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running lean '76 280z


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Hi guys,

I am looking for your advice before I open my AFM unit and start messing with it.

Symptoms: when the car is cold, it runs very lean, bogs down, popping noises, lack of power. When it warms up, more manageable to drive, although it occasionally makes the same popping noises. it also smells lean from the exhaust.

What has been done:

1. Remanufactured AFM, sealed up. Adjustments to the air bypass screw don't help richen up the mixture.

2. I went through all the FI Book tests, the air temp sensor in the AFM/water temp sensor in the t-housing, all pass. Cold and warmed up.

3. Throttle positive switch - adjusted, at idle, middle position and at wide open per FSM.

4. The fuel pressure gauge shows 30 psi and idle, 36 psi at acceleration. New fuel MSD-2225 fuel pump with a pre-filter, clear gas, no rust in the tank.

5. Vacuum gauge shows 14-15. I am guessing this is a low reading.

6. Idle - 850/900 rpm, 11 BDTC

7. Some time ago I installed an inline pod resistor for the t-stat water temp sensor. If it is at 0 Ohm, the car runs lean, revving up from idle produces lean popping. If I set it at 1000-1200 Ohm, the car can be revved up.

8. This is a CA car, came with a functioning EGR valve, while testing I disconnected it.

9. New intake/exhaust gasket, valve cover gasket, no air leaks around the oil pickup/oil filler. Removing the oil filler cap, reduces the RPMs, engine wants to stall.

10. Air regulator was tested, it closes up after 10 minutes. Pinching the hose produces engine stumbling.

11. The injectors were replaced with brand new units a while back.

12. The timing chain was replaced a while back. I haven't done any valve adjustments.


I am wondering if there is anything else I missed before I open up the remanufactured sealed AFM and start messing with its CCW movement to richen up my mixture. I also have another used ECU unit I can swap. What bothers me is why the AFM's bypass screw doesn't make any changes?

The car spent all its life in CA and was a joy to drive. Once we moved to WI with colder climate and mornings, I found myself adjusting the in-line pod resistor to add 1000-1200 Ohm. This week-end we had temps in the 40s, I went to fill it up and noticed this leaning mess.



Edited by darom
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31 minutes ago, darom said:

7. Some time ago I installed an inline pod resistor for the t-stat water temp sensor. If it is at 0 Ohm, the car runs lean, revving up from idle produces lean popping. If I set it at 1000-1200 Ohm, the car can be revved up.

Why don't you just keep it at 1200 ohms?  Not clear what the problem is.

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Even with 1200 Ohms added to the water temp sensor circuit, the car still runs lean, bogs down until is fully warmed up. When it is warmed up, I still can hear lean popping noises through exhaust.

I don't have to smog it any more, maybe bump the BTDC to 15-16 degrees?

Edited by darom
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More ohms.  You should be able to kill the engine with the potentiometer.  

Although, "bogs down" doesn't sound like the lean problem.  The lean problem is usually popping back through the intake manifold.  And bucking like a horse when you ease in to the throttle.

If you can't fix it with the potentiometer you won't be able to fix it via the AFM.  


This part, below, makes me thing that you might have a vacuum leak.  A simple test that I found for vacuum leaks, after I got my engine all figured out, was that I could kill the engine by closing off the idle speed screw.  Just drop idle speed until the engine dies.  If it never dies then you have a vacuum leak somewhere.  Not the AFM idle mixture screw, the throttle body idle speed adjustment screw.  The one with the big head that you can turn by hand.

Besides that, the screw in the hole on the AFM is only for low air flow adjustments.  It shouldn't do much once the engine starts pulling a lot of air.  It's just a screw in a small vane bypass port.

2 hours ago, darom said:

What bothers me is why the AFM's bypass screw doesn't make any changes?


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I ran the engine with the potentiometer on max (I might have a 5K unit), the engine ran on fast idle at 1500 RPMs. I couldn't kill it.

Tomorrow I'd lower my idle speed to check for any vacuum leaks. Thanks, this is a good idea.

Just for laughs I swapped my ECU with 1 spare unit I had.

Same part number as my stock unit, A11-600 000, it has less pins than the one that was in my car. The can ran rich (I could tell by the exhaust smell), I turned off the potentiometer completely since it didn't make any changes. The fuel pressure at idle was 32 psi (vs 30 psi before), the vacuum was 12 (vs 14 before). No more popping through the exhaust when I was revving the engine. (in the picture: the bottom is my stock unit).


It is good to know that if a problem doesn't go away with a potentiometer, an AFM adjustment won't help much. I'd hate opening it up and messing it up even worse.

I put my old ECU back to look for vacuum leaks tomorrow.

Appreciate your help.

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I did some testing tonight, and @Zed Head you might be onto something. I couldn't kill the engine with the idle speed screw. I could lower the engine's RPM's to 700s until the screw's end of travel, but it would still stay running.

Another test with the potentiometer. Mine is a 5K unit, I couldn't use it to kill the engine either. The most noticeable difference is when it is at 0 Ohm (runs bad at idle when it is cold, when revved up, there is popping) and 2K Ohm - the RPMs will pick up, the engine vacuum will go up to 14. Putting it at 5K shows no difference.

I can however kill the engine by pinching the Air Regulator's rubber hose. I bought a unit brand new years ago from a Nissan dealership. Per atlanticz.ca's site, it appears the valve never completely closes. I have another spare which I  confirmed works by closing its plate completely after 7-8 minutes.

I removed the AFM, the door is not stuck, opens/closes smoothly with slight pressure. No cuts/holes in the rubber AFM/intake hose.

I ordered a small smoke machine from Amazon, it will be here next week. Hopefully I will be able to pin point the source of a vacuum leak (unless it is the AAR's which is feeding too much air?).



Edited by darom
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The PCV hose was removed and checked, no cracks. The PCV valve is not stuck. Thanks.

I am thinking my stuck open AAR might be the culprit, what bothers me is that pinching it I can completely shut the engine off. Basically, per Fuel Injection bible AAR provides more air when the engine is cold to increase the RPMs (speed up the warm up time). Once it shuts in 8-10 min, its function is over (there is still going to be some air escaping, but the amount is negligible). In my case, it is still open adding more air thus creating lean condition. I have a used AAR from a 300zx (?) with a removable cover. I can install it to test the theory.

Edited by darom
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I forgot about the AAR.  So, my proposal would only work on a hot engine, closed AAR.  But that is still air that has passed through the AFM.  A stuck AAR won't cause a lean condition.

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check for vacuum leak by blowing smoke in thru a tube attached to the brake booster port on the intake manifold. use a stryofoam cup to block off the front of the AFM. you can have leaks around the intake/exhaust manifold gasket as well. Just look for smoke.



Edited by Dave WM
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@Dave WM Thanks! I will run these tests next week. This was a great video. I will check the intake/exhaust bolts torque specs.

I replaced the injectors, AAR, AFM, timing chain/front cover gasket along with intake/exhaust one (Fel-pro) 5 years ago. The car passed smog in CA.

If this is an intake/exhaust gasket, any preference which brand to get? Rockauto lists some Fel-pro, Beck and other brands. Can I double stack these to get a better seal?

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