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3.9 v-6 fuel issues

3K views 16 replies 6 participants last post by  gohammergo 
#1 ·
So, thanks in advance for any assistance here. Have a bit of an issue, as follows:
1994 Dakota 4wd, 3.9, auto trans.

Bought this truck 5 months ago with 94000 miles on it. Ran great. Started great. Good mileage, around 18 ish.

Every once in awhile when I started it, it would buck and misfire for a bit until it warmed up good. Seemed like if I ran it for a few minutes, shut it off, and then started it up it would run good again.

Then I noticed that when I hit around 150 or so miles on a tank, it would run out of gas? If I added gas, it would run good again. Once it would hit around 150 miles it would buck and stumble just like it was out of fuel. I always carry a can of fuel with me, and if I would pull over and add a couple of gallons in it, it would smooth right out and run fine.

It seems like the problem is getting worse. Nearly every time I drive this, it seems to take longer and longer before the engine smooths out. At first, it took around 9-10 miles on the highway and it smoothed out. The last time I drove it, I went almost 70 miles and it was still slightly missing when I shut it off.

I put new, plugs, cap, rotor, and wires on it. Changed the oil and filter. I noticed that when I pull a trailer, that the engine runs pretty hot too. There is no oil in the antifreeze, and no antifreeze in the oil. Doesn't use oil. Doesn't smoke any. Doesn't use antifreeze. No leaks of any kind on the truck. No engine lights come on.

Not sure which way to turn with this. I have another parts truck that ran good. I thought that maybe I would take the gas tank from the parts truck and put it on this one, but they are both have skid plates under the tanks and look like they would be a pain to change.

Any advice would be most welcome! Thanks again.
 
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#4 ·
1994? Fuel pressure gauge on the Schrader on the fuel rails. Unless it doesn't have a Schrader, then by T'ing into the fuel line to the fuel rail. I don't have a picture, but the factory service manual LOOKS like there's one on the driver's side, up from the rear of the engine. Of course, that's also a "Typical fuel rail", so ...

As to swapping the pump - I'd not SWAP, but I'd suggest you may need to consider pulling the bed to get the pump swapped. It sounds like it's running dry early - although "150 miles" doesn't mean as much as "when it's down 8 gallons on my 22 gallon tank".

That may be easier than removing the skid plate.

RwP
 
#5 ·
So I need an actual fuel pressure gauge for that? What should the pressure be? I'll look for the valve today when I get home from work.

You are right, you worded it much better than I did. It runs dry at about 150 miles. Then, when I fill it up, it takes only around 8 gallons or so.

Any idea what could cause it to do this? Someone else suggested that when the tank was full that there was more pressure on the pump from the weight of the fuel? Someone else suggested also that this is a pressurized system and that maybe the line going back to the tank may be cracked and not letting the pressure get high enough. If that's the case, would there be fuel leaking or at least fuel odor when the truck is running.

Again, thanks for taking the time to answer this.
 
#6 ·
So, the problem continues with this. Work has been crazy and I haven't had a chance to work on this at all. Until last night.

I did cut a hole in the box and took the pump out. The inside of the tank looked pretty clean, however, when I took the pump itself out of the assembly, there is like a cloth filter thing in the bottom. It looked and felt like there was a layer of smooth black grease in it. I blew it relatively clean with my compressor, and then cleaned the fuel filter that's at the bottom of the assembly as well. Started up and ran better than ever! For about a 1/2 mile or so, then started running the way it was before.

Today, I took the fuel pump assembly out of my parts truck, and put it in my driver. It does seem to run somewhat better, but not quite the way it should be. At least the gas gauge works now :)

Has anyone else encountered the grease issue? I thought maybe it was something from the pump?
Is there any other filter between the pump and the rails?
Some kind of pressure regulator that could be dirty?
I tried taking the line off where it goes into the rail. I took the clip off of the line and tried pulling it apart but to no avail. Any tricks? Pull harder?

Thanks in advance for any help! :)
 
#7 ·
Did you test the fuel pressure?

Be sure to test the volume as well. You should be able to pump a pint (a soda bottle) or so in 30 seconds. Just use the bypass on the fuel pressure gauge (or just pay to have someone test the system thoroughly if you are not comfortable messing with your fuel system. Fuel spraying inside your engine compartment is not a good thing).

Low fuel pressure or volume can cause the engine to run lean, explaining the overheating.

You might be dealing with a kinked or damaged line if you swapped the pump and got the same symptoms. I'd do the diagnostic work before I changed any more parts.
 
#8 ·
Thanks for the reply. I have not tested the fuel pressure. I'll have to get a gauge for that.

What you say does make sense too, since when the truck is running good, the temp is normal. It's when it runs rough that it heats up.

I looked at the fuel lines and where it comes out of the pump, its rubber or plastic. Somewhere it changes to metal, and the metal lines are pretty rusty. I'll have to give it a more thorough check tomorrow.

Thanks again, John
 
#9 ·
I've been working on this and still can't get it.

After I drove with the new fuel pump for a short while it started running just like it did before.

I put new injectors in it, ran the same.

Checked the compression, 5 were right at 110 and one was at 107. The plugs smelled kind of gassy, so I put new plugs in. Ran the same.

When I put the new injectors in, it actually ran worse, and the check engine light came on, so I did a code check. Showed 27 I think, related to the injection system. I thought maybe I had messed something up but I had just forgotten to put one of the plugs on the injector. Started it up again and no check engine light, but still ran rough.

I unhooked the battery so the codes would clear and let it sit for awhile.

When I went back out, I crawled under to check the fuel lines. I moved them around as I was looking at them. They actually look pretty good, not as much rust on them as I had previously thought.
My wife had suggested that maybe the catalytic converter may be bad, so while under there I tapped on the bottom of it. Sure enough, it was rattling. So I hooked the battery back up to drive it onto my ramps. It started up and ran as smooth as could be!???

I put it on the ramps and cut the converter off, and started it back up. Still ran smooth, better than ever. So, I put a muffler on it, started it up and drove it off the ramps. Still smooth. Took a ride to town, to get some ice cream for my wife for suggesting the converter........ started off running, again, just as smooth as can be. About a 1/4 mile down the road..... a miss..........then another.......... by the time I drove a mile, it was running just as bad as ever. I took the ruck home and used my wife's truck to get the ice cream.
Checked the code when I got home, nothing. Just the one saying the battery was unhooked.

Now, this is the second time that the truck has run super smooth for short whiles. The only thing in common is that I was moving the gas lines around. The first time was when I cleaned up the original fuel pump, and the second was the one just described. This has me puzzled pretty bad. The exhaust smelled kind of gassy too. I thought maybe with the bad converter, that maybe if I ran it a bit it would clear up.

Could there be something in the distributor that is making the plugs not have enough spark maybe? Even though the cap, plugs, rotor, and wires are new, I realize that part of it could be bad.

I'm stumped...... any ideas?
 
#10 ·
The other thing you did was remove the battery terminals. Maybe take a *very* close look at them as well as all the grounds you can find.

If the truck is not getting a steady supply of 12v who knows what kind of trouble it could cause, I've read several accounts of poor battery connections causing rough running. How old is the battery?

This is a mystery all right.
 
#11 ·
Thanks for the reply. I actually have the battery unhooked right now. After the work I did yesterday my wife found some info regarding the distributor, so I put a new sensor in that too.

I'm going to check the grounds right now. The battery is about a year old and is very heavy duty.
 
#12 ·
Maybe our problems are related for different reasons?

I put new plugs, wires, cap on my truck, ran GREAT.... for about a month.

First fire of the day, it started running rough, REALLY rough until it began to warm up. The kind of rough that causes anxious moments.

I thought for sure it was a temp sensor going out or something along those lines. Turned out the new plug wires I had installed were garbage (autolite lifetime warranty). At night, you could actually see a bit of a glow around the wires.:eeksurprise:

I swapped on a set of plug wires from Summit racing and an MSD coil while I was at it and it cured the problem. I actually have a bit of a traction issue now, and driving in the rain is whole new experience.

Maybe in your case its the heat causing these components to crap out?

Just because something is new, don't assume its good.

Good luck.
 
#15 ·
So, I just wanted to say I have the truck running good, at least as of now. Lessons learned again, always check the basics first!

I think there were multiple problems with this vehicle, but after I changed the fuel pump, injectors, new cap, rotor, plugs, and wires, the problem turned out to be bad plug wires after all. I noticed some glowing from the coil wire, so I replaced it with one from another truck that was running good. Still glowing, so I took a set of wires off of my parts truck that had only been used about 2 months. Still no change. I had heard about issues with the distributor being worn and was about ready to change that. Then on the way to work, Sat73 suggested plug wires and coil. Easier than changing the distributor, so I picked up the best set I could find locally, changed them , and it runs like a top now. It's been a week now and still running great!

And that must be why when I bring something to a shop to get worked on, the first thing they do is change plugs and wires!

Thanks to all who gave advice, and remember, to always check the basics!
 
#17 ·
Yeah, that's very true. Diagnostics is an art in itself. Unfortunately, I have no decent mechanics to call on. The ones in my area are sadly lacking in both skills and ethics. The last three times I brought vehicles in to be repaired, I ended up re-fixing them because the "pros" didn't do a good job. I did speak to one guy about it, and he said basically that they could take a look at it, but couldn't promise much in the way of repairs.

This truck had more than one issue in the running problems. Even though it did need new wires, I don't think that the wire issue had anything to do with the fact that after putting on 150 miles or so that it would basically run out of gas. Once I put gas in, it would run good again for about the same duration. At least I don't think that wires could cause that? This truck had obviously sat somewhere for awhile, based on the year of it and the miles on it.

Another example of this. Last week, the brakes started making noise. I could feel that the front brakes were starting to "grab", so I got new pads and put them on. The old breaks were pretty bad, but not down to the metal yet. The brake squealing continued, so I pulled the back drums off. All four of the little coil springs that hold the shoes on were gone. Just a few pieces left. On the passenger side, the lower horizontal spring was not where it should be either. It was wedged into the upper springs. So, now I have new brakes all around with new springs.

Now it's time for new shocks, and then I'll be in pretty good shape.
 
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