Cylinders 1 and 2 Misfire Caper
 

Hey friends, I'm new here. Got a real mystery in my 2000 RAM B3500 Van 3.9L with 178K and don't know where else to turn.

She's been throwing P0301 and P0302 misfire codes at me for some time now despite getting spark, fuel and compression. Even more curious, the cylinders are both in the front and misfire consecutively in the firing order.

Aside from the codes, the other obvious symptoms are a rough idle and the smell of raw fuel from the exhaust, so yeah.. misfires.

Other than that, she runs really smoothly (mostly as a V6?!?) and has been a solid workhorse for the entire 110k I've had her.

RECENT DIAGNOSTICS:

SPARK: Plugs, cap, rotor and wires have been replaced TWICE in the last 100mi (2nd time with better parts by mechanic to rule out spark).

COMPRESSION: Testing (by mechanic) showed levels within range (lowest was 145ish).

FUEL: Injectors and connections swapped (by mechanic) from 1&2 to 3&4 and 3&4 worked great but 1&2 still misfired.

MECHANICAL: Just pulled the valve covers off and measured the lift of the push rods and all were within 0.05 inches of one another. (1&2 were in the middle of the range) so worn cam lobes don't seem to be the culprit.

Also, PCV Valve and air filter just replaced. Fuel pump replaced in last 30K.

Even though it runs smoothly without the front 2 cyl's misfiring, I'd hate to do any damage to the engine.

Bad sensors, computer, or electrical connection somewhere? There is a leak in the water seal above the hood so moisture from rain could be an issue but what should I check?

Any ideas out there of what could be causing these very specific misfires??? Has anyone else had this problem?

Thanks all!

Take a look down the throttle bores, and see if you see oil in there.

Unplug the front O2 sensor, and drive it that way for a day or two. Run any better?

Burn any oil??

How did the plugs look when you changed them?

What is the unladen airspeed of a Swallow?

welcome to DF! :D

Bad intake manifold gasket, worn cam bearing perhaps.
Did he do just a compression test or a leak down test as well?

Thanks for chiming in here gents. Got some interesting news...

I found some oil pooling in the intake manifold I finally got around to replacing the plenum plate and gasket with the Hughes kit last weekend.
Total success! However, aside from the little bit of oil in the plenum pan, there was no visible break in the gasket.
I suppose it all needed to be done sooner or later so I'm glad it's done.

That said, the idle is still rough and she's running pretty fumy so I imagine as soon as I hit the highway with a load, misfire codes will come back.

There was only a compression test done, no leak down... yet. Not a bad idea but I don't have a leak down tester as of now.
Also, I don't recall how the plugs looked and I didn't pull the O2 sensor either yet. Could be worth a try, BUT...

When I let her run a while after the plenum repair, I hear a knocking noise. And it wasn't a new one - I remember thinking I heard something before but now it's very apparent.
It's somewhere under the driver side and clearest under the van. It's only at idle and after she's been running for a few minutes.

Timing chain slack? Think that would be causing the cyl 1&2 misfires?

Timing chain slack would be more likely to give you random misfires on all cylinders, not just two..

Try unplugging the front O2 sensor. See if anything changes. (this WILL set a code.)

I spent years chasing a misfire on 3, 5, and occasionally on 1 from the day I owned the van. I changed plugs, wires, cap, rotor, injectors, intake gasket, plenum gasket, cam sensor, and the crank sensor twice. I resigned myself to it being a cracked cylinder head where the misfire would come and go based on heat changes. Then my 2nd crank sensor started acting up so I changed it. After that I have not had it misfire even once. The problem the whole time was crank sensors that just were not manufactured with high enough quality. So either keep swapping out crank sensors until you get one that makes the problem go away, or bite the bullet and get an OEM one.

Also, what brand/type of plugs did you put in?

I disconnected the upstream O2 sensor and ran it for a while but nothing changed for the better. It actually just made for even more fumes which I'll take to mean that the 02 sensor is still good.
Thoughts?

As for the timing chain, I initially was getting misfire codes for the 1st and 2nd cylinders AND a multiple/random cylinder misfire (P0300). I was told that P0300 was being thrown because the 1st and 2nd cylinders were misfiring so I didn't think to mention it. Now I'm thinking maybe there are other misfires occurring besides those 2 cyl's but less frequently and that the timing chain is partially responsible.

Could this be a possibility? Either way, slack in the chain alone can't be good.

Maybe in combination with a worn distributor gear, and even a faulty/confused crank or cam position sensor, a specific rhythm could come about where the timing could be thrown off mostly when the first two cylinders should be firing. Just wish there was an easy way to test without a scope.

I recently did another compression test to be sure that compression wasn't the issue. Attached are the results. Looks like they're about 165 to 135 and the misfiring 1st and 2nd cylinders are in the middle.

Is there any way to test for a faulty crank sensor?

Champion RC12LC4