I used to run a single Walbro 255 in-tank with the stock harness. When it would get cold outside, it would lean out, the colder, the leaner.

I just finished building my fuel system and it consists of dual Walbro's, dual -6 lines out of the tank to a big Y-block. -8 line to magnafuel filter, to the rails, then to a Fuelabs regulator with stock feed as return.

I also installed 2 hotwire kits with 10awg wires all the way to the pump's relays.

Now, it does the SAME thing (on 1 pump). Leaner when it gets hotter. The car has a 408 that makes 530-550rwhp MAX N/A. When it's warm outside, and I get it running perfect, it will start running super lean as it gets cold and I'll just have to command really rich to compensate. On 1 pump commanding fat, it runs 13.0:1, with both on, it runs right at 11.8 or so (what I'm commanding). I pulled the fuse on the primary pump, and ran it just on the spare pump, and it runs exactly the same as the primary pump alone. I figure this rules out anything being wrong with the primary pump itself. I tried to spray it for the first time last night with just a 50 shot to work out the kinks. I thought the spray was working because the A/F would change under WOT, turns out it was just because both pumps were on.

What is the deal? I have always thought that a single Walbro 255 could support 600rwhp? I thought for sure it would run even better with the hotwire/true return system. I don't really want to run 2-walbro's constantly, but will if need be. I get 13.4 volts on the pump side of the relays, which use 2' of 12awg. I have them grounded to one of the swaybar end link mount bolts, which goes directly into the framerail/unibody. I even grinded down the bolt head so they would make good contact with it. Could this be part of the problem?

 

Almost sounds like a tuning issue not a pump. What makes you think it is the pump? I put a fuel pressure guage on my car, it helps diagnose any problems.

 

I can click on the second pump, and it runs perfect. Doesn't matter which single pump I have on, it still runs lean if only running on one. I have a gauge, but it's under the hood.

I thought it was a tuning issue too, and I've been jacking with it for MONTHS with no end in sight. Night before last was the first time I've tried spraying it, and the first time I've run it on both pumps. I set the FJO's trigger to turn the nitrous on at 10k rpms so it wouldn't spray and I could just run it N/A on 2 pumps.


It may be worth noting that I'm at SEA LEVEL and it's like 40* outside. Uncorrected it's probably making above 550rwhp.

 

You need a gauge on it to see what is happening. You may just plain be running out of fuel.

Nate

 

It's definitely running out of fuel, just seems like it shouldn't be on one pump while on motor. The Fuelabs regulator that I bought from you works awesome and looks great too. I can switch on/off my second pump and the pressure at the rails stays rock solid at 60psi.

I do notice that with one pump on, if I just watch the gauge and rev it, it dips hard to about 40psi then comes back up. If I turn the second pump on, it BARELY moves with a rev.

Maybe my car makes a lot more power than I think it's making.

 

take it off and see if you can blow through it. I had a filter get stopped up after 2 months of use..it just took 1 bad tank of gas.

 

The fuel filter is a brand new Magnafuel -8an. It's got about 400 miles on it.

 

My sinlge Walbro would only support about 525rw, 600 with a KB BAP....

 

I had a buddy that used a BG Filter(NEW)...He kept having psi drop..come to fine out it was the filter.

 

If you are using a Y and it goes lean with only 1 pump on, sounds like the check valve in one of the pumps does not work.

 

Your regulator is probably boost referenced and having to put up more pressure, sacrificing a little flow. Im guessing this was with an old setup? Your new one probably makes 525 on 1psi lol.

Thanks for the info though, I may just have to call it running out of fuel and run 2 all the time. The car is a weekend warrior and I have a lean shutdown switch on the nitrous kit (and fpss) so i'm not that worried about unknowingly loosing a pump and hurting something.


I have run both pumps independently with the same results. Check valves were my first thought. This would have to mean that both of them are broken.



One is a walbro 340 other is a 341, not sure if that matters, but I got the 341 for free from a friend. The 341 has a little brass "venturi tube" that I just left open. I figured I had any problems associated with that stuff ruled out by running both by themselves with the same results.

 

Nate spec'd out the filter, and its supossed to have very little flow impedence. The regulator being downstream of everything should decrease its effect even more.

The pressure bleeds off in the system pretty soon after the car is turned off. How long should it take?

 

Unless you are somehow adding fuel in your tune because of IAT, cold starting or ambient temp changes, your car will run leaner in colder weather, given no fuel system changes. Running two pumps, more pressure, yes then because your effective injector flow rate changes, you will see richer AFR's when compared to one pump. It is really cold here in the PacNW right now... and when I drive car, unless the IAT's are up high and the car is fully warmed up, I will idle and cruise .75 to 1.5 AFR points leaner than if it was 65-70 degrees outside. For example, 65 degrees ambient, warmed up (82c) my car idles at 14.6:1. Today, 29 degrees outside, warmed up (82c) my car idled at 15.8:1.

Now once youre under load, the intake manifold and underhood temps are up, the difference will be much less. But don't be surprised if cold weather makes things change a little.

 

The airflow is calculated based on the MAF response and the IAT. It should change with the weather. Mine idles consistently and drives at 14.6/7 regardless of temp if I'm just cruising. It runs consistant no matter what temp it is. Under WOT when it's cold, it's flowing more air and needs more fuel. The injectors are flowing LESS than normal because the single pump can't maintain pressure.

When the second pump comes on, it's only seeing the pressure it SHOULD, which is stay at 60psi, I have a real return system on the car. When one pump is on, the motor is consuming all the fuel from the single pump, which is just enough to run it 13.0 when it's 40* outside. The regulator actually CLOSES during WOT on 1 pump.

 

I suppose you could be running out of fuel... you are still running a MAF? Can you log data to see whats happening to your fuel? The IAT doesnt play that much of a role on the calc of airflow on the stock tune with stock MAF tables, unless you have changes your Blending tables. Either that or you have too much feed line for a single Walbro? I have a single Walbro, stock feed, -6 return and an SXP Reg and I have never ran out of fuel because of the pump, pushing 30# and 42# injectors. Especially hard wired. Hmmmm.... Is the reg hooked up properly? (regulating on the return side, with the bottom port toward the tank) I assume it is not boost/vac referenced? Something hinky is goin on. With regards to the ground, I would grind away somewhere and expose body metal and installa ground lug. Then ground the relays and pumps to that. Do you have a digital multimeter that you can use to check the integrity of that ground?

 

Yes, I'm still running the MAF. It hits the second to the last cell in the table, I had originally thought it was just maxxing out when cold outside, but it's not (amazingly).

Everything is hooked up right. Regulator's boost port is open to atmosphere. I do have a Fluke I can check the ground with. There's about 2' of -6 off of each pump, then a big Y-block with 1/2" NPT x -AN threads, then probably 10' of -8 to the back of the passenger side rail. It's then got a -6 cross over and about 18" of -6 to the regulator.

I can understand a spike drop in fuel pressure due to the initial surge, but it stays down. I can stand on it from 3k to 7k and it will start leaning out around 5k.

 

Are you running billet rails? (FAST, or Nasty, or other) If so, you should have a -8 crossover. The crossover needs to match the ID of your fuel rail or larger. Never smaller. This isnt necessarily the cause of your problem, but restricting the crossover will cause uneven pressure when fuel demands spike like on hard acceleration. Fuel system plumbing is like plumbing sprinklers in a building... the reduction in inside diameter of the pipe (line) need to happen in a decending order. Never ascending. So, from the pump to the front of the car, the smallest restriction needs to be last; which is usually the regulator.

Also, double check the fittings or hose clamps in the tank, on the pumps. The smallest leak in the tank will dramatically affect fuel pressure. Check that ground(s) for sure. Also, when you check the ground, leave the meter on for a few seconds and watch the display. A less than adequate ground can take a few seconds to show in the display. The number should be as close to zero as possible (the Fluke is like the Craftsman in that they smooth out jitter in the measurement).

Also, it may be that a single Walbro may not be able to adequately maintain your pressure on a -8 line with the demands of your injectors. Remember, pressure and volume are inversely proportional.

Lastly, (I am not telling you to do this, but) switching my car to full Speed Density was the best thing I ever did for my tuning headaches. Just my opinion, many will disagree, but I fought wild AFR curves for weeks and once I went SD, then tuned AutoVE and RAFIG... it was all butter.

Sorry about the long post, just trying to help you out bro, these kind of problems always **** me off too and I have had my share.

 

One more thing I just thought of... as a test.

The rail that has the outbound -6 line to the regulator. Cap it off at the rail. Making it a deadheaded fuel system. Then turn on the key and see what the fuel pressure runs up to. It should skyrocket to well over 65-68 psi. If it doesnt, then you have a pump problem or a leak somewhere in the tank. This will not hurt the Walbros, they should be able to take running deadheaded.

 

Yes I'm running Nasty's billet rails. I completely understand all the flow schtuff, I'm about 1" away from my mechanical engineering degree.

The wideband is on the passenger side bank, which is the same bank that has the -8 plumbing to it directly from the tank, so if anything, it should be running RICHER on that side. The crossover is a -6. I figured with the passenger rail sucking down 300+hp worth of fuel, 450hp worth on gas, a -6 crossover would suffice, plus I already had it. My N20 system's fuel feed comes off the back of the passenger side rail via -8an x -8an x 3/8" NPT TEE right before the main feed hits the rail. When I first installed the regulator, I had it screwed in too far and the pressure went up to like 80psi with just turning the key on and having a single pump come on.

I can't run an SD tune on my '98 because it has a secondary VE table that is only 1/2 resolution. I also can't do a handy-dandy auto tune either lol.

A leak in the tank did cross my mind. I have -6 female x 1/4" NPT male adapters in the tank on the bottom of the -6 bulkheads, then 1/4" NPT female x 5/16" barb fittings on the bottom of those, with 5/16" high pressure hose running down to the pumps (about 8" worth).