Weird LS startup issue - fuel or electrical?

Hey guys -

So I have the new FiTech surge tank/pump installed but I’m having startup issues, not sure if it’s all fuel related or if there’s something electrical going on too.

First issue - fuel pressure fails to stay at 60psi after my pumps prime. Cycle the ignition, pumps prime to 60psi, then shut off. Pressure should stay high and it bleed down for a while - I deleted the Bosch 044 pump and I believe this pump was keeping pressure in my system.

I installed a new check valve between the new HP pump and regulator but pressure still fails to hold. I pulled the new check apart and it’s a pretty wimp rubber flappy valve - I can blow into it both ways. So it’s basically a crappy valve and my pressure not staying up is likely that. I’ve checked for leaks and there are no external leaks right now.

OK - so fuel pressure is too low to start the engine. I have been able to jimmy the engine to start by cycling the pumps, waiting a few seconds, then engaging the starter. I’ve been catching the pumps right as they hit 60psi and the engine will start and idle just fine if I do this sorta 2-step start sequence. Doesn’t run rich, no surging, idles normal. ISSUE. The throttle is non-responsive when I do this startup routine. The engine idles and giving it gas does nothing. The ETC position readout on my dash says 0% which is what it reads when I don’t have my foot on the pedal.

Ok, so potentially an issue with ETC wiring right? Well if I just cycle the ignition and don’t try to start the car I can modulate pedal position and get ETC readings like normal and I can hear the throttle blade opening and closing (when the engine isn’t on).

During this whole fuel system troubleshooting thing the only electrical thing I did was to re-wire my fuel pump signal.

Old setup - fuel pump wire from fuse box, split into 2 wires, fused, then into the each pump.

New setup - fuel pump wire from fuse box, goes into 2 relays in parallel then signal passes they a roll-over/shock sensors, then ground. So the only load that wire sees is from the relays. The relays draw power from a +12V junction block, 1 wire into each relay then on to a pump.

I’ve taken the shock sensor out of the equation by pulling the harness and jumpering the leads, no change in how the car behaves during startup.

So 2 potential issues happening here, I’m wondering if the ETC not responding when the engine is running is a clue. I have a new check valve on the way, hoping that addresses my peiming pressure issue. But even if I can get 60psi to the rail at startup I’m concerned the ETC issue will continue to plague me.

Low voltage? I’m seeing 12V on my dash before cycling the starter, don’t know what it’s dropping down to during the start cycle. Have the battery on a tender right now.

Is the low load on the fuel pump lead coming from the fuse box an issue? Can’t think of why the system would care about low load on the wire but throwing it out there.

Anything else to check for?
Have you also checked voltage on the relay when its under load? then check voltage on the fuel pump. I'm thinking at some point your losing voltage and this is why your seeing issues.
Can someone tell me when the pumps see +12V? They prime once the ignition is cycled, then they come back on once the car is running. Does that second time happen during the cranking phase or only after the engine has fired and is idling?

Unfortunately I’m doing the work solo so difficult to watch the meter at the back and hit the ignition button at the same time.
When you cycle the ignition on, the fuel pump should come on and run for about 2 seconds and then shut off. Should also run while cranking. If you install a new check valve and the fuel pressure still won't hold, you might want to check for a leaking fuel injector?

With the symptoms you're describing, it sounds to me like a battery voltage issue. If the battery voltage drops off too far during cranking, it causes strange things to happen, like the no throttle response you're seeing. We see this type of thing a lot here also due to all of the same circumstances that I'm sure you have there.......the battery has been sitting forever while you work on the car and particularly if it's a gel cell battery like a lot of builders the battery slowly goes to crap from sitting and when you finally need it, it's toast. We've worked on over 30 GTM's now over the years, and I'd say of the ones that had gel-cell batteries installed, probably 90% of them required replacement before the car was completed....they simply would not take a charge or produce enough current to start the engine without the voltage dropping off to cause all of these strange symptoms you're seeing. I'd say a good test of this would be to get a known good battery and use jumper cables to hook that battery up direct to the starter and then try starting and running the engine and see if you have the same symptoms. If the symptoms go away, then you have a battery voltage issue.
Thanks guys - great info! Yeah the battery has me concerned; it was low while I was running my tests so it’s been sitting on the battery conditioner for the last 2 days. I figured once the engine caught and was running at idle the alternator should be providing the necessary voltage/current but perhaps low voltage during startup caused the ETC and pump issues.

The fuel pressure issue is most likely the crappy check valve I got - I pulled it apart and it’s just a rubber flap over a hole, I could blow through it in both directions. I’ve got a new one on the way, hopefully that’s a better design with more positive sealing.

I’ll reconnect the harnesses just in case to verify a wire hasn’t popped loose somewhere.

Fingers crossed, I’ll have some more time to work on the car tonight. Thanks again for the suggestions!
Status update:

The battery tender got voltage up into the 13.X range so I primed the pumps and before pressure could drop I cycled the starter and the engine caught just fine. ETC control was also normal - so +1 for a potential battery issue with the ETC weirdness. Fuel pumps also ran just fine.

I shut things down and tried repeating but couldn’t get the engine to catch again - voltage reading on my dash was about 12.3V. I suspect voltage may be dropping too low during cranking that the ECU is having a freak out.

I went out and picked up a new battery, maybe not necessary but this will give me some peace of mind.

On the fuel pump issue, I let the battery sit on the tender while I got the replacement and when I came back I hardwired the pumps to run continually. Engine fired right up and ETC control was normal.

So it may be a combination of bad check valve and dying battery. At least now I have a way to start the car and move it.

New check valve came, Chinese writing on the package ... blow through the check and it holds tight! Definitely superior to the vibrant performance check. So ... +1 for the Chinese on the valve and to amazon for getting it out to me in 2 days.

New check, new battery. Fingers crossed that gets me all the way over the line! Back at it tomorrow.

Thanks again for everyone’s input!
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Something weird that I’m observing ... I’m feeding my relay +12V on the input (85) and I’m only measuring 6V at pin 86. What causes the voltage to drop like that? I’m going from +12V source through 2 relays wired in series then to ground. It’s like the relay has a massive resistance at the coil but I only measure 90ohms. On the second relay in the series I measure 6v at the input and nothing in the output. Should relays be wired in parallel?

Howard Jones

GT40s Supporter
You can't wire relay coils in series for you purpose. What you get then is a voltage divider circuit where 1/2 of the applied voltage gets dropped across the first coil (90 ohms) and the other half gets dropped across the other equal 90 ohm coil. This results in 6 volts on both coils and 1/2 the current through both of them compared to one 90 ohm coil alone.

Coils should be wired in parallel (if you want them controlled in unison) so that both see the same control voltage. Be aware that you will now see double the current in the full coil circuit so fuse/protect wire size accordingly.
Lol thanks Howard - seems that’s exactly what I’m seeing! OK, thankfully there’s an explanation ... I’ll re-wire and do some more testing. This whole fuel system re-do has been seemingly cursed from the outset!
WOW. I'm stunned. No words. This thing is cursed.

So I have an explanation for my weird startup issues and it's such an incredible story I feel the need to share it. There are nuggets of knowledge sprinkled herein but most of it is just me sobbing at this point.

So a few people know, most don't, but I had entered the car into the La Jolla Concourse d'Elegance this weekend (I should be in the car driving over as I type this).

But I'm not.

Rebuilding the fuel system in my car was prompted by several factors, one of which was wanting to delete the Bosch 044 pump from my setup, another of which was to get the car ready for this show. I've been busting tail trying to piece together and troubleshoot this new setup and it seems like every which way I turn I've been met with some type of bad luck. So let's start ...

There's something about the Bosch 044, that when you have it plumbed into your system and prime the pumps it's able to maintain system pressure. I originally thought it was because it has a built-in check valve - it must be secondary to the check valve typically included on the discharge side, genuine or knock-off. Anyway, after setting up my new fuel system I noted that it wouldn't hold pressure after the pumps cycled. The only real fluid side changes I made were to delete the Bosch and surge tank, replacing them with the integrated FiTech unit. So the only real change is that the HP pump is no longer the Bosch (I don't think the surge tank makes a difference, it's just a big container with lines coming to and fro). So ... I figured well crap, the Bosch can hold pressure because it's got a check valve and this new pump doesn't. OK, time to search for a check valve. Being in CA, apparently some check valves are OK for sale here and others aren't - and of course the local race supplies place that has everything hydraulic you can imagine doesn't carry check valves!

So I ordered a Vibrant Performance one, thinking that it was a brand name valve and it ought to work right? WRONG. The check valve didn't do jack. I installed it and pressurized the system and pressure just bled back down like I hadn't done a thing. I removed the valve and blew through it in both directions - it didn't make a difference. I cracked the valve open and the internal is literally a rubber flap covering a hole. I'm sure a fart could have blown through the check. D-day for the show was 4 days away so I Amazon Prime'd my ass the next best thing - an obviously Chinese manufactured no-name check that had positive reviews. I'd have gone the Jets or Summit route but I needed it right NOW and I couldn't stomach paying $30-40 for shipping alone. The valve arrived and holy smokes, it looks good! I could see a metal structure and a spring by looking down the opening - great, this is the kind of positive sealing I was looking for!

Install the valve. Nothing. Nada. WTF. I had installed it in the INLET side of my fuel pressure regulator. You see where this is going right? Yeah, so the fuel pressure regulator doesn't explicitly have a check, but by the very nature of what it does, it's a check valve. It stays closed until it hits the desired hydraulic pressure then the diaphragm inside cracks open to bleed pressure - but only enough to bring it down to whatever it was set to before seating again. Well ... at this point I supposed that my fuel pressure regulator was on its way out. The diaphragm is damaged and the check wasn't doing jack because all the pressure was bleeding out through the return port. OK, I'm going to jury rig this thing so I can get the car to the show then we'll fix it later. See where this is going yet?

I move the check valve over to the OUTLET side of the fuel pressure regulator and wouldn't you know it? I prime the pumps and pressure on the gauge bleeds down, but this time the engine starts running when I crank it! A-Ha, so the check is actually working, it's really maintaining rail pressure, and the engine is starting now - WOO!

Except the engine doesn't want to keep running. :(

Thanks to Howard, it hasn't been since college that I've had to think about Ohm's Law, but he reminded me about voltage drop across components in series and in parallel. It didn't occur to me to think about this until I pulled out the voltmeter to figure out why the car was having issues staying on. As part of my redesign effort I wanted to take the load off the fuel pump wire coming from the fuse box; previously I had run both pumps off the single wire, splitting it just before getting to each pump. That worked fine because the pumps were wired in parallel and both were seeing a full +12V. In my new setup I used that same wire to serve as a signal wire, running it through 2 relays IN SERIES. Once tripped, they would draw +12V from a nearby junction block; one relay per pump. So ... every time I cycled the engine to prime the pumps they would run - because the battery had *just* enough voltage that the relays would trip and power the pumps. It must have been on the very edge of when the relays would trip because I could drive the car around for a bit then it would randomly quit - likely because the relay running to the HP pump tripped open, shutting the pump down. OK - figured that out, I'm going to hard-wire the pumps to my kill switch, I'll re-wire the relay setup later. SWEET.

The Concourse show is tomorrow and it's 5PM. I'm tired and frustrated but at least I've finally gotten it put together enough that I can drive it and enjoy a day at the show, looking forward to finally driving this car - it's been about a month since the car died on me and I embarked on this whole fuel system re-do thing.

OK, check valve in, getting 60 psi at the fuel rail, fuel pumps hard-wired, I got everything I need to limp this bitch to the show before I have to get back into it and address the regulator and wiring. I took the car out for a ~20 mile drive just to test things outs. GOD it feels good to drive the car again! I'm having a ton of fun but I notice my rear view monitor is flickering. It would go out intermittently. Hmm... bad connection? Weird, I didn't touch any wires in my ceiling or behind my dash panel. Keep driving ... sometimes I instinctively go to push DOWN on my lift because I'm paranoid that I haven't fully dropped the car and I just happened to do that - whoa, weird, my monitor just went out again. Monitor comes back up after a few seconds. Hit the lift pump - monitor goes out. WTF? How the heck could they possibly be related? They're on totally separate circuits.

Uh-oh ... cycle over to the diagnostic screen on my AIM dash and system voltage is low. It's in the 12.X which is unusual, it should be 13V+ with the alternator spinning. OK, bad battery (I LITERALLY JUST REPLACED THIS 2 DAYS AGO!!!), bad alternator (I LITERALLY JUST REPLACED THIS ~200 MILES AGO!!!!), or blown fuse (from the alternator to my battery). Damn, I'm about 8 miles from home, take it easy but at this point I know the clock is ticking. I turn everything I don't need off - fan, fog lights, radio - lol, not much else in the SLC that can use power! By the time I pulled into my driveway my battery voltage had dropped low enough that the rear view monitor was dead (it has a safety features that shuts it down below a certain voltage) - but at least I made it home! OK, open up the back and wouldn't you know it? The fuse between my alternator and battery had blown. WTF. I don't know what could have possibly blown a 50AMP fuse. I checked the connections between the alternator and battery and everything looks good. No short. All my vehicle loads are going through the kill switch so the only thing between this fuse is the battery and alternator. Father-in-law thinks it's a defective fuse. I'm putting this in the "Future nightmare to come" category for now.

It's 3:30AM, day of the show. I didn't sleep very well and now I can't get back to sleep. Gates open at 6AM and it's 30 minutes away. I've got another hour and a half of sleep to go but I'm nervous. I don't like driving the car all janky rigged like this but I made a commitment to bring the car to the show and want to follow through. Lay in bed for the next hour and a half and it's finally time to load the car. Get everything loaded ... I have to play musical cars and I don't want to wake my neighbors. Starting the SLC is going to be a commotion so I decide I should start the car up with the garage door closed then take off as quickly and quietly as possible.

Car packed. I'm ready. Flip the kill switch, pumps whirr to life. Let's go!

Except we're not.

Engine cranks cranks cranks, nothing. WTF.

I wanted to cry.

Open up the back of the car and look at the fuel pressure gauge. 20 psi.



I typed that twice, it's not a typo. My eyeballs aren't wrong. Gauge reads 20 psi with the pumps running. Pull out the voltmeter. Yep, +12V on both pump terminals. Touch all the lines. No fuel leakage. 20 psi. Pull out the wrench and adjust the fuel pressure regulator. NOTHING. 20 psi. Turning it one way or the other and the pressure is constant. 20 fvcking psi.

That's it. I realized the diaphragm must have let go overnight or as I tried to start the car. 20 psi must be the non-regulated restriction with a fully blown open fvcking regulator. Can't blame a knock-off for this one, it's an Aeromotive unit I purchased from Summit.

I left everything in the car, turned off the garage lights, and sat down to type out this miserable accounting of my last few days. It's a good thing I did, I can see the humor in everything now. As they say, writing can be cathartic.

But I'd rather be driving my SLC.

Howard Jones

GT40s Supporter
Circuit protection rated at 50 amps is marginal for the alternator charge circuit. Even a 50 amp alternator can deliver more than 50 amps for a short period. I would recommend you protect the alternator output circuit at the alternator output rating and add another 20 percent for spikes. This will of cource require a wire that can conduct at least 150% of the max alternator output. I used 1/0 for my 100 amp alternator.

Here is a good chart: You are very interested in voltage drop. So use the 3% scale and maybe upsize one step. At 150 amps you can see that 1 awg or 0 awg is about right for 10-20 feet in length.


Here is a nice circuit breaker rated at 150 amps. I like Bussman made stuff. You can select anything from 30 - 300 amps rated circuit breakers from them as well as a lot of other suppliers. Get water proof.
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Thanks Howard, I was thinking along the same lines. I’ve got a 140amp alternator, I’ll double check the wire I’ve got in there now. I ran 0 for my kill switch and the alternator got maybe 2 sizes smaller. I don’t recall now how we arrived at the 50amp fuse, might have been one of those “you should never exceed this” kind of things. But going slightly over what the alternator is capable of makes sense.
I did some gardening this morning and found a moment of clarity.

I also came across this YouTube video on fuel pressure regulators:

It was mostly the YouTube video that gave me clarity, the gardening just got me out of the house for a bit.

I disassembled my fuel pressure regulator and to my surprise, there didn't seem to be anything wrong. The diaphragm was intact and the ball and seats looked good. Spring looked like a spring. After watching the video it's clear to me now the only way for fuel pressure to drop down to 20 psi is for the ball to not fully seat against the bleed hole, resulting in excess flow going to the return line (which gets plumbed back to the surge tank). It didn't seem to me like there was any way for the ball to get hung up.

So what else could cause low psi? filters ... I had just installed a new 40 micron filter and the surge tank and pump are new, maybe there was a bunch of crap in the surge tank and it got shoved into the filter, clogging it. I removed the filter and ran alcohol through it going in the reverse direction but it came out clean.

Ruh-roh ...

So I re-assembled everything and left the filter element out of the HP filter housing. So now there's no restriction between the FiTech pump and the FPR. I then put +12V to both my pumps and observed that I was now seeing ~15 psi. I put my hands on the low pressure pump and verified it was running; I also verified I could feel flow moving from the low pressure pump and there was flow leaving the surge tank - the surge tank is most definitely being topped off by the LP pump. With the engine off and the pumps cycling, I'm essentially dead-heading the pump - it should be making max pressure.

So ... my current conclusion is my FiTech pump pooped the bed.

Knock-off FiTech pump anyone?

I'll give them a call tomorrow to see if they can help me troubleshoot but at this point there's not much else left in the system to pull apart and diagnose - Gaaaah!

Anyone else have any theories for me to chase down? Seems like the low pressure side of the system gets taken out due to the surge tank being full. With the tank topped off it's just the FiTech pump, some lines, and the FPR. I'm fairly confident there isn't blockage between the FPR and the pressure gauge as; 1 - the car failed to start this morning, and 2 - it's a 1/8" NPT fitting to the gauge, would have to be something pretty enormous to cause blockage at the gauge!
Have you opened up the Fitech surge tank? I had one where the tube that goes from the pump in there to the outlet broke. It would then only supply 20 psi.
What is the output of your alternator? Is it more than 50 amps?