I'm trying to figure out what my car is doing (I need it to run well enough to get me to a meeting this evening about 30 minutes worth of highway driving away). I think I might be having fuel delivery issues, and I think it might be the fuel pump (pretty much the only part of the fuel system I haven't yet replaced).

I got a fuel pressure tester (FLAPS freebie rental), depressurized the fuel system as per the manual (start it up, unplug the fuel pump connector in the rear, let it die), and opened the gas cap to relieve any tank pressure/vacuum.

I hooked the pressure tester up teed off between the high pressure fuel filter below the DSF shock tower and where it connects to the hard line there on top of the block. Then I reconnected the fuel pump plug, started it up (it started almost immediately), and read it off.

At idle (warm, not hot) it read ~30 PSI. If I revved it up, it got up to ~35 PSI. I shut it off, and it held ~30 psi for a reasonable amount of time (it didn't dip below that, I just depressurized the tester and moved on after a while).

Then I depressurized the system (pulled the fuel pump plug) and put everything back together.

Lacking whatever special tester specified in the manual, I'm want to make sure I know what I'm seeing before I drop the dough on a new fuel pump. According to the manual, "fuel pump pressure" should be 64-85.3 PSI, but "fuel line pressure" (emphasis mine) should only be 28.4 PSI. The tests are slightly different and involve things I don't believe I have or have access to. What did I test, and is it right on (or even a little above) spec, or is it less than half what it should be?

 

Adding a bit of WTF to the equation, I did some more research and found some conditions for various fuel pressure tests.

- I hooked everything back up and checked it at idle. Still ~30.
- I hooked up the check connector. Still ~30.
- I disconnected the vacuum line at the boost sensor. Still ~30.
- I disconnected the vacuum line at the fuel pressure regulator. It jumped to ~38.
- I pinched off the line between the tester and the engine, and it jumped to about 70.

At this point, I'm thinking it's not the fuel pump.

In an attempt to thoroughly rule it out, I rig the pressure tester up so that I can read it while I drive, and also hook up my multimeter so I can watch the voltage across the relay. Guess what. It's behaving like normal again!!!!


IT'S BACK TO NORMAL AFTER NOTHING! W! T! F!???

 

aren't cars fun? LOL

maybe your relay is going bad and getting sticky? usually relays either do or don't work, but you never know

 

The same could be said for ANY electrical component... Mechanical components at least have the decency to fail slowly and with dignity or suddenly and without question... This is ridiculous. I took off work today to get this fixed, and now it's fixed itself, undoubtedly to go bad again when I don't have the ability to do anything about it... This may be my last new car (new to me is post-60s)...

FTR, I double (or whatever multiple I'm up to) checked the TPS, ditto timing, ditto whatever else. Nothing remotely out of spec. Of course it wouldn't be, because it's magically fixed itself for the time being. As soon as I return the fuel pressure tester, it'll be back to running like crap. It ran like crap on my way to get the thing, but it looks like whatever part got scared and decided to clean up its act.

 

sounds like me whenever frustration starts to set in. the rotary/anti-rotary conspiracy theory ....

i know, right?

 

I love cars. This is the newest thing I've had since college, and I go through at least one a year. Buy it, fix it up, drive around enjoying the hell out of it, then I typically get bored and sell it. It's exceedingly rare for me to find issues I can't handle quickly and easily, but this electronic stuff is kicking my butt... I had a set of electronic points in a distributor once that went bad in about the manner whatever is causing me these troubles is going bad. Just bad enough to ruin your day, but not entirely strand you, and erratically at that. Took forever to track it down, and I swore off electronic points after that. This thing is just one big set of electronic points after another. Why the hell would you cancel the turn signals electronically? Seriously? That doesn't make sense no matter how you look at it!

I figured since I took the day off to try to fix this thing, I'd at least try to get something fixed, so I broke out my resoldered CPU and set about trying to figure out why it's so unhappy. Maybe if I solve the known issues and get everything else happy, it's somehow tied into it and will follow suit.

 

you know, i just read your signature and i like it. every once in a while i get to marvel close up at a diesel engine (mainly tractors), and maybe someday i'll get one - not a tractor, but diesel truck. however, i am absolutely dying to get my hands on an air cooled engine, Porsche 6 preferably, but i wouldn't turn a Type IV VW engine away though. mechanical masochism ... i like that.

 

I've got a few TIVs kicking around. The simplicity of a good air cooled engine is a beautiful thing. Very sensitive to tune, but if you speak air cooled and give them the attention they need, they'll treat you right. Very similar to rotaries in that regard (the engines themselves, not all the unnecessary complexity they're wrapped in).

Back to the subject at hand though, since whatever is going on is so intermittent, let's pretend I didn't do any tests; at the ECU or otherwise. What could cause it to act like this?

What would a knock sensor do if it failed? What about a crank angle sensor? I still lack familiarity with all the billions of sensors that don't seem to actually do anything but wreak havoc when they fail, so I'm at a loss here.

 

well, before i took you off on that tangent, you never really stated what the car was doing/not doing. you merely said you think you might be having fuel delivery issues and you think it might be the fuel pump. you stated it's starting and running now. in post 4 all you said is it ran like crap, but what exactly was it doing before?

you've replaced your fuel system except the pump, does that include the filter in the tank?

 

https://www.rx7club.com/2nd-generati...ssist-1084772/

 

okay. read through it.

just a thought ... it may mean something, it may not. since the car starts, idles and for the most part runs well, i will assume the AFM will test out well using the FSM procedure. however, is there a way to see what it's doing while driving at high RPM?

 

so.. you described how you( correctly ) checked the fuel system but you don't speak of the symptoms. it's often easier to search the symptoms then to search for the "suspected" problem.

 

Read through the link two posts back. I needed real time help with the fuel pressure test, the overall diagnostic is a different, though obviously related subject.

 

I tested it out of the car with a multimeter. Essentially, I made sure the temperature was reading within spec (I didn't try it at different temperatures though), then made sure the resistance changed according to flapper position, etc. It checked out. I work from home, so I don't drive a whole helluva lot; total miles with the new AFM are on the order of maybe 500. I've only put three or four tanks of gas in it since I've had it.