Fuel pressure issue

[rickpaw]

So I just completed a LM7/4L60e swap in my 67 Firebird. I had about 300 miles on the car after the swap, mostly around town with 30-40 minute trips. Last week I took it to Cruisin the Coast (CTC) from Houston and ran into fuel pressure (FP) issues 4 times.

Here's my fuel set up:
-LM7/4l60e combo. Tank is Tanks Inc FI tank with Aeromotive 340 pump. Corvette fuel filter/regulator mounted just in front of the tank. Brian Fich duel vent tank vent, along with stock vented fuel cap.
-Fuel line is -6AN from tank to stock 3/8 hard line on the frame rail. 6AN from pass side rail in front of firewall looping along the top of firewall to the driver side fuel rail inlet.
-Stock LS1 intake/fuel rail/injector. Stock LS1 ECM with mail order tune from PSI Conversion.
-Voltage at the pump: 13.5V. Alternator puts out 14V. Pump is triggered by ECU and powered thru 30A relay directly from the battery in the trunk. 10awg pos/neg wires.

The issue:
-The first 250 miles of the trip the car ran great. The last leg of the trip (Houston to CTC), the weather got warm so we had the AC on. After about 2 hrs driving with AC on, the car started stumbling and lost power. Pulled over and the car barely idled. I hooked up FP gauge and it only showed 20 psi. Took the pump out and found some debris blocking about 50% of the screen at the pump inlet. Cleaned it out and put everything back together. Got 40 psi of FP (voltage at pump=13.4V). Not enough but enough to get the car running to CTC (we were 30 mins away). During CTC it died 2 more times with FP of 20 psi. All while AC was on and in stop and go traffic. Both instances we took the pump out and the inlet was clean. The fuel filter/regulator was removed and checked for debris. It was clean also.
-We ordered/overnighted a 2nd Aeromotive 340 pump and changed it out on Sat, figured the 1st one was bad. :G-Dub: Drove the car around Sat with no issues (AC was not on). Drove back to Houston from CTC Sunday. About 160 miles into the trip (after the AC had been on for 1.5 hrs), the car stumbled and died again. FP was at 20 psi. Let the car sat for around 15 mins, and FP jumped up to 40 psi. One of the old school guys in our group mentioned the engine sounded like it was struggling for air. We removed the air cleaner and it ran better so we cut a hole in the K&N filter to allow more air in. We drove the car back to Houston without the AC the rest of the way without any issues.
-Let the car sit over night and start the car. FP shows steady 58 psi.

I ran the car in the driveway yesterday with the AC on full blast, weighted the gas pedal down so the engine revved to 2500 rpm (cruise rpm when it happened). Obviously there's no load on the engine since the car was just sitting. Ran the car for 30 mins with the hood closed. Got it pretty warm to the point of the coolant overflow was puking, temp got to 242. Everything in the engine compartment was hot, but the fuel pressure showed a steady 58 psi. I think that's just ruled out vapor lock.

Any ideas/tips would be appreciated. Also during the trip, I had intermittent code of "knock sensor bank 1 low voltage input" showing up.

[DaleTx]

This might be a stretch... but I'm just throwing out ideas.

One time I had a Holley electric fuel pump that would drop pressure... and then stop working completely when it got too hot (when sitting in traffic). It was like it would seize up. When I let the car sit for a while and let the pump cool down it would start working again.

One other idea would be to check the tension on the serpentine belt to make sure the belt is not slipping when the air conditioning compressor turns on, causing low system voltage.

It's no fun when you have glitches on a new build... but we've all been there :)

[rickpaw]

This might be a stretch... but I'm just throwing out ideas.

One other idea would be to check the tension on the serpentine belt to make sure the belt is not slipping when the air conditioning compressor turns on, causing low system voltage.

It's no fun when you have glitches on a new build... but we've all been there :)

Hi Dale,

Voltage drop came to mind when it first happened. We measured voltage at the pump when FP was in the low 30s psi, and it showed 13.5V (but then AC was not on). The belt tensioner is within recommended range, and the belt is tight with no slack. One of the guys in our group had some issues with Aeromotive pumps in his LS1, and switched over to ACDelco pump and had no issues since. He suggested I do the same. I wished I had data logged it on the trip back.

One positive thing during the trip is the gas mileage. I put in 2 times less gas than the rest of the group, and that put me in around 30 mpg range.

[mikels]

Most likely due to pump overtemp during low load (high bypass) operation. Classic problem with fuel pump sized to deliver required fuel @ max power being asked to operate in low load conditions where most fuel is bypassed and returned to tank.

Best solution: Vapor Works or equivalent where pump is only operating at enough load to deliver required fuel (no bypass). This mimics what OEM's are doing for very same reason (along with reduced parasitic loss for increased FE). Carl's system works awesome!

Next best: Staged pumps where only operate on one pump under low flow conditions with secondary pump coming online during increased load conditions. BTW, Vapor Works can also be used with staged pumps for those with greatly varied fuel delivery requirements from idle to max power.

I've fought this very issue so many times and it is extremely fustrating to be stuck on side of road due to such a seemingly simple issue.

This is another reason OEM's mount fuel pumps in tank - for cooling effect (as well as noise supression). But even that is not enough for most aftermarket high flow pumps operating at the now nominal 400 kPa (~60 psig) fuel pressure under high bypass conditions for extended periods of time.

Easy check can be after low speed cruising around in your car (conditions where you noticed fuel pressure fall off), can you even touch your external regulator without buring several layers of skin off your fingers?

[hifi875]

I have the tanks inc. setup in my 69 Camaro running the holley terminator system and have put over 4000 miles on it(power tour and other trips+autocrossing) with no issues. Mine only has to run at 43psi though. seems weird to have trash in a brand new tank.

[rickpaw]

I've searched on here and there were some instances where a few were left stranded due to fuel pump over heat on long trips (ie power tours). The overheated pumps were Aeromotive pumps. Makes me wonder whether the Aeromotive pump I have in there now is too big for my application, and overheated.

[rickpaw]

Easy check can be after low speed cruising around in your car (conditions where you noticed fuel pressure fall off), can you even touch your external regulator without buring several layers of skin off your fingers?

The filter/regulator was warm, so was the fuel tank, but not hot to the touch.

[CarlC]

The Aeromotive Stealth 340 is capable of supporting approximately 900 FWHP naturally aspirated @ 60psi. That's a lot of pump for most applications running at battery voltage.

For most N/A applications running 550hp and below (includes a good safety factor) and running a traditional pumping arrangement the Walbro 255lph (GSS340) is a good economical choice that has much less power demand than the Aeromotive 340.

All pumps however are subject to damage when run dry.

The pressure scenarios you mention are typical of a pump that is on it's last legs. When cool they will run for a short time, then pressure will begin to decrease. The "short time" gets shorter and shorter until complete failure arrives. If you have a fuel pressure gauge it's very apparent. I went through multiple GSS340's before I gave up on the traditional systems.

If you're going to stick with either of these pumps I suggest keeping the tank at least 1/2-full. This allows the pump to be partially submerged at all times and allows for a larger mass of fuel to absorb heat.

[rickpaw]

Thanks Carl for chiming in. In all occasions when the pressure dropped, the tank was at /2 mark, in fact it happened after we filled the tank. I'm very certain that the pump was submerged in fuel the whole time. The pumps are new. First one had 600 miles. The 2nd one has 400 miles.

I'm going to switch the pump over to a mid 90s Chevy pick up pump. That way I can get replacement at local part store. In MS, I walked into the local part store and asked for a walbro or an aeromotive pump, and they had no idea what I was talking about.

[Che70velle]

Thanks Carl for chiming in. In all occasions when the pressure dropped, the tank was at /2 mark, in fact it happened after we filled the tank. I'm very certain that the pump was submerged in fuel the whole time. The pumps are new. First one had 600 miles. The 2nd one has 400 miles.

I'm going to switch the pump over to a mid 90s Chevy pick up pump. That way I can get replacement at local part store. In MS, I walked into the local part store and asked for a walbro or an aeromotive pump, and they had no idea what I was talking about.


As a suggestion, check your required fuel pressure. If your thinking about going to a mid 90's TBI pump, I'm not sure that would work, as they are 15-18 lb. pumps. The LS engines require almost 60lbs to work properly. I think the later year 96-98 chevy truck "vortec" had more fuel pressure than the TBI engines did, but how much more I'm not sure.

I'm not a fuel system expert by any means, but have you checked your tank venting? Seems like a long shot, but could this be a venting issue, causing a pump to overheat? Sometimes the biggest problems have the simplest fixes.

[rickpaw]

As a suggestion, check your required fuel pressure. If your thinking about going to a mid 90's TBI pump, I'm not sure that would work, as they are 15-18 lb. pumps. The LS engines require almost 60lbs to work properly. I think the later year 96-98 chevy truck "vortec" had more fuel pressure than the TBI engines did, but how much more I'm not sure.



You're right about the fuel pump from the truck. Actually it's from 97 vortec truck (AC Delso EP381). The same one being used by Rick's tanks, besides the Walbro 255 for stock LS engines.

We checked tank venting as well. It has Finch's dual vent set up. When we removed the fuel assembly, there was not any indications that the tank is not venting (ie no whooshing sound).

[rickpaw]

Well, I dropped an ACDelco EP381 in the tank. Hope it holds up in long drives. At least I can get a replacement pump at local parts store if that one fail on road trips. One thing I noticed was that the fuel pressure held steady at 58 psi after I shut the engine off (40 psi after 12 hr period), whereas the fuel pressure would immediately dropped to 0 with the old pumps (Aeromotive 340).