Low fuel pressure

So I have a new built motor, I'm doing the tuning myself. I'm having an issue where my afrs are lean under wide open throttle. I suspect the problem is the fuel pressure dropping off up top.

 

Fuel system mods include:

Dw 1200cc injectors

Aeromotive fuel pressure regulator

Aem 325lph fuel pump with a new filter

 

I have 1500 miles on this set up with no issues, all of the sudden I'm not hitting my target afr up top. I have a gauge on the fuel pressure regulator but I can log it so I can only see it at idle. I had it set at 43.5 psi at idle but I've been noticing it goes lower sometimes. I re adjusted it to 43.5 after a drive (it was at 39 at idle) I haven't tried it out yet.

 

I looked for boost leaks and I don't see anything obvious so I'm going to take another in depth look.

 

The only reason I can think of is that maybe the fpr loosened up or broke in after the 1500 miles and I just needed to re adjust the fuel pressure. I'll go out and test it and see if that was it, but I'm hoping someone else might be able to give me some insight.

 

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It shouldn't be 43 at idle, it should be around 34. 43.5 is the nominal fuel pressure at atmospheric reference (ie with the vac reference off).

 

If you've been running a lot of fuel pressure, it's possible you popped an o-ring in the pump bucket. It's happened to me a couple of times, had to stack a bunch to keep it from happening. I'd at least take a look in there. Also, get an electronic sensor and wire it to one of your TGV ports, you'll be happy you did. That way you don't need to suspect anything, you just know.

 

Exhaust leaks upstream of the O2 sensor can also wonk readings.

 

I'd also do a proper boost leak test in case you haven't. Finally, make sure you're not running out of MAF (if you're still running one). I was on the edge and as soon as it got cooler out I started clipping the threshold.

It shouldn't be 43 at idle, it should be around 34. 43.5 is the nominal fuel pressure at atmospheric reference (ie with the vac reference off).

 

If you've been running a lot of fuel pressure, it's possible you popped an o-ring in the pump bucket. It's happened to me a couple of times, had to stack a bunch to keep it from happening. I'd at least take a look in there. Also, get an electronic sensor and wire it to one of your TGV ports, you'll be happy you did. That way you don't need to suspect anything, you just know.

 

Exhaust leaks upstream of the O2 sensor can also wonk readings.

 

I'd also do a proper boost leak test in case you haven't. Finally, make sure you're not running out of MAF (if you're still running one). I was on the edge and as soon as it got cooler out I started clipping the threshold.

 

I'm not running out of maf.

 

I double stacked the o rings from the filter to the pump if thats what you are talking about

 

Right now I have my egt sensor wired into my tgv, but I hardly log it so I will be switching over to a fuel pressure sensor.

 

So if I pull the vacuum reference line off of the fpr I should see 43.5 if properly adjusted? With the line on it should be around 34 psi at idle?

 

I'm definitely interested in looking into the above fuel pressure statement, and i will check to see if my adjustment fixed it but I will likely pull the pump again to check the o rings.

 

I thought since the injectors are flow tested at 43.5 psi thats what they should be set to at idle. Hopefully you can go a little more in depth on this subject

 

Thank you

 

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Where are you tapping your reference line for the FPR?

 

 

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Where are you tapping your reference line for the FPR?

 

 

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Right on the fuel pressure regulator

 

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I went and drove around and did a wide open throttle pull and my afrs were 4% leaner than target. Before this issue the afrs we spot on target all the way through the wot runs.

 

Like you said I think the problem may be an o ring in the basket from running too high fuel pressure.

 

I'm going to buy some o rings from subaru and then pull the fuel pump again and hopefully that is my problem because that is a cheap fix. I'm also going to get a proper sensor that I can data log with and lower the fuel pressure at idle.

 

Hopefully that will fix the problem, ill keep yall updated. If anyone has any other ideas of possible causes feel free to post them, I'm still open to other possibilities.

 

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Yep, you had the pressure right in your reply above. The injectors get bench tested at 43.5 psi, but most fuel pressure regulators are designed to be at that pressure at atmospheric reference, and track linearly at 1 psi fuel pressure per psi of manifold pressure minus 43.5. That's how the OEM nonadjustable FPR works, my Radium nonadjustable also. Of course with the adjustable you can set it to whatever you want, but you shouldn't need to increase it from the discussed values unless you're running out of injector. Probably goes without saying, but if you lower your fuel pressure to what it should be you're going to have to rescale your injectors at the very least.

 

On the subject of o-rings - I double stacked the first time but they popped a few months later again. Probably a little extreme, but the following stack has been holding just fine.

Right on the fuel pressure regulator

 

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I’m asking where are you getting the source from to the FPR.

 

 

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I’m asking where are you getting the source from to the FPR.

 

 

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The fpr is on the return line

 

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The fpr is on the return line

 

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Vacuum line.

Where are you get the vacuum from the intake?

The stock port above cyl 4?

 

 

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Vacuum line.

Where are you get the vacuum from the intake?

The stock port above cyl 4?

 

 

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Vacuum refrence is from stock location on the intake manifold

 

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I pulled the pump and all of my o rings look to be in good condition 1 of the orings stayed in the assembly when I pulled the pump out the other was still on the pump. Is there anything else I should check while I have the pump out. I'm considering putting the stock pump back in to see if I got a faulty pump maybe. Or do I just put it back to gather and hope all is good.

 

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I'm trying to think if I did something to cause this issue, the only thing I can think of is i messed around with the tip in enrichment, but that shouldn't have anything to do with the wot fueling. The problem started probably a day or two after I did that anyway. I haven't done anything that would mess with the airflow.

 

The car is acting like the maf scaling is incorect up top. What I'm thinking now is to put the pump back in, I have a fuel pressure sensor coming tomorrow so I will be able to tell if the problem is fuel pressure related. Correcting my mistake with the base fuel pressure, and going out and logging it to see if it is still leaning out.

 

If the pressure is accurate and it is still leaning out, i may just need to re adjust my maf scaling. There are no boost leaks that would cause this much of a discrepancy, i know because i once accidentally left my wastegate hose unplugged and it didn't cause any fueling issues like this.

 

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Try tapping into the bpv vacuum source for the FPR as well. I’ve heard of vacuum isolation with the stock FPR port.

 

 

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Sounds like a reasonable plan. Nice your pump came with the little o-ring spacer, mine didn't.

I also did some more research last night, idk all the ins and outs yet, but it seems like people are hardwiring the fuel pumps to the battery with a relay kit. The idea behind it is the stock wires don't supply enough amperage up top to allow optimal flow. But i feel like if that was the case the problem wouldn't happen all of the sudden. I could be wrong but ill look into hardwiring my pump because it is recommended for a big pump and big injectors. My injector duty cycle is 47% under wot so I'm not at the limits of my fuel system.im thinking maybe the pump could have broken in a little and now its just not enough amperage.

 

I also went down a rabbit hole last night. I was looking at all my data logs to pinpoint when the issue started.

 

On 9/8 my tune is solid, but I wanted to do some 4th and 5th gear lodging to make sure nothing wonky was happening in different load situations. So I did some 80-120 and 1 80-140 and my logs looked fine afrs were dead on.

 

9/9 probably the worst log i have seen 1st - 4th afr never hit target.

 

9/10 I thought the issue was caused by low fuel level in the tank so I filled up and it seemed to fix the problem for the rest of the nights driving.

 

From then on the problem is back.

 

Idk if the problem happening after a 140 pull is just coincidence but I pulled my intercooler, intake, spark plugs, did a compression test. Didn't find anything wrong put it back together ill put my fuel pressure sensor in today and take a look at what that tells me but I think the next step would be to hardwire the pump.

 

I found a th r ead from a while back were a guy had the same issue and he replaced the pump like 3 times, the fpr 2 times, and none of that fixed it, the problem ended up being the pump needed to be hardwired. So I'm going to do more research on that and I'll update if I figure anything else out tonight.

 

Here is spark plug pics if anyone is interested

1-4 in order, only enough cylinder 4 looks richer than the rest. These plugs only have 1500 miles on them and they are 1 step colder.

 

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Agreed on the hardwiring point. Doesn't make sense to me in your situation. I had to size up my fuel pump wiring to reduce the voltage drop, but I only encountered the issue at about 75% IDC on 1050cc injectors. Based on your quoted injector size and IDC, that's almost 200 cc/min more at rated pressure than you're flowing. My pump voltage was 10.5V which is way low. Also, my pressure drop definitely didn't just show up all of a sudden, it came on immediately after a change that increased airflow.

 

In any case, you will see if you have a pressure problem once you log fuel pressure, as your fuel to manifold pressure rise rate will fall below 1:1. If the FPR is good and the bucket is confirmed not leaking, then maybe I'd take a look at pump voltage to see if it's way low.

Edited September 14, 2020 by awfulwaffle

Here's a follow up question. How do your MAF readings look at the same RPM/manifold pressure portions during your pulls, before and after the issue started? I've noticed MAF reading shifts on the order of a couple percent with large changes in humidity before.

Here's a follow up question. How do your MAF readings look at the same RPM/manifold pressure portions during your pulls, before and after the issue started? I've noticed MAF reading shifts on the order of a couple percent with large changes in humidity before.

The maf readings are dead on before and after the issue

 

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I'm wondering if maybe an alternator about to go out could be a problem as well. I doubt the wiring will do anything put it needs to be done at some point so I might as well do it now while I'm in there. I remember seeing a lower battery voltage when I looked at my learned values in romraider.

 

Normally my battery voltage is 14 v without accessories

On Friday my voltage was 12.56v without accessories like before. Could this be a possible cause for my lean condition, my latencies are probably not correct because my fuel pressure is wrong

 

 

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Edited September 14, 2020 by samuel082799

12.56 is OK for engine off voltage, but way low with the alternator turning. You may be on to something there - lowest I've seen is 13.8V, which led to 10.5V at the pump. Even if your latencies are off I wouldn't expect that significant a shift in WOT unless they're waaaaay off, but I'd think you would have noticed that when dialing in CL fueling. But pump undervolt looks more likely if your alt output voltage is that low.

 

In a pinch, you can splice a diode into the voltage sense line of the alt harness to trick the onboard regulator into raising the output voltage. Could do that to see if the leanout is related to pump undervoltage. That said, 12.56V with the alt spinning is a sign that something needs fixed.

 

Out of curiosity, how far off of ID's recommended latencies did you end up?

Edited September 14, 2020 by awfulwaffle

I have the latencies 5% higher than the spec sheet so not much.

If I were to take it to auto zone would they be able to tell me if the alternator is on its way out, or does it have to be worse?

 

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I think they should be able to pick it up. If I'm recalling correctly, the minimum spec voltage for our alternators is around 13.6-13.8V. Edited September 14, 2020 by awfulwaffle

If the alternator is the problem would i be able to get away with buying an auto zone remanufactured one or do i need to get an oem pump to be sure it will provide the voltage needed?

 

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As long as you buy a reputable brand there should be no issue. Denso is the OEM for subaru, try not to go rebuilt as they are hit and miss if you are not installing it yourself.

Napa sells quality parts, my other GT had an alternator from Napa.