I need a place to monitor fuel pressure. Some have told me that the circled part of the engine is used by techs to monitor fuel pressure, is this true? If so, would it be possible to install a pressure sensor (similar to one used to monitor boost pressure) to monitor constant fuel pressure and is the pressure at this location accurate?


Is there any place better for monitoring fuel pressure?


http://www.dieselplace.com/forum/uploads/97C_DSCF0080.gif

That fitting is used to measure suction pressure/vacuum. Later! Frank

On a stock truck (ie. no modified fuel system with a lift pump), there is a vacuum at the point you show in the picture. I have the Kent-Moore vacuum gauge that attaches to that point. What is measured at that point is the vacuum at the input to the low pressure fuel pump (which is part of the High Pressure fuel pump assembly). The vacuum will vary depending on throttle position and acceleration. With a new fuel filter and the truck parked, the vacuum will vary from 2.5 inHg to 3.0 inHg from 600 RPM to 3200 RPM. Put the truck in gear and drive around and it will go from 2.5 inHg up to 8 inHg (depending on acceleration).


I am not sure why you would want to constantly monitor fuel system vacuum. This test port can be used to detect when it is time to change a fuel filter. You can do that by occasionally hooking a gauge up to this location.


Hope the above helps,


AlanEdited by: OC_DMAX

I am looking for a place to monitor fuel pressure. The place referred to above doesn't seem to be it.


Monitoring fuel pressure seems to be something that becomes more critical in high hp applications (which I am slowly getting too). It may not be critical, but knowing the fuel pressure has got to be a good thing and since I have a gauge to monitor it, I would like to. With all the recent talk about fuel/lift pumps and maintaining constant pressure, it got me wondering where and how these people are testing the pressure to see if it's constant. I think most of them are using a Tech II or something of that nature. But there has to be a mechanical way to install a pressure sensor. I know that it can be monitored, I just don't know where or how or if there is something already on the truck to make it easy...?

So you want to measure the common rail fuel pressure - 23,000 psi. You can get this information from the OBDII data port under the dash. I have monitored this using a Predator. The Predator samples pretty slow, maybe once per second. However, I have seen plots posted by DieselPower that came from a Tech II. The sampling rate looked a lot higher than what the Predator is capable of. I would be a little nervous messing around on the high pressure side of things. Maybe some of the programs like Autotap (which run on a PC) and are a lot cheaper than a Tech II can monitor the data that you want?Edited by: OC_DMAX

yeah, your gonna need something that can read from the data port. A tech 2 can do a live graph that is handy also, or a snapshot so your dont have to watch it, just start the snapshot, then do your thing, and you can download it to puter and replay it frame by frame

So the pressure you guys are referring to is the common rail fuel pressure (high pressure). Is this the pressure that some people are worried about maintaining by adding fuel pumps? I thought I had read in a couple of dicussions something about maintaining 9-10psi (or close to that)... After a little more reading, I believe this is referring to the pressure in the fuel lines prior to the injection system, though I'm not certain. Can one you of you clarify this?


I don't have access to a tech II so accessing the common rail fuel pressure isn't going to be possible. Is it something that will flucuate? From everything I've read, it's not the common rail fuel pressure that people are worried about but rather simply the pressure to get the fuel to the injection system... I believe this is the pressure I would like to monitor as it is the one that I think some people are attempting to help with a fuel pump.


I apologize for the questions, but I don't fully understand the way the fuel systems on our trucks work. And I've been trying to piece together pieces of information from about twenty related discussions in an attempt to figure out the whole thing. I just know that a number of people have stated a fuel pump is needed and I have seen proof (enough for me anyway) that one is need on high hp trucks.


Thank you guys for the information so far... It's helping me out!

The bottom line is this:


The Engine Control Module (ECM) commands a certain common rail fuel pressure for a given condition (throttle position, engine rpm, etc). The fuel pressure control loop within the Bosch fuel injection system tries to maintain this commanded pressure. On a stock truck, if you run full throttle, the ECM will be commanding 23K psi (159 MPa). On my stock truck, I will get just slightly more than the commanded. As stated in my first post, this information can be monitored at the data port (I use a cheap tool - the Predator,,,, others more fortunate like Dmaxallitech have access to a Tech II). The information monitored is 1) Commanded Fuel Rail Pressure and 2) Actual Fuel Rail Pressure. This monitoring capability is built into the truck (magic of electronics).


Some of the individuals on this forum who are running the large HP tunes (>200 HP increase) have monitored Commanded Fuel Rail Pressure and Actual Fuel Rail Pressure. What they noticed was, the fuel pressure control loop within the Bosch fuel injection system was not able to maintain the commanded fuel rail pressure (ie. actual is not equal to commanded). Remember, they have modified the duration that the fuel injectors are actually injecting fuel into the combustion chamber (extra milliseconds of injection = more fuel = more power). The problem is the fuel pressure collapses so (extra milliseonds of injection = less than desired fuel = less than desired HP). From what I have read, this condition does not seem to occur on every truck.


As a result of the above, some people have explored adding a lift pump between the fuel tank and low pressure fuel pump (which is in the High Pressure Fuel Pump Assembly). The theory was the low pressure pump was not able to supply the High Pressure Pump with sufficient fuel during the high demand period. By adding the lift pump, the measurements now made at the Fuel System Test port go from a vacuum (~ 5 inHg) to a pressure (~5psi).


What you need to determine first is whether the fuel system on your truck has sufficient capacity to supply the fuel pressure commanded by the engine controller. If it does, then there would be no immediate need to add a lift pump for extra HP. As John Kennedy has indicated, he has not seen an increase in HP by adding a lift pump.


Others on these DMAX forums have added lift pumps to try and increase the efficiency of their fuel filters and to remove trapped air from the vacuum side of the fuel system (more specifically fuel filters). This is a completely different issue than what is mentioned in the above paragraphs and do not confuse the two.





Hope this helps,


AlanEdited by: OC_DMAX

Thank you Alan. That information helped tremendously. I'm not sure I am running into a situation where the low pressure pump is not able to keep up with the high pressure system. I'll have to gain access to something to get that information. Is the predator as accurate as the tech II for this type of information?


I am in the works of getting a high hp pcm and another individual that has it already highly recommends a pump. Not necessarily for a hp gain (though one would be nice) but more for keeping the fuel system from starving. He noted surging conditions at hwy cruising speeds indicating a lack of fuel and by adding the pump, it cured the problem.


I'm still not entirely sure where to monitor fuel pressure with my guage though. Monitoring the common rail pressure isn't an option (gauge doesn't go that high and I doubt there would be a way to even measure that). What type of pressure do people normally monitor with a fuel pressure gauge??