280z afm testing +other ?s

[ll77]

hey guys.

so ive been having smog problems recently. i got a tip from a smog station that my afm might be the problem b/c its running lean at idle and very high on CO at speeds above idle. so i took the afm and ran some tests w/ an ohmmeter and it failed both potentiometer tests. ***EDIT*** these 2 were from the fsm

when i ran this test (as seen on http://www.atlanticz...fm/index.html):

 

Measure the resistance across pins 7 and 8 while moving the flap. It should change. (though it may be jumpy)

 

Repeat this but between pins 6 and 7 while moving the flap.

 

the resistance only dropped a little bit when i began pushin the flap back, but stopped dropping about a quarter of the way in. resistance went from continuty to approx. 0.4 ohms. it checked out on the rest of the tests. my mechanic friend tells me he doesnt trust ohmmeter tests b/c he's seen parts that fail resistance tests work fine, but I was thinking it would make sense that the potentiometer was bad b/c that could make the flap open, but not all the way. so at low engine speeds, it could get too much air, making it lean, and at higher speeds, too little, making it rich. is that good logic? also, it seems my car will not rev past 3500-4000 rpm, and its got a rough idle. perhaps related?

 

also, any ideas of what else could be causing my car to run rich would be helpful. the technician mentioned a couple things, but i didnt write them down ( i know, i should have). i think he said my cold start valve shouldnt be the problem, i didnt understand why. also, my fuel pressure is registering about 40-41 psi at idle, and jumps to about 43 when i rev it, and i know that the fuel pressure should be at about 36 psi. so im thinking about the fuel pressure regulator too. ***EDIT*** could also be that the gauge is faulty or inaccurate, it is installed on the fuel feed line, but i have heard that this doesnt indicate the "true" fuel pressure as that should be measured from the fuel rail?

 

also, i am in the market for an afm, and i might make a trip to see a guy with a couple of them tomorrow. he has 2 280zx afms, but only 1 280z afm, all used, not tested yet and have been tweaked for air/fuel ratio adjustment. so i was wondering do 280zx afms work in 280z's too? i know they both have l28's so they should right? but if not, i think i'll go for a remanufactured one instead off zccjdm.com.

 

so what do you guys think???

thank you soo much!!

[MegatherioN]

In case you don't have the factory service manual for the 280z you can download it from xenons30.com that might help you with your problem.

[Skib]

In case you don't have the factory service manual for the 280z you can download it from xenons30.com that might help you with your problem.

 

 

^^ very good source

 

 

if there not up shoot me PM, I have a few Z FSM's on file in PDF

[ll77]

yes i have three of them, one of them complete and for my year. i also have the efi manual, and a chilton's manual, but none of these can reason this out and explain for me. i guess my main concern right now is should i get another afm?

[ll77]

so today i put on the afm w/ the connector off to see if it was actually working or not. it started up, but would die immediately. i plugged in the connector, and it started right up. so i guess the afm is working. however, i'm thinking that it is not a 280z afm, because I noticed that the holes on the bottom of the afm did not match up with the holes in the mounting bracket. I also think that 75-76 models had a blowback valve in the flap of the afm, and mine does not. So, I'm thinking that the previous owner tried to use a 280zx afm on my car. Do you guys think this adds up? would using a zx afm in a z explain the problems i'm having right now? Pics coming soon!

[datzenmike]

Inside the AFM is an arm that wipes across a resistive material and depending how far open the flap gate is from the volume of air entering a varying voltage is fed to the ECU. The ECU uses this info to compute the injection pulse.

 

After many years this resistive coating gets worn away and there are dead spots on it giving the ECU a faulty reading of how much air is entering the engine. Think of a windshield that has a curved scratch on it from a wiper arm.

 

The AFM can be opened and the wiper arm moved to a fresh position on the resistive material. In the picture below the black saw tooth thing is the resistive covering and the copper U shape is the wiper that swings in an arc rubbing against it to vary the voltage. This may not be exact but yours will be close.

 

[Skib]

i'm thinking that it is not a 280z afm

 

 

they varry threw all the years of the L28

 

check the number on the AFM (theres a sticker on it) and check the one on your ECU, they should match.

[datzenmike]

but I was thinking it would make sense that the potentiometer was bad b/c that could make the flap open, but not all the way. so at low engine speeds, it could get too much air, making it lean, and at higher speeds, too little, making it rich. is that good logic?

 

Missed this.

 

The other way round. The air volume entering pushes the flap door open, this moves a wiper across the potentiometer and varies the voltage output. This tells the ECU how much air is entering so it can compute the fuel ratio needed.

[ll77]

Thanks a lot for the tips guys. I thought the afm controlled how much air was going in! Thanks a lot for clearing that up for me.

The technician said that the air flow meter might be causing the problem, but i guess he could be wrong too. But if the potentiometer is registering a lower resistance than it should at engine speeds, then the ecu would read less air than in reality, and in turn inject less fuel than necessary, and make the mixture lean at above idle, right? My reasoning is based on the results of the test I had described above, from atlantic z.

And I'm going to try that Mike, do I just bend it a little and make sure its contacting the black part?

Skib, I checked the numbers and the only numbers that match are "L28." Maybe because it has been remanufactured? Yeah, and the other weird thing is that it has a fairly newly rebuilt afm that was only purchased 2 years ago!

[ll77]

I've called motorsport auto, which was where the AFM was bought, and they told me that there are 3 different cores for my year, and the number I gave them does not correspond to my year. but it is also very hard to know which one was originally used if it has already been replaced. Their staff seemed quite knowledgeable, and they told me that it is possible that my car could run rich if the wrong core was used, as in my case. Now I need to know which core I need. Perhaps the numbers correspond between the ecus and afms as you mentioned, Skib. So I found these 3 numbers on my ecu:

6426

A11 600 000 (this does not match any number they gave me for the afms, but the format is very similar)

L28

[Skib]

A11 600 000 (this does not match any number they gave me for the afms, but the format is very similar)

 

 

these are the numbers you want to match

[ll77]

Which ones should match? None of the afm numbers they gave me match up to it, but one of the numbers' last 3 numbers match up... none of their afm numbers match it, really. Here they are: http://www.thezstore.../SFC03A/11-3040