pan also. There are water leaks in several places and the water pump bearing sounds dry. With 176000 miles on the clock and what looks like jack leg repairs carried out it's time. Steve
 

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Looks like you got thread-jacked a little bit there, Trailblazer.
Anyway, getting back to YOUR Dak: your symptoms kind of sound like what I have heard described when a cat fails. Except I would think you would get a specific code (maybe in the P0420s) & I wouldn't think a failing cat would affect your vacuum readings, but I don't know for sure.

I took a look at interpreting vacuum readings in one of my books, & I'd put what you describe into the "large fluctuation" category, & it is saying to perform a leak-down test to check for a weak or dead cylinder or a blown head gasket.

The sort of intermittent nature of your problem kind of reminds me of a carburated 4 cylinder bike I once had that would intermittently run like shit as you describe & then would straighten out a bit for a while. It turned out that one of the intake rubbers between one of the carbs & the engine had dry rotted & cracked & there was a massive intake leak, but at times it would seal itself back off as, i guess, the angle of the bike changed.

Totally different animal, I know, but it makes me think more & more of your intake manifold plenum leaking. Vacuum leaks can be kind of self-sealing sometimes, & it might be that at times the vacuum that you are sucking down there sucks up some gasket to temporarily seal some of the leak.

Just taking some guesses.
 

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I guess I need to apologize for posting about the same engine and running issues I'm having. I have not spoken with anyone who has used a vacuum gauge in an effort to sort the issue. I did not realize it was a you and Matt only conversation. I will continue to post information on my own threads. They are based on facts learned as I continue to work and study the 3.9 V6. My old engine came out Sunday and I have begun to discover more about it's time with a jack leg mechanic. Steve
 

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Any chance you checked the OBDII codes? Is the check engine light on? How many miles are on this engine? The timing chain guides are plastic and break at some point. Can you look at the timing mark with a timing light as you rev the engine? Have you run a compression test?
 

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01 Dakota 3.9 SLT 4x4. If I unhook the battery & reset the ecm it will run basically perfect until it gets to operating temp. Then it slowly starts to misfire and the vacuum gauge goes crazy. Needle goes from top to bottom of a round gague constantly very fast. It will misfire so bad u can barely move.
U can be riding 30 mph while the engine is misfiring badly and all of a sudden it will kick in like nitrous and run perfect for 10 seconds. Because the pedal is pushed halfway down. Then it will start misfiring from some to basically all cylinders. Then kick in again. Only code that comes up is a o2 sensor. The sensor ripped from the exhaust while trying to remove it using mapp gas and a wrench. Now I have to replace the y pipe. Fun times.
Could a 02 sensor cause the computer to adjust the fuel so much that it causes the insane misfires and vacuum readings? I've replaced fuel pump, injectors, filler neck & gas cap. All the ignition items other then distributer shaft is new. New valve cover gaskets. Crank sensor. Bench tested throttle sensor. The fuel pressure gauge bounces when its misfiring. It keeps steady pressure while driving. Could the evap system cause the vacuum and misfires.? If I pull any vacuum hose off of the evap system there isn't any changes on the vacuum gauge. The gauge is hooked onto the intake where the evap system leads to. It idels a little low on vacuum but when u drive it the vacuum reads in the green on the gauge like it should. When it misfires the vacuum readings are bouncing all over the place. With the vacuum issues while its misfiring and reseting the ecm making it run fine until it gets hot makes me think its more then a 02 sensor. I'm replacing the y pipe next along with the rest of the exhaust. Any info would be great. I need this truck to run.
My 97 Dakota 3.9 would run fine for about 120 miles on the freeway and then start backfiring. Long story short, I had a failing PCM that would take that long to heat soak and fail. If you shut it off for 15 minutes you could then drive 5-10 miles until it did it again. I nursed it home 300 miles by opening the hood to it's safety latch and running a ratchet strap around the hood for safety. That gave more airflow across the PCM, and along with a light foot on the gas petal watching my vacuum gauge, I was able to limp home. A replacement PCM fixed it all up. As far as the fuel pressure fluctuation, that could also feel like a misfire. When these electric fuel pumps shut off the engine stops running almost instantly. You may have an intermittent break in the wiring to the fuel pump.
 
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