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1979 Datsun 280zx base manual
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Discussion Starter · #1 ·
Hi all, thanks to everyone’s help here I’ve gotten my 79 280zx really close to being road worthy again.

I’ve got one last hurdle.The engine sounds great and revs freely in neutral, but once I put the car in gear and try to accelerate, it stumbles with anything more than 1/4 throttle. Seems like an extreme lean mix issue. I’ve read previous topics with similar questions and read through the FSM and EFI bible but I’m still stumped and have a question.

Question: This is either a too much air or not enough gas issue, so what is happening differently with the vehicle in gear vs neutral? Is the fuel pump needing to push more through but can’t? Just trying to understand the underlying reason this isn’t happening in neutral.

I think it’s either a tired fuel pump or bad AFM. Other than that I’m at a loss.

Things I’ve checked:
-All EFI bible/FSM electronic checks
-Fuel pressure at idle ~28psi, at full throttle ~35psi
-Dropped fuel tank and replaced with spare that was cleaned previously.
-Fuel pump sounds fine with key turned to On and can hear the fuel pressure regulator working.
-Spare ECU makes no difference.
-Opened AFM and glue blob has not been broken so I can assume it hasn’t been tampered with?
-Turning idle adjustment screw of AFM all the way rich on both sides doesn’t solve issue
-Adjusted throttle position switch to be at full throttle at 1400 rpm and wired new connector
-All electrical connectors except AFM have been replaced.
-PCV valve new
-Have not ran proper vacuum test but car stumbles if I unscrew the oil fill cap or remove dip stick.
-Replaced all vacuum hoses with silicone
-New hoses on both sides of AFM.
 

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#7 Humm, no response to idle speed adjustments would indicate there's false air being introduced downstream of your idle air bypass pathway and closed throttle valve. Once under load (in gear) off idle the lean popping and stumbling is exacerbated. You most highly likely have a vacuum leak somewhere, that is where by air is bypassing the AFM flap/potentiometer. You can isolate the leak path using an aerosol can of starting fluid by lightly spraying or misting the ether next to hose connections, mating flanges while listening for a change in idle speed due to the "additional" fuel source. If you have a Mighty-Vac brake bleeder you can use it as a vacuum gauge or attach a simple vacuum gauge avail at most auto parts stores hooked to any manifold vac port to see where your at as far as your manifold pressure (in-Hg).

My $0.02 ¢
 

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I agree, too much air. Could be lots of places. Intake manifold to head. EGR valve. Aux Air Valve stuck open are places people tend to forget. I have read where the internal port (inside the intake maniflod) has deteriorated and causes a really bad exhaust leak into the intake. The engine stumbling when you remove the oil cap or dipstick while running is normal because you are pulling vac on the engine crankcase (valve Cover) when running. The "in gear" stumble is because of the load on the engine compared to neutral. The cylinder head temp sensor is what turns the rich/lean up and down. The higher the ohm value when cold the richer the mixture. Low ohm value when the engine is hot will lean the fuel.
 

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1979 Datsun 280zx base manual
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65 Posts
Discussion Starter · #4 ·
I’ll have to try to do a proper vacuum test. The air bypass being stuck is an interesting one actually…not sure how I can check that because I know they are pricy. I know there are no manifold leaks because I had them all ground down flat. And the water temp sensor is new and passes all checks. Still thinking it might be an AFM issue. The previous owner did a bit of swapping with components from 280z so maybe he plopped in the wrong AFM and it’s giving fits? It’s a 5 prong connector but I don’t know the model number as it’s worn off.
 

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The Auxiliary Air Regulator Vlv (AAR) can be diagnosed by simply removing the two hose clamps and then the two bolts securing it to the int manifold. Your AAR maybe mounted differently. They can become clogged with sludge from crankcase gases and the bi-metal can no longer function properly. Remove and check out the aperture to see if it is still open like a crescent moon or fully closed.

I had a 76 Coupe that was a handful and come to find out it was due to cracks hiding in the accordion folds of the AFM to TB Boot. Flexation between the body mounted AFM and Engine places stress on an already 40 yr old Boot.

Any air entering the induction system past or downstream of the AFM will have a leaning affect. Just thought I’d mention the Boot as an additional place to inspect and check for leakage.
 

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1979 Datsun 280ZX
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I have a 79 280zx n/a that had the same problem. I replaced my fuel pump with a new one and the problem was fixed. The original pump that is on the 280zx's have a little screen filter inside the inlet barb. Mine on my old pump was filled with sediment and clogged so that might have been the issue. Make sure to put a cheap $10 in line filter between your pump and gas tank so Yours doesnt get clogged with sediment lik mine did. Hope this helps you :)
 

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1979 Datsun 280zx base manual
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Discussion Starter · #7 ·
So I got MOST of the problem solved. Turns out some of the vacuum line connections were mixed up and my timing was back to 0 degrees instead of 10. I guess I either didn’t let the engine warm up all the way the last time I checked or it was before I reinstalled my freshly milled head. But I can now at least get the car going.

It’s still lean though, gurgles a bit below 2000 rpm but then gets going pretty good. The vacuum test showed right around 14 in pressure, so in that sort of iffy area. I’ve also got the idle adjust screws on the AFM screwed all the way in (rich) in order to get it running the best I can. Maybe I’ll take it to an auto shop and see if they can test the air mixture at the tail pipe to get an idea of how far off I am.
 

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1979 Datsun 280zx base manual
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65 Posts
Discussion Starter · #8 ·
Replaced the fuel pump and rubber hoses, and removed the old rusted out fuel damper since it seems like we don’t “need” it (also wanted to eliminate it from causing the stumbling). Still stumbling from 1500-2000. Honestly, the last thing is the AFM right? Do I actually need to adjust it like I’ve read some have needed? I’ve also read quite a few stories about not touching it, but I genuinely don’t know what else it could be.
 

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84 300ZX fuel injection system is an easy swap for this vehicle.Gets rid of all the fringe sensors on the original 280zx.The MAF system is far superior better performance and better mileage.Less than 5 wires to hook up.Why is no-one recommending getting rid of that older mechanical flap style system.Once you switch youll see why nissan, toyota,Bmw, etc. did also.
 
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