Hi Yall

Been driving myself crazy with a FPR issue. Posting in 'general' as I think this applies beyond a specific model.

Working on a 99 c3500 7.4 vortec. Fb dually.

Had low fuel pressure (48) and leaky injectors. Swapped both out but FFuel pressure still only 48 (holds pressure much longer though). As spec is 58ish I'm still not where I need to be. Im on my 2nd new fpr and same issue. When i pinch the return fuel line pressure goes up, when pinch is removed it drains back to 48 fast.

I started with ac delco 217-3073. Site said oem match part and after install I found that it was rated 36 psi. So I got a delphi fp10021 which was rated 58. Both are stamped 400kpa on bavk which is 58psi.

My logic says if fuel pump is bad pressure wouldnt rise when return line pinched. And it holds pressure well. So i dont think is fuel pump or check valve but not sure.

What could be wrong here?

2 bad fpr's (3 counting original)?
bad pressure gauge?
any other tests to perform?


thx in advance!
matt

 

if the fuel pump isn't supplying enough fuel...not much a good fpr can do. when you dead head the pump by pinching the return line...the pressure should skyrocket instantly not just go up. before installing a pump... make sure the voltage to the pump is good and the pump has a good ground.

 

Tech2, thanks for the advice! Would you mind if I ask a clarification question, hope not and thanks in advance (again)!

The pump is definitely weak (or bad ground etc), and I plan on replacing that as well now too, thanks. What I just can't seem to get my head around though, is why the FPR holds at a higher pressure when the return fuel line is pinched. KOEO, pressure goes to around 50 psi. If I pinch the return, the pressure will build to 65-70 or so. When I release the pinched line it immediate falls back to 50. If I don't release the pinched return line it holds pressure for a while (making me assume the pump and check valve aren't leaking, even though pump is indeed weak). My logic, which is often flawed, is that once the line is pressurized the FPR should hold that pressure (any pressure below its rating of 60ish that is). But instead of holding the pressure immediately drops when I unpinch the return line. It seems that even with a strong pump, the extra pressure is just going to leak past the FPR.

What am I missing? Just trying to better understand how this stuff works.

Other assumptions of mine:
the fuel check valve is on the supply side of pump (as it seems like it would be redundant to have on the return side since that's what the FPR is for).
a fully functioning fuel pump would get the psi up to around 85

And I am also second guessing my cheapo pressure gauge too.


Again thanks for the help and I hope all these questions aren't too annoying!!

-Matt

 

restriction creates pressure...the fpr regulates pressure by returning fuel back to the tank through the return line...if the line is pinched off when koeo, the only thing your testing is the fuel pump, not the regulator

hard for me to comment as i do not know the symptom; only what you think the problem is.
you had low fuel pressure...there had to be a symptom that made you want to check fuel pressure...what is it. low power? long crank? hard start after a hot soak? fuel system lean dtc???

ie. i have a bad alternator...what can the problem be? = going down a wrong diagnostic path...post symptoms...my lights are dim, flicker, slow crank

 

As far as symptoms... seems under powered when towing and hard starts once hot. Leaky injectors may explain both but thats what led to pressure test. And even with the low pressure its running rich based on fuel trim data, which i attribute to leaky injectors.

The reason i keep blaming the fpr regarding pinched return is that I am assuming the fpr shouldnt allow any fuel to enter the return line until pressure exceeds 60ish psi. Isnt that the purpose of fpr? It doesnt open to the return line until pressure exceeds the set value. I could very likely be confused!

From what I read if i pinch the return and pressure still drops its a pump issue assuming fpr and injectors are holding (which can be confirmed by pinching both lines).

Is there a better/different way to test the fpr? Its not leaking at vacuum port.


Thanks yet again!!!

 

if the fuel trim is rich...its adding because the fuel pressure is low. easiest thing to do at this point is to do a volume test of the pump

if you pinch the return line...where/ how is the fpr going to dump the pressure.. it can't you blocked the return...the pump won't drop pressure at this point...you added a restriction...pressure will go up. a good pump will double pressure immediately...a bad pump slowly.


if you want to test for leakdown you do it engine off. install you gauge, prime the fuel rail by cycling the key on...engine off...till you hit the max pressure. let it sit 10minutes. if the pressure drops the leak down is from the check valve in the pump, injectors or fpr.
-repeat test this time blocking off the supply line after priming....no leakdown the check valve is bad....leakdown check the injectors and fpr

hard start after hot soak is indicative of leaking injectors or ruptured frp diaphram if the fpr vac line is dry, check the injectors. the fuel rails are easy to pull out of the manifold...remove it and put cups under each injector...watch for leaks

imo you need a fuel pump...just make sure the fuel filter isn't the problem. whenever you change a fuel pump...the filter must be replaced or no warranty on the pump

 

----

Thanks again for all of the continued advice, its really appreciated and very kind of you!

I could test fuel volume, this may make sense.
I've run some more pressure tests with a NEW tester and have what I think is better insight now. With the previous tester I couldn't actually see it until I got back out of the truck.

All of this is key on engine off stuff:
When pump is running pressure jumps to about 60, which I'd assume is really 58 (best I can see)
As soon as pump shuts off, pressure immediately drops to 52/53.
It drops to 50 in 10 minutes seems to hold at 50 for quite a while.

I've run tests with both supply and/or return lines pinched including pinching the return while pump is on. With return line pinched psi will go to 70-80 and drops 5-10 psi in 10 minutes when keeping the return line pinched or both the return line and supply line pinched. However, I don't feel 100% confident that my line pinchers completely block the flow. As FPR has no gas in vacuum line and the new injectors don't seem to be leaking (rail is currently installed), I kinda think that maybe some pressure seeps past my clamps.

As I can actually see the pressure at ~58 with the pump running I feel much better also given that its holding around 52 after 10 minutes vs. 40 like that previous pressure gauge showed. And granted this was the old gauge but there was still 30ish psi after sitting overnight.

At this point I think I'm going to slap her back together and change out the pump this coming weekend. Unless you think I should do volume test or whatever else first.

And to answer this, this was my logic. Probably makes no difference at all at this point, but I at least wanted to answer/explain. if you pinch the return line...where/ how is the fpr going to dump the pressure.. it can't you blocked the return...the pump won't drop pressure at this point...you added a restriction...pressure will go up. a good pump will double pressure immediately...a bad pump slowly.
-- My logic was, that once pressure was built (in this case built by pinching return line), once the return line was released that the FPR should still hold pressure at 58psi. What I now think, and still could have no clue what I'm talking about!, the FPR holds pressure at 58 when under constant pressure/flow from the pump but drops back down to 52ish whenever the pump isn't on. I was expecting the FPR to hold a constant 58 with pump off.


Also, regarding the fuel filter, I changed this about a month ago when I first started messing with this issue. Should I change again when I change the pump?, seems like a waste but I'd rather do it right!

Based on my new ramblings, with pressure showing 58 while pump is running, would you slap the truck back together and move onto the fuel pump, or should I run a volume test before putting the plenum back on and all of that?


Thank you thank you thank you (again), you rock!

 

fuel pressure regulatorI noticed that on the oring back up there is a slight indentation (on at least two of the three, as the third is installed). I've seen no mention in manual or online about aligning this in a specific way. Assume if this was important it would be mentioned somewhere...

 

if you have 58psi and spec is 58, i wouldn't change the pump.

i assume the hard start after a hot soak issue was solved when the injectors were changed and now you are chasing a lean mil p0171 p0174 dtc. Is this correct?

 

Not correct but good guess! I'm tackling both at once. I've put up with the lack of power under load issue, but it seemed to become worse once the hard starting occurred. Could just be my mind. Anyhow, once the hard starting occurred is when I looked at fuel pressure and realized there was an issue with low fuel pressure too. There have actually been no codes thrown. Just pulled the running rich info from the fuel trim. I added a snapshot of some fuel trim info I had handy, just in case you're curious.

This is all fairly new to me, I'm somewhat used to working on old tractors but nothing with data (as if you could not tell, lol). Replacing all O2 sensors as well as I get some mode6 errors. I just use this truck around the farm and hauling cattle, bought it a couple years ago with 174K miles and have put about 3K on it. It was in decent shape when I got it, but spent about as much as I paid repairing stuff, which has been a good learning experience.

If you think the pump is fine I'll leave as-is, or until trouble occurs. Gotta imagine that's coming soon. Makes me a little nervous as you said pressure should double when return is pinched, so it does seem weak. Main goal is for it not to go out with a trailer full of cows in tow!

Thanks again!!!