Hello everyone, maybe somebody can help me? My 1977 280z is running rich at idle and the idle mixture on the AFM doesn't seem to help.

I spend the last few days cleaning the electrical connections and timing the ignition again as it was out of spec. The car runs way better but the AFR gauge I temporarily installed still shows 11,5 at idle. I'd like to bring that down (or up depending how you look at it) to 14,7 as the car stinks of gasoline. The previous owner mentioned that the car running rich was the reason it didn't pass inspection (auction company didn't mention it had no valid inspection on it, APK over here in the Netherlands).

After cleaning all the connections and checking the timing I tried messing with the idle air mixture screw but that didn't help. When turning clockwise a bunch the car did start to run richer at some point but when turning anti clockwise it didn't get past 11,5 AFR.

Can anybody help me diagnose the problem? I would really like to drive the car on the road before the winter stop (oldtimer insurance). Also still waiting on some parts from Zservices EU but some are on backorder.

In my quest to make the car run as it should I did notice the ignition coil is a 1,5 ohm unit with a total resistance of 2 ohm with the external resistor. I believe it needs to be closer to 1 ohm total so I'll probably replace the coil with a 0,5 Ohm one. Can somebody confirm this?

Edited after coffee kicked in...strikethrough does not show in the snippet.

The idle air screw only adjusts idle RPM, not the fuel-air ratio. It's difficult to lean out the mixture on the EFI engines. Hope that you find something wrong somewhere. Edit - sorry, just looked at your picture and see that you meant the AFM screw.

One possible reason for a rich mixture is a leaking fuel pressure regulator. The diaphragm can crack and let fuel directly in to the intake manifold through the vacuum reference hose.

Another possibility is a lack of the vacuum on the FPR vacuum port. A disconnected or cracked hose or a connection to the wrong vacuum port. The FPR reduces fuel pressure in proportion to intake vacuum. Check the hose for fuel to see if the FPR is leaking, and make sure it's connected directly to an intake vacuum source.

Sticking injectors is a possibilty.

A maladjusted AFM is a possibilty. Generally, the AFM spring should not be messed with. But sometimes it's too tempting. See if the glue blob on the adjustment wheel is intact.

I had also found that my aftermarket AFM was set to run rich. I had to use an old original AFM to pass emissions inspection in Oregon.

Edited yesterday at 06:21 PM1 day by Zed Head

I have already ordered some new vacuum lines just in case as some of them seem a bit hard and if one cracks it can cause a lot of problems. But as far as I’ve seen my vacuum should be fine. My AFM unit did seem pretty dirty on the inside so I was thinking of cleaning it in a ultrasonic cleaner with an electronics friendly cleaner. Also somebody did have the AFM open at some point in time but I don’t know if they messed with the gears inside.

I do have a pretty good understanding of the 280z EFI system and the inner workings of the components so I’m not afraid to touch the AFM unit but I would still like to ask for some tips and directions to look at.

All in all the entire engine needed a tune up as many contacts where slightly corroded or out of spec and I suspect the car was "tuned" at some point as it has an aftermarket O2 and crank angle sensor, neither one hooked up to anything. The car runs way better now but it’s still running too rich. My vacuum lines should come in this week and I’ll do some more component testing and cleaning before I start messing with the AFM itself.

Edited 11 hours ago11 hr by Mitchel0407
adding some details

Your car should not have a crank angle senser, that is for a turbo car. Your car is also pre 1980 when factory O2 sensers were just coming into play. Someone may have installed the O2 senser to try to adjust the AF ratio, but it is not factory.

Have you checked to see if your Throttle Position Switch is adjusted correctly to let the ECU know the engine is at idle?

What does the AF ratio do when you drive the car?

The car is as far as I have seen completely stock with the exception of aftermarket exhaust headers (explains the O2 sensor bung) and a aftermarket cold air intake filter. The crank angle sensor sits on a (I believe) homemade bracket and isn’t connected to anything. The Electrical harness (both car and engine) also aren’t modified so that lead me to the conclusion that the might have been used to “tune” the stock system.

I am happy with the aftermarket exhaust headers as they sound good and I can watch the AFR via the O2 sensor and the temporary AFR gauge. I would like to monitor the AFR when driving but unfortunately the car doesn’t have an valid inspection so I can’t drive it on the road. And guess the reason it didn’t pas inspection….

So now I can only try and retune the car while stationary on my driveway, which should work fine enough for now. I did check and adjusted the TPS as the two prongs were a bit too close to each other and there was no middle ground between idle and wot positions. I set the timing to 16 btdc and the idle to 800rpm btw. I also blocked off the fuel supply to the cold start valve to eliminate that as a possible problem (don’t really need it anyway in my case).

Only possible options I can think of now is to measure and check the electrical sensors (yes I read the EFI bible) and clean the inside of the AFM unit. Might also clean the injectors while I’m at it.

The saving grace for my 78 model that was running extremely rich was changing every connector on the EFI harness. Cleaned all of the male connections. Finger nail file works great and reinstalled the harness. This was after months of trouble shooting. Doesn’t mean you don’t have other issues but you have a good base line to start from.

7 hours ago, Mitchel0407 said:

The car runs way better now but it’s still running too rich.

Lean misses can cause rich exhaust. And California cars did have 02 sensors and catalytic converters through 1975 and up. Where did you get your car? Does it have a "Catalyst" sticker on the driver side door jamb?

Looks like it has the proper AFM. Are the injectors original?

Doesn't look like you pulled the vacuum hose the FPR to check for fuel. Pulling and inspection of each spark plug is worth doing, you might have a bad cylinder. Check and set valve lash, it's the first item in the Tuenup chapter. Check that your timing advance mechanisms are working correctly, both vacuum and mechanical. 16 is kind of high for idle RPM, the system is designed to retard timing at idle for cleaner exhaust by using ported vacuum timing advance. Check the charcoal canister to see if it's loaded with fuel, and check the operation of the control valve on top. Maybe somebody modified it and you're pulling intake air through the gas tank.

The tiny things add up.

On 9/1/2025 at 1:16 AM, Mitchel0407 said:

here in the Netherlands