EFI Datsun Z - only runs with throttle slightly pressed

[Zed Head]

3 hours ago, milligan21 said:

• turned down the pressure regulator 25psi > done nothing different
• turned up the pressure on the fuel 40psi > no different. 
• watched the afm opening while i had my foot semi down on the throttle ( could be opening too far? to be investigated )
• found TDC on the piston > which on the distributor cap pointed the rotor to spark lead #1 as it should be > checked the harmonic balancer ( it was at 20 - not sure if this is right or not? have to get a timing light tomorrow but if the car needs to be warmed up then i cant check the timing?  )
• still only runs with my foot a little down on the pedal and will only do so when the TVS is disconnected and wired like in the photo for some bizarre reason. Still no revs and usually cuts out if i push the pedal too much. 

Racer X has a good point.  You have so many different issues though it's hard to figure out where to start.  On the EFI throttle bodies there is a screw with a big round head on it that you can turn by hand.  That is the throttle blade bypass screw that controls idle speed through air supply control.  You could try just opening that up, (unscrewing it to let more air in), and see if that will allow you to start the engine and let it run by itself.

On the EFI engines there is also the auxiliary air regulator (AAR) that lets air past the throttle blade when the engine is cold.  On a properly set up system the AAR keeps the idle up when cold, and the idle speed screw determines warm idle RPM.

If you know that the piston is at TDC then the mark on the damper pulley should be at zero.  If it's not you might have a bad damper.  The rubber degrades and the pulley slips.  So a timing light won't help you if that is the case.

 

I'll circle the two parts I mentioned from your pictures and you can check them out.

Idle speed bypass screw -

 

AAR -

[milligan21]

7 minutes ago, Zed Head said:

Racer X has a good point.  

Idle speed bypass screw -tried adjusting both ways different number of rotations and it made no odds 

AAR - it's opened as I checked it before. 

WOuld it not make sense to rotate the distributor a few degrees CCW to reduce the 20 to 10 on the pulley marker?

 

 

[Zed Head]

3 hours ago, milligan21 said:

found TDC on the piston > which on the distributor cap pointed the rotor to spark lead #1 as it should be > checked the harmonic balancer ( it was at 20 - not sure if this is right or not?

Maybe I didn't understand what you were saying here.  Did you assume that the piston was at TDC because the rotor was pointing at spark lead #1?  Or did you use a piston stop or at least something in the spark plug hole to determine TDC?

The spark jumps across a gap as it gets close to the terminal so you can't really use the rotor to guess at timing.  The leading edge of the rotor terminal is what you would want to look at if you're going to guess.

[milligan21]

13 hours ago, Zed Head said:

Maybe I didn't understand what you were saying here.  Did you assume that the piston was at TDC because the rotor was pointing at spark lead #1?  Or did you use a piston stop or at least something in the spark plug hole to determine TDC?

The spark jumps across a gap as it gets close to the terminal so you can't really use the rotor to guess at timing.  The leading edge of the rotor terminal is what you would want to look at if you're going to guess.

Yes I used piston stop and also checked the rotor in the distributor and then checked the harmonic balancer to see where it was at. Anyway.... I retarded the distributor and it made 0.00 difference  😕

[Zed Head]

You can re-mark the damper pulley since you're using a piston stop to determine TDC.  A 78 should have the degree marks on the tab on the cover.  Make a new mark and then you can use a timing light to set timing.  Assuming that somebody just put a damper from a different model on the engine.  If the damper rubber is bad then it might just move again.

Anyway, that's kind of how working on the EFI engines goes.  A long list of small things that need to be corrected.  Once you get them all correct it will run like a top.

[milligan21]

21 hours ago, Racer X said:

Don't these have an idle air bypass circuit?

 

21 hours ago, Racer X said:

Sorry, I'm not knowledgeable enough about this particular EFI, but usually the throttle butterfly remains closed at idle (including starting unless one depresses the accelerator slightly while cranking to start, which shouldn't be done with EFI systems), so some provision must be in place to allow sufficient air to enter the engine for starting. The GM throttle body fuel injection systems used in the early days of electronic fuel injection well into the 1990's all used an idle air bypass to provide startup and idle air for engine air/fuel management.

Hmm ok so.. IF the throttle body vane / plate was stuck open it might be feeding too much air in at idle and choking the car out.... might MIGHT be a possibility as the 76  efi system apparently had issues with the valve cover pipe clogging the throttle body up. I was going to check this before but then got sidetracked with fuel and afm and this and that and forgot 😄 So   tomorrow i will tear it down and check 😉 Thanks for the reminder 😄

[Zed Head]

If the vane (blade) was stuck open there would be too much air and the einge would rev too high at idle.  The whole point of the blade is to keep the air out so the engine does not blow itself up.

If you can keep it running with your foot on the throttle pedal (it's called a throttle because it throttles the air from the engine) then you should be able to keep it running by opening up the idle speed screw.  Once you get it to idle you can mess around with timing and the AFM vane and other things.

You mentioned the AFM and it sounded like you thought that somebody had messed with it.  Removed the glue blobs maybe, and adjust the spring tension.  That would be bad.

[milligan21]

1 minute ago, Zed Head said:

If the vane (blade) was stuck open there would be too much air and the einge would rev too high at idle.  The whole point of the blade is to keep the air out so the engine does not blow itself up.

If you can keep it running with your foot on the throttle pedal (it's called a throttle because it throttles the air from the engine) then you should be able to keep it running by opening up the idle speed screw.  Once you get it to idle you can mess around with timing and the AFM vane and other things.

You mentioned the AFM and it sounded like you thought that somebody had messed with it.  Removed the glue blobs maybe, and adjust the spring tension.  That would be bad.

Ah ok.. so before i disassemble the throttle ill have a lookie inside it with a mirror thing! 

The idle speed screw.. I already tried that but will maybe open it more than 10 rotations. When I turn it it makes a clicking noise for some reason so maybe its just turning and not screwing anything... Also to be investigated 😄 so many things to do as well as re assembly , windows rubbers etc. On the windows ( you might know ) do I need to use a glue between the rubber seal on the windscreen and metal of the car or is good enough just pushing it in? 😄   

[milligan21]

& sorry yes someone was messing with the AFM but all the checks in the bible check out ohms etc etc and i calibrated it again using the atlanticZ method as I said 

[Zed Head]

On 1/26/2023 at 6:05 AM, milligan21 said:

It starts and runs as long as I keep my foot down a bit on the throttle and have the TPS disconnected

 

On 1/26/2023 at 6:05 AM, milligan21 said:

Fuel pressure is good at 36psi and all that

I forgot a bunch about what you were doing.  I see also that you already had the idle screw opened up.

There are other possible reasons why it won't run unless it gets lots of air.  Maybe there is a big vacuum leak and you need more AFM vane movement to supply more fuel.  You could test that by propping open the AFM vane.  Maybe ignition timing is off.  Maybe valve timing is off.

If it was mine I'd probably prop open the throttle blade after I got it started, or before I started it, so I could keep it idling while I messed around with ignition timing and the AFM.  Either have a friend hold the throttle open or prop it open na dbready to shut the key off it starts to rev too high.  Once you get it to idle so that you can work in the engine bay then you can move the AFM vane more or less open to add more or less fuel through the injectors.  The effect will be a clue.

 

Actually, if it was mine I would take the valve cover off and verify cam timing, then adjust valve lash, then set ignition timing as close as possible, check the air flow path including the filter for obstructions (mice, etc.), then start it, let it idle, and start fine tuning.

Lots of ways forward.  

[milligan21]

3 minutes ago, Zed Head said:

 

I forgot a bunch about what you were doing.  I see also that you already had the idle screw opened up.

Makes 2 of us 😄 haha I said I was gonna be methodical from the start and track everything I done.. but I didnt obviously. 

 

Anyways, I believe while I was away one of the times my dad checked the timing on the cam ( or checked valves - have to ask what he checked but he had that cover off ) 

I tried most things at this point but tomorrow im going to go check a few things and will report back 😄 I MIGHT run through all the EFI ecu checks again and write down the results so I know where I am at there 😄 Thanks again for the replies .. helps me think where to go next 

[Racer X]

As mentioned, I have limited experience with the system on the Z, but quite a bit with domestic, and some other import brands.

You commented about the crankcase vent allowing the carbon buildup behind the throttle plate. This is common, I've "fixed" lots of cars for poeple by simply removing the throttle body, cleaning the carbon, and reassembling.

Cost?

My time and some carburetor cleaner.

So it might be something to look at, and insure the idle air bypass is clear too.

 

Regarding the timing.

I would set the engine to number one TDC (the positive stop method is perfect), then ensure the timing mark is at zero. If not, correct it, then set the static timing, start the car and set the timing after warming the engine and attaining a stable idle. 

Also, while looking at the initial timing, have a good look at the damper rubber. When it fails, bits begin to come out. If there is any doubt, replace it.