AFM Adjustment

[UR2H]

One possibility is your water temperature sensor. Resistance in the circuit will lead the ECU to adjust for a cold engine and dump more gas.

Another possibility is your FPR. My 1976 car's FPR regulated pressure high when it got hot. Measure fuel pressure when the engine and FPR are hot, but be aware that the oil-filled gauges can be inaccurate when they're hot so make sure the gauge is cool.

My fuel pressure sits just slightly under 30 psi when warmed up nice and hot at 800rpm. The fuel pressure gets really high when I remove the vacuum to the FPR. I was told that is how to test for a faulty FPR.

[Zed Head]

Fuel pressure just under 30 at idle sounds okay. Why didn't you give a number for no vacuum hose, instead of "really high"?

The 1977 FSM Adobe file is harder to use than the other years since it's not broken in to chapters, but if you can find the Engine Fuel section, there are some great diagrams of where to place the probes at the ECU connector (with a numbered pin picture), how to set your meter, and what resistance values you should see. The 76 and 78 test methods are the same so you could download one of those FSMs to make it easier.

Practice measuring resistance across a light bulb filament or similar if you aren't familiar with using a meter.

There might be a water temperature switch up there also (there is for 76, not sure about 77). Make sure that you have the right bullet connectors. The switch is either open or closed based on temperature and determines which pickup coil the ignition system uses (if you have the dual pickup distributor you should have the water temp switch).

[cygnusx1]

If your plugs all look like that marshmellow, then you are losing mileage due to another reason. Dragging brakes, low tires, driveline drag, or a fuel leak.

The little screw in the corner of the AFM is a BYPASS to allow air to go around the flap without getting metered. The more air you let sneak through that screw (counterclockwise), the leaner it idles.

Just an FYI: If someone opened up the idle mixture screw in the AFM, all the way, it might idle lean. To compensate for that, someone may have erroneously turned the AFM tooth wheel to the rich side, making it idle perfectly. However, that would make it too rich everywhere else but idle. OTOH, toasty brown plugs would indicate that the combustion mixture was OK..so I refer you back to my first lines above. If I remember correctly, each tooth in the AFM represents about a 3% change in fuel mixture.

PS. What kind of driving are you doing? A perfect Z can get 12mpg if driven hard. If it gets that on the highway at 55mph, then you have an issue.

Edited September 25, 2011 by cygnusx1

[FastWoman]

Dave, I couldn't manage to drive my '75 Z's mpg all the way down to 12 mpg even when I was young and stupid and knew only two throttle positions (WOT and closed). I agree that something else must be amiss -- maybe leaking fuel or dragging brakes, as you suggest. Plugs show firing of all cylinders at a good mix. Fingering the AFM shows proper mix.

Question about how to measure resistance: I know nobody reads instructions, BUT there should be some instructions handily enclosed with the multimeter. You have to plug the two electrodes into the indicated sockets, and turn the selector to resistance. You'll want a range of maybe 5 kOhm. Touch the two probes firmly together, and you should get a reading of 0 kOhm. Hold the two probes apart, and you should get an infinite reading (however that's indicated on your meter). If you get these two readings, then you have everything set up right. Now touch the two probes firmly to the two points you want to measure the resistance between (e.g. the two connector blades on the coolant temp sensor), and take the reading. These are very general instructions. Every meter is a bit different, so it's hard to get more specific.

[UR2H]

Fuel pressure just under 30 at idle sounds okay. Why didn't you give a number for no vacuum hose, instead of "really high"?

The 1977 FSM Adobe file is harder to use than the other years since it's not broken in to chapters, but if you can find the Engine Fuel section, there are some great diagrams of where to place the probes at the ECU connector (with a numbered pin picture), how to set your meter, and what resistance values you should see. The 76 and 78 test methods are the same so you could download one of those FSMs to make it easier.

Practice measuring resistance across a light bulb filament or similar if you aren't familiar with using a meter.

There might be a water temperature switch up there also (there is for 76, not sure about 77). Make sure that you have the right bullet connectors. The switch is either open or closed based on temperature and determines which pickup coil the ignition system uses (if you have the dual pickup distributor you should have the water temp switch).

When I removed the vacuum line I leaned over to look at the gauge and saw it was close to 45-50psi but I kinda got scared so I hurried and plugged it back rather than checking to see what the exact reading was.

[UR2H]

If your plugs all look like that marshmellow, then you are losing mileage due to another reason. Dragging brakes, low tires, driveline drag, or a fuel leak.

The little screw in the corner of the AFM is a BYPASS to allow air to go around the flap without getting metered. The more air you let sneak through that screw (counterclockwise), the leaner it idles.

Just an FYI: If someone opened up the idle mixture screw in the AFM, all the way, it might idle lean. To compensate for that, someone may have erroneously turned the AFM tooth wheel to the rich side, making it idle perfectly. However, that would make it too rich everywhere else but idle. OTOH, toasty brown plugs would indicate that the combustion mixture was OK..so I refer you back to my first lines above. If I remember correctly, each tooth in the AFM represents about a 3% change in fuel mixture.

PS. What kind of driving are you doing? A perfect Z can get 12mpg if driven hard. If it gets that on the highway at 55mph, then you have an issue.

The mechanic I saw had a machine that measured CO and my CO% form the exhaust was very very high. He said that is caused by running rich. I think I will hold off on the AFM adjustment for now since there can be other possible reasons.

I don't drive hard...sometimes but hardly especially these days. When I'm cruising I try to stay right around 2500-3000rpm and I shift around 4000rpm. Like I said I'm, usually never at WOT.

[UR2H]

Dave, I couldn't manage to drive my '75 Z's mpg all the way down to 12 mpg even when I was young and stupid and knew only two throttle positions (WOT and closed). I agree that something else must be amiss -- maybe leaking fuel or dragging brakes, as you suggest. Plugs show firing of all cylinders at a good mix. Fingering the AFM shows proper mix.

Question about how to measure resistance: I know nobody reads instructions, BUT there should be some instructions handily enclosed with the multimeter. You have to plug the two electrodes into the indicated sockets, and turn the selector to resistance. You'll want a range of maybe 5 kOhm. Touch the two probes firmly together, and you should get a reading of 0 kOhm. Hold the two probes apart, and you should get an infinite reading (however that's indicated on your meter). If you get these two readings, then you have everything set up right. Now touch the two probes firmly to the two points you want to measure the resistance between (e.g. the two connector blades on the coolant temp sensor), and take the reading. These are very general instructions. Every meter is a bit different, so it's hard to get more specific.

My multimeter didn't come with a manual but I did look over the one this link provides -

http://atlanticz.ca/zclub/techtips/electricalchecks/cp7678.pdf

Also when touching the probes together I get a very low reading and apart gives an infinite reading so the multimeter seems to be working fine. I just don't get why I didn't get a reading when I was testing at the bullet connectors. Maybe I just need practice? Well I will give it a go on my next day off which is Thursday unfortunately.

Thanks again to everyone for the advice. Really appreciate it.

[Zed Head]

When I removed the vacuum line I leaned over to look at the gauge and saw it was close to 45-50psi but I kinda got scared so I hurried and plugged it back rather than checking to see what the exact reading was.

I'm not sure how the FPR mechanism would allow your fuel pressure to jump that high when you remove the vacuum hose, maybe some debris in the orifice, but that is very high fuel pressure. If that reading is correct, that is the pressure the injectors would see whenever you press the gas pedal to accelerate.

It won't hurt anything to let the pressure sit at 40-50 psi. You should recheck it with the vacuum hose off to confirm. Without the hose it should be in the vicinity of 36 psi.

I would check the FSM procedure for testing the water temp sensor and test it at the ECU. For some reason they split the tests up in to one description for continuity at the ECU, and another with actual readings at the sensor. Just take the resistance readings at the ECU and see if they fit the temperature chart in the FSM.

Edited September 25, 2011 by Zed Head

[FastWoman]

You might not have gotten a reading because you weren't pushing the probes firmly enough into the metal of the connectors. Use sharp points and edges! ;-) It's possible the connector on the temp sensor isn't making contact (old and corroded), but I would think you'd be blowing black smoke out the exhaust if that were the case.

[UR2H]

I'm not sure how the FPR mechanism would allow your fuel pressure to jump that high when you remove the vacuum hose, maybe some debris in the orifice, but that is very high fuel pressure. If that reading is correct, that is the pressure the injectors would see whenever you press the gas pedal to accelerate.

It won't hurt anything to let the pressure sit at 40-50 psi. You should recheck it with the vacuum hose off to confirm. Without the hose it should be in the vicinity of 36 psi.

I would check the FSM procedure for testing the water temp sensor and test it at the ECU. For some reason they split the tests up in to one description for continuity at the ECU, and another with actual readings at the sensor. Just take the resistance readings at the ECU and see if they fit the temperature chart in the FSM.

The fuel pressure increased to 38psi with the vacuum hose removed. I guess I didn't get too good a look last time.

[UR2H]

You might not have gotten a reading because you weren't pushing the probes firmly enough into the metal of the connectors. Use sharp points and edges! ;-) It's possible the connector on the temp sensor isn't making contact (old and corroded), but I would think you'd be blowing black smoke out the exhaust if that were the case.

So I got a few readings. I didn't get the readings from the ECU though, was having trouble removing the connector. But at the bullet connectors I got a reading of.285 when warmed up. For some reason I couldn't get a reading with the engine cold so I ran the engine for maybe 1 minute or so (~100 degrees) and it read .8. One thing I noticed is when I was getting the readings, the numbers kept dropping. Is that normal? So when I got the .285 reading, it then read .284, .283,.282, and so on....

I should also mention, the bullet connectors looked very clean.

So I was going to hold off on asking this until this situation at hand was figured out but I'm thinking that it may pertain to the problem. So I notice the idle drops to around 500rpm them stumbles back up to 800rpm whenever I come to a stop or press the clutch. The engine never dies but it does do this practically every time. Is this something that would be affecting my gas mileage?

[grantf]

That is normal, no? as the temp increases the resistance decreases and vice verse.