I have been chasing a fuel pressure issue on this truck since the built motor was installed. At first I thought it was a tuning issue. I ended up tuning the truck in open loop all the time and ran it a bit richer and it seemed to work well. I still wanted to be able to run it in closed loop though so started working on that tune.

I first put a fuel pressure gauge on the rail and saw that at wot I only had 35 psi. So I dropped the tank and found that the plastic "Y" for the pumps was crushed on one leg. I initially "repaired" it as nobody stocked one, and then found a stainless unit from McMaster-Carr for about $15. It is 3/8" o.d. so it requires going to a bit larger rubber hose to work, but works great. I now had 60 psi at wot, re-did my tune and continued with the closed loop. But the surging I had under load initially came back, worse than ever. This is a big surge of the a/f under load like climbing a hill or passing a car without going wot. If you backed off the throttle it was fine or floored it things were fine but the in-between was an issue.

Next move (should have been my first) was to monitor my fp all the time. Since everything was new I assumed it was right; that was a big mistake and I know better than that. At any rate, I saw that the surging directly followed the fuel pressure, which was bouncing around 10-20 psi back and forth. Back off and the fp stabilized, push harder on the pedal and it jumped to 60 psi and steady.

These trucks have what some people call 2 speed pumps. Actually the truck has a resistor that in closed loop cuts the voltage to the pumps to about 7-8 volts and slows the pumps down; slower = less pressure and cooler running. Once you get into open loop like wot it switches via a relay to full voltage and speeds the pumps up, plus the vacuum line to the regulator is now under boost and adds a few more psi. I think some of these trucks have problems controlling the aftermarket 255 pumps some of us run on the low speed side.

Here was my final solution that fixed everything. I replaced the fixed stock pressure regulator with an adjustable unit. The 97-98 LS1 Vette unit is a direct replacement, so order an adjustable regulator for that exact year/model. The later ones won't work as they quit bypassing the fuel to the tank. I cut the two wires to the resistor and soldered them together. The resistor is behind the front bumper, a white porcelain deal that attaches to the intercooler pump bracket. I adjusted the regulator to about 38 psi with the engine idling and vacuum hooked up to the regulator. I still get the low/high pressure the factory intended but the resistor is eliminated which gave my low side issues. It now stays perfectly steady and slowly rises the harder you push the pedal. The high load surge is completely gone.

Sorry for the length, but couldn't really make it shorter without missing some key details.

Jody

 

Glad to hear you got it fixed...Solving problems on our L's can cause major headaches sometimes...

 

Jody,

Since the resistor is effectively bypassed, how are you getting a "low" circuit? Now that the resistor is bypassed, its a direct ground, so the voltage to the pumps should be the same regardless of which position the high/low relay is in.

It seems like bypassing the relay under the hood would have had the same effect?

Glad you figured things out!

-Dale

 

Yes and no, at least on mine. Mine has 11-12 volts now so it is a bit less than 14 like wot. Maybe mine is different. I did try bypassing the high/low relay a while back but for whatever reason I could not get it to idle at 38 psi, always was 60 psi even with the regulator turned all the way down. At 60 psi all the time the pumps did get hot in just a few minutes of driving. Not happening yet and I have driven longer than needed before to heat things up.

I tried to keep the low speed side in, but it would not maintain the pressure until I bypassed the resistor. Actually JLP told me to try that and it's working. I did order a new resistor and it will be in today as maybe mine is bad. When it arrives I'll install it and rest the pressure to 38 at idle and see if the problem is still gone. I'll re-post what I find. But before bypassing the resistor I monitored the voltage into and out of the resistor while driving; the fuel pressure was going up and down but the voltage was steady, so that made little sense to me. So I bypassed it and reduced the pressure due to the increased pump speed and it solved it.

I think most cars don't have a pump controller either; they just have the vacuum line on the regulator to reduce the pressure under vacuum. They run the pumps at full voltage all the time.

Jody

 

How is it working the same? If you bypassed the resistor, now you have full ground, so there is no high/low. Also, if you bypass the resistor, and then set your base fuel pressure, you are going to have LESS fuel pressure at WOT.

 

i just wired in a second resitor in parallel with the stock one to bring my fuel pressure up. i am still trying to find a better solution.

 

I have 38 at idle and 60+ at wot. I have 11-12 volts to the pumps with the resistor bypassed and 14 at wot. Even with no voltage change how would you have less psi at wot when you have the vacuum (now boost) pushing on the top side of the regulator increasing the pressure?

Jody

 

I wouldn't have expected those results. The point of the high/low voltage setup is to maintain normal fuel pressure on the lower voltage setting, and then get the required higher voltage needed for more fuel pressure at WOT. Normally the truck would be at 39 psi base with only 7 volts, then jump to 60 at WOT. I would think that if you are starting at 39 already on the high side, it wouldn't jump as high. But apparently your truck disagrees .

 

Ok #1, Great to hear Jody, I know this has been a real hair puller for ya and it sounds like you got it licked

(If I continue to melt plugs, I will DEF have to monitor mine)


Ok and #2
Someone want to tell me why the hell we have a bunch of half naked fruit cakes dancing half naked to the left of this box ???
WTF kind of advertisement is that

 

I would understand the no pressure increase if the voltage remained the same, although mine does change a couple volts. I don't understand why you would not expect a pressure rise from adding boost to the top of the regulator diaphragm. That always works that way, whether it be a mechanical fuel pump or any boost referenced regulator that I've ever had. On the carbed centrifugals that's how you maintain the fuel in the carb by running a boost line to the regulator or top of the mechanical fuel pump; one psi of boost pressure adds one psi of fuel pressure. Since I'm running 20 psi of boost added to my 38 psi base pressure plus a handful from the extra 2 volts (for whatever reason that may be happening) I end up with over 60 psi at wot.

Drove the truck for a couple hours today; fuel pressure was perfect, tank didn't get hot, and the truck is running perfect. I do have a new resistor coming and may try it again just to see if it'll run correctly with it.

By the way, I appreciate your help in solving this.

Jody

 

THOUGHT I WORKED ALL MY PROBLEMS WITH MY TRUCKS PERFORMANCE OUT BUT APPARENTLY NOT...

THE "IN BETWEEN" ACCELERATION SUCKS ON MY TRUCK AND STILL CAN'T FIGURE OUT WHY.. IT'S LIKE WHEN I GO TO PASS CARS OR CLIMB HILLS ALSO THE TRUCK STRUGGLES.. IF I QUICKLY DEPRESS THE PEDAL, NO GO AND THE S/C SOUND SOUNDS LIKE ITS SICK.. WHAT YOU'RE SAYING ABOUT THE FUEL PRESSURE DOES SOUND LIKE IT COULD BE POSSIBLY BE MY PROBLEM ALSO...I JUST WISHED I WAS AS KNOWLEDGABLE ABOUT THIS STUFF A LOT MORE ON HOW TO FIND OUT...WHEN I TAKE THE IDEAS YOU GUYS GIVE ME INTO THE FORD DEALER THEY THINK I'M CRAZY (I.E. CLEANING THE INTERCOOLER) WHICH I DID MYSELF B/C FORD CLAIMED I SHOULDN'T HAVE TO DO THAT..ANYWAY DOES ANYONE KNOW OF ANY PLACES IN GA THAT DO WORK ON LS AND ALSO LOOK AT PROBLEMS LIKE THESE B/C FORD DOESN'T..

 

Yep
You'd want to talk to James Parker (Runnin with the Devil)

Site
http://runninwiththedevil.com/

Forum
http://www.diablosporttuning.com/forums/

BTW
TG I see the Boys in Pink are gone