Getting around 11 MPG, running rich, need help

[Zed Head]

The FPR can fail open (low pressure) or it can fail (clog) closed (high pressure). The stock fuel pumps have a bypass that kicks in at 43 psi. So you can have a clog in a return line or a failed (clogged) FPR and the car will still run, just really rich.

If you have an aftermarket fuel pump, you could get some very high fuel pressures, since many don't have a pressure bypass.

Fixing old posts with new knowledge - The stock Datsun fuel pump reliefe valve (bypass above) had a range of 43 to 64 psi, not just 43.

[Z Tyler Z]

Update, car drives perfectly...lets hope this lasts, we really don't know how we fixed it but we did. All we did was cleaned and re check the gap on all the plugs. Check the fuel pressure, which is a steady 30 psi at idle and jumps up to 36 ish with any throttle. When we hooked up the fuel pressure gauge a lot of air came out of the lines, maybe this was the cause?

Also we re-checked the timing was only at 2 degrees advance now its at 10. Car idles at 800 and has plenty of power, lets hope the gas mileage improves too. Just filled up so I'll get back to everyone in a couple of days and tell you guys how it is. Other than that thanks for all the help, lets hope it drives perfectly from now on.

Also we tightened up my crankshaft dampener bolt it was pretty loose (Dave says scary loose).

Edited October 23, 2010 by Z Tyler Z
Grammar

[Zed Head]

It's about time! Timing is everything! Good times! How's the mileage?

I advanced my timing a few degrees at a time until I was at 7 degrees over FSM recommended. The engine got more responsive each time. 2 degrees is pretty retarded. A doggy engine is a typical sign of retarded timing. Probably should have asked about it.

Don't forget to followup, it's a good thread.

[olzed]

Pleased you got it right . Retarded timing can make an engine struggle.

[Z Tyler Z]

Sad news.. Just filled up and did the math only came out to 13 mpg this was like 90% city driving.

Where do I go from here? Car still drives good bcdd sticks every now and then but other than that it drives great. I smell raw gas here and there though...while driving.

[Zed Head]

Z Tyler Z, I forgot one important thing about the 1976 model, that probably hurts gas mileage. The 1976 model has a switch in the transmission that controls a solenoid valve that opens the vacuum advance line to the distributor. If everything is connected properly, you only get vacuum advance in 4th gear.

It's described in the Emissions section of the FSM, so I'm guessing that retarded timing gives the government mandated mixture of combustion byproducts. It's only described in the 75 and 76 FSMs.

I bypassed my switch (just ran the hose directly to the vacuum diaphragm) so I have vacuum advance in all gears.

[jonathanrussell]

Hey Zed Head. I have been thinking of doing this exact thing to my 75. Care to share more details about the re-routing you did? When you say "just ran hose directly to vacuum diaphragm" do you mean from the vacuum advance diaphragm on the distributor to a manifold vacuum source?

Thanks.

[Zed Head]

No, I just bypassed the solenoid valve. You want to keep ported vacuum. There should be a T off of the ported vacuum source. One hose goes to the vacuum canister and the other goes to the solenoid valve, which then passes through to the diaphragm. Run that one straight to the diaphragm.

Another way to do it is to just unplug the wire to the solenoid. It is normally off. The transmission switch supplies power in gears 1 - 3, to close the valve. That is probably the easiest way to do it if you're not sure you want to run that way and want to try it first. Be aware that you'll have a dangling wire that is occasionally hot, although it does have an insulated end. Same thing at the transmission, you could disconnect that wire (anyone who does a 5 speed swap on a 75 or 76 probably has that switched hot wire dangling under the car).

There is a good diagram and explanation on page EC-9 in the FSM. It doesn't show the three way vacuum hose T though. Check it out then stare at your engine for a while and follow the hoses and wires.

Edited November 13, 2010 by Zed Head

[jonathanrussell]

Thanks for the explanation.

[FastWoman]

Cozye and I have been grinding out teeth at nights, trying to get our '78 EFI systems to work right. My most recent efforts are documented under the "purs like a kitten" thread. In short, I went through my entire fuel/intake system, doing some work that definitely needed doing, checking out all the components, refreshing, refurbing, etc., and I still ended up with a system that was running much too lean. I even acquired a spare ECU and tried that. Same thing.

Also the same story with Cozye. He's verified EVERYTHING -- at least twice. He's been more thorough than I have. His EFI is running lean.

All my sensors are spec, fuel pressure is right, injectors are new, connectors are new, everything checks, and I have no vacuum leaks, so the most likely problem, in my view, is the ECU (or more accurately, BOTH of my ECUs). Transistor circuits from the 1960's and 1970's don't have the same sorts of negative feedback controls as their linear IC successors, so they aren't as stable. Of course neither is as stable as the digital circuits that followed. Semiconductor performance fades and drifts over time, particularly in the old stuff, and the circuits may not be performing within their original design parameters. Anyway, that's what I think has happened to our ECUs: They've faded and drifted over time. While they might still be responsive to changes in sensor readings, they might be out of whack overall.

Add to this that our systems weren't designed for ethanol gas, which requires a richer mix. Modern lamda-type EFI systems are able to adjust the mix dynamically, based on exhaust O2, but not our systems.

The solution that I used to get my system running pretty well was to put a variable resistor in series with the coolant temp sensor for adjusting the fuel/air mix. Note that this approach (adding resistance) richens the mix, so it's only useful if you're lean to start with. If you're starting out rich, the best approach might be to adjust the AFM to a higher spring tension, per the instructions on Atlantic Z. Although this is a bandaid approach, it seems to work pretty well (so far).

Edited November 15, 2010 by FastWoman

[olzed]

Carburetors are the way to go then. No more problems

[FastWoman]

Olzed, I just went for another spin today. My engine runs beautifully. Now that I've learned the EFI and have figured out how to make adjustments to the mixture, I don't think I'll have any more headaches. I've certainly beat my head against the wall about carburetor problems before. In fact I have a Holley carb I have to rebuild and put back on our boat. It's a royal pain in the... er... boat!