Truck has random misfire when hot

[DaveS]

Hi, 

Currently I'm dealing with a misfire when hot. Mostly noticeable at idle, lower rpm and slower speeds. It will also make a crackling sound when driving at low rpm. It often hunts for the right idle rpm if I let off the throttle. If I snap the throttle, there's a delay of like 1 second and then it will blow some white smoke like the timing is off. When I go to take off, if I push the throttle too fast at low rpm, there's like a short flat spot in the apps where the truck runs weird and crackles almost like it's choking out. 

 

I have tested and replaced almost all of the fuel related components including injectors 4 times, the pump, and all the sensors except IAT. When cold the IAT reads the same as coolant temp so I don't think that's the problem. I tend to believe the problem is electrical because I did the hot wire to the pump test and the misfire seemed to go away BUT I'm not entirely ruling out some kind of mechanical issue either. I did reset the valves and a few were not quite right so that did help some but I can still hear the rough running when it's warmed up. There is no air in the fuel, per the clear hose test. The apps signal appears to hold steady on an analog meter. I have done the WT wiring mod on my truck, the alternator shows almost no noise and just to be sure I completely disconnected it. Nothing helped. 

 

Until recently, there have been no codes. It finally threw a PO122. So currently I'm doing the testing in Mopar man's article for that code. Everything checked out until step #6, which is to check for continuity between the blue/white wire on the apps harness connector and pin 31 on the ecm connector. Is there supposed to be a certain level of ohms there instead of just continuity? There is continuity there, if I'm doing it right that is. Both harnesses disconnected, key off. 

 

My question is this. In the article you state that if there is continuity, the ecm needs to be replaced. That's where it gets a little confusing. It states if there is no continuity, repair the open. If there's continuity, replace the ecm. So it sounds as if either way it's a problem. Earlier I swapped the ecm with my brother's truck, which is exactly the same as mine. It didn't change a thing. BTW, I'm also running a timbo apps which is probably 4 yrs old now. I had also swapped the apps with my brother's with the same result. 

 

I hope you can help. 

Thanks, Dave 

[Mopar1973Man]

First off the top on the injectors are you running VCO or SAC tips? I've SAC tip crack rather easily. I would pull them and inspect the injectors for cracked tips. I've seen a set of 140HP injectors in a 12V crack a tip in less than a 1,000 miles. Then pop pressure that gets too low will cause it to miss. When it starts dropping down in the 280 bar realm it can start a miss at idle hot. 

 

Just talked to another member just a few days ago and found out he sent his injection pump (VP44) in for testing and found out the PSG had issues and there was no timing advancement, and didn't throw a P0216 code. After having the injectors re-popped back to 320 bar (custom set) and having the VP44 replace the PSG and calibrated again fixed a bunch of his issues.

 

As for the P0122 most likely the APPS sensor that failed but... Wiring wise to make it simple with both ends unhooked you want 0 ohms from end (APPS) to end (ECM) for wiring. Then the other test is red probe to the wire and the black probe to the block or battery ground. It should be infinite ohms being there should be no wires shorted to the block. This will get you pass the Dodge FSM stuff that is confusing. 

[DaveS]

I've had some real problems with injectors. A little over 2 yrs ago the stock injectors got tired and I replaced them with a set of 7x10 vcos from Weston Shupe. I loved them. They made great power, low smoke, and ran extremely smooth. At 30k miles, I was on a road trip running empty and a nozzle cracked. Weston warrantied them but it took a month for him to finally send me the replacements. I still don't know what kind of injectors he sent but they ran horrible. The holes were not in a straight row but offset. Those lasted only 18k before they started peeing. After that I went thru a set of nozzles from power driven diesel, a full set from DAP which also had 1 fail after several thousand miles. Mitch warrantied those and I got him to make me a set of 7x10 vcos, popped at 310. I've put on 7k miles on those now and they seem to be doing ok. When I put them in, the truck ran cleaner and crisper but it didn't solve the misfire. 

As for the injection pump, it's from power driven diesel with new electronics. It also made no difference in the misfire when I installed it. 

 

I did the testing you suggested. There is infinite ohms between the wiring and ground but between pin 5 on the apps harness and pin 31 on the ecm harness, it showed between .3 and .4 ohms with the meter set at 200. What does that mean now? Ecm bad? If so, why didn't it run better when I put on my brother's ecm? 

[Mopar1973Man]

14 hours ago, DaveS said:

I did the testing you suggested. There is infinite ohms between the wiring and ground but between pin 5 on the apps harness and pin 31 on the ecm harness, it showed between .3 and .4 ohms with the meter set at 200. What does that mean now? Ecm bad? If so, why didn't it run better when I put on my brother's ecm?

 

Nope the wire is good. If the ohms were greater than 5 ohms then the wire would need replacing. That small bit of resistance shouldn't pull the voltage enough to set codes. Being even stock you can be as low as 0.4 volts at idle and still pass. I would try a different APPS sensor if possible first before considering the ECM. Even when that time comes I would say to call @Auto Computer Specialist and have them look it over and verify.

[DaveS]

I'll get another timbo and try that

[Mopar1973Man]

Do keep in mind the APPS sensor will not create a misfire. This will only possibly correct the P0122 code. 

 

As for the misfire hot that would be still be between the VP44 and the injectors but most commonly the injectors more so. 

[DaveS]

So it seems like I may have multiple problems going on. The po122, the flat spot and hesitation is most likely the apps sensor. It does run better since I reset the valve lash but it's still not completely smooth. It's just hard to believe it's injectors after replacing them so many times. It misfired immediately after installing them. And it was there with my old pump and also the new one. Hard to believe both pumps would have the same problem 

[Mopar1973Man]

Now reaching out a bit. Now there is a older member here @jlwelding that had a misfire issue after warming up and his problem was a cracked piston. As the piston warmed up it would open the crack up and drop compression. Now I'll say being that yes you could do a blow by test but one bad cylinder will not make it fail. Now a compression test would show a cracked piston. Of course I'm not going to head this way until I know for sure all the simple stuff is for sure covered before heading deeper down the rabbit hole. 

[DaveS]

Yeah I thot of that too. I did a compression test cold and they all checked out 450 psi+. And they were all within 10 psi of each other. But it could be possible that it wouldn't show up so much cold like you said. It has very little  blowby at any time. 

[jlwelding]

Man that eng still haunts me to this day. Close to 9k in tracking down that miss fire. You could start that truck , leave it in the driveway running all day and it sounded like the best running eng you ever heard, drive down the road 250 yards and it would start. It was opening up the crack with added heat. I literally had an entire pallet heaped up of parts I had bought. It is practically impossible to diagnose a hairline crack in a piston. Im going to tell you from my point of view, I will never go down that road again. The amount of money and man hours and others man hours I could have bought a really nice truck. Just buy another eng and call it a day, it would have been way cheaper and the amount of stress not just on me the whole family. I know Michael was stressed too, he got to where he answered the phone, well what did you try today? 

[DaveS]

I hear you, jlwelding, that sounds exactly what I've been doing, throwing money at it and replacing parts. And I wouldn't want to call Mike all the time either lol.

 

It's got 422k on the rotating assembly and it was in great shape when I pulled the head about 2 years ago. Not even a top ring groove and the cross hatching was still all there. But I like upgrading things so I'm seriously thinking of pulling the motor and putting in new pistons, rings, and bearings. I've got some new technology I want to try on that. And reseal everything too. I just hope I find the problem if I go to all that work. 

 

For me, I've been trying to track it down for close to a year now. It wasn't bad at first and I just lived with it but it's been gradually getting worse. Which to me does point to a mechanical problem 

[Mopar1973Man]

Well just the simple requirements of a diesel. Needs fue land compression. As you stated your compression is 450 PSI +/-10. That's good. Being that we have test numbers there I would say move back to fuel. But even that has been checked. 7x0.010 Injectors at 310 bar 7k ago, and changed the VP44. The only thing I can say is send the ECM to be checked.

Edited October 6, 2022 by Mopar1973Man

[Great work!]

Easy things:

 

Check battery terminals 

Swap fuel relay in pdc with another

Re-seat and clean engine connectors by the brake booster.

Swap IAT with a buddy 

Disconnect any tuners

Using a non contact temperature gauge check for exhaust manifold temp deviations, see if it's just one cylinder missing. 

[DaveS]

Great ideas all. I have cleaned all the grounds, checked the cylinder temps with a non contact thermometer, and returned the truck to stock tuning. Battery terminals were replaced with the batteries abt 4 yrs ago. They are clean and in great shape. Truck starts instantly. The only thing I didn't do yet was swap the fuel relay and the IAT sensor. So I can do that. And I could also send off the ecm to have it checked. 

 

One thing I did notice lately is that the injectors are starting to haze more, even when hot. Especially since I don't pull anything heavy very often anymore. Mostly just driving around so it's pretty light load. I hope they're not going out again 

[Mopar1973Man]

1 hour ago, DaveS said:

One thing I did notice lately is that the injectors are starting to haze more, even when hot. Especially since I don't pull anything heavy very often anymore. Mostly just driving around so it's pretty light load. I hope they're not going out again 

 

Yup. The hazing is not a good sign. Thor (2006 Dodge 3500) has this issue currently and I need some time to pull them down and send back to DAP for re-popping and flow matching. I need to just get the free time to pull it apart to start but time is very limited these days. 

 

1 hour ago, DaveS said:

I have cleaned all the grounds

The other body grounds have zero to do with any ECM or PCM those should be either passenger side battery to after the W-T ground mod on the gear case. The other grounds on the fenders and such are all aux stuff like lights and such. 

 

1 hour ago, DaveS said:

The only thing I didn't do yet was swap the fuel relay and the IAT sensor.

Fuel relay if its bad will throw a P1689 code. The IAT you already verified since it matching the ECT sensor. Changing that wont do much good. IAT only changes timing after it drops below +80*F then the stock ECM adds timing. If your already on the Quadzilla this is meaningless being the timing is control from Quadzilla and the IAT sensor only going to control grid heater function then. 

[DaveS]

Gotcha. So sounds like the most likely culprits are either a cracked piston, bad injectors, or something with the ecm

[IBMobile]

1 hour ago, Mopar1973Man said:

The other body grounds have zero to do with any ECM or PCM those should be either passenger side battery to after the W-T ground mod on the gear case.

How to Repair the grounds for the PCM.

https://mopar1973man.com/cummins/articles.html/24-valve-2nd-generation_50/51_engine/electrical/pcm-ground-splice-repair-r675/?tab=comments#comment-115

[DaveS]

I'll do that wiring mod once and see what happens. If nothing else it's an upgrade. 

[Mopar1973Man]

Basically what it comes down to for this issue. Being compression is good cold, I'm going to lean back on injectors because of the haze problem and possibly the ECM.

[jlwelding]

I second the injectors, when you get like I was you should have 3 sets of spares. You tried your brothers ecm and no change Im leaning injectors for the last hope.