Backfiring through AFM....lean from low fuel pressure? (76 280Z)

[RPM Resto & Custom]

Hey guys...I'm over at my parents right now looking at my father's 76 280Z. The car is completely stock with no engine modifications and I believe it still has the original fuel pump in it. He has been having an issue with "backfiring" for a while now and after we looked at it together today I can hear it actually coming from the intake side through the AFM. From what I've read using the search function this can indicate a lean condition. The backfiring only happens when you jab the throttle to accelerate...which I also believe would be consistant with a lean problem.

All of the vac hoses seem to be in good shape as does the boot between the AFM and throttle body. I also slightly adjusted the timing and idle...about 12* and 800 RPM's.

I have a homemade fuel pressure gauge for Z cars that I use....I put it inline between the outlet of the fuel filter and the inlet of the fuel rail. With the car running at idle I'm seeing right around 30lbs of pressure and if I rev it up and hold it the pressure will increase momentarily and then come back down to around 30 again....which should indicate the pressure regulator is working correctly right? Is 30 psi too low and possibly causing the lean condition and backfire through the AFM?

[RPM Resto & Custom]

After researching the topic some more it seems that 30psi may be an acceptable fuel pressure and that it is most likely related to the signal from the temp sensor and the way the computer has worn over time and receives the signal. Is this the more likely scenario if you rule out vac leaks?

I have an ECU out of a 78 with the same trans that I can attempt to swap over just for sake of testing the theory. If the 78 doesn't have the backfiring through the AFM then it should stop in the 76 if that is in fact the issue right? I will doublecheck first but I'm almost certain the ECU is the same P/N for both applications.

[Wade Nelson]

Suggest you start by putting a vacuum gauge on it to make sure you don't have a sticking valve or other mechanical problem.

[RPM Resto & Custom]

Yeah that was the one diagnostic tool I forgot to grab from my shop before I headed over today. Not that it's an accurate measurement but there shouldn't be any major vac leaks because the idle is pretty smooth and when I had the fpr disconnected I felt a strong pull from the hose. But I do understand that even a small amount of unmetered air after the AFM can throw off the a/f ratio substantially

[Zed Head]

The intake backfiring is also a sign of a "bad" AFM. But I've "fixed" two AFMs that were lean by using the potentiometer in the coolant temperature circuit (just installed another spare AFM last week, and the potentiometer saved it. It was stumbling and on the edge of backfiring). You might try that. You could also measure resistance on the 6, 8, and 9 pins. If you don't get 180 and 100, the AFM might be going bad.

It may be a higher ethanol content in gasoline making a slightly lean situation worse. Ethanol effectively dilutes the mixture, making it lean. For example, E85 engines use huge injectors and sometimes double up to get enough E85 in to the engine. Modern engines compensate through the O2 sensor, old systems are stuck with the factory algorithm. Pontificating...

I've used a 78 ECU in my 76. The part number were the same. They performed identically.

And 30 psi at idle is good. It should jump to ~36 if you remove the vacuum hose to the FPR.

[RPM Resto & Custom]

Zed what do you mean by the potentiometer in the cooling temp circuit? Do you mean the resistor mod that some guys use, specifically the adjustable dial type that can be installed? Using a "good" ECU would have the same effect at least until it went bad as well right? Or are you referring to the adjustment that can be made inside the AFM itself?

[Zed Head]

Yes, the resistor mod. It's easy, pretty quick and effective for richening the mixture across the whole air flow curve.

I've read several accounts of people finding resistors in the EFI harness that looked like they were put there professionally. Installed and wrapped for a permanent fix. It seems like the system was designed on the lean side and people have been fixing it for many years by adding a resistor to the CTS circuit.

I don't know if the ECU's are going lean, or the AFM's, or the ethanol in gasoline is putting things over the edge. If the ECU swap doesn't work, I would try the resistor next.

[EuroDat]

The temp sensor mod is used to correct ECU drift. All the old ECU's have it to some degree and it could be whats causing your problem if combined with E85 fuel.

Having said that, if you do the ECU swap and it doesnt fix it. The temp sensor mod will probably improve it, but you are then fixing a symptom and not the cause. Something is telling the ECU to use less fuel and your telling it to use more.

On a different note: We have a couple of old process plants using old ABB Bailey Network 90 control systems. The parts are NLA and good spares are hard to find. We were having problems with drift. We had spares, so we vacuumed all the dust off and decided to clean them with ultra purified water. Literally dropped them in a bucket of water and then dried them in an oven for 8 hrs at 50degC. They have never worked better.

The dust gets in and changes the resistance which causes drift. "Washing" them solved this problem. I have done this on mobile phones. My sons phone has been through the washing machine twice and this saved it.

If I had a spare ECU, I would consider trying it. Even though it has worked on 16 of these ABB units without fail, I am still reluctant to try it on my ECU. Just cant get use to the idea of dropping it in water.

[bartsscooterservice]

many times I had it be the ignition ( bad condition or timing to advanced ). Valves are not closed all the way @ TDC when it ignites the mixture, so it travels back into the intake and through the afm.

[RPM Resto & Custom]

Well I got my father to come over to my shop today for a while and it looks like we solved or circumvented the problem. I swapped ECU's between the 2 cars and it made no difference so I basically ruled that out. This left me with either the resistor mod or trying to recalibrate the AFM itself. Rather than start trying different resistances I decided to pirate the AFM off of the good running 78 here and sure enough the 76 started running like a champ with it installed. No more backfiring through the intake and tons more power all around. Now the AFM out of the 78 had some non original silicone around the cover but not sure if it has ever been adjusted or not so it's a bit of a mystery there.

We opened up the original AFM and everything inside looked brand spanking new...no corrosion, lots of shiny connections, etc. It is the original one from the car as my father has had it since new. I loosened the set screw and we started playing around with the adjustment. Just 3 teeth looser made the backfiring through the AFM stop. After a number of trial and error runs around the block with the car a total of 12 teeth looser seemed to make the car run happiest. We will check the plugs after some driving and make sure nothing looks unusual and go from there. If the condition comes back again over time we will most likely just get a new AFM at that time.

[Willoughby Z]

Wow.

12 teeth?

IMO, something is waaaay off somewhere. Before you trash the AFM, check it out thoroughly.

http://atlanticz.ca/zclub/techtips/afm/calibration/index.html

http://atlanticz.ca/zclub/techtips/afm/index.html

[EuroDat]

12teeth? I didnt know the spring had that much tension.

Willoughby's tips are a good idea. I would use the drink can and string test to check the spring.

12 teeth sounds like it might be binding or something like that. Your lucky you had a compatable car to swap parts, that made it easy to eliminate possible problems.