What tool do I need to do that? I've only ever used the schrader valve.

I was thinking since this only happens at or above ~90F, I figure I'll drive it around to get it warm then come back home and point the heat gun at a couple components, one at a time. See if I can get a reaction out of the fuel pressure.

Also, does this line warrant replacement? It's the line that feeds into the filter. This is 100% my fault, I keep forgetting to unscrew the bracket whenever I change the filter.

 

If you want to go the custom fuel route:
http://www.lonniesperformance.com/

Guy used to post here and CZ28.com regularly. Very knowledgeable.

 

For some reason I was thinking that the regulator was on the front end of the system, but it's on "back" side. I thought it was pump, regulator, injectors, return. I had to pull up some pics of the LT1 fuel rail as it's been so long since I've dealt with it. Looks like the pump feed goes around the passengers side, comes to the front, crosses over, goes down the driver's side the hits the regulator with the return exit off the regulator.

So it's pump, injectors, regulator, return. The path I was going down was trying to isolate if the pump is the issue or the regulator by seeing if the pump could keep up....but with the setup different than I thought, that complicates my thinking a little as the return flow I expect will change with engine fuel demands....and since I think it's going to be open ended to the tank, it really shouldn't have any pressure(or at least very little).....at least I think that's how it works. Shows my lack of understanding on fluid mechanics as the injectors couldn't hold pressure if the regulator was before on a return system. A "duh" moment on my part.

Can you snap a picture of your regulator setup and how it ties into the LT1 fuel rail?

The crimps in the lines, via your pics, aren't great, but I don't think that's related to the issue you are seeing.....unless they are collapsing over time and pinching off the fuel flow. To me this comes down to one of three things(provided something in the lines aren't giving you grief)....pump, regulator, or bad injector(s).

When this problem happens are you seeing the low pressure with the engine running? What happens to the pressure if you turn off the engine then turn the key to start without actually starting the engine. Is your FPR vacuum compensated? Have you pulled the vacuum line during the low pressure stints to see what happens to the pressure?

Off the wall suggestion...is it possible the fuel filter is collapsing with the higher pressures of the LS1 style system? I wonder if you could make a temp adapter to remove the filter and run a straight line in it's place.....just a temp test.

 

Huh, that's what I thought too.











Yes the low pressure happens while the engine is running. It seems to get worse the slower I go, regardless of rpm. In other words, less air flow = lower fuel pressure. If I turn it off then turn the key on to prime the pump it will hit 60psi. I'm not using a vacuum line on the FPR. Not sure about the fuel filter collapsing. I've only used Wix filters and I've had maybe 3 in the years since I noticed this problem.

 

Is it just mean, or are you not attaching engine vac to fuel reg?

 

Correct, I just make the curve in HP Tuners

 

Looks like you have the vacuum port capped off....have you tried pulling the cap off the vacuum port when the pressure drops are happening?

 

No, I'll try that when I troubleshoot this week.

 

I am running a Quantam 255 venturi pump in the stock GM venturi pump housing on my ~500 hp 383 in my Express van on E85. Using a 19v Boost-a-Pump. Just driving around it runs at a normal voltage. When you put your foot into it the vacuum drops to the vacuum sensor in the unit and it bumps the pump to 19v. Back off the throttle and the voltage drops. Been running this for a year. At 19v the 255 feeds a 500 hp engine on E85 and has no problem holding 58 psi. In normal driving the smaller pump is quieter and does not heat the fuel as badly as a 340 lph pump would. Also draws less amps. Using a Racetronix harness.

Have you verified the tank is not building a vacuum when the pressure drops? When the pressure drops remove the fuel cap and check for vacuum. Mine was building a vacuum years ago and I had a dirt dobber nest in the vent hose going to the vapor canister air inlet.

 

Attempted some trouble shooting today. I drove the car for about 10 miles to get it warmed up then parked it in the driveway and let it idle. Initially I tried to introduce heat to certain areas. I used my heat gun set at 500 degrees and pointed it at the alternator, fuel pressure regulator, and the relay + bulkhead connectors above the rear end. Pointing it at the FPR for a couple minutes got the pressure to drop by 1 psi although that could've just been lucky timing. Eventually after 40 minutes of run time, the pressure started dropping on its own. Here's a video of the initial pressure drop including some under hood footage and some rear tire well audio so you can hear the pump.


Next I attempted to cool things down. I initially tried stuffing a floor fan under the fuel tank in hopes of cooling the bulkhead connectors and relay. I felt like it wasn't really working so I put the fan on the engine in hopes of cooling the FPR. You can see that it seems to work.


Then I pulled the fan off and the pressure dropped like a rock so I threw the fan back on. I checked the amps on the fuel pump harness at all pressures and there didn't seem to be a rhyme or reason to it. At 58psi the amps could be 12 or 14. At 34psi the amps would also be anywhere from 12 to 14.


Eventually not even the fan was bringing the fuel pressure back up. The car had been running for over an hour at this point.


I pulled the vacuum cap off the FPR and nothing changed. There is no vacuum or pressure build up in the tank.

 

How old is the FPR?

 

About 5 years.

 

You measuring 14 to 18 amps with that clamp on the AC setting?

 

I need to mention I got my numbers wrong. It was 12 to 14 amps, not 14 to 18 amps. I'll edit my post.

Yes I'm measuring amps with that clamp on the AC setting. I looked through the instructions on how to read it but it didn't have anything. When I set the dial to "2-20a" AC it shows as a decimal that you can see in the vid. Usually 0.7 or 0.65 ish. I figured it was displaying amps as a percentage of 20 since 20 x 0.7 is 14, the amp draw the fuel pump is supposed to have at 60 psi.

 

What you are seeing is the amps as a direct reading.....if you see 2.5....that is 2.5 amps AC. Should be very little AC in the system if the alternator is working right. You need a DC amp clamp to measure DC current.

 

Here is a decent inexpensive one....

 

Ah, shows how much I know! So the readings are good? 0.65-0.7 on the fuel pump wire and 2.5 on the battery wire. This particular clamp has a DC function, I'll try that. Any thoughts on the fuel pressure behavior? How it can move so slowly or quickly?

I forgot to mention that after I pulled it into the garage I primed the pump and it sounded awful. There wasn't a high pitched whine like usual, it sounded like it wanted to make noise but someone was choking it. The gauge only read 6 psi. I tried it again and it sounded about the same but the gauge hit 55 psi. I did not try starting it again.

 

We don't know what the pump current readings are because you don't have a clamp that can measure DC current. I've looked over the specs and it can't do it. I posted a decent inexpensive one above.

 

Shoot. Well this problem has spanned 2 pumps and 2 alternators and this one is brand new. Would DC current even be helpful? If there was an issue it would have to be in the hotwire kit: The wiring or the relay. Can wires and relays deteriorate like this with heat?

 

If the alternator and wiring are doing their job and the pump is in good condition there should be a correlation between pressure and amps.
To rule out completely the electrical you need amps (you can measure this anywhere in the line feeding the pump) and volts directly at the pump (this is not possible due to the pump being inside the tank) but you should measure voltage as close as possible to the pump. There could be a voltage drop along the hotwire kit due to a loose connector or bad relay.




Regarding the fuel pressure regulator:
Have you tried adjusting the regulator when the pressure drops to see if you get any response from the system at all?
another approach would be to insert a piece of fuel injection grade hose in the return line coming off the fuel rail and pinch it during a pressure drop to see if line pressure rises.