Possible bad Fuel Pressure Regulator?

[TritleZ]

1976 280z stock

I'm having troubles with my engine not wanting to stay running. I was suspecting my FPR to be bad. I hooked up a pressure gauge and I was only able to build pressure up to 28psi. I read in the FSM that stock fuel pressure is 36.2.

 

It took awhile to build the pressure to 28psi. I had to crank the engine for a good 30 seconds to get enough pressure to get the engine to fire and start. The engine started once it hit around 20psi. After it ran for a few seconds it built pressure to 28psi before slowly falling back down to around 20psi. From there it started to sputter and chug before the pressure dropped to 15psi where the engine died and shut off.

 

A couple questions now:

Is this signs of a bad FPR?

 

Also, shouldn't I build pressure with the key in the "ON" position? I don't build any pressure until I start to crank the engine.

 

Would that be signs of a bad or weak fuel pump?

 

Any suggestions would be greatly appreciated. I have another fuel pump/dampener that I will be swapping in/out tomorrow evening.

 

Thanks!

[Daeron]

More likely indicative of a bad fuel pump. To check, try pinching off the fuel return line out of the FPR and see if that increases your fuel pressure. If the pressure regulator fails, it usually either allows too much fuel to flow past (and presents you with low fuel pressure) or deadheads, and allows too much pressure. Sometimes they leak, too. Anyhow, if you pinch it off and pressure builds up, then you are right.

 

However, odds are at least 60/40 that it is your fuel pump that is the problem. There's probably about five or ten percent to be fudged there for an "other fuel delivery problem" section.

[TritleZ]

More likely indicative of a bad fuel pump. To check, try pinching off the fuel return line out of the FPR and see if that increases your fuel pressure. If the pressure regulator fails, it usually either allows too much fuel to flow past (and presents you with low fuel pressure) or deadheads, and allows too much pressure. Sometimes they leak, too. Anyhow, if you pinch it off and pressure builds up, then you are right.

 

However, odds are at least 60/40 that it is your fuel pump that is the problem. There's probably about five or ten percent to be fudged there for an "other fuel delivery problem" section.

 

 

Well, I replaced the fuel pump today with a known working one. Same results. Fuel pressure takes awhile to build up. Have to crank the engine over for a good while. Once the pressure gauge hits around 20psi the engine will start. From there it SLOWLY raises to about 26ish psi. If I try to give it any more throttle (other than idleing) it will bog and "POP". After running for a little bit (under 1 minute) it will start to lose psi and end up dieing out.

 

I tried pinching the return line with hose pliers. I was able to get the fuel pressure to increase, but it VERY SLOWLY raised to around 26-28psi, but never higher. Eventually it will go back to sputtering and will die out after the fuel pressure drops again.

 

So, I guess I can rule out the fuel pump to be the cause? Now I'm leaning towards the FPR? Any other suggestions or things to check would be greatly appreciated.

[2eighTZ4me]

Throwing ideas out here. I'm assuming the "known working" pump was used also. Did you pull both of them apart and check the screen in the fuel pump? Those things clog and could lower pressure. Also - your fuel pump is triggered by your AFM on a 76 model. You won't get the pump to kick on until you crank it, as the AFM flap has to move off its' resting position to trigger the pump. What happens when you pull the vacuum hose off the top of the FPR when the motor is running? Any change in pressure?

[TritleZ]

Throwing ideas out here. I'm assuming the "known working" pump was used also. Did you pull both of them apart and check the screen in the fuel pump? Those things clog and could lower pressure. Also - your fuel pump is triggered by your AFM on a 76 model. You won't get the pump to kick on until you crank it, as the AFM flap has to move off its' resting position to trigger the pump. What happens when you pull the vacuum hose off the top of the FPR when the motor is running? Any change in pressure?

 

I didn't take the pump apart. I will try that tonight while checking the fuel tank for debris. I will also report back and tell you what happens with the vacuum line pulled off the FPR. Thanks for the suggestions on things to check.

[S30 SPL]

Pull your fuel filter and check the actual fuel quality. I bought a Z that had some rust in the tank and I can get fuel flow, but not enough pressure. Lots of debris in the fuel. If you are getting debris in the front, that means that you are sucking it up at the pump and pickup blocking flow, this lowering pressure.

[TritleZ]

Yeah. I pulled the hose off from the gas tank to the fuel pump. Barely any gas came out, then it stopped. (I had over 2 gallons of good gas in the tank) I used some compressed air and blew into the gas tank through the hose. Gas flowed out of the hose, then stopped. This led me to believe that I have quite a bit of debris in the tank clogging the hose going to the pump. I drained the remaining gas and pulled the tank out. I could not believe my eyes. I've never seen so much rust and particles/chunks inside a gas tank before.

 

I sloshed around some gas in the tank and tried to get as much of the junk out as possible. I was able to get some, but not nearly enough to feel comfortable to put the tank back in and try to start the car. I need a good way (or shop) that can clean a gas tank. I will try and post a picture tonight of how rusty the inside is.

[2eighTZ4me]

Any respectable radiator shop should be able to vat the tank out for you. I would highly recommend having them seal the tank too once it's cleaned. This will prevent further rust from forming and breaking off and clogging things up. It's a little pricey ($200??) but WELL worth it. There is the POR15 tank seal, but I've seen a tank after a radiator shop has done it, and after a POR15 treatment - I'd just trust what the rad. shop did over a DIY POR15 job. My $0.02 worth. They're pros and not "reading instructions for the first time" - you get what you pay for.

 

Well - knowing that the tank is full of crap - better pull those fuel pumps apart and get that screen out and clean clean clean!!

 

Glad to hear you found the issue.

[Daeron]

Blast out the supply and return lines as well, and just replace the fuel filter.