Surging rail pressure with cruise

[Tinman]

For quiet a while now the 06 has had an usual behavior of "jerking" while coasting down hill with the cruise on. Feels kinda like towing a heavy load. I recently noticed that the rail pressure is the cause. While set above 65mph, the rail pressure will drop to it's "coasting" pressure while going downhill. This is about 11K psi at 70mph (higher at higher speed). As soon as it gets to this pressure, it starts to surge, bounce, oscillate (... whatever is the correct term), at a frequency of about 1-2x/second. It's bad enough that I don't use cruise and more. I replaced the FCA and monkey'd with the fuel pressure to the CP3 without change. Truck otherwise runs perfect. It is not related to the Smarty as it will do it with stock SW. I'm at a complete lose as to where to look next.

 

[MChrist]

I also have the same jerking sensation going downhill with cruise on. I'll have to check my rail pressure when it's happening. I thought it had something to do with aftermarket trans. I notice you also have aftermarket trans.

 

[TCDiesel]

Check MAP Sensor...

 

[Big Hamp]

My '07 5. 9 does the same thing and it sure is annoying. I have a g56 6 speed. So you think it could be the map sensor? What makes you think that is the problem??

 

[EDankievitch]

Funny you post this today, I was just about to post a thread on the same topic. I've got an 05 4x4 with the G56 6 speed that's doing the same thing. What does your over head Mpg meeter say? mine has slowly been getting better readings, it's now up to 42 Mpg after a weeks worth of driving around town. No, I'm really only getting 18. 9 mpg or about 520 miles on a tank. My engine will also miss fire if you do a high idle, hold the throttle at 2400 rpm and will miss worse as the rpm increases. Under load there is No miss all the way up to 3,000 rpm. I also changed the FCA with No change. And my fuel pressure will surge just as yours does when I do a driving test with my scan tool hooked up and use the cruse controll. I've got about 126,000 miles on the stock injectors and Cp3 pump. I'm not running any chips or programers. Any one have any thoughts??? Before I spend money on a MAP sensor. Thanks for any help.

 

[Tinman]

I can't see the trans doing it as it only does it with the TC locked.



I assume I'm looking for a dirty MAP sensor. Or am I looking for voltage/ resistance?

 

[EDankievitch]

Ok,

Now you've got us all wondering what are we looking for with the MAP sensor???

 

[TCDiesel]

The Map sensor default with engine off is 14psi. . it should read 0 @ idle in P/N if it fluctuates at all it needs to be replaced.

 

[EDankievitch]

Ok, that make sence thanks will check it out soon. I'd be nice to have this thing run smooth again.

 

[MChrist]

I paid attention to my rail pressure last time the surging happened. I was towing our camper up a slight hill, crested the hill at around 17-18,000 psi, then as I started descending, the truck bucked a little, and I watched the rail pressure go from 18,000 to 8,000 in about 2 seconds as the load was removed from the motor. I would assume that would be normal, since cruise was set, I was traveling 70 and rpm was 1700. Also, I get my rail pressure readings via my OBD II port and Edge Monitor.

 

[TFucili]

I had made a post quite some time ago about this surge while coasting, but did not have the rail pressure connection. Thanks for the information. Is there a means to test the MAP sensor without a scan tool? Can it be pinned out for a reference voltage?

 

[AH64ID]

While there may be an underlying issue causing the surge to get more abrupt I beleive some of this is normal, to an extent.

If it happens only on occasion on a specific hill (I have on near me that it will do it at 52 mph every time, ~5mph faster or slower and it stops). What I think it is the fine line between zero throttle and minimal throttle. The incline is just at the virge of being steep enough to allow the truck to coast at the set speed, but still just short enough that it needs fuel, probelem is that the minmum fuel is enough to get the truck past the set speed, so it goes back to zero fuel, and back and forth. I beleive this is the reason for the rail pressure change as well.

A slow to react FCA, or bad MAP, or other sensor may enhance this condition.

Now if it happens on most inclines, then I would say its more than the normal slight surge.

 

[Tinman]

I can see it being normal in certain circumstances but mine does it anytime the cruise is on and on any hill that will let it coast to it's coasting RP. I don't want to use my cruise so something needs to change. I'll get that MAP checked.

 

[AH64ID]

I would agree that you have something out of whack, and the the MAP is a good place to start.

Do you run a oiled filter? If so check the sensor in the intake tube.

 

[EDankievitch]

Ok, So I hooked up the scan tool the dam thing only reads in KPA. With the engine off I was showing 102 bouncing up to 105 when the engine was started it showed 112 bouncing up to 115. it never stayed constant. So I replaced the MAP sensor now reading constant with no flux or bouncing. But the truck still bucks when running in cruise on a slight down hill run. Also the engine will miss at high idle (above 1900). I did a driving test watching the fuel rail pressure at Idle it shows from 48,000 to 68,000 KPA when driving up hill wide open throttle it will maintain 39,000 KPA. When you coast on that slight down hill where the truck bucks the fuel pressure will go from 68,000KPA then drop to 190KPA then back up to 58,000KPA if you watch long enough it really bounces all over the place quite quickly. soon as you add aload it will steady out and stay there. The FCA is new, I've done the outboard oil test wih no change. I know my injectors are up there in miles (if 126,000 is high) and my clutch is about shot (yes it's the G56 6 speed with the dual mass flywheel) Any thoughts where to go next with figurein this out? I'm sure I'm not the only one with this suge/bucking and miss fireing.....





05 4x4 Quad with the G56 6 speed all stock

 

[EDankievitch]

I should add that the engine will start to miss just as you wind out before a gear change... ... . Soon as the load is removed from the engine. And that there seems to be a slight sruge in power when you start to let off the throttle Thanks again for your Ideas

 

[Tinman]

I checked my MAP sensor with an OBD and sure enough it wasn't steady. My OBD measures in mmHg and it would jump around from 28. 1 to 28. 4 to 28. 7mmHg. Replaced MAP/IAT sensor. OBD reads steady 28. 7 now, but no change in the bucking problem. Didn't help (although I think the truck runs smoother now). A member has PM'd me about another the possibility that it could be alternator noise with the rail pressure sensor, so I'm gonna pursue that next.

 

[EDankievitch]

I changed the fuel rail pressure sensor and the rail blow off valve, No change. I also did change my clutch to a Southbend unit. That made a huge diff as my dual mass flywheel was very worn out and sloppy, but still not right. When the engine is ice cold it runs geat nice and sooth soon as it starts to warm us I get the miss above 1500 rpm (out of gear, no load on engine) and also while coasting. The fuel pressure on the rail bounces when the miss happens... ... The truck runs too well under load or when cold for me to think that the Injectors are bad or failing.

 

[steved]

I installed cruise on my truck at around 100k (which is simply two steering wheel switches) and immediately had the bucking/surging cruise. I questioned it, and was told by everyone it was normal... I couldn't have imagined buying a new truck that bucked like this, but whatever. So I lived with the surging, but I eventually lost an injector at 145k. The injectors were rebuilt and F1 tips installed.

With the injectors fixed, my surging cruise was gone and; at 230k, it has not came back. The theory that was presented was that the injectors were worn, and therefore leaking, which was causing the rail pressure to be low and fluctuate which caused fluctuation fuel delivery during cruise (when the cruise was expecting a steady supply of fuel into the cylinder).

My cruise is absolutely smooth and non-surging regardless of load, coasting, etc. .

 

[AH64ID]

Thats an interesting observation Steve. It seems that the original injectors didn't have the efficiency of your rebuilt ones. It completely makes sense, and goes in line with the theory of being between zero and min fuel, if the injectors are failing or dirty then that line gets wide, and surging occurs.

I wonder if it will start to come back as the injectors age?