OK, I've been trolling around here looking for answers and coming up short. Found some good info but nothing exactly like the probems I have and nothing to fix it yet.

I was going about 70mph on the interstate, then abruptly no power, rapid backfiring noise. Truck is still running but has intermittent power. Maybe 10 seconds of strong power followed by 20-30 seconds of no power, like flipping a switch. I limped it all the way home on the shoulder with the check engine light on. About 20 miles of this. It would run all the way up to as fast as I could get it going and then it would loose power and basically be coasting while I tried to just keep it running. Although keeping it running is almost impossible because when it is running poorly it has no throttle response/registration... sometimes.

Got it home barely after a few stalls and lengthy restart attempts. Read the codes and it had a P0300 and all the P030Xs. Last summer I did all the plugs and coils after having some misfire issues. While completing this, 6 of the 8 broke off in the engine, just my luck LOL. Since then I have replaced coil #4 which was intermittent misfiring still, and that seemed to fix the problem.

I can look at the live data using the torque app on my phone and can see the fuel trims and O2 data. Thought fuel delivery right away, but the pump makes good pressure, and the driver module seems to be good to go. MAF is good and looks brand new. Can't find any vacuum leaks. After trying a bunch of stuff and unplugging the PCM multiple times the codes have returned in different variations. At first it was all the misfire codes like I mentioned, then it changed to misfires only on cylinder 4 and 6 with the multiple P0300 code, and now the codes are just P0172 and P0174 (rich bank 1 and lean bank 2). At this point the most effective way to get it to started was if you floor the gas pedal, which I think is actually cutting the fuel.

The fuel trims reflected the P0172 and p0174 with a LTFT1 in the negative teens and a LTFT2 pegged at positive 29%. The odd part is with the O2 data. The O2 sensors upstream bounce between .1 and .8 or so kinda like they should, and when they do the downstream sensors are around .9 (maybe a little too consistent though). But here is the weird part, remember how I said it was intermittent power, well when the truck runs OK the O2 sensors are reading what I just stated above; then all of sudden like the flip of switch there is now power at all, cant hardly keep it running and the O2 sensors all flat line like they are not giving any feedback. I've checked the fuel system status while this occurs and it remains in closed loop.

My last best guess was a plugged cat, and by my assumption maybe the passenger side, bank 1, would be worse than the driver side. I checked the vacuum (at the brake booster) to look for exhaust restrictions and the vacuum was bouncing between about 2 and 5 or so, pretty terrible. I unbolted the passenger side header from the down pipe and was able to open it up enough to slip a socket extension in there to hold it open. Vacuum changed to about 5-7 with less oscillation and now it starts with no throttle input and can barely idle with no throttle pedal input. Also the LTFTs are now hoovering around 0%, but both STFTs shot up to about positive 55%. I'd say an improvement, I guess, but not as much as I expected. Vacuum readings were observed while trying to maintain about 2000 rpm.

Not sure where to go from here? I am poor, LOL, and my wife just finally got fed up waiting for her car back and bought a newer jeep grand cherokee. Now I really need to get this one going so I can sell one of the two!

Any suggestions? A lot of possibilities here but what could possibly explain those crazy O2 readings? At this point if I had a new PCM to try, I think I'd give it a shot and just see what happens LOL.

 

Could perhaps unplug O2s or MAF to force it into open loop, and see if that changes anything.

 

I've unplugged the MAF, no change in running. I haven't unplugged the O2s, that might be part of my next step. Unless somebody has a better idea, I think my next step is going to be to remove both cats, down pipes, and y pipe and get it all out of the way, and then see how it runs...

 

My SWAG is that either a bank has jumped time or the VVT is going nuts.


Why not start with a compression check to make sure its not mechanical?

 

Haven't done compression check because the last time I pulled the spark plugs 6 of them broke off in there, so to be honest I'm a little scared to touch them LOL.

Low compression would definitely tell me some clues about timing issues but wouldn't the thing be throwing some other codes if the timing really was off? Also, wouldn't it be consistently poor performance instead of the intermittent condition it has now? I don't know, timing is up there on my possibility list but I'm not sure.

 

Check the back center of the intake manifold for a large rubber vacuum elbow. Take it off and inspect if for damage.

 

I think you'd get some specific code for timing issues if it jumped a tooth, but these codes aren't fool proof, sometimes the PCM can fail to set certain codes. With a scantool you can check what the VCT system is doing, if it's trying to correct timing that is way out of whack. In order to rule out sticky or bad VCT solenoids, you can simply unplug the VCT solenoids to see if that changes anything.

 

I might be shooting a different direction here, but might be worth-while to check the TPS PID's and see how everything looks. Make sure the voltages are where they should be, as well as make sure the percentages of things like throttle body blade position, accelerator pedal position, and TPS sensor position all jive with how much throttle you are giving the vehicle via the accelerator pedal. Torque won't be able to read but a couple of these PID's.... you'll need to get the paid version of FORScan to see more than just a couple of the PID's I mentioned.

If you need the voltages for the different sensors let me know and I can upload the PC/ED troubleshooting steps for you.

I was experiencing some intermittent power issues, and the vehicle was going into limp mode at times even. Luckily I got some TPS codes to get me in the right direction, and the problem in the end turned out to be the PCM. One of the TPS pins on the PCM had apparently been pushed in at some point, and when that happens, it breaks awake from the flexible plastic circuit board. I guess it was barely making contact all this time and eventually stopped making enough contact. It wasn't really repairable (can't solder anything back together on those pins) so the new PCM fixed the problem. But you could have a degrading TPS and still possibly not be getting any codes just yet, so worth checking out if the vacuum lines and all seem good.

 

Well, I've just about all but given up. I am leaning towards a timing issue, for any number of reasons and for some reason not getting any timing/cam related codes... or a bad PCM? I am gonna haul it in to a friends shop in the next day or two and hopefully they can get a better look with a real scan tool. I will be sure to update the thread once we solve the problem.

 

It started snowing too much for me to want to keep messing with it. My friend stopped by and with his car hauler and we towed it to his shop. He called me the next day and said two bad cats was the issue. He said after pulling out both up stream O2s it ran perfect. Put a scope down there and could see they were melted down and clogged up. I bought both cats/pipes for both sides for $383 shipped off ebay (steal!) and he will be installing them next week when they arrive. He also said that sometimes they end up needing to replace at least the upstream O2s with cat failure, we will see. I'll update again when it is actually back on the road!

 

Hey, Thank You for the update.

More importantly, better determine WHY the cats went South.
You don't want the new cats todo the same.

 

Yep, a usual culprit is running too rich, misfires or running it on leaded fuel by mistake (is leaded still available in the US?)