I've seen this on a few trucks I've tuned, and I recently had a friend contact me to ask if I could help him with this problem on his truck. Here is what I do to fix it. This tip is for an LBZ or LMM. I haven't had to fix this issue on an LB7 or LLY. The first thing you want to do is try raising the fuel pressure limiting tables. These are {B1058}, {B1057}, and {B1073}. I've attached some screen shots below so you know what to look for. In table B1058 and B1073, I raise it to the max value of 250. In B1057, I usually just raise the cells that are 180 up to 250. This just makes sure that the actual rail pressure doesn't go outside the max values and freak everything out. On my truck, this was all I had to do to fix my jumpy rail pressure.
If those changes don't fix your issue, the next modifications will have to be to the PID control tables. If you compare them, you will see that the LMM and LBZ tables are very different. What I have done before on an LMM that worked was to copy and paste stock LBZ tables into the tune. If that doesn't fix it, or you are already working with LBZ tables, you will need to start increasing the values in these tables. The tables can be looked at in two groups. The first group is the "P" and "D" control tables. These tables are {B1068}, {B1069}, {B1070}, and {B1062}. They all look very similar to one another and I adjust them using the same technique. If you look at the graph for these tables, you will see that all of them have a spike toward the left side of the graph at the 1200 RPM value. What I do is highlight everything above 1200 RPM and increase it by 10%. If that doesn't get rid of the rail pressure spikes, you will need to increase it a little more, but remember, your numbers are bigger now, so if you go up another 10%, that will be a bigger increase than the first 10% because the value is now 10% larger than the original adjustment. I would try 5% and see what that does. Here's a screen shot showing what the tables look like.
Now, when you do your first 10% adjustment to the "P" and "D" control tables, you also need to adjust the "I" control tables. These are tables {B1063}, {B1064}, and {B1065}. If you look at the graph of these tables, you will see the values sort of stair step down as the RPMs increase. What I like to do is copy a value of a step and paste it into everything below that. If you look at your datalog, you will probably notice the rail pressure spikes get larger as the RPMs increase. You need to see where they really start to get pretty large and start there on this adjustment. I've attached a couple of screen shots showing the before and after. I used the value of the 2400-2600 RPM step. In most cases, this will get you close to where you need to be, but if not, you can always go up another level.
If you keep playing with these tables, you should be able to smooth even the roughest rail pressure spikes, but always make changes in small increments to keep from creating another issue in your quest to fix your original problem. I hope you find this helpful. If you have any questions, please don't hesitate to ask.
If those changes don't fix your issue, the next modifications will have to be to the PID control tables. If you compare them, you will see that the LMM and LBZ tables are very different. What I have done before on an LMM that worked was to copy and paste stock LBZ tables into the tune. If that doesn't fix it, or you are already working with LBZ tables, you will need to start increasing the values in these tables. The tables can be looked at in two groups. The first group is the "P" and "D" control tables. These tables are {B1068}, {B1069}, {B1070}, and {B1062}. They all look very similar to one another and I adjust them using the same technique. If you look at the graph for these tables, you will see that all of them have a spike toward the left side of the graph at the 1200 RPM value. What I do is highlight everything above 1200 RPM and increase it by 10%. If that doesn't get rid of the rail pressure spikes, you will need to increase it a little more, but remember, your numbers are bigger now, so if you go up another 10%, that will be a bigger increase than the first 10% because the value is now 10% larger than the original adjustment. I would try 5% and see what that does. Here's a screen shot showing what the tables look like.
Now, when you do your first 10% adjustment to the "P" and "D" control tables, you also need to adjust the "I" control tables. These are tables {B1063}, {B1064}, and {B1065}. If you look at the graph of these tables, you will see the values sort of stair step down as the RPMs increase. What I like to do is copy a value of a step and paste it into everything below that. If you look at your datalog, you will probably notice the rail pressure spikes get larger as the RPMs increase. You need to see where they really start to get pretty large and start there on this adjustment. I've attached a couple of screen shots showing the before and after. I used the value of the 2400-2600 RPM step. In most cases, this will get you close to where you need to be, but if not, you can always go up another level.
If you keep playing with these tables, you should be able to smooth even the roughest rail pressure spikes, but always make changes in small increments to keep from creating another issue in your quest to fix your original problem. I hope you find this helpful. If you have any questions, please don't hesitate to ask.
