Hi Everybody,
I've got a frustrating check engine code showing up, more or less randomly. My truck seems to sound great and gets good mileage so I think its a "phantom code"

Any help would be greatly appreciated.

1998 Dodge Ram 5.9 V8 Two Wheel Drive Automatic 128000 miles

I get a random occurrence of code of 0304. I know this is engine misfire cylinder 4. The codes seems to trigger when the truck is first cranked up. It also happens in the winter, not the summer.

I have changed the distributor cap, spark wires, spark plugs, coil, air filter, air charge sensor, idle air control valve, both O2 sensors, throttle position sensor, cleaned the throttle body, decarbed the engine with seafoam, and installed a new catalytic converter. I also changed a few low gauge wires that looked worn (alternator/battery).

I doubt there is a vacuum leak, there is quite a bit of vacuum after the engine is off and I pull the brake boost line.

The truck runs great and gets good mileage, but still the code. The only things left I can think of are the MAP sensor and the cam/crank position sensor.

Any ideas of what the problem is?

Thanks-

 

I have no trouble clearing the code, either with a code reader or by disconnecting the battery. The CEL code is not constant, persistent or common. The problem is the code shows up from time to time, almost randomly and it is not associated with any noticeable engine misfire.

 

Thanks for the replies!

Unfortunately, I don't think my code reader will monitor like that, but I'll look into it.

As far as the plugs go, they looked fine and all looked the same. I replace them every 6 months and use champion -- I always have, since long before this problem. Does anybody have another recommendation?

I think the spark/coil wires are routed correctly. I even tried rerouting them according to TSB 18-48-98.

http://dodgeram.info/tsb/1998/18-48-98.htm

I have read that the plenum gasket may be the culprit or possibly a rocker arm adjustment. I just figured that those problems would trip the code consistently.

Does a leak in the plenum gasket lead to increased/heavy oil burn also? I don't really see that. Maybe 1/2 qt/ 3000 miles or so.

 

I am an automotive service manager and can tell you through experience with this problem on my truck and many others that the plenum gasket on the bottom of the intake is most likely the cause. It is very common on these trucks. The other thing that will give you more of a clue that this is happening is pinging under a light acceleration with a load on the motor. If you look down the throttle bore and find any signs of oil on the bottom, it is time to pull the intake and put new gaskets on it.

 

Thanks for all the input so far.

It turns out there is a minor leak/weep in the valley pan gasket.

After a compression test it looks like a valve issue for cyl 4., 90 v 120 for the other cylinders. Upon inspection the valve seat is hammered.

Given the overall good condition of the other cylinders and reasonable compression, I guess its time for 2 reconditioned heads.

 

I looked at some upgrades on http://www.hughesengines.com/Index/index.php. They are sweet.

Honestly, I'm worried enough that these rebuilt heads are going to blow the rings.

I hope I never have another problem with this truck. But when I do I'll check this forum out. You guys are cool.

 

Just to give an end to the story. I had MOPAR reconditioned heads installed, it included the plenum gasket, valley pan gasket manifold bolts/studs. They also replaced the coolant temp sensor.

The heads were $490 each and have a 3yr/100,000 warranty. I got 10% off for being a UF employee and another 3% off because I bought the truck from this dealership. Total $1900.

They said the lower parts of the engine looked immaculate, and that I should not worry too much about ring blow-by. Other than watching for oil burn and blue smoke, any other recommendations for things to watch?