Chasing a problem

[N2BRK]

I'm long out of talent, and and can't seem to lick this one. My buddy's truck build became the biggest CF in history. He finally got it back and it shook like a paint mixer - shop improperly balanced. Otherwise it drove perfectly. The truck was driven for a couple months and then torn back down. Again a long time in between CF ensued. Now it's all back together and under light throttle it drives just fine, but under load it will cut out and come back. It's violent! We thought it was the trans at first in a tie up - that bad. I put it in M5 and T/H and rolled in on the highway and got it to do it while locked in 5th - so it is not trans. I logged it one night and the fuel, load, and actual/desired RP all stay constant but the MAF, MAP, Baro all take an instant dive and come right back. It's a super short event. You can see it in the throttle position too, yet there is no actual movement of the throttle.

No codes. Nothing. Only codes we were getting were misfire during cold temps (now gone) on high idle warm up. When it cuts out and comes back there's never any code. Fuel is fresh and AD filters changed.

Aside from going through every wire in the harness, we also swapped MAP, MAF, and ECU with my truck - no luck. Last night I swapped the vane pos sensor and that also did not fix anything. Cam position sensor? Throttle? Lug nuts? WTAF! LOL. I'm at a loss. This guy is on a year and a half without a working truck and he's obviously disgusted and about to set it on fire.

Any SWAG??

 

[Dallas S]

What mods are done to the engine? Sounds like a similar issue I am having with a compound LMM...

 

[darkness]

Does it do it at any certain rpm? Or as soon as you mat the throttle?

 

[N2BRK]

Fingers cast Pistons, valve springs, rods, o-ringed block. LMM.

RPM isn't a factor. I can do it at 3000 or down below 2. It seems to be load dependent. With a small blip of the throttle it will do it, with a steady throttle it will do it as you get a loan on her, or if you mat it you'll cry thinking Uncle Rodney is going to come through the block to say hello.

 

[Chevy1925]

I’d be looking at the bail connectors. Engine torquing up might be pulling on the connectors. Verify grounds on the block as well

 

[N2BRK]

Thanks - checked them and they ohm out good. I will check again. Grounds were cleaned and reattached already. Will triple check.

 

[Dallas S]

I have two LMM that do very similar. They both are load/airflow dependant IMO. Both compound turbo. Gear doesn’t matter. I will post up some logs for you to compare. Logging and tuning on one of them today. Overlay dyno runs of the two and they look identical(other then power output, 650hp vs 950) cut comes 2.6 seconds into the pulls. Not rpm dependant. Not b3400-b3403 related.

 

[Ron Nielson]

With your symptoms, it sure sounds like an electrical problem, something is shorting out momentarily. Our trucks are getting old and the wiring harnesses are showing weakness. In my truck, I have replaced the wire loom on every harness that I could get my hands on. Many of them just crumbled. I know MAF and MAP are physically close to each other but I haven't looked at a wiring diagram to see if there is a common circuit between the two. Wiring diagrams available here: https://www.gmupfitter.com/pages/body-builder-manuals. And if you can ever get a code out of it, this is a resource that publishes all the GM advisory letters pertaining to codes: https://www.dtcdecode.com/. I have recently seen some relating to wiring harnesses grounding at various points in the engine and under the dash.

Another thought. There was a poster a couple of years ago on another forum that had a problem where the engine would just randomly turn off (completely) while driving, then would start right back up and run normally for a while, then turn off, again and again. He finally traced it to the battery + cable connector that feeds the fuse box. It was just loose enough that it would randomly lose its connection. That sounds different than this problem. Even so, I would take a very close look at that fuse box connection, look for green crusties anywhere in the fuse box, pull the fuses and make sure they are seated correctly, etc. If this truck lives in salt country, it is a real possibility.

 

[juddski88]

I'm long out of talent, and and can't seem to lick this one. My buddy's truck build became the biggest CF in history. He finally got it back and it shook like a paint mixer - shop improperly balanced. Otherwise it drove perfectly. The truck was driven for a couple months and then torn back down. Again a long time in between CF ensued. Now it's all back together and under light throttle it drives just fine, but under load it will cut out and come back. It's violent! We thought it was the trans at first in a tie up - that bad. I put it in M5 and T/H and rolled in on the highway and got it to do it while locked in 5th - so it is not trans. I logged it one night and the fuel, load, and actual/desired RP all stay constant but the MAF, MAP, Baro all take an instant dive and come right back. It's a super short event. You can see it in the throttle position too, yet there is no actual movement of the throttle.

No codes. Nothing. Only codes we were getting were misfire during cold temps (now gone) on high idle warm up. When it cuts out and comes back there's never any code. Fuel is fresh and AD filters changed.

Aside from going through every wire in the harness, we also swapped MAP, MAF, and ECU with my truck - no luck. Last night I swapped the vane pos sensor and that also did not fix anything. Cam position sensor? Throttle? Lug nuts? WTAF! LOL. I'm at a loss. This guy is on a year and a half without a working truck and he's obviously disgusted and about to set it on fire.

Any SWAG??

I can't even count how many threads you've got on this truck's problems, I do feel for you guys. I'm sure you've answered this before, but you have tried other fresh TIS OS files correct?

 

[N2BRK]

I know it’s just been such a cluster. I feel terrible for him. I’ve been trying to help here and there at night after work. We didn’t try different files but I swapped my known good ecm and it didn’t help.
I’m stuck on thinking/agreeing that it’s a wire because it sat apart so long at his mechanic’s that there could be corrosion. My friend went through and cleaned and tightened every connection - and he found some were bad and a ground not connected. But it didn’t do it either.

They just extended my busy season by a month, so I’ll still be working 7 days a week through mid May. I hope it’s solved by then, otherwise I’ll give it all I can to help in May. But I’m just an enthusiast not a trained mechanic, so I dunno. What dumbfounds me is that it’s bad enough to make your left nut jump in pain but won’t throw a code! I thought these smart trucks were supposed to throw codes, lol.

He was gut punched when we showed up to pick it up and it’s idling in the lot with the CEL and throwing misfire codes. They spent three days trying to get it to start. They went through all the wires and found nothing and then it fired up. Ugh. So much money and time in this.

 

[Yngdmax92]

No joke I have had the same issue as you speak of on my personal lb7. I posted about it with logs and videos. Tried everything under the son. And I mean everything. I changed ecm, tcm, ficm, app sensor, engine, engine harness, cp3, injectors. Everything !!!

I was young and dumb and blamed Kory willis ppei tunes to. He gave me money back and I tried other tunes, different os files. Couldn't get it. Literally sold the truck. Pissed me off so bad! I hope you can help this guy out

 

[N2BRK]

oh man. That’s sad. I get it though!!!

No joke I have had the same issue as you speak of on my personal lb7. I posted about it with logs and videos. Tried everything under the son. And I mean everything. I changed ecm, tcm, ficm, app sensor, engine, engine harness, cp3, injectors. Everything !!!

I was young and dumb and blamed Kory willis ppei tunes to. He gave me money back and I tried other tunes, different os files. Couldn't get it. Literally sold the truck. Pissed me off so bad! I hope you can help this guy out

 

[Ron Nielson]

What year truck are we dealing with?

I logged it one night and the fuel, load, and actual/desired RP all stay constant but the MAF, MAP, Baro all take an instant dive and come right back. It's a super short event. You can see it in the throttle position too, yet there is no actual movement of the throttle.

Was this captured with a scope or something else so that you can look at the waveform now?? Did the signal drop out almost entirely? If so, what happened on the high-speed LAN at the same time, if you know?

Ivan of Pine Hollow Diagnosis is working on a Cummins that 'drops out' when you set the cruise control. See here"

Source: https://youtu.be/TF0IPyzY1eg
As in your situation he knows what is happening, at least in part, but hasn't found the cause of the dropout. So far, he thinks bad ECM, but hasn't found the right test to make sure. Maybe some help with your truck when Ivan reveals the answer to his problem.

 

[2004LB7]

I logged it one night and the fuel, load, and actual/desired RP all stay constant but the MAF, MAP, Baro all take an instant dive and come right back. It's a super short event. You can see it in the throttle position too, yet there is no actual movement of the throttle.

Aren't those all from a common 5 volt circuit? I think maybe even the oil pressure sensor too. I thought I seen a thread where the oil pressure sensor was shorting out and killing the power to the maf, map and some other sensors

 

[N2BRK]

I am working on borrowing a better scanner to do just that.

I think they are on the same reference. I'll check oil pressure sensor too. thanks

 

[2004LB7]

Yeah, maybe just need to log the 5v supplies. I think there are two or perhaps three of them. If you see any changes then maybe that will help direct you to the problem

 

[clrussell]

Make sure the 5v wire (and other wires) going to the oil pressure sensor aren’t squished between the power steering drive bracket and the block.

 

[N2BRK]

Hopefully I can make some time this weekend. This time of year I work around the clock 7 days a week. With what little time I have, it's better to ditch the honey-do list and work on trucks anyway, LOL.