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Fluctuating Fuel Pressure

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Old 06-12-2018, 04:57 PM
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Pwnage1337
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Default Fluctuating Fuel Pressure

Guys,

Need a little help diagnosing an issue.

I have a FAST EZ EFI 2.0 system running a HSR on my 383. I have the fancy regulator from Kirban, thinking it would solve my issue but it did not.

While running down the highway, I am seeing fuel pressure swings from 38-45 PSI, while at a constant throttle. It's making the car stumble a little bit under light throttle/cruise. Under WOT the pressure is rock solid. I believe the issue to be real because of the slight stumble, and also the wideband is showing a lean swing every time the fuel pressure is jumping around.

I have a wideband in each header - drivers side is feeding the ECU, the passenger side feeds a pillar gauge. The issue only seems to exist on the drivers side.

Do I have a bad injector? The system is batch fire.

Thoughts?


Jeremy

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Old 06-12-2018, 05:17 PM
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Kevova
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Fuel filter? Monitor vacuum on fpr is it stable or does it also vary?
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Old 06-12-2018, 06:40 PM
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billschroeder5842
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Fuel filter would be my starting spot too. Even it it is not the culprit, it is always great PM
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Old 06-12-2018, 11:08 PM
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I changed the fuel filter last year because that was my first thought. Do I need to have a special filter for a high flow pump? 255 lph
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Old 06-13-2018, 05:55 AM
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Is the regulator vacuum referenced? Also can you data log the fast system to see what its own wideband is reading? I do not think the issue would only exist on the passanger side, I would assume this is related to both banks. Is the FAST controlling the timing, if so how is it being done?

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Old 06-13-2018, 08:10 AM
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Originally Posted by bjankuski View Post
Is the regulator vacuum referenced? Also can you data log the fast system to see what its own wideband is reading? I do not think the issue would only exist on the passanger side, I would assume this is related to both banks. Is the FAST controlling the timing, if so how is it being done?
Yes it's vacuum referenced, and it still does it with the vacuum disconnected . I'm unable to datalog this system which sucks, but had someone else drive the car while I watched both widebands. It could be that the passenger side just updates way faster?

The FAST is controlling timing. There's two parts, the normal timing curve which is just an "initial timing" (at idle) and then a Max timing. You set an "all in RPM" that you want to achieve your max timing at. I'm currently 18 degrees at idle, 35 degrees Max, all in by 3000 RPM.

The second part of it is a vacuum advance based off the MAP sensor. Right now when this issue is happening the car is asking for about 35 degrees of timing under light load, about 8% TPS. 2000ish RPM
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Old 06-13-2018, 08:23 AM
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Originally Posted by Pwnage1337 View Post
Yes it's vacuum referenced, and it still does it with the vacuum disconnected . I'm unable to datalog this system which sucks, but had someone else drive the car while I watched both widebands. It could be that the passenger side just updates way faster?

The FAST is controlling timing. There's two parts, the normal timing curve which is just an "initial timing" (at idle) and then a Max timing. You set an "all in RPM" that you want to achieve your max timing at. I'm currently 18 degrees at idle, 35 degrees Max, all in by 3000 RPM.

The second part of it is a vacuum advance based off the MAP sensor. Right now when this issue is happening the car is asking for about 35 degrees of timing under light load, about 8% TPS. 2000ish RPM
OK, what is the vacuum doing when the fuel pressure is moving around. I know you said the fuel pressure still moves around when the vacuum is disconnected, that seems like an issue. What I am trying to do is determine why the engine stumbles, I do not really think the fuel pressure change is causing that, but it may be.
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Old 06-13-2018, 10:32 AM
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Originally Posted by Pwnage1337 View Post
I changed the fuel filter last year because that was my first thought. Do I need to have a special filter for a high flow pump? 255 lph
You do not. I run the stock filter with my Walbro.

My first question is what the vacuum is doing while the FP is moving.

When I had this problem it turned out to be the wiring on the sending unit in the tank though. Have you already upgraded that?
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Old 06-13-2018, 10:46 AM
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Originally Posted by vader86 View Post
You do not. I run the stock filter with my Walbro.

My first question is what the vacuum is doing while the FP is moving.

When I had this problem it turned out to be the wiring on the sending unit in the tank though. Have you already upgraded that?
I got the hotwire kit, but not the upgraded in tank harness. Looks like I probably should have lol. With the vacuum reference disconnected, the fuel pressure should be solid, right?
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Old 06-13-2018, 11:33 AM
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Originally Posted by Pwnage1337 View Post
I got the hotwire kit, but not the upgraded in tank harness. Looks like I probably should have lol. With the vacuum reference disconnected, the fuel pressure should be solid, right?
Yes, it should be stable. Maybe you have some sort of fuel tank cavitation problem. Is the discharge of the return line causing air bubles that cause cavitaion at low fuel usage and high return volume?
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Old 06-13-2018, 11:50 AM
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If the fuel pressure varies when the vacuum line is disconnected, it sounds like it could be the regulator.
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Old 06-13-2018, 12:01 PM
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Originally Posted by bjankuski View Post
Yes, it should be stable. Maybe you have some sort of fuel tank cavitation problem. Is the discharge of the return line causing air bubles that cause cavitaion at low fuel usage and high return volume?
How would I verify that? I do have my chatcoch canister eliminated and the line capped off if that means anything.

Originally Posted by 383vett View Post
If the fuel pressure varies when the vacuum line is disconnected, it sounds like it could be the regulator.
That would be a bummer. I thought the old regulator was **** so I upgraded to the Kirban. It's brand new last September
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Old 06-13-2018, 12:02 PM
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Originally Posted by Pwnage1337 View Post
With the vacuum reference disconnected, the fuel pressure should be solid, right?
It should be relatively stable either way, varying with vacuum, which is an easy, valuable check; hence is my starting point, being the control. With proper control, a continued variance points elsewhere, like fuel supply or return line pressures.

That pump seems like overkill unless your engine is way more radical than what works well on my 550+HP 409.
Strange is the L/R difference, with the cross pipe connections. Assume there's no fuel odor anywhere?

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Old 06-13-2018, 12:16 PM
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Originally Posted by whalepirot View Post
It should be relatively stable either way, varying with vacuum, which is easy and my starting point, as it is the control. With stable control, a continued variance points downstream, like fuel supply or return line pressures.


That pump seems like overkill unless your engine is way more radical than what works well on my 550+HP 409.
Strange is the L/R difference, with the cross pipe connections. Assume there's no fuel odor anywhere?
From my gathering, most guys who are running upgraded pumps just go to the 255 from racetronix, which is what I did. I have no fuel leaks, have put 3000 ish miles on the car with this issue. It's not a hard misfire, but enough to notice a little chug when the converter is locked up. I thought maybe I was "lugging" the engine and put the car in 3rd gear to get my RPM up around 2500-2600 but the issue still exists at light throttle.

I haven't given thought to the return filter on my car. There's a small filter on the return line... Don't know where I'd even get one
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Old 06-13-2018, 12:36 PM
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Originally Posted by Pwnage1337 View Post
I got the hotwire kit, but not the upgraded in tank harness. Looks like I probably should have lol. With the vacuum reference disconnected, the fuel pressure should be solid, right?
The FP should be stable then yes.

It is the in-tank stuff that will get fried at the connectors.
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Old 06-13-2018, 12:51 PM
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Originally Posted by vader86 View Post
The FP should be stable then yes.

It is the in-tank stuff that will get fried at the connectors.
So you had a similar issue that was remedied by the upgraded bulkhead harness from Racetronix?
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Old 06-13-2018, 01:29 PM
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Originally Posted by Pwnage1337 View Post
So you had a similar issue that was remedied by the upgraded bulkhead harness from Racetronix?
In my case the car stumbled because the wires inside the tank were melting and resulting in intermittent power to the pump, and ultimately failed and left me stranded.
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Old 06-13-2018, 02:18 PM
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Originally Posted by vader86 View Post
In my case the car stumbled because the wires inside the tank were melting and resulting in intermittent power to the pump, and ultimately failed and left me stranded.

That's good to know. I think I'll order the bulkhead harness now lol
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Old 06-14-2018, 08:39 AM
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Originally Posted by Pwnage1337 View Post
How would I verify that? Add a hose to the return line in the tank that directs the flow away from the pump inlet and instead towards the driver side or passanger side of the tank.

One other question does this surging and fuel pressure issue change with fuel tank level?

One other note is I do not think you have a return line filter, I have never seen one and what would be the purpose?

See above

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Old 06-14-2018, 10:19 AM
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Originally Posted by bjankuski View Post
See above
It appears that it's a dampner and not so much a filter. But it is there on my car.

Issue doesn't seem to be any different with tank level but I haven't paid it that much attention

https://www.corvetteforum.com/forums/c4-tech-performance/2532982-fuel-line-filter-in-return-line.html
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