Air Flow Meter Fails Some Resistance Checks - Repairable?

[sscanf]

HI All,

I haven't been here in a long time but I have been gradually working on my 76 280z. Lots of new parts including all new injectors (this car came to me in pieces and with some missing pieces, including the entire exhaust system). Anyway, it runs but runs rough when cold, not bad when warm but still seems like it should be smoother. I was running through some of the resistance checks in the EFI manual. I had suspected the air/water temp sensors because it is running pretty well when warmed up but those checked out OK, at least the cold measurments did. Anyway, I was surprised to find problems when checking the AFM as it looks like the guts are new. Here is what I am getting:

Pins 6/8 - 230 ohms. Good, I think (spec says approx 180 ohms)

Pins 7/8 - open. Out of spec, should be "small resistance"

Pins 8/9 - 120 Ohms. Good (manual says "approximately 100 ohms).

Pins 7/8 - Voltage tests seem OK as I vary the opening of the flap. No Jumps. Good.

Pins 36/39 - Continuity always. Out of spec but I don't think this matters.

6/7/8/9 to body all open. Good.

Given that it runs pretty well when warm, are the out of spec measurements of any consequence here? It seems like the pins 7/8 being open are the only really problematic issue but I have no idea if this is really a problem.

The AFM looks like someone put a kit into it or something. The little board in there looks brand new could it have been installed incorrectly?

Let me know if you have any suggestions.

Thanks,

Dan

Here she is:

Edited November 10, 2013 by sscanf

[FastWoman]

Your pin 7 is the wiper on the potentiometer. If it is open with respect to pin 8 (and therefore all other pins), then your wiper must just be floating. I frankly don't know how your engine would manage to run that way. Anyway, look at the spring-loaded wiper that follows the semicircular carbon trace as you open the flap. That's not making contact somehow. Perhaps the trace is dirty? You can clean the trace by lightly wetting a paper towel with some WD-40 and wiping it down.

Your contacts 36/39 are your fuel pump shutoff. They should open up when the flap is completely closed. This is a safety feature, so that your pump will shut off if your engine dies (e.g. in an accident). You'll see a wire that opens the switch contacts. You can loosen a set screw to rotate the wire a bit, so that it engages the switch when the flap is closed.

Pretty car!

[sscanf]

I pulled it out again last night and found that it still read as an open circuit when the flap was closed but could get some readings out of it when I moved the flap (which jives with the voltage readings I was getting for the powered test). It was very jumpy so I cleaned it with deoxit. Made no difference at all. I also noticed that the fuel pump shut off switch actuator arm was bent. I straightened it and it seemed to be right but I have come to the conclusion that this AFM has been messed with. I just ordered a rebuilt one from the Z store. $400 core charge - ouch!

I am going to bench test it before I install it and and will post the readings from a new one here for future reference.

Thanks for the help.

Dan

[FastWoman]

Your getting occasional readings from the wiper answers my question about how your engine can even run, roughly though it may be.

That's quite a lot of money (at least for some of us) to spend. You might be able to fix the old AFM, even if it has been messed with. There is spring calibration info on the Atlantic Z site that I used to calibrate mine. You can also alter the coolant temp sensor circuit's resistance to fine-tune your fuel/air ratio. (The ECU can drift, although this is more a problem with '77/'78 vehicles.)

I have to wonder whether the wiper is making firm contact with the semicircular resistive trace. The part is delicate, and maybe it got bent somehow. You might also have a bad contact at the wiper's pivot point and might try some Deoxit there too.

[sscanf]

Your getting occasional readings from the wiper answers my question about how your engine can even run, roughly though it may be.

I have a feeling that the application of a voltage to the system changed the electrical properties enough that it conducted better than when not loaded. Once warm, it actually ran pretty well though I don't really know how well it could run. I chalked up some less than smooth operation to the header and free flow exhaust. Anyway, this is part of the reason I want to bench test the new one first. I want to see what a good one looks like and might try to tweak up the old one before I give up on it. Maybe it doesn't really matter that it measures open when the flapper is closed.

quite a lot of money (at least for some of us) to spend. You might be able to fix the old AFM, even if it has been messed with.

It actually costs $169. I get the $400 back when I return the core. I also ordered wheel center caps while I was at it.

There is spring calibration info on the Atlantic Z site that I used to calibrate mine.. You can also alter the temp sensor circuit's resistance to fine-tune your fuel/air ratio. (The ECU can drift, although this is more a problem with '77/'78 vehicles.)

If I manage to rehab the old one, I'll take a look at that.

have to wonder whether the wiper is making firm contact with the semicircular resistive trace. The part is delicate, and maybe it got bent somehow. You might also have a bad contact at the wiper's pivot point and might try some Deoxit there too.

I went over all of that. It appeared that the contracts on the wiper were in touch with the trace. I tried pressing on the wiper while taking a measurement (no help on the dead spots). I also took measurements from inside the case rather than at the pins (to take them out of the equation). I also checked continuity from the wiper to the arm that the pivot point (good). I think there is a tiny crack or two somewhere along the carbon trace. I tried taking a measurement from end to end on the trace and it measured open.

Anyway, thanks for taking an interest in this. I really appreciate the help. I have been working on this car off and on for the past two years. Its time to finish it!

[FastWoman]

I tried taking a measurement from end to end on the trace and it measured open

.

Ah, well, there you go! There's no rehabilitating that board. The only thing you could possibly do is to short across the break in the trace, but then you would be getting uneven readings and therefore inaccurate fuel/air ratio.

[sscanf]

Quick update: New AFM arrived from MotorSport yesterday. Really nice looking rebuilt unit - the body is cleaned and coated - looks better than new! I put it on the bench and quickly ran through the tests - everything is to factory specs. Installed and fired up and the car now runs MUCH better, including a smooth idle when cold (and it was really cold last night, something like 30 degrees).

I have not adjusted anything else but it did seem a little rich still after warm up (based on smell) but I need to go over everything again now that it has a properly functional AFM. It does already have a new coolant temp sensor and fuel pressure is good (I installed a gauge permanently) I also replaced the aux air unit (because I damaged it while attempting to adjust it - don't go there its not pretty - I was running it with a ball valve from my plumbing spares for a while - manual but effective).

I didn't run it for very long either so it might have sill been cold. I should be able to spend some time with it over the weekend. I just put new wheels on it too:

[FastWoman]

Congrats on the new AFM!

You might not be running as rich as you think. I don't know how old you are, but many people have no memory of cars prior to catalytic converters. Exhaust nowadays is almost odorless -- not somewhat gassy, like back in the day. That said, plugs should read about the same as they always have. If your plugs look sooty, then you're running rich. The insulators ideally should be a nice, light shade of mocha -- not black (rich) or chalky white (lean).

If your engine is running rich, the most likely culprit is a faulty connection to the coolant temp sensor. Those connectors in the thermo housing area take a lot of abuse. The best place to check the resistance of the coolant temp sensor circuit is right at the ECU connector, as that will tell you whether you have any faulty connections. (Or sometimes some clever mechanic might have inserted a resistor inline with the CTS wiring and wrapped it up in the harness, making the mixture richer -- as was the case in my car.)

Like the wheels!

[rcb280z]

Just keep in mind that the F.I. on these cars are Temp and Resistance sensitive. Just a little out of the norm on either and you can easily have a lean or rich running condition. Like FastWomen said, check resisitance at connections. I usually check them all. Living here in CA it is hard to pass emissions testing and each time I have mine tested I know if I had not cleaned all electrical connectors I would probably fail testing. So in a way it is good because it forces me to check them often. It is a chore to keep my Z running clean enough for CA. I have to say it is not always happy running this way too.

Dan, does the old AFM have an anti-backfire valve in the flap? And was the AFM the original one to the car? I have a 77 and mine has the anti-backfire valve and there has been discussions here as to when (year) the valve showed up. I'm trying to find out if mine is original or not.

BTW, nice Z.

Edited November 23, 2013 by rcb280z

[sscanf]

Dan, does the old AFM have an anti-backfire valve in the flap? And was the AFM the original one to the car? I have a 77 and mine has the anti-backfire valve and there has been discussions here as to when (year) the valve showed up. I'm trying to find out if mine is original or not.

Unfortunately, given the giant core charge that MotorSport charges for the AFM ($400!!!!), I had to get it right into the mail for them so no longer have it available to check. But I don't know if the AFM I had was original. It looked to me like it had new electronics but the rest looked old (unlike the beautiful AFM I received from MotorSport). I have a feeling someone put a kit into it and messed it up - either abused (and cracked) the board or somehow messed it up. MotorSport did ask for the part number on my existing AFM and the new one came with the same part number so it should be an exact match - I can check this one if it helps your cause. Just let me know what to look for.

BTW, nice Z.

Thanks!

[sscanf]

Congrats on the new AFM!

You might not be running as rich as you think. I don't know how old you are, but many people have no memory of cars prior to catalytic converters. Exhaust nowadays is almost odorless -- not somewhat gassy, like back in the day. That said, plugs should read about the same as they always have. If your plugs look sooty, then you're running rich. The insulators ideally should be a nice, light shade of mocha -- not black (rich) or chalky white (lean).

I have a feeling this is the case (i.e. I forgot what the 70's smelled like). It definitely does not smell as rich is it did before the AFM. Much more subtle. I'm going to put new/clean plugs in it (my old ones were always sooty) and see how they go.

So - I did get a chance to run it a little over the weekend. It starts and idles really nicely and idles nicely once warmed up but the idle was set high. I took it for a spin around the neighborhood and it ran really well. Lots of power, sounds great and ran great but there are three concerns:

1. I had a sudden cut out for a split second somewhere along the line - I have a feeling this is associated with the fuel pump - a replacement installed by the previous owner with questionable looking wiring. Very occasionally when I turn on the ignition switch (without cranking), I don't hear the fuel pump. Off and on again and its good. I once had to smack it with the handle of a screw driver but I'm not sure that really is what fixed it. I think I will uninstall it, inspect it, then re-install properly (for some reason he ran the ground wire all the way from the battery).
   
2. 
   
3. When I went to adjust the idle (by ear), it consistently stalled. It sounds to me like its at something like 1200 RPM but running really well there. I need to put a tach on it (the dash tach has problems - see next point) - maybe its just my ear. It was also really cold out yesterday (25).
   
4. 
   
5. The tach is very jumpy, especially at low RPM. Sometimes it just reads 0. Could be the tach itself or the ignition module or something to do with the distributor upgrade done by the previous owner. I need to figure this out.
   

If your engine is running rich, the most likely culprit is a faulty connection to the coolant temp sensor. Those connectors in the thermo housing area take a lot of abuse. The best place to check the resistance of the coolant temp sensor circuit is right at the ECU connector, as that will tell you whether you have any faulty connections. (Or sometimes some clever mechanic might have inserted a resistor inline with the CTS wiring and wrapped it up in the harness, making the mixture richer -- as was the case in my car.)

I'll do exactly that. It does have a new temp sensor.

Like the wheels!

Thanks!

[FastWoman]

A SUDDEN dropout of power doesn't sound like the fuel pump. If your pump suddenly stopped pumping, your power would fade over maybe 1-5 seconds, depending on whether your engine is under load. SUDDEN dropouts of power sound more like ignition to me. Adding to this hunch is the fact that your tach sometimes reads zero. You might have a weak/fading ignition that is only barely adequate to trigger the tach and that sometimes drops out. Also remember that the ECU is triggered by the ignition. If the ignition pulse is so weak your tach can't read it, it might be so weak your ECU can't read it either. In that case your fuel delivery would suddenly drop out, as the ECU wouldn't put out any injector pulses.

If I were you, I'd check over the ignition system. The problem would be a low-voltage one -- somewhere from the ignition module to the ignition coil, but not beyond. It might also be in the wiring from the (-) post of the coil to the ECU and tach.

Sometimes our ignition modules fade away. Mine did. You can either install a "refurbished" unit (meaning that they cleaned it up, I think), or you can retrofit with something else. I did a GM HEI retrofit on my car, which was quite cheap ($11 for the chip, plus some wire). It continues to work very well. You can search for that mod on this forum.