Help needed with diagnosis

[Pilgrim]

I just found a brand new air flow meter at a great price - it's on its way and I should have a report within a week IF the weather holds. I'm not changing it out standing in snow...

[Pilgrim]

OK, just got a chance to install brand new AFM. Unfortunately, it did not fix the problem. DANG!! The problem has gotten worse over the past few months - now it doesn't happen every few days, but constantly, and even when idling after the car warms up for a minute or so. Darn thing isn't driveable until I get this fixed. :mad:

However, this means that I have a perfectly good (and tests good as per factory service manual) 1983 280ZX Turbo air flow meter for sale!

Meanwhile, we may get another heavy snowfall tomorrow so the car will be under its car cover for a while.

I haven't checked around for vacuum leaks yet - that may be the next item. If I could get it to idle for a while, I could spray starting fluid around the hoses. That would probably be the only way to find a leak in the big hose between the AFM and intake manifold.

Here's a list of what I have done:

- ECU has been replaced and all connections coated with Deoxit to assure contact.

- Fuel filter has been replaced.

- Fuel pressure was been tested last summer after the problem appeared and was OK.

- Air flow meter has just been replaced with new AFM.

- Head temp sensor has been removed and tested - it passes.

- Cat converter has been checked for clogs (I removed it and sighted through it) and it's not plugged.

Edited February 7, 2010 by Pilgrim

[Pilgrim]

Here's the latest idea....On a turbo model, there is a fuel damper mounted near the fuel pump outside the fuel tank, and there is a fuel pressure regulator located on the fuel injection rail.

I'm familiar with the fuel pressure regulator and have changed it in the past, although it has been a few years (more than 3).

What about the damper or the regulator? The regulator is easy to reach and replace, and I'm sure the damper is manageable. It does make sense that it would be something in the fuel system, as the engine is clearly losing fuel but movement of the gas pedal restores it immediately.

According to the manual (p. EF & EC-49) the damper's purpose is to damp pulsations from the fuel pump, and the pressure regulator's purpose is to return surplus fuel to the fuel tank.

Logic tells me that pulses from the fuel pump would be unlikely to affect the car in the way I'm experiencing (and there's not much to fail in the damper), but it seems to me that a malfunctioning regulator might be returning too much fuel to the tank and letting the engine starve.

Does that make sense?

[Pilgrim]

Latest news flash - I've gotten some feedback that the fuel pump control modulator can cause this - it's an electrical box located above the ECU on the outside of the driver's side footwell. Right now it's top of my list.

UPDATE: I found this old thread on zdriver.com which seems to describe the exact problem, plus a way to test it by grounding the wire that sends a signal to the relay, thereby keeping the fuel pump on:

http://www.zdriver.com/forums/showthread.php?t=28820

This seems well worth following up as a test when the weather improves here.

Edited February 8, 2010 by Pilgrim

[Pilgrim]

Final discovery - it is the fuel pump control modulator. After reading the other thread, I went to the connector and found TWO white wires - but one was all white, the other was white with a yellow trace. I unplugged the connector and cleaned it with Deoxit on the off-chance that it was a bad connection - no luck. Then I stuck a jumper wire into the back of the connector on the solid white wire, re-connected the connector and grounded the other end of the jumper. I can hear the fuel pump run constantly and the car does not stumble or die. The fact that it continues to run seems to prove a bad modulator.

The modulator is in a rather difficult place to reach - but I'll order one and change it out. Until it arrives I'll leave the jumper in place, let the fuel pump run when the key is on and the fuel system will return the excess fuel to the tank. Not ideal, but it will only go on for a few days and on short trips.

[Pilgrim]

I replaced the modulator yesterday and that has corrected the problem. Now I have a rough, low idle (500 RPM or less) when the ZX is cool, but it's correct at 750 RPM when warmed up. I think the culprit is the temp sensor on the passenger side - I pulled it planning to replace it, but had the wrong part. As I was re-installing it, the sensor came apart. I managed to get it back together and re-install it but it's probably not working right. I'll get that fixed.