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

Fuel filter? Monitor vacuum on fpr is it stable or does it also vary?

Fuel filter would be my starting spot too. Even it it is not the culprit, it is always great PM

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

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

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.

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?

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?

If the fuel pressure varies when the vacuum line is disconnected, it sounds like it could be the regulator.

How would I verify that? I do have my chatcoch canister eliminated and the line capped off if that means anything.

That would be a bummer. I thought the old regulator was shit so I upgraded to the Kirban. It's brand new last September

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

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?

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

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

The saga continues guys.

Car was off the road for about a month, blew a head gasket. That was my fault for beating on it in 100+ degree weather. I retarded my cam and ran a little thicker gasket to get my DCR into a more acceptable range since you can't buy 93 around here anymore, and you could when I built the engine. Also installed a 160* high flow T-Stat and a FlowKooler WP.

Initially my thought for the lean drivers side bank could have been the HG starting to let go. I was wrong. After putting the car back together, the issue persists. I've tried a couple things...

- Swapped injector harnesses, maybe a bad connection at an injector is causing one to not fire. - No Change
- One at a time, isolate vacuum to the FPR, climate control/cruise control, PCV system - No Change
-Pulled the plugs, and they all look pretty similar. Not like one is pure white while the others are nice and brown.

Thoughts of what I need to do/can do to diagnose:

-Move the fuel pump return in tank, what Bjankuski suggested.
-Check header primary temperature to see which cylinder could be running lean - ordered one of those thermal guns
-Swap injectors from bank to bank and see if the problem moves
-Swap the 02 sensors from bank to bank and see if the problem moves

Right now my order of business is:
Move the 02's - if the problem moves, I think i can be confident that i have a bad 02
Move the injectors - if the problem moves with the injectors, pretty safe bet that i have a plugged/faulty injector.
Burn the car to the fucking ground and start over

Other questions I have:
Is it possible for injectors to "partially fail" The car runs dead even at WOT, it's just under light throttle that the issue exists, and gets noticeably worse as the underhood heat gets up there.
Would an injector failing to fire cause any disruption in the fuel pressure and make it fluctuate? Doesn't seem likely but I'm not sure so that's why I'm asking. I could see maybe if the entire bank was failing to fire. If an entire bank was failing to fire i doubt the car would even run.

Edit: I haven't ruled out a Vacuum leak. I have tried finding one (for an hour straight) and can't find anything. Unless its pulling air from the lifter valley...

Thanks in advance

Jeremy