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I have an 86 Mazda Rx-7 that was swapped with a 95 LT-1. I have had it over 10 years now and recently I have been running into a fuel issue that I can't solve/understand. I have had an Aeromotive in tank pump previously and one morning I came out to start it and noticed it was hard to start. I have a fuel pressure gauge in my dash, and noticed that it wasn't holding pressure. I figured it was the check valve in the pump, so I replaced the pump. After replacing the pump I noticed that it would only prime to about 35-38 psi, but it would hold steady. I drove it a few times and it felt good until I did a couple of hard pulls and noticed the pressure would go to 43 and then drop to 25 at which point I was losing power and immediately let off. I also ran into a few problem where the engine would shut off because the pressure started dropping to the point of zero. After a cool down of 30 minutes or so, it would usually prime back up to 38 and it would fire up again. Today I decided to swap the fuel pump again as that was the only change I made, suspected maybe a bad pump. New pump is the same model and it primed to 42 and then dropped to 40 with key on. I took it for a drive and as soon as I got on the gas it dropped pressure again.
The fuel pump is run off of a relay that is activated by the PCM and fused. The relay is 12v 120 amp (overkill) and it's fused using a 30 amp Megafuse. I havn't today, but I can log the computer while it is running and everything seemed normal except for the idle seemed lean which I suspect is because of the inadequate pressure.
Can the stock Fuel Pressure regulator break in this manner? I have tried running it without the vacuum hose attached and get the same results.
Ideally I would like to get rid of the factory regulator and use an external adjustable version, but I am running into a problem getting the car high enough to run the appropriate lines from the tank. I have all the parts, but I could really benefit from a lift.
What is pressure at idle, with the vacuum compensation line attached?
What is the pressure at idle when you remove the vacuum compensation line from the fuel pressure regulator (holding finger over line to prevent a vacuum leak)?
Is the fuel tank vented, to prevent significant vacuum buildup?
What is the flow rating of the pump? Excessive flow being returned to the tank can cause fuel temperature to increase (particularly if return. Line is close to exhaust), which can cause fuel to vaporize in the pump suction. Is there a filter sock on the pump? Is it clean?
Pressure at idle with vacuum - 34
Pressure at idle with no vacuum - 42
I tried taking the car for a drive around the block with the vacuum removed and it stayed at 42 until I started to accelerate hard at which point the pressure went promptly to the 20's and i let off and it returned to 42.
Fuel Tank is vented to the atmosphere and makes no noise when the cap is unscrewed
Pump is an Aeromotive 11541 (340 stealth series) -- at 40 psi the 340 boasts more than 340 lph
Pump is brand new and has an included sock, the also fairly new pump that came out also had a brand new looking sock on it.
The only item that changed from when it was running fine and started experiencing hard start was an Aeromotive 11141 (discontinued) to an Aeromotive pump 11541, both are standard replacement for the rotary and should still flow more than enough for the LT-1. That's not to say something hasn't degraded or broken, but that is the only change I have made.
340 LPH is wayyyyyy more than enough for a basically stock LT1. Aeromotive claims that pump is good for 850 HP on a normally aspirated, gasoline fueled engine. That’s a believable figure. My base pump is an in-tank 208 LPH. Easily supplies the engine's 500 flywheel (engine dyno proven) HP. I have a 2nd identical pump mounted outboard, that only runs when the 300 HP nitrous system is armed. Also have an outboard fuel pressure regulator.
Fuel filter clean?
Is the tank vented by way of a vented fill cap? Do you notice excessive fuel vapor odor when the cap is removed, indicating hot fuel? If the tank pulls a vacuum, and/or the fuel is warm, you run the risk of vaporization in the pump inlet at high flow volume. Try a WOT throttle pull with the fill cap off.
When you say “stock” FPR, is it actually the correct GM part? The Aeromotive LT1 AFPR is not reliable, for example.
Mine feels like it has a -300 shot when the fuel pressure drops.
Fuel filter was brand new when i swapped the 2nd pump, probably has maybe 100-200 miles on it now.
tank has a vent line that runs to the engine compartment which i used to have hooked up to the evap solenoid, but i have removed that a while ago and it’s open now.
it seems to be getting worse as now I can’t even make it to the end of my street (8 houses) without it cutting out on any type of acceleration, so I don’t suspect heat as a culprit in that sense.
i talked to aeromotive today and the tech guy seems to think it’s the K&N filter not flowing well enough and thst i should use an aeromotive version. I have a summit racing
(SUM-230123B) that i will try to plumb in, just need to figure out the AN fittings.
Last edited by evilpaul; 08-31-2021 at 02:13 PM.
Reason: mobile post
I had looked up that filter. It showed applications for a lot of GM light trucks, generally 4.7L or less. That looked suspicious, but there were a couple with the 6.0 liter V8, and even a diesel with a larger engine.
Are you sure you are getting a consistent 12+ volts at the pump?
What size is the fuel supply line? But I run enough to make 800 HP through -6AN supply and return lines. System operates at 58 PSI, and the Bosch pumps are “high pressure “ versions, but for your engine a 3/8” or -6AN line should be fine. Is it possible something caused a crimp in the supply line?
On the vent line, is the car's original carbon canister still in the line?
I'm having almost the exact same problem with a completely stock 95 LT1, my post should be up soon with #'s to compare as soon as it's mod approved.
Did you notice any increase in engine temp?
I checked the voltage and it's 14.5 coming off the fuse to the pump when it's running.
The fuel supply line is a combination of 5/16 (original rx-7 steel tube) and 3/8 flex line, but worked previously to this new problem.
Vent line has no carbon canister at the moment .
I am wondering now if maybe a rock put a dent in the line underneath the car, can't see anything out of shape on any of the visible lines.
Still looking at replacing the stock fuel pressure regulator with an adjustable version, just need to swap some fittings and get some new injector o-rings.
Small update. I put the car on some ramps and was able to examine the fuel lines and everything looks pretty good as far as no dents or kinks. I did find some things I am not happy with though for the future. Previous owner stepped down the lines from 5/16 hard line to -4 braided lines at the tank, which I believe is 1/4", so that sucks. I imagine the return is not so much of an issue, but the feed is definitely hampered by that.
The plan going forward for now is to swap out the factory regulator for a stand alone and run all -6 lines in the engine compartment.
In the future I will upgrade the hard tube feed line to 3/8 and -6 coming out of the tank. I'll also change the hardline adaptors coming off the return line to 5/16/ and -6 which should be plenty for the motor that's in there and not choke out the pump so much.
Stock nylon and metal supply lines are 3/8” (-6AN). Only the return line is 5/16”
On the other hand, I run all -6AN braided lines, supply and return. And they can handle supplying 800 HP, at a BSFC of greater than 0.50 #/HR/HP on nitrous.
But somehow 1/4” just sounds too small. With that huge pump it has to be choking flow. A large part of the flow is getting returned to the tank.
Yeah I agree 1/4" is too small. I guess my local hydraulic shop are a bunch of idiots as they told me the line was -4. I ordered a -4 to -6 adaptor for the interim to see if I could bypass the stock fuel regulator and of course it doesn't fit, it's to small. Apparently those lines are a -5 (5/16) which seems really hard for me to source adaptors. Currently I am going to plumb everything in the engine bay as -6 with an external regulator and then adapt that to the existing 5/16 lines from the tank. If that solves my current situation, then I will drop the gas tank and finish plumbing the rest of the system with 3/8 lines as feed and return.
Any idea if the port that I have plugged on the driver side of the manifold next to the power brakes can be used as a vacuum source for the fuel pressure regulator?
I am not running an EGR system. I have an external Fuel regulator mounted on the driver side and was thinking of using that port versus running a vacuum line around to the passenger side of the manifold, but wasn't sure if it was the same vacuum source or not.
I was using the correct port previously, you can see it here capped when I was testing the factory regulator. You can also see the new external regulator on the driver side.
08-24-2021, 07:55 PM
Fuel pressure dropping, not adequate
