PRV replaced, fuel pressure still tests low while off

Hello!

While troubleshooting a misfire, I ran a Fuel Pressure Relief valve test and it tested low. From my understanding, after shutting off the car at operating temp, the heat of the engine will heat the fuel in the rail and the pressure should rise to ~1800 PSI before the pressure relief valve opens. The pressure should remain about there before everything cools and decreases in pressure.

My first test got the fuel pressure to 1200 PSI

my second test didnt increase fuel pressure at all


I then fixed my misfire with a new set of plugs and the car runs ok as far as i can tell but figured maybe i should replace the PRV in case it was on its way out.

again, the fuel pressure didnt increase at all. this time i logged engine coolant temp too


-am i just doing the test wrong?
-i dont see/smell fuel in the engine bay so i dont think i have an external leak, but maybe its a small one?
-i had the injectors cleaned and tested maybe 20k miles ago but i guess they could have started leaking into the cylinders... how would i test/check that? could i just look though the spark plug hole with a borescope? idk fuel evaporates quick...
-maybe the pressure is leaking out of the rail back into the HPFP? is that even possible?

thanks guys

 

@Enki fuel wizard should be able to help you out

 

Hmm... Yesterday I checked my fuel pressure after getting home from work (like 30 miles) and it actually went up a bit before slowly dropping.

I had to drain my fuel rail when replacing the PRV, maybe there's an air pocket trapped in there messing with my test? Is there a way to bleed this thing? That would not explain the bad test prior to changing the PRV though, so I'm not sure.

I will have the intake manifold off for another job, maybe I could get my heat gun on the fuel rail and log fuel pressure to see what happens?

 

Follow up.

I don't appear to have an external leak. Either an injector is leaking into the cylinder (which I don't want to believe as I had them cleaned and tested recently) or this little check valve at the bottom of the HPFP is bad

I can't seem to find a mazda/Ford part number and the number printed on it doesn't come up with anything. Does anyone know what this part is?

 

Not sure it's part number however @Rokusek runs a scrap yard and might have a spare one

 

Mazda doesn't sell that alone, only with a complete HPFP.

I might have one of those laying around...

 

Thanks for the reply. Is this a common failure point? Since they don't sell one separate, that makes me think probably not... A friend says he might have one from an old pump housing he would give me, if not ill PM you and set something up!

 

To my knowledge I do not believe this to be a fail point. I could be wrong, but not something I have seen.

Just let me know I have three.

 

I didn't think there was a check valve in that fitting thought it was just a male to male adaptor. I also have never heard of it being a failure.
The check valve that fails is the PRV on the fuel rail this is what I would have thought of had you not shown the part.
SKU: L3K9-13-2J0
https://edgeautosport.com/mazda-oem...7-2013-mazdaspeed-6-2006-2007-cx-7-2007-2012/

Here's the test procedure

Edit: looks like I missed reading a post, you already replaced it

 

Not sure if this problem has been resolved but I'm also currently running into this issue and was wondering if maybe you had some time to test more @Sand.

 

I am also curious, he got the lower valve I sent him, haven't heard anything in regards to if it fixed the problem or not.

 

I will be replacing it this weekend as part of a bigger job on the car and will report back!

There's also a chance that my testing may be flawed. The car seems to run OK as far as I can tell. What I think might be happening:

.. I run the car to get the engine hot
.. Pull in the garage or a parking spot or something (which takes a bit of idling time)
.. .. In that time the fuel in the rail is warming up
.. I shut the car off and wait for the engine heat to transfer to the fuel to raise the pressure
.. .. Since there is already lots of heat in the fuel, the pressure only increases a bit till the temperature of the fuel equalizes with the hot rail, block, etc.
.. .. That would explain why in my original post, the first test showed like 1200 psi and the second showed nothing (fuel was already hot)

Again, there's only a few ways pressure could leave this part of the system
.. External leak (checked, looked and smelled OK)
.. PRV (already tried replacing that)
.. That one way valve fitting thing at the bottom of the HPFP that the high pressure line threads on (what I currently plan on replacing)
.. Injectors (would get them re tested if my schedule this weekend allowed)
.. (something else I didn't consider?)

 

Sorry for the weird formatting, typing stream of consciousness style on my phone

 

• PRV
• Injectors

I've had both fail.

When the PRV fails, you can typically blow through them or hit them with compressed air and they'll leak. Another way to test is to just pop the yellow clip line off of the PRV and look/LISTEN for fuel coming out.

When injectors fail, it will have a nasty misfire and you'll go rich at idle (negative fuel trims relative to normal). You might be able to pull the plugs and look to see if they're wet.


I refuse to get injectors cleaned at this point. I've had several cleaned injectors fail, and I don't trust them...

 

I do have a popping at idle like a misfire but no cel, the weird thing though is I'm running lean at idle currently. I'm pulling my car in tonight to do a smoke test though to check for a vacuum leak. Normal driving the car is perfect. Stayed out of boost completely till I deal with this though.

 

phate
Good idea about the prv and good point about the plugs

Also, do you think used injectors are more reliable than old rebuilt ones? I guess cleaned injectors are different from rebuilt... Also when the injectors leak is it usually like a gradual leak till they get bad or do they usually go down hard

Cody
Good luck, let us know if you figure anything out! From my understanding, a little burble at idle is normal, but sounds like you may have more going on.

 

I've had the best luck with reused injectors, sans cleaning. [I did buy all new injectors for this latest build, though.] Honestly, the cleaning has never made a noticeable difference other than showing they're better in the graphs they supply.

When my injectors have failed, it's all of a sudden, no warning or gradually getting worse that I noticed.

 

Well I absolutely fucked something tonight. Found a few vacuum leaks, corrected those and now my stft and ltft are stuck at -.16. They will not change. Traction control light is also stuck on. The car is basically undrivable currently. Everytime I hit a bump my afr shoots to around 8-9 and it tries to die. Adjusted my intake and it seems a bit better but still not right. Currently throwing a p0103 code now as well

 

Roommate just got home and happened to have a spare MAF in his garage. Swapped mafs and my car is running and driving again but my stft and ltft are still acting weird. Ltft is still stuck on -.16 then will jump to 20 then work for a second then go back to -.16 then back to 20 again. I'll have to do more testing tomorrow. Reset my battery as well so that may have something to do with the weird readings.

 

Check the wiring to the MAF