Purs like a kitten...

[FastWoman]

I can pull between 18.5 and 19 with the right fuel/air mix. The engine has about 160k on it, and the timing chain is almost worn enough to move the sprocket to the #2 position, so that's probably about all I could want in engine vacuum.

I had a bit of time today and did a few easy things:

(1) I pulled off the AFM and plugged the AFM-to-throttle boot with a yogurt cup. I pulled off the vacuum booster line to the brakes and attached a hose to blow through. I pressurized the intake with nothing but moderate lung power, held the pressure with my mouth, and got a feel for how long it took for the pressure to leak away. I then pulled the little HVAC vacuum control line (adjacent to the power brake vacuum line) and repeated the test. I found that the pressure took maybe 5 times as long to dissipate with the HVAC line in place than with it open and blowing air. Pulling that one little vacuum line is not particularly consequential to the fuel/air mix, and any vacuum leaks that might exist (but probably don't) would be far less consequential than that. The small amount of pressure dissipation I could feel was probably the result of air leaking around valves, rings, etc. -- i.e. the air leakage through the entire engine. I highly recommend this method for satisfying yourself as to whether your intake is tight. It's quite quick and easy to perform.

(2) I re-confirmed my fuel pressure. I had previously only checked the pressure regulator with compressed air, so an in-situ test removed that shade of doubt as to whether I'm dealing with a normal fuel pressure. Note: I'm still using a cheap gauge, but it agrees very well with a couple of other cheap gauges I have.

(3) I readjusted the spring tension on the AFM to factory'ish specs, per Atlantic Z. I had been running the engine 7 teeth richer than the original setting (which gave me a vacuum of maybe 15 in). However, when I carefully did the aluminum can and water stuff, I found the spring actually had to be wound up 8 teeth tighter (leaner) than where it was originally set. I'm going with that setting and am closing the book on all of my sensors being to spec. I'm now going to work this problem from the fuel delivery side: ECU, fuel pressure, injector flow rate.

(4) I also finished flushing my coolant and refilling with fresh 50/50. Of course this has nothing to do with the fuel mix issues, but it was on my priorities list with the weather getting cooler and the coolant flush sitting in the engine/radiator.

Injector flow testing tomorrow, hopefully.

[cozye]

Interesting. I also get about 15.5" of vacuum when I have the AFM set to 6 teeth richer.

FWIW, when I did the leak down test, The motor cooled off quite a bit while I got my setup right and figured out the procedure, at that time I got 20-22% leak down. I did it again today with the motor up to operating temp and got more like 12-15%. Should be well in line with a 100k mile motor. Anyhow, I thoroughly checked out where the air was going, it's all going past the rings, nothing by the valves or through the intake/exhaust. So thats why I ruled out head, etc..

My fuel pressure has also tested good each time I've tested it. Although I do have the same hot start issues that you've mentioned.

[FastWoman]

As I said, I doubt it's the age on your engine that keeps it from running smoothly. It might burn a bit of oil or lack the full power of a fresh engine, but those are different issues.

I don't have a hot start issue anymore. I suspect the issue you're having is from air and/or fuel vapors in the fuel rail, which have to be purged before fuel will spray. I solved my hot start problem with a fuel pump primer switch (which I operate for a few seconds before firing up the car) and insulation on my fuel rail. The primer switch simply delivers +12 to the fuel pump relay coil. I wish I had figured out that solution on my old '75. At a mere 8 years of age (when I bought it), it was a devil of a beast to start and restart.

[Z train]

Fastchick,the check valve on your FP was failing and causing the hot start issues.

[cozye]

my fuel pump comes on as soon as i turn key to "on" and I let it prime before starting, however the FPR doesn't allow the fuel rail to circulate prior to getting a vacuum on the FPR ? Am I incorrect in this assumption ?

Z train, check valve ?

Sorry about the thread jack, I've just wondered if the hot start and lean conditions were some how related.

[FastWoman]

Ztrain, I think it might have been both the check valve and dribbly injectors, perhaps combined with heat. Even with everything new, a quick prime does help make that restart instant. Without it the engine cranks a couple of times.

Cozye, the check valve Ztrain is referring to is inside the outlet nipple of the fuel pump. The nipple unscrews from the pump. The part is currently bordering on NLA, so you might not be able to find it. However, you could probably install an inline fuel check valve (from Ebay) right before your fuel filter:

http://cgi.ebay.com/ebaymotors/5-16-Check-Valve-Diesel-Gas-8mm-One-Way-Fuel-Flow-_W0QQcmdZViewItemQQhashZitem3cb2622af0QQitemZ260690815728QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

You're correct that the FP only operates briefly when the key is first turned. My primer switch is a pushbutton that sends 12V to the relay coil, bypassing all the other circuitry. When I push it, the pump will run, even if there's no key in the ignition.

BTW, it seems I've contracted some sort of bug from the college kids. Honestly, I think germ sharing in college is worse than in preschool. Anyway, I haven't yet decided whether I'm energized enough to test injector flow rates today.

Edited November 10, 2010 by FastWoman

[Z train]

my fuel pump comes on as soon as i turn key to "on" and I let it prime before starting, however the FPR doesn't allow the fuel rail to circulate prior to getting a vacuum on the FPR ? Am I incorrect in this assumption ?

Z train, check valve ?

Sorry about the thread jack, I've just wondered if the hot start and lean conditions were some how related.

There is a check valve attached to the fuel pump.Between it and the FPR is what keeps the rail pressurized.I have a hot start issue on my DD 810.ANd the FP bleeds down rather quickly(definately over night).I have another 810 that has held pressure without running for over 6 months now.

And unless you've altered it,the pump is only on when the motor is turning/cranking?This was Datsun's safety interlock(of sorts) to cut FP power in the event of a wreck.

[cozye]

There is a check valve attached to the fuel pump.Between it and the FPR is what keeps the rail pressurized.I have a hot start issue on my DD 810.ANd the FP bleeds down rather quickly(definately over night).I have another 810 that has held pressure without running for over 6 months now.

And unless you've altered it,the pump is only on when the motor is turning/cranking?This was Datsun's safety interlock(of sorts) to cut FP power in the event of a wreck.

Might have been modded by PO. My fuel control relay is tied to the ignition circuit, which comes on with the key in the "on" position. When I do a hot start, I can hear all kinds of bubbles in the fuel tank coming through the fuel return.

Thanks for the tip on the check valve guys.

Fastwoman, Another suggestion that's been made to me that seems plausible is "cam walk". Apparently there have been cases where the timing chain tensioner isn't doing a good job keeping tight due to partially blocked oil gallery. I'm checking in to it.

My injectors have been flow tested. Tonight I thought that maybe I was sucking air into the fuel pump, so I installed some clear fuel line just before my fuel rail and looked for bubbles while running. There were not any. I'll be pulling the valve cover off and checking for play by rocking the cam back and forth. I tried looking at it with a timing light while running, and it does appear to move a bit but my timing light is a cheapy and it's strobe doesn't seem consistent during this test (although it does when I'm checking timing on the crank)

[Z train]

Might have been modded by PO. My fuel control relay is tied to the ignition circuit, which comes on with the key in the "on" position. When I do a hot start, I can hear all kinds of bubbles in the fuel tank coming through the fuel return.

QUOTE]

Sounds like the PO had a bad check valve and went the cheap way out.

[cozye]

Might have been modded by PO. My fuel control relay is tied to the ignition circuit, which comes on with the key in the "on" position. When I do a hot start, I can hear all kinds of bubbles in the fuel tank coming through the fuel return.

QUOTE]

Sounds like the PO had a bad check valve and went the cheap way out.

I honestly don't know if the PO was smart enough to diagnose it. I can tell you that the fuel pump safety switch on the AFM module is not operational or wired in on the 78. There are no pins in the harness for it. That would be the only way I could think of that the fuel pump wouldn't run in the key "on" position since the fuel pump has to run in the on while the car is running.

[Zed Head]

I honestly don't know if the PO was smart enough to diagnose it. I can tell you that the fuel pump safety switch on the AFM module is not operational or wired in on the 78. There are no pins in the harness for it. That would be the only way I could think of that the fuel pump wouldn't run in the key "on" position since the fuel pump has to run in the on while the car is running.

The '78s used an oil pressure switch to run the fuel pump when the key is at On.

[FastWoman]

Almost... The '78's fuel pump will run when there's oil pressure or alternator voltage (or both). The AFM's fuel pump shutoff switch is retained in the '78, but simply isn't connected.

I'm feeling a bit better today, but I don't know if I'll be up to going outside and working on my car.