Question: Rail pressure question

[07keo02]

I noticed lately, on my bigger tunes, I get this very slight intermittent surge under acceleration. The truck still pulls really hard. It's just like this tiny repeated surge under acceleration. I ran a log with balance rates and desired + actual fuel rail pressure. The only thing I noticed was my max pressure ended up being around 24,600, and it only ever desired like right at 23,100. So it, at one point, put out like 1500 PSI more than it asked for. Could this cause the issue? Also, average actual pressure was about 100 psi than average desired. Number 7 on the balance rates is high but is within spec once warm. The injectors have less than 20K (bosch reman's) and no smoke etc. I also replaced the Cat filter I have on the frame with no change. I tried to attach the log but it just says invalid file. Sometimes I wonder if it is my axle intermittently Wrapping under the higher torque, but the roadmaster active suspension is acting against it causing that surge feel? Any thoughts?

 

[x MadMAX DIESEL]

What does the rail pressue do when the surge happens? You should change it to ask for more pressure if it will keep up, it will make more power.

 

[07keo02]

My log was kind of long because I started it idle before I even took off. I didn't notice anywhere it was doing anything abnormal. It does normally lag behind a bit doesnt it? How inoved is changing the pressure? Ive messed with throttle based injection quantity but all and all dont really know how to tune. Truck also has a new fpr. I wonder if my lift pump is still kind of high on pressure. The cp3 seems really healthy thats why this is kind of confusing. I guess i need to get a log of a couple wot runs only to get a better idea. Also, the fprv can be weak on an lb7 with no codes correct?

 

[x MadMAX DIESEL]

I wouldnt mess with the fprv on your truck. They usuallyare fine, and withthe pressure your holding it looks like it is fine. I was wondering if it is bouncing up and down while its surging, guess its not or you would notice it.

 

[07keo02]

I will try and get a better log to make sure. I'm also going to replace the 2 micron cat at the engine just in case, and run the injector cleaning process as soon as i find someone here in town that carries 1/2 inch fuel hose. Will post results whenever I get time to get it done. Thanks for the feedback.

 

[07keo02]

I did three more quick logs, two on the race tune and one on the stock tune, Duplicating the problem. I still see no hiccups in rail pressure that I can catch while watching. Is the boost reading correct though? It showed it hit like 37 psi on the race tune and 32 on stock tune. Also was reading 13 psi at idle, I have to assume it cannot read accurately. If anyone wants to look at the log, please let me know if there is a way to post it or I can email it etc. Thanks again.

 

[x MadMAX DIESEL]

Boost is close enough. Subtract atmosphere pressure from whatever it says and that's the actual boost

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[07keo02]

Oh I see, I was starting to think maybe the waste gate was stuck or something, but figured I would get an over boost code. Any other issues you could think of causing this surge like feeling while accelerating, possibly like weak leaf springs or issues in the rear end??? I also ran it with the lift pump fuse pulled today. Cp3 seems plenty strong, it pulled through both cat filters on the race tune.

 

[x MadMAX DIESEL]

Does your stock tune do it

 

[07keo02]

Yes but to a much less degree, have to really be feeling for it on the stock tune.

 

[x MadMAX DIESEL]

If it was me i wouldnt worry about something very slight. But its not my truck

 

[wdino73]

Can u feel the surge when your driving?

 

[07keo02]

Its always has been a "surgy" truck like the bucking low speeds but i can only feel this intermittent hesitation or jerk feeling under hard acceleration.

 

[07keo02]

Still trying to figure this out. I made the DSP5 tune I have out of one tune I had built specifically for my truck by Nick at duramax tuner. Then I used 3 others I got from a tuner library and then my stock tune. I copied all the tables one by one to complete the DSP5. Is it possible the truck is trying to defuel and I'm getting this feeling in my ZF6? Just a thought. Thanks.

 

[x MadMAX DIESEL]

Ask nick what to log, otherwise just use the standard duramax pods and send him logs

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[Hot COCOAL]

Call nick and ask what your pilot timing is set at, and if maybe it should be adjusted, he will want some data logs, i had some bucking and surging around 3200rpm after i added a bigger turbo and several other air flow mods, and nick adjusted the tune for me(pilot timing, i think) somehow, idk, but the issue is resolved now...smooth as stock, but way more powerful of course...just watch your cylinder pressures. Yea, something to to with pilot timing or pilot injection iirc he said it will cause higher cylinder pressure but it would still be in an acceptable range

I'm sure others can chime in on this comment, I know nothing about it, just that I was experiencing a similar little gremlin and its fixed now, only my gremlin occurred at the top end of the rpms and the surging was like a feeling of getting a huge power increase, and if I let of the throttle it would buck some, so usually I kept it pegged:thumb:

 

[plumber802]

How much fuel pressure are you running from the lift pump? Too much can cause surging as well.I run mine at 8psi.

 

[07keo02]

I had to bottom out the set screw when I installed it, was getting surge at idle and a code. I would like to get a gauge so I can get it right at 8 though. I have a smooth idle now. The bucking at low speeds I seem to get is from the un-linear throttle pedal over bumps and taking off with the South Bend in the ZF-6. I can't find anything in the log. I'm not convinced that this isn't the roadmaster suspension trying to give up under heavy acceleration. I will still get a hold of Nick on Monday though and see what he thinks.