View Full Version : Fuel pressure dropping at WOT. Help!
04-21-2003, 02:00 PM
Thanks for the response.
I see even with the 340 on your system you are still seeing a 7psi drop at WOT. What causes that?
If I could get my fuel pressure up to 62-65 with a 7psi drop at WOT I would be happy.
Having the same performance with the 340 really sucks!
Is there an adjustable FP regulator that will fit where the existing 3-way junction block is now?
04-21-2003, 03:59 PM
About an adj FP reg back at the junction - I'm not sure. I've been searching for a few weeks and cant decide on how I want to go about it. Putting an adjustable back there would be the easiest route but I'm not sure it would be the most effective. Since I'm staying NA, a constant pressure set up adjusted from the rear should be fine.
Any info out there on this?
04-22-2003, 01:13 AM
I have a 98 Z28 with headers, PT4000 TC, TR224 cam and a few other bolt ons.
With the stock fuel pump the fuel pressure after about 50% throttle would drop from 57psi to 50psi. This past weekend I replaced it with an in tank Walbro 340 and took it for a test drive.
To my surprise I had the same problem. <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" /> The fuel pressure would drop instantly from 57psi to 50psi after 50% throttle. The fuel pressure holds steady at 50psi.
I have checked / replaced the fuel filter. I have visually inspected all the fuel feed lines for damage. I have blown through all the fuel lines to confirm there is no blockage.
I have checked for excessive voltage drop across the power feed for the fuel pump. The drop is ~.15 volts with the pump off so I do not think that is the problem.
I do not know what else to check. <img border="0" alt="[bang head]" title="" src="graemlins/gr_banghead.gif" />
Please offer some suggestions.
04-22-2003, 01:32 AM
Its got to be something with the stock regulator set up. There seems to be no "constant" pressure with a Walbro 340 and stock fuel lines. I've installed 4 now and have different readings on 3 of them. Mine runs at almost 80 psi!! <img border="0" title="" alt="[Eek!]" src="gr_eek2.gif" />
When warm at the track it will level out around 76 and drop to about 69 under WOT. I have verified this with a direct reading gauge to rule out my in car FP gauge.
I'm still trying to decide what adjustable FP regulator to go with and how I want to run my feed and return line. I need to do something soon though, as I'm tired of tripping Bank 1 and Bank 2 RICH codes!! <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />
Bottom line though, I feel the pressure output from the 340 just reacts differently on our crappy stock in-tank regulators.
04-22-2003, 02:28 PM
Further thoughts on my dropping pressure:
I did more driving and testing and as soon as I hit 4000rpm the pressure starts to drop. By the time I hit 5000rpm the pressure is at 50psi.
The only possibilities are
1. the existing fuel pressure regulator is bad
2. the new fuel pump is bad
3. low voltage to the fuel pump
I really do not think the regulator is bad. It is working at idle and if it is bad it is usually bad all the time.
It is also unlikely that the new fuel pump is bad. It is acting exactly like the stock one I pulled off.
Low voltage to the pump- I am getting 12.3v to top of the fuel tank. At the battery I have ~14v. Maybe the pump needs more than 12.3v to operate correctly.
04-22-2003, 02:30 PM
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by red ws6 99:
<strong> You can tee in an adjustable regulator at the back. A lot of the forced induction kits do this. You will just need to convert to quick connect fittings that return line uses.
Gary </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Gary,
What brand/model number would we need to use?
04-22-2003, 04:07 PM
For constant pressure at the injectors you would
want the blowoff regulator up at the engine with
a return line back to the tank. I thought '98s
were this way but '99-up had the regulator back
by the pump? This leaves the forward fuel line's
flow-related and acceleration (g-force) related
pressure drops as subtractors from the regulated
Some have mentioned the marginality of the fuel
pump wiring and improvement from beefing it up
(pump pressure is current, pump flow is voltage,
substantial drop in applied pump voltage will
bring in the flow*pressure line closer to the
bone). I bet this is only worse with higher-
I've seen this question pop up before but never
seen any good info from anyone who's fixed it,
on which of these, or other factors are really
04-22-2003, 04:53 PM
98 and up Fbodys have the regulator in the tank. 99 and up have a plastic tank and the 98s have a steel tank.
As pump volume increases (Walbro 340 over stock) the current flow to the pump also increases.
As current flow increases voltage drop increases.
I will upgrade my power and ground wire to the fuel pump and see what happens.
04-22-2003, 07:35 PM
i got money on the stock regulator sucking.. mine did the same thing (and was a bad regulator)
04-22-2003, 09:25 PM
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by MIGHTYMOUSE:
<strong> i got money on the stock regulator sucking.. mine did the same thing (and was a bad regulator) </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">What regulator do you recommend going with?
red ws6 99
04-23-2003, 01:15 AM
You can tee in an adjustable regulator at the back. A lot of the forced induction kits do this. You will just need to convert to quick connect fittings that return line uses.
04-24-2003, 07:58 AM
Not sure if knowing this helps but Chris and I put in my Walbro this past weekend and although I don't have my FP Gauge yet I did take a Static reading at idle with a handheld gauge and it was at 64-65psi. As soon as I get my gauge I'll be able to see if it's dropping.
04-24-2003, 03:23 PM
I was looking at the Aeromotive A1000-6 Injected Bypass Regulator ($130) or the Aeromotive 13301 ($110) which includes a high pressure spring to increase pressure range to 20-65 psi from the 3-20 psi range.
See link below.
I would plumb it into the return line. The cost of the fittings/adapters and line would be about $70.
Regulator link (http://220.127.116.11/products/view.phtml?f_cat=Regulators)
04-24-2003, 03:27 PM
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by StealthStalkerLS1:
<strong> Not sure if knowing this helps but Chris and I put in my Walbro this past weekend and although I don't have my FP Gauge yet I did take a Static reading at idle with a handheld gauge and it was at 64-65psi. As soon as I get my gauge I'll be able to see if it's dropping. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Thanks for the information. So the 64-65psi was just from replacing the fuel pump and still using the stock regulator?
What was you FP with the old stock pump?
When you turn off the ignition what does the fuel pressure drop off to?
04-24-2003, 09:38 PM
If memory serves my idle FP before was 59. That was with the T-Rex Inline and the stock FP.
The 65psi is Idle with the Walbro and the stock regulator.
I cannot recall what the pressure was when I turned the key off. I can easily check for you tomorrow though..
red ws6 99
04-25-2003, 01:11 AM
Weldon, Aeromotive, CAS. None are made for our setup. If your going to go to the trouble install it on at the engine. It will react better. If you put it at the back the biggest hassle will be adapting it to the stock lines. The one I have came with the Incon kit. It has the correct quick connector built into it. Its the same unit as the small aeromotive regulator for the LT1. I cant recomend it because it junk.
05-03-2003, 09:20 PM
Well I replaced the regulator and still have the same problem. At idle the fuel pressure is at 57psi. As soon as I go WOT the fuel pressure drops to 50psi. :bang:
The only other possibility is that the new Walbro 340 pump I put in is bad. :(
What is strange is that this is exactly how the fuel pressure was acting before I spent the $200+ on parts to fix it. :( :mad: :barf:
Any suggestions would be welcomed!
05-04-2003, 12:51 AM
Hmm.. Dam John that sucks. Any chance yours might be an easy fix like Crunch who had a problem with his dropping PSI because the fuel supply hose from the pump was not tight enough?
05-04-2003, 12:15 PM
When I pulled it apart yesterday I was hoping that I would find a loose clamp but no such luck. I checked all the hose clamps and all were tight.
Considering the problem now is the exact same as it was with the original pump and regulator and the only thing I have not changed out is the in tank connection to the regulator then maybe that quick connect fitting is leaking.
05-05-2003, 09:26 AM
Are you able to determine whether:
- The pressure drops linearly with fuel flow,
or right from 57 to 50 just when flow starts,
or whatever? The nature of the pressure drop
would implicate different things. Anything
nonlinear at low flow would indicate the
regulator, anything that falls off abruptly
at the top would indicate pump motor or pump
electrical, to me.
- The at-pump and at-regulator pressures are
substantially the same, or grossly different
from the far-end-of-line pressure (which
would specifically point to the fuel line
flow-pressure drop; this would also be a
pretty linear pressure drop with flow).
I saw a chart the other day that said a 600HP
car wants 1/2" fuel line. What's under ours,
like 5/16" or 3/8"? I'd say that the ideal
for fuel pressure stability would be to run
some 1/2" up to the motor, use a front-mounted
blow-off regulator, reuse the former forward
line as the return (reconnect it to the return
tube at the old regulator). I also think that
the fuel rails could be fatter and the feed
should be at the back of both rails ("Y"),
and return / regulator should take off from
the front of both ("Y") so that both sides are
equal-pressure at high flow (possibly helping
with the bank-bank imbalances often seen in
these engines) and both sides get "swept", not
dead end (which is bad for collecting crud in
the injector screens).
05-05-2003, 01:37 PM
The fuel pressure drops with relation to the throttle position. If I go WOT it drops to 50psi instantly.
I am using the stock regulator in the tank and reading the fuel pressure on the fuel rail. I expect to see some drop but not this much.
05-09-2003, 02:22 PM
I had the same problem, and it initially turned out to be that the hose that connects to the in-tank fuel pump came loose. But when we fixed that, the pressure STILL dropped to 50PSI at WOT. I think low fuel pressure is what did my stock motor in. I had a good tune with no detonation and no knock retard. After the rebuild we'll have to revisit this immediatley so this same problem doesn't burn up my brand new bottom end. :(
05-09-2003, 08:45 PM
Consider this (is this a feasable explanation...?):
At higher RPM the injector duty cycle goes up
(the injectors are open for higher percentage of the time), and so the pressure will be less since there is less injector "restriction";
The regulator is at the pump and regulates the pressure there (at the pump), but there is nothing to regulate the pressure at the fuel rails (...9 feet away from the pump...);
05-09-2003, 10:35 PM
That is something I did not think of! I guess that is possible. How could I test for that?
I have another idea as well. I cut an access hold to get the pump out. I tried to pull some slack the plastic/nylon fuel feed line and it would not budge. Maybe when the car was assembled the fuel tank was installed against this line causing a restriction.