I'm thinking about sensor failure. Is it pulling timing because of bad software or bad sensor input?
CK5
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Or what if it's setup for the wrong sensor type(s)?
that's basically the idea behind the data logging however we haven't been able to pin point the cause. Every sensor has been replaced, some more than once and I'm on my second PCM from Howell. The fuel pressure is still an issue however I'm not confident it's the cause of this particular problem.
I'm heading to the mountains in a couple days for the Sierra Trek offroad event and I'm planing on bringing the truck. Luckily it runs well enough at lower rpm to hit the trails which are typically ran in double low all day. I'll probably park it for the fire trail runs unless it's fixed....
I will put this out here as the confusion is thick. You've got two issues to focus on. One is fuel pressure. Two the timing drops under throttle. To my understanding, one is not driving the other or vice versa. The ecm will chase itself with low fuel pressure like Larry said, trying to richen up the mixture. But that's not going to cause the timing to fall off under acceleration when it really should go the other way at that time.
With the timing falling off under load being consistent between ecms and not having and Knock sensor input is the thing I can't wrap my head around. I'm thinking out loud here, but on one hand the timing going negative is consistent enough to believe it's in the tune. But on the other hand, it's also behaving like it's actually getting a knock sensor signal even though it's not showing it in the data. I'm curious if you'd see any Knock input if you were to tap on the block while it's running and data logging. I know the sensors have been replaced, but I'm just wondering if the ecm would see the input or not.
That still won't explain why the timing is dropping under load, but at least we know the knock sensors and circuits are functioning. If you don't see the input by tapping, I'd start looking at the circuits. It might not be pinned right at the ecm connector. Could be a bad crimp. Not sure, but it doesn't make any sense that it wouldn't pick up any knock value.
Again I'm thinking out loud. Something is going on where it's behaving like there is a knock input to retrard the timing. I just can't put my finger on it. But the fact that the ecm has been swapped with no change, multiple sensors swapped with no change really tells me you need to look at components that haven't been changed that are the common denominator. That may be a deeper dive into the wiring. Simple continuity tests between the sensor and ecm would prove the terminal position in the connectors and if the circuit is complete. I've not had a problem with either one of the harnesses I got from Howell, but that doesn't mean somebody could have an off day and transpose a couple of circuits at the ecm side considering how many there are. I think you are down to that point where it's just a crazy error in something like that. It would be the only explanation given the amount of parts swapping you've done without any impact on the problem.
With the timing falling off under load being consistent between ecms and not having and Knock sensor input is the thing I can't wrap my head around. I'm thinking out loud here, but on one hand the timing going negative is consistent enough to believe it's in the tune. But on the other hand, it's also behaving like it's actually getting a knock sensor signal even though it's not showing it in the data. I'm curious if you'd see any Knock input if you were to tap on the block while it's running and data logging. I know the sensors have been replaced, but I'm just wondering if the ecm would see the input or not.
That still won't explain why the timing is dropping under load, but at least we know the knock sensors and circuits are functioning. If you don't see the input by tapping, I'd start looking at the circuits. It might not be pinned right at the ecm connector. Could be a bad crimp. Not sure, but it doesn't make any sense that it wouldn't pick up any knock value.
Again I'm thinking out loud. Something is going on where it's behaving like there is a knock input to retrard the timing. I just can't put my finger on it. But the fact that the ecm has been swapped with no change, multiple sensors swapped with no change really tells me you need to look at components that haven't been changed that are the common denominator. That may be a deeper dive into the wiring. Simple continuity tests between the sensor and ecm would prove the terminal position in the connectors and if the circuit is complete. I've not had a problem with either one of the harnesses I got from Howell, but that doesn't mean somebody could have an off day and transpose a couple of circuits at the ecm side considering how many there are. I think you are down to that point where it's just a crazy error in something like that. It would be the only explanation given the amount of parts swapping you've done without any impact on the problem.
I’m leaning towards a short in the harness like I mentioned a little bit ago. It just doesn’t make sense anymore.
I agree it can happen.
I bought a brand new genuine CAT harness for a 1998 c12 CAT engine and after changing every sensor and still no start we started testing the harness and found 2 wires were swapped, one was power to the injectors.
Everything is on the table now
What version of HP tuners are you using? I'm gonna guess it's one of the newer ones- like 5+?
Somewhere the newer versions were cutting out options because the EPA was getting after them. You get a chance, and you feel like it, upload your hpt file. Curious to see how yours looks in my older version. (I refuse to update).
Somewhere the newer versions were cutting out options because the EPA was getting after them. You get a chance, and you feel like it, upload your hpt file. Curious to see how yours looks in my older version. (I refuse to update).
I wonder if it’s time to start the data log and pulling on the harness to see what happens.
The knock retard kicked in when the MAF was unplugged therefore I don’t have any reason to believe that isn’t working properly.
I still have a hard time believing the tune is wrong but crazier things have happened.
The harness or some other wiring issue is always a possibility however I thought I would see something on the data logging to narrow it down.
I’m using the current version, whatever that is. I can upload the file or email it. I uploaded a copy last week.
I’ve done that while it’s stationary, however the truck needs to be moving and under load.
No and I don’t know what that is.
It's a calibration of the crank position. Probably on your first PCM, it came with the engine and so was already done.
You do it in the scanner, not the editor. I think it's under Vehicle-->Controls & Special Functions. It doesn't really sound like your problem, but I'm surprised Howell hasn't mentioned it. I don't know if it's possible to read the values out of one PCM and copy them to another.
https://ck5.com/forums/threads/quadrasteer-8-1l-suburban-tow-rig.339146/page-13#post-4171696
You do it in the scanner, not the editor. I think it's under Vehicle-->Controls & Special Functions. It doesn't really sound like your problem, but I'm surprised Howell hasn't mentioned it. I don't know if it's possible to read the values out of one PCM and copy them to another.
https://ck5.com/forums/threads/quadrasteer-8-1l-suburban-tow-rig.339146/page-13#post-4171696
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I’ll check that out, thank you.
The first PCM didn’t come with the engine btw. That was sent in as a core after I ordered the harness and PCM from Howell.
On a tech 2 it’s called a crankshaft variation learn procedure. Odd thing is, after putting my new to me 8.1 in earlier this year both Larry and I tried to do the crank relearn as it should be be done with the install of a new engine with old pcm or with a freshly calibrated pcm.
I’ve done the process a hundred times at the dealership. But no matter how I did it the tech 2 would abort the procedure. Because the base tune is based on a 6.0 (which still requires the same procedure done) I built the vehicle up on the tech 2 as a 6.0 2500 truck, wouldn’t do it. I built it up as a 8.1 truck for different years. Still nothing. I tried every year of a 800 truck from 2001-2007 with a 6.0. Same results.
Besides it being procedure for a fresh engine install, mine has the annoying habit of not wanting to fire up on the first crank (cold or warm). Sometimes it will let out a big backfire through the exhaust like the timing is way off. Cycle the key off and try again and it starts right away like it should. It never had that habit with the old engine and I’m sure because we couldn’t get the crank relearn done. But it hasn't caused any other timing issues at all. The engine runs strong like expected.
While I think it’s important to do the process, I’m not sure that’s why the timing is falling off under a load. I’d see if you can get it done with your software and recheck.
I’ve done the process a hundred times at the dealership. But no matter how I did it the tech 2 would abort the procedure. Because the base tune is based on a 6.0 (which still requires the same procedure done) I built the vehicle up on the tech 2 as a 6.0 2500 truck, wouldn’t do it. I built it up as a 8.1 truck for different years. Still nothing. I tried every year of a 800 truck from 2001-2007 with a 6.0. Same results.
Besides it being procedure for a fresh engine install, mine has the annoying habit of not wanting to fire up on the first crank (cold or warm). Sometimes it will let out a big backfire through the exhaust like the timing is way off. Cycle the key off and try again and it starts right away like it should. It never had that habit with the old engine and I’m sure because we couldn’t get the crank relearn done. But it hasn't caused any other timing issues at all. The engine runs strong like expected.
While I think it’s important to do the process, I’m not sure that’s why the timing is falling off under a load. I’d see if you can get it done with your software and recheck.
I've been reading about similar issues for other users. I'll try it tonight after work.
Yesterday I knocked a few other items off the list.
Hood vents installed.
Installed new ORD Kevlar bushings and bolts at the front shackle hanger (both spring and hanger)
also not pictured, I wired the derale PS cooler fan and installed 8 rock lights for the night runs.
Hood vents installed.
Installed new ORD Kevlar bushings and bolts at the front shackle hanger (both spring and hanger)
also not pictured, I wired the derale PS cooler fan and installed 8 rock lights for the night runs.
I have been tempted to install some vents, after I got rid of the cowl hood I could tell a difference.
I hope you get this fixed, I really think it’s something silly with timing and the harness.
I hope you get this fixed, I really think it’s something silly with timing and the harness.
Per the instructions, the PCM wants to be wired to both the parking brake and the brake pedal switch. I don't know if either of you has either of those things. I linked to my old post because I couldn't get HP Tuners to do it until modifying the instructions. In my case I was using a 6.0 PCM, but the vehicle application was a valid combination (8.1L in 2500 Suburban). I had just segment swapped the 8.1 into the factory Suburban file.
I agree that it's probably not the cause of these issues, it just struck me as odd that PCMs are getting swapped without any mention of CASE learn.
The parking brake and brake pedal were my concern as well since I'm not wired for it.
