Fuel pressure drop

[AustinQuattro]

Did it run right after if was tuned? Seems like they should have checked the fuel pressure. Is the fuel pressure correct now? If it is, it's probably some sensor like your MAF sensor or O2 sensor. Can you check your sensors with a scanner or DVM?

By the way, my 502 is running great now. I went ahead and hooked the vacuum port back up because I figured it was there for a reason. Yeah, I could let it learn without it, but my concern was that if I plug the regulator vacuum line off, and it blew again, it could spew fuel all over my engine and headers, and maybe start a fire. I'd rather have it dump into the engine.

Yeah, I'm going through all the sensors now. Seems the guy I had wire it may have not tightened a few things. Yesterday was a bad day. The guy who helped me get the engine & trans in apparently didn't put the fill plug on the trans...so when I made a small trip around the block checking things out, it got up to temp enough for it to come out and onto the headers and start a fire. All is ok, no damage, but lots of leaked fluid and scary!!!

It did not run right after the tune. Still wouldn't start without starter fluid, would rev only after coming off idle a bit, and in gear would not rev much at all.

It's obviously a fueling issue, just not sure what yet. Pressure is there from the fuel pump and the line is pressurized even after sitting, thinking it's something with the wiring, or a sensor not being grounded well, maybe even a bad crakshaft/camshaft position sensor or ground. I'm just going through everything now. Thanks for the response. BTW, I'm running C4 suspension front and rear like you, on a NerdRods chassis, with double adjustable coilovers all around. I really can't wait to drive this thing!

[chevynut]

Did it run right after if was tuned? Seems like they should have checked the fuel pressure. Is the fuel pressure correct now? If it is, it's probably some sensor like your MAF sensor or O2 sensor. Can you check your sensors with a scanner or DVM?

By the way, my 502 is running great now. I went ahead and hooked the vacuum port back up because I figured it was there for a reason. Yeah, I could let it learn without it, but my concern was that if I plug the regulator vacuum line off, and it blew again, it could spew fuel all over my engine and headers, and maybe start a fire. I'd rather have it dump into the engine.

[AustinQuattro]

Older thread but should be relevant. I swapped an LS3 into my 57 Belair and I'm having issues. It's a crate LS3, Texas Speed & Performance put their PRC cnc'd heads on and stage 2 cam, and it's not running right. One of the biggest initial issues is it won't start without spraying starter fluid into the intake. Then it idles and revs and runs. Off Idle, if I stomp the pedal, it will stutter a bit, but if I ease in just a little, then I can stomp it and it revs quickly and smoothly. I had a shop tune it, and they've tuned this exact setup before (except it was a camaro with the same heads and cam, not a swap). I'm at a loss. It's obviously a fueling issue, I have fuel pressure even after it's been sitting off for quite a while. I'm wondering if it could be a bad fuel pressure regulator or maybe a sensor isn't grounding properly? Not sure where to look...

[chevynut]

The problem you had is probably a freak. Lots of miles put on these regualators without failure.

I actually did a web search and found a lot of instances of this happening with these regulators. It's probably a small percentage, though.

I don't think Holley intends for it with Terminator X. whether it's LS or not.

I don't blame you for not wanting to have that failure again.

I think I'll just put a rubber cap on the manifold vacuum fitting so I can put the vacuum line back on later if I think I need it. If it's not necessary I'd rather not have it. Thanks for the input on this.

[Rick_L]

Biggest thing here is what one is starting with both hardware and fuel map.

Many OEM GM setups used vacuum reference - TPI, LT1, Ramjet do. If you're starting with one of these and an existing fuel map it probably is much easier to keep it. The problem you had is probably a freak. Lots of miles put on these regualators without failure.

The Edelbrock Pro Flow 4 uses vacuum reference. Since it's not as adjustable as some others, if one had one of those it might be best to keep it.

On the other hand, LS engines don't use vacuum reference. I don't think Holley intends for it with Terminator X. whether it's LS or not.

I don't blame you for not wanting to have that failure again.

[chevynut]

First off, you have it backwards. More manifold vacuum (less manifold pressure) on the pressure regulator the lower the fuel pressure.

I meant that as manifold VACUUM drops, fuel pressure rises. I corrected it.

As a general statement, the effect of having or not having a vacuum referenced pressure regulator can be dealt with in the fuel map, and with the fuel trims using the O2 sensor.

That's what I've been told, but there are conflicting opinions on various forums. To me it shouldn't make any difference what the actual pressure is as long as the fuel map and O2 sensor deals with it. Some guys are of the opinion that the vacuum line makes the regulator act like a "accelerator pump" by increasing the fuel pressure when the vacuum drops. Not sure I buy that, especially with 60 PSI of fuel pressure. I think I'll plug the vacuum port under my intake and remove the vacuum line. I don't ever want my engine flooded again and I don't see the downside to not having it.

[Rick_L]

First off, you have it backwards. More manifold vacuum (less manifold pressure) on the pressure regulator the lower the fuel pressure.

As a general statement, the effect of having or not having a vacuum referenced pressure regulator can be dealt with in the fuel map, and with the fuel trims using the O2 sensor.

[chevynut]

So looking back a year ago it looks like this could have been the start of my fuel pressure regulator problems. My brother came to CO in April, and I fired up the engine for him and it was popping out of the exhaust and running rough. I didn't do much with it after that until I tried to get it running better to charge the A/C. Now I know the fuel pressure regulator vacuum line was pouring fuel into the intake.

That brings up a question. I have been told to remove the vacuum line and that you really don't need vacuum to the regulator. I know the vacuum changes the fuel pressure with respect to ambient pressure, but keeps it tied to manifold pressure. So as manifold vacuum drops, the fuel pressure rises. But it seems like a constant pressure relative to ambient would be better, and you can just tune the system to the correct for the added pressure at idle.

I'm not sure whether to hook up the vacuum line or not, and I read conflicting things online about it. if I didn't hook it up it would prevent the problem I had with the broken diaphragm and flooding of the engine which would be nice.

Any advice or comments on this?

[55 Rescue Dog]

You could always add a momentary pushbutton to prime it before cranking and use it to test the pump too. My engine primes automatically for about 3 seconds after turning key to the on position. When filling my whole empty fuel system, I just turned it on for a bit and back off several times and had to once after running out gas from a bad level sender. I had to empty the tank to fix that, and just clipped my remote starter button on the terminal strip for the pump to quicky drain the tank dry. I would think you could add a check valve to the fuel line if that's the issue. My 18-year-old truck engine starts in less than 2 seconds after sitting all winter.

[chevynut]

Ignore these guys Laszlo. They are wasting your valuable time and energy.

I ignored him as long as I could. I'm done with his bullshit. Read his posts on my other threads.

Maybe there isn't a check valve in the system, and its designed that way to allow pressure to bleed off when the pump is not energized (ie. when you turn off the ignition key.) Maybe thats a safety thing, so that the fuel system is not constantly pressurized when the vehicle is off? Maybe that increases the life of your injector heads/seals , since they are only pressurized when the key is on? ie. not 24/7

I did a lot of research on this and MOST cars maintain fuel pressure. Then I found out that SOME Walbro pumps have a check valve and some don't. I don't think it matters as long as you allow the system to pressurize before cranking, which I usually do. I'm not going to do anything at this point and the long cranking (it's not that long) may be due to something else, like timing.

[WagonCrazy]

Ignore these guys Laszlo. They are wasting your valuable time and energy.
Back to your question about fuel pressure.

Random thoughts here
Maybe there isn't a check valve in the system, and its designed that way to allow pressure to bleed off when the pump is not energized (ie. when you turn off the ignition key.) Maybe thats a safety thing, so that the fuel system is not constantly pressurized when the vehicle is off? Maybe that increases the life of your injector heads/seals , since they are only pressurized when the key is on? ie. not 24/7

[chevynut]

Funny, thing those C 4 Corvettes with antiquated electronics are getting impossible to repair while the C1. C2 and C3 carbs keep on going.

You're just too stupid to understand that the only C4 Corvette parts on my car are the suspension and differential. Maybe if you learned something about electronics and modern cars you wouldn't have to use that obsolete 60's junk that you have in your junkyard.

[markm]

My fuel pump is a genuine Walbro GSS340 and I see conflicting info on the internet about whether it should have a check valve or not. Some say it does, and others say it doesn't. I replaced the fuel hose on the pump a while ago and I wonder if I somehow lost the check valve that's supposed to be in the outlet inside the old hose when I took it off. I don't remember hearing the return fuel flow before I replaced the hose. But most guys say it's no problem as long as you have pressure when running.

The ECU is working just fine but I haven't had a chance to do much tuning with it since I can't drive the car yet. I went through the benefits and downside of replacing it some time ago and decided it wasn't worth the effort. Any ECU just fires injectors and plugs and it's all about tuning which I believe I can do to my satisfaction. I don't need any more inputs and outputs and I have a WBO2. It probably works as good as an "antiquated" LS1 ECU and a helluva lot better than an antiquated carburetor.

Funny, thing those C 4 Corvettes with antiquated electronics are getting impossible to repair while the C1. C2 and C3 carbs keep on going.

[Rick_L]

To get started on troubleshooting your cranking time problem, pull the plugs. You will see if your tuneup is providing too much or too little fuel. If it's a tuning issue, they all should look similar. If it's injector problems, individual plugs will follow what the injectors are doing, as it's unlikely they all failed the same way at the same time - unless they are full of fuel line chunks.

[chevynut]

My fuel pump is a genuine Walbro GSS340 and I see conflicting info on the internet about whether it should have a check valve or not. Some say it does, and others say it doesn't. I replaced the fuel hose on the pump a while ago and I wonder if I somehow lost the check valve that's supposed to be in the outlet inside the old hose when I took it off. I don't remember hearing the return fuel flow before I replaced the hose. But most guys say it's no problem as long as you have pressure when running.

The ECU is working just fine but I haven't had a chance to do much tuning with it since I can't drive the car yet. I went through the benefits and downside of replacing it some time ago and decided it wasn't worth the effort. Any ECU just fires injectors and plugs and it's all about tuning which I believe I can do to my satisfaction. I don't need any more inputs and outputs and I have a WBO2. It probably works as good as an "antiquated" LS1 ECU and a helluva lot better than an antiquated carburetor.