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Most pervasive problem I have ever had in any vehicle I have owned - Can Anyone Help?

I’ll try to keep this as brief as possible, but this has been a problem that has been going on for 2 years that I have been unable to fix and it has limited where I can drive the durango. Luckily, it is a utility-use vehicle and not our daily driver (any longer, but because of this problem). I have never experienced anything like this before and have stayed away (mostly) from throwing parts and really tried to diagnose as much as I can. So here goes.

2006 Durango, 4.7 SLT, ~ 160k.

I won’t get into the life history, but here is what I cannot resolve.

If I am driving around town (less than 45 mph), it drives perfectly fine. No issues at all. If I am driving on flat straight road at 45+, it drives fine, no issues at all (95% of the time). However, say I am driving on a highway that has some flat stretches then some hills. Not super steep, but steep enough to make it drop down a gear if I hold throttle steady and, of course, if I press harder on throttle. I can be cruising along at 45-60mph+ on flat stretch, then go down a hill, but have to go up a hill, same speed, I start losing power, CEL comes on (for multiple misfires, usually bank 1) At the same time, I hear what sounds like afterfire in the cat area or some other area of exhaust on driver side. I can feel it in the floor as well. It’s constant as I am trying to head up the hill pop, pop, poppopopop, pop pop, etc. Once I get to the top of the hill and it levels out, it is still struggling, but usually it clears itself up. I assume there is some recovery time. So long as I am driving flat again, it drives like nothing happened. Again, if I am below highway speeds, I can climb hills no problem at all and this doesn’t happen at all - THIS is what is driving me nuts. It is VERY specific - highway speeds/under load.

Here is what I have checked and what I have replaced:

Fuel pressure is spot on
Cylinder pressure is within specs for dry and wet, with no concerning deviation between neighbors
I replaced ALL plugs (Champion Copper, also tried NGK and a few other brands, but Champion has been best)
Replaced ALL coil over packs
Replaced cylinder 5 and 7 injectors (they seemed to be the cylinders throwing codes the most when specified)
Also replaced cylinder 3 injector as it had a crack on the side that concerned me.
Post Cat O2 sensors have been replaced (due to CELs at various times over the years), though I know this is not really relevant.
I will admit, this thing does have pre-cat O2 sensor problems. I have had to replace bank 2 sensor 1 once before and now need to change it again due to a heating problem. This, however, is the opposite side where I have the problems, it seems.
New Battery (within the past year - had lots of corrosion)
Alternator tests perfectly fine.
Changed air filter and all that good stuff.
Ran Seafoam in crankcase, intake, and gas. It helped it run smoother, but did nothing to fix the problem.

No coolant contamination in the oil at all. Although, I do think I have a leak somewhere as I get white smoke when I first start it that is more than just condensation, but it clears up pretty quickly. I have to put in enough coolant from min to max no more than 1 time a year, so it is slow, whatever it is. No signs of hose leaks or any other leaks underneath.

It starts fine, every time, no matter the temperature. It idles mostly fine. If I concentrate hard enough, I can feel a little teeny bit of shakiness on idle, but not sure if this is normal for this vehicle and age.

From the wires I have inspected, they are all still sealed up, no rotting that I can see, though the one thing I haven’t done is really get in and diagnose electrical, though no other signs of electrical problems.

I just don’t get it. Except for this one very specific problem that happens at a VERY specific point in time and is repeatable every time, it runs great.

I love the truck for a good 4 wheel drive, towing, and the getting-supplies-from-the-lumberyard vehicle, but this problem has limited me in using it.

Thanks in advance!

Does the Check Engine light ever start flashing?

Tester

You said that you checked the fuel pressure, but I did not see reference to fuel volume. Most of this sounds like classic plugged fuel filter symptoms.

Another possibility, though less likely, is obstructed exhaust. Has it ever burned a lot of oil that may have filled the catalyst with ash, or has it overheated one of the cats from misfiring?

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I agree with @Manolito. It sounds like a fuel delivery issue. I would be checking the fuel filter, fuel pump, and fuel lines. And don’t kid yourself. Fuel pressure can look great in the driveway, but it’s a world of difference when the engine is under load.

The conditions you cite are related to engine load. The faster you go the higher the wind resistance and more engine power it requires. Then add in a hill and it can’t handle the demand. Either one alone is not enough demand to reveal the weakness.

I’d be interested in hearing more about the specific codes and their circumstances. You mention which cylinders are indicated most often so thinking you have more detail in this regard.

I too agree with the fuel volume. You would need a fuel pressure gauge that you could leave in place and read from the passengers seat, then drive a little ways until you come to an open road with little traffic. Slow down to about 30 and then go WOT (wide open throttle) and have the passenger watch the fuel pressure as the engine goes through 5000 RPM. If it drops, this would be the issue.

If that isn’t the problem, then remove the exhaust manifold from the side that seems to be most affected. Look up into the warm-up cat (aka pre-cat) to see if the matrix has melted and is blocking exhaust flow. This can happen even without oil burning. If the engine has run too rich too often or sometimes just the proximity to the exhaust valve can cause this to happen. If so, replace both exhaust manifolds and pre-cats.

You have narrowed it down to the two prime suspects for me as well. If I had to put money on it, it would be hard to do as it could be either; however, I would lean more towards possible exhaust blockage. Good thing is I have to replace the front O2 sensors tomorrow (one is bad and I like to replace in pairs. So since they are an absolute pain in the tail on this truck, I’m certainly going to take the opportunity to pressure test the cats on both sides.

As far as volume, again, a possibility. My check valve is in such a position where I can actually tape the gauge to my window securely and see it while I’m driving. I thought about this when i tested the pressure the other day but didn’t do it.

Interestingly, the 2 cylinders I have the most problem with are the last 2 at the end of the fuel rail. However, I’d think fuel is getting to the cylinders because of the stereo fire in the exhaust that I know is happening.

I do feel (hope) that after a couple years of the problem I am close…hopefully not just wishful thinking.

Wouldn’t a pressure test in the front sensors (and subsequently the rear if I find a problem to rule out muffler) do the trick, or be good enough? Removing the O2 sensor is pain enough but the manifold removal is a bit much for a “just to see” activity . If I discover a blocked cat , I can then replace the manifolds at that time. I’m glad you brought up oil. It IS burning oil and I forgot to mention that. Doesn’t seem to be too bad though. About half quart between oil changes. I am nearly certain this is due to a leaky valve seal. I would have to get this done by a pro and with the quoted cost, I’d be better off just replacing the motor in reality. Thanks for the response.

Put me in the column of a fuel problem too. Pump, filter (sock or in-line), pressure regulator. Do what I did and tape the fuel pressure tester to the windshield and see what the pressure is when it’s acting up, then go from there.

You might just drop the exhaust pipe and use an inspection mirror and flex head flashlight to look up into the cat. Be sure to replace the doughnut afterwards.

If you know someone with an inspection camera, you could look through the front O2 sensor hole.

No, you cannot check the pressure in the exhaust manifold because the engine won’t run properly with the O2 sensor missing. You need to check while driving under similar conditions.

Good luck, because with my imagination, I can fathom a scenario where it’s a transmission issue.

Drive this vehicle at night and make it do what it does… As soon as it produces these symptoms pull over and look at your Cat or Exhaust headers… They may be glowing red. Sounds like lean out conditions probably caused by a fuel pump that cant keep up with demand/pressure correctly.

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I’m thinking the fuel pressure is dropping for some reason when this symptom is happening. That’s where I’d start, have your shop rig up a fuel pressure gauge so you can see it when you are driving. If it drops coincident with this symptom you’ve found a good clue. If anything the fuel pressure should increase as you start up a hill b/c the manifold vacuum will decrease (toward more positive pressure) so the fuel pressure regulator should increase. The fuel pressure regulator’s job is to keep the difference between the fuel rail pressure and the intake manifold pressure constant.

After that I’d do fuel trim measurements. This all assumes there are no engine codes besides the misfire codes of course.

After that I’d temporarily modify parts in the exhaust system so the cat is bypassed, as a test, checking if it is caused by a plugged cat.

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I plan on doing the test like this. By the way, on this truck, the fuel filter is not inline or I would have changed it long ago. It is all one piece with the fuel pump…in the tank.

Some day a throttle position sensor will be the culprit. I have only seen one go bad myself and it was cracked. It was also on a 1986 GM car, so good luck with that one.

Sure it will, just needs to be cold and running in open loop mode…

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