First I'd like to apologize for you're struggles so far. Ram hates us. P0191 code is tricky especially if you have aftermarket repair data like All Data or Mitchell.
P0191 in Mitchell reads the code as "FUEL RAIL PRESSURE SENSOR CIRCUIT PERFORMANCE"
Being a trained Ford technician whenever I see the word "Circuit" in a description I immediately think electrical. I mean, that would make sense right?
Ram doesn't make sense so forget everything you've learned thus far.
It's actually a really easy code to fix and is also ALWAYS accompanied by the Informational P2BAC code.
P2BAC is only informational telling the tech that another code is present that effects emissions and that the EGR system has been disabled. It does not require repair and will eventually clear itself out. You can also clear it with a scan tool.
TECH NOTE: Be sure if you are working on a 2018+ RAM you have a scan tool capable of communicating with the security gateway.
Moving on.
*** WHAT THE BOOK DOESN'T TELL YOU ***
P0191 set conditions are simple. IDLE, HOT.
Get it to operating temp and let it idle.
The monitor runs once per drive cycle and takes 2 cycles to trip.
No need to drive 100 miles under loaded conditions, you're just wasting time and fuel.
As per Tech Authority to diagnose a P0191 code you must first check the fuel pressure sensor. Basic stuff. Check for zero at Key On Engine Off (KOEO), they say anything under 870 PSI is acceptable but if you're reading anything over 100 PSI I'd be replacing that sensor for bias.
TECH NOTE: We recommend doing a fuel quality check on all fuel codes. Biofuels such as Shell HPR, any Biodiesels, Biomass Diesel and basically anything that is NOT #2 diesel can cause fluctuations in fuel pressure.
If everything looks ok with the sensor they want you to check injector return flow.
This is actually pretty easy.
Just find the return from the back of the head and put a test banjo #9012 in the fuel drain tube at the rear of the filter housing.
9012 TEST Fitting
Unplug the Fuel Control Actuator and injector harness at the valve covers to prevent the engine from starting.
Crank for 10 seconds to purge air from the lines.
Drain graduated cylinder and crank for an additional 10 seconds.
Return flow should be less than 40 ml.
If your flow is greater than 40 ml you could have injector tubes leaking or bad injectors. We recommend torqueing the injector tubes to spec and retesting.
TECH TIP: To isolate a bad injector remove the fuel lines one at a time and cap at the rail using test plugs. They are available at Freedom Racing Here
Re-run the test until your return flow drops to find the suspect cylinder.
Injectors and tubes should always be replaced as a set. New injector get a new tube and should not be moved from hole to hole. That's how bad things happen.
You can find individual injectors or complete sets on our website in the 4th Gen RAM Fuel Injection Section
We always recommend using OEM Bosch Blue Box parts. They are the best stock replacement on the market. You can also choose to get increased flow like the Industrial Injection Cummins 6.7L CDT +10% which are legal for sale in all 50 states. Good for CA buyers.
One last thing to check for is a leaking High Pressure Relief Valve. Though they commonly cause High pressure fault codes it's still a good thing to check for.
It's easy to do. Just use that same 9012 fitting and see if any fuel leaks out at idle. It shouldn't have any flow until max fuel pressure which is around 26k psi, give or take a couple thousand.
Relief Valves are cheap. Buy them Here. There is only one for the 07.5-18 Ram. It's a Bosch 1110010013.
We don't recommend installing a fuel relief plug unless you are racing or sled pulling that requires higher pressure than stock. All that does for stock trucks is wear out your regulator because you are relying on the regulator for all fueling adjustments. Large and small.
That should be all you need to fix the P0191 code. If you have questions or anything to add to this tech article please feel free to call us at or send us a message [email protected].
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