Ok so I have a 98 durango with a 5.9l and the cel came on and I read the code and is a p0307 and says cylinder 7 misfire detected but the engine runs fine and accelerates with out hesitation and idles fine so what's going on?

 

And i cleared the code last night and the light went off then this morning it came back on

 

Random misfires can be a PITA. I've never had one on the Durango but had them on other cars and can take a while to diagnose if the first obvious things are not the root cause.

The P030x codes can fire without any noticeable engine performance issue because the misfire can result in an excess of unburnt fuel in the catalytic converter, which is bad for the cat and bad for emissions.

Diagnosing it is figuring out which of the air/spark/fuel triangle is the problem. I found this page http://www.random-misfire.com/ helpful. Could be something as simple as a bad plug or plug wire, or a leaky fuel injector. Things get more complicated if it is a vacuum leak, bad compression, or a bad valve. Lots of good 'how to' vids on YouTube too.

 

I I was wondering because it's not constantit's just kinda random cause I reset it this morning and it hasn't come back on yet

 

And the light came on the first time I was getting on it alittle so could that be because the fuel pump isn't strong enough to bring all the fuel up I know the fuel pump is going out and doesn't hold fuel in the lines up to the injectors so I have to wait for the fuel pump to prime so it starts right away and doesn't crank alot

 

Intake manifold plenum gasket failure I would say is the #1 cause of random cyl. misfires, assuming your plugs, wires, cap/rotor have all been replaced.


Turborusty

 

I would go in this order, personally.

1. Replace you plugs with cheap champion copper gap .040 before install.
2. Replace the wires.
3. Replace the distributor cap and rotor (upgrade to brass)
4. Grab a bottle of Seafoam and run it in the tank if needed. I don't like fuel additives but each to their own as you could have a dirty /leaky injector.

IMHO....If it were a plenum gasket failure I would assume he should be getting random codes as you stated, at this point he hasn't. Point is I'm sure a tune up is in order.

As for the fuel pump, it doesn't sound like a bad pump but rather a bad check valve in the pump itself. But I've never thrown a ignition misfire code with no gas, it's usually electronic. Could also be a Crankshaft sensor or camshaft/pick up sensor.

 

I know I will catch Hell for this, But replace the cylinder heads with Reman's. This is a common problem with the 3.9,5.2 and 5.9 engines. Go ahead a do your compression test, wire set, dist cap, dist rotor and spark plugs. But if you get to the point of taking the intake manifold off to replace the gasket on the belly pan. You may as well go ahead and replace the heads.

 

I'd have to get my service manual out to confirm, but I believe the P030x only triggers if it happens in two drive cycles, so you may not be out of the woods.

Fuel pressure can certainly be a cause.

There is lots of sophisticated testing you can do with fuel pressure gages, multimeters, etc.. An extremely useful tool to start with would be a Bluetooth OBDII adapter and the torque app on a smartphone. You can see the behavior in real time and start figuring out when it is happening (cold or warm, idle or at speed, so on), which will point you in the right diagnostic direction.

Before any of that, a simple place to start would be by swapping parts one at a time with a cylinder that is not reporting a misfire.

 

Yes you will.

It's a great idea considering those particular engines are prone to cracking between the intake and exhaust valves because they were never originally built to run at 195+ continuously because of emissions. I always drop them down to 180F T-stats. I was told by a very credible machine shop and others agree that about 95% of these heads are cracked when they get sent in.

But this would be considered a last ditch effort because of the issue. I would first point you into the tune up, maybe switch injectors and see if the cylinder moves, but yes it could be a head issue.