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Pac_Man

Low Fuel Pressure (76 280Z)

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Hey everyone,

 

I've got a 1976 S30. It's effectively stock as far as the EFI system goes, all I've done is change the plug wire connectors for the injectors and I've replaced the fuel send lines with new hose. Of course I also put a new filter on it after I got it as well.

 

During the summer, it started having an issue where it would randomly lose engine power entirely for brief moments- a number of seconds, really. Then it would pick back up and drive normally. I thought it was just crud working its way through the lines from the gas tank and I wasn't too concerned about it. Shifting up a gear would usually make it stop.

Then it started to bog down on the rare occasion. It would run rough, misfire, had no power, and wouldn't rev over 1500 rpm (or if it did, it went kicking and screaming). I would drive it for a minute and then it would start running normally.

 

On my way to my first day at a new job last month, it died in my neighborhood and wouldn't start again. My dad and I were trying to push it back that night but it was tough since it was on a slight slope. He went to get the truck to pull it back and I decided to try starting it again to see what it would do. It caught a couple times and then finally coughed back to life. I changed the fuel pump thinking it was the old one going bad. I used an Airtex pump from O'Reilly (I know, you're probably rolling your eyes now) and installed a Fram G3 between the tank and the pump to keep tank crud from messing up the pump. It worked fine for a couple days and then the issue came back with a vengeance.

 

As it sits now, the car idles fine. Driving it, it starts to stumble around 2500 rpm and the power cuts completely at ~3000 rpm. It's almost undriveable unless I'm really careful with how high I rev. Even then, it can be fussy.

 

The general manager of the auto shop I work for knows the car really well so he helped me diagnose it on Saturday. He checked and adjusted the TPS and AFM. We tried a new computer since we had one laying around and there was no change. He checked for leaks on the intake and found none. He removed the vacuum from the FPR on the fuel rail and fuel pressure rose accordingly which, if I've read correctly, means that the FPR is functioning properly.

 

The last thing we did was check fuel pressure (probably should have done that first but hindsight is always 20/20). It was running at 32 psi @ idle and under load (revving the car) it would dip to ~30 psi.

 

If I'm correct about the FPR working properly, I've narrowed it down to 3 suspects:

 

- The G3 filter I installed between the tank and the pump

- The new pump

- Fuel damper near the pump

 

I read about people installing the G3 so I thought it was a good idea. The pump is new, but it is a random O'reilly brand.

 

Since I was having issues before and after replacing the pump, that leads me to believe that the issue does not lie in the pump setup I installed. My old OEM pump was probably still working alright for its age.

 

As I type this, the most plausible cause in my eyes would be the fuel damper. I'm not sure what the symptoms of a bad fuel damper are, however.

 

Anyone have any suggestions? I'm going to try running it without the filter when I get the chance, but I've been obscenely busy the last few days.

 

 

Thanks.

 

Pac_Man

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You haven't even mentioned the most basic pressure measurement, pressure without the FPR vacuum hose attached.  The base pressure.  Rig something up to see the pressure when the problem happens.  You could have done both of these in the time it took to write your post.

 

There's a story they tell in philosophy courses about the old philosophers sitting around looking at a horse trying to deduce how may teeth it has in its head, based on how and what it eats.  When all they need to do is open up its mouth and count.  Seems like you have more of a philosophical than a practical leaning.

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As much as I appreciate the lecture, I already stated that the fuel pressure rose with the vacuum line removed from the FPR. As for the actual number, I don't remember exactly but I believe it was 36 or 38 psi.

 

I would love to go work on my car instead of posting here, but I don't have the necessary equipment at my house to do so. I had to drive my car to work on Saturday and work on it there so I could properly diagnose it. On top of that, I am also rather busy with working on my college degree. I can't work on my car whenever I fancy, and I'm trying to be productive in the meantime.

 

I'm simply trying to gather more information for reference from a forum that was designed to expedite the exchange of information. I appreciate your time and knowledge, but if the majority of your effort is spent on criticizing me, you may be better off ignoring the thread.

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I'm just trying to have some fun while making a point.  You obviously had a gauge connected but didn't measure the most important number.  Almost every thread you'll see on measuring fuel pressure mentions 36.3 psi.  Hard to understand why it wasn't done.  I think the criticism was deserved and well-delivered.  Good luck and carry on...

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The person who was helping me said it was normal which is why I didn't commit the number to memory.

 

I can ask if he remembers the number, and if not I can retest it on Saturday if I have the opportunity to work on it at the shop.

 

Sorry for being ornery, I took it more personally than you intended.

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