Well, Tore it all apart this weekend. Completely replaced all plugs & wires with new ones. Boy did I get a surprise when I pulled the wires off the Distributor cap. Holy hell. the cap was toast and so was the rotor. Turned out to be an easy fix and she now runs like a champ! Thanx for all your help guys.

 

Hey poison squirrels. This is my first post in any forum ever. I've read a bunch of the ones in here and it has helped me troubleshoot the problem, but I still have not stumbled upon the solution.

20K miles ago I had a tuneup on my Durango, ran like it was brand new. New plugs, plug wires, rotor & cap with what looks like gold or maybe brass contacts. About 5K miles ago I started to have a stuttering problem with slight loss of power, and eventually threw the P0420 code.

Bought a new O2 sensor from AutoZone, no difference so I took it back to the store and lived with the issue as I only drove it to the airport & back.

I've been throwing parts at it, thinking "it's fuel, spark & air - how hard can this be", but still haven't fixed it. It currently has 150K miles. So far here is what I have completed/verified:

1) Fuel pressure test showed initial pressure at 50, then after a few seconds ranged between 45-47, which is within the acceptable range. I also swapped out relays thinking there might be something, but both relays tested the same. So I conclude that both relays and fuel pump are good.

2) Injectors -I can feel them all clicking, and have metered all of the coils, and all of them check out in between 12.1 - 12.4, so I conclude they are good. I have also ONLY run Chevron Supreme since I bought this truck, and I'm the original owner. I had to replace the intake manifold around 80K miles (thermostat bolts rounded off and I couldn't drill the out), and my original intake was extremely clean. I cleaned out the replacement intake, and peering in through the throttle body, it still looks pristine. Throttle body itself is also very clean.

3) replaced TPS sensor

4) replaced Idle Air control

5) replaced MAP sensor

Hard to start, seems like it's running lean as it seems to me to rev higher when it is trying to start. Idle is at 600 RPM, and is rough, like it's missing fairly badly. Pressure coming out of the exhaust is rough as well, inconsistent and breaking up.

When I do get it running, I have to feather the gas pedal to keep it running at stops signs/lights most times, and my air stops blowing cold if I don't as well. If I romp on the gas (my preferred method of driving of course!), then she almost dies, then backfires, and I have to feather the gas again to get higher rpms.

I fear that the leaning out is dumping gas, most likely ruining my cat in the process. The cat could already have issues, and possibly I've fixed the original problem by throwing parts at it, but now the cat is the issue. I am worried that if I replace the cat and have not fixed the original problem, then I'm just going to ruin another cat.

I don't have plats (do have Champions).

One more thing...when I take the vacuum line off of the intake, she jumps about 400 rpm for a few seconds, then drops back down. I've seafoamed it with limited improvements. I've added seafoam to the gas.

Any additional insight is greatly appreciated...have a great weekend!

Best regards,
Matt

 

Matt,

As a side reminder, only use OEM stock replacement O2 sensors. Aftermarket will tend to fail again quickly.

Take off the air hat, open the TB with a screwdriver propped in there and take a look down inside with a flashlight. If it is wet that's bad. It should be bone dry. Wet and you have found part of the problem.

The TPS is best left to a stocker OEM part too. I personally have a 6 week Borg Warner TPS coming off tomorrow in favor of a stocker. Lesson learned. While the TPS isn't as addicted to being stockers like the O2, just play is safe for the long run.

Check the battery under load; check the battery terminals for uber tight and clean. Any issues there, replace the battery.

IndyDurango

 

I always heard that 5.9L engines doesn't like the platinum spark plugs, but the Iridium's are fine... Is this right? I mean I've been using iridium spark plugs on my 5.9L for 3 months and there is not a single problem but I'm curious about long term effects. Will there be any?

 

Don't worry about the IR plugs. I am currently using the champion IR truck plugs and I have over 50k on them. They really outperform the plats and do not interfere with the older style ignition system (5.2/5.9 engines). They are supposed to be good for 100k miles. so I am keeping watch of the plugs. So far, the tips are as clean and not fouled up at all. Starts super easy and idles smoothly.

 

Hey guys & gals,

The problem was a plenum gasket. I had a local trusted mechanic shop fix it for me, and it is driving like the day I bought it again. FUN to drive!

I hope that this helps someone else, as this was a challenging one to figure out.

Drive it like you stole it,
Matt

 

Matt, exactly what my first mention to look at was. Bingo.

Just had that fixed with the Hughes kit myself on my R/T.

Nice to hear you got it fixed.

IndyDurango