Fuel Rail Pressure is Low - Now What?

This may or may not cause an MIL (check engine light). This is a very common problem that we hear about from our customers. Actual fuel rail pressure is not keeping up with desired fuel rail pressure. This may only happen in high fuel demand situations like towing or racing but it can happen with everyday use.

Symptoms:
May include one or all and not limited to; Low power, poor shifting quality, excessive smoke, engine limp mode, reduced mpg.

Causes and repairs:
1. Dirty fuel filter is the most common issue we see when fuel rail pressure is low. The filter should be changed in the 8,000-10,000 mile range if good fuel is being purchased and running mostly low HP tuning, much sooner if running competition/race style tuning. When in doubt, change the filter! This is about a 30 minute job that can be done without any special tools. Cost; Less than $50.00

2. Leaking fuel filter head, or cracked water separator. The filter head is the part that the fuel filter screws into and has the primer plunger incorporated into it. There are O-rings and seals that break down with age and use. ATPTrucks.com sells an inexpensive rebuild kit complete with with filter and instructions to repair this important part. The water separator screws into the bottom of the filter itself and must be replaced if cracked or damaged. Common hand tools are all that is needed. Approximate time is 1 hour. Cost:  $109.99

3. Weak fuel pressure relief valve (fprv) for 2004.5-2010 model years. This can be tested by doing the “bottle test” This is an easy, cheap way to see if the fprv is leaking. If any fuel is leaking past the fprv, then the internal spring will need to be shimmed. ATPTrucks.com sells the shim kit, including the proper parts needed to bring the spring pressure back to acceptable level to stop leaking relief valves. You will need an 18mm wrench and a bench vise. Approximate time is 1 hour. Cost: Less than $25.00.

4. Collapsing or cracked fuel line between the filter head and the high pressure fuel injection pump (cp3). This is usually caused from a plugged fuel filter causing a major vacuum between the filter head and the cp3. If not cracked, the problem is usually fixed by replacing the filter. The fuel line can be replaced if the filter change doesn’t fix the problem. Remove the fuel line and find a suitable replacement at the local auto parts store, making sure the line is rated for diesel fuel.

5. Blocked or broken fuel tank pickup. We have seen this many times. The common cause is a foreign object gets down the filler neck of the fuel tank. The most common thing we see is the paper seals of diesel fuel additive. This is not a fun job but is not too difficult. Run the fuel level down as much as possible, then the tank will have to be dropped. The fuel sending/pickup unit will then need to be pulled out. Inspect the unit as well as the fuel tank and remove the offending particles or repair the separated pickup tube. Tools needed: fuel line quick disconnect kit, can be the cheap plastic ones sold at the local parts store. Approximate time is 3-4 hours. Cost: is less than $20.00 unless the pickup/sending unit is damaged.

6. Weak CP3. We see this start to rear its ugly head at an average of 175,000 miles depending on the history of the vehicle. If the truck has had a lift pump (electronic pusher pump) and/or correct fuel filter changes, then this can be greatly extended. This CP3 is under high demand, sucking fuel from the tank through the filter and then converting this to pressures up to 26,000psi.

It is a mechanical pump and does wear with use. This is a big job to do as the CP3 is gear driven off of the camshaft and is located in the valley of the engine. I recommend having a qualified mechanic diagnosis this pump if the above repairs have not fixed the low rail pressure problem. The cost for parts alone will be $800.00-$1000.00 without installation. The mechanically inclined person can do this job, no specialty tools are needed but patience and time. The CP3 doesn’t need to be timed to the cam. Approximate time for the DIY’er would be 6-10 hours. Installing a lift pump may help fix the symptom and is a good modification for the future even if a new CP3 is needed. If a mechanic is out of the question, then installing a quality lift pump would be my first recommendation.

7. High injector return rates. This is also an expensive, time consuming job so I recommend having a qualified mechanic diagnosis the problem if you are at this point. Cost for parts and labor will vary depending on model and year. Parts costs can vary from $2000.00 to $4000.00 and labor from $600.00 to $1500.00.

To summarize, normal maintenance goes a long way in helping your truck survive in high demand situations. Most fuel pressure issues are not caused by tuning if the tuning was done correctly for the current modifications on the truck. Quality lift pumps will allow the engine to handle higher fuel demands and offer the benefit of cleaner fuel being supplied to the expensive fuel system. This holds true for stock or modified trucks.

Hopefully this will help you with diagnosing your truck's fuel rail pressure issues.

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