yeah, sorry for the sucky info. To identify your transmission, you have to look on the side of your transmission casting to identify it. It'd have a P20 and then either A000 for (CVDL)x and B000 for Ex/Si.

The gas mileage problem existed before the head was replaced. And I *assume* (but you know what they say about assuming) that I got a correct replacement head. I do know they ordered in a replacement from an out-of-area machine shop that they've worked with for a long time.

Is there a way of knowing for sure (without pulling off the head) if it's the correct one?

jadz, thanks for the quick response. Hopefully it's OK with you if I try to learn some more.

"the VX LAF sensor will be at negative voltage when read from the D14 and D16 pin outs when accelerating"

OK, that's more-or-less what I thought. But are you sure a DSO (digital storage oscilloscope) is needed? I realize the nature of closed-loop mode, in general, is the O2 sensor voltage fluctuates rapidly, and this could be hard to follow on a DMM (whereas a DSO would be perfect, because it's inherently designed to track a pulsating voltage). But I thought those rapid fluctuations are a characteristic of conventional O2 sensors, not the wideband sensor in the VX. Also, when you apply a large throttle opening and the system suddenly snaps into open-loop mode, shouldn't the voltage then be fairly steady (as long as open-loop mode is sustained)?

I'm not trying to analyze the waveform in detail; I just want to be able to detect whether or not I've entered open-loop mode. Shouldn't a DMM be adequate for this purpose? Especially given that this is a system with a wideband sensor?

There's some long posts in this thread so i'm having trouble keeping track of it all, but if it hasn't been said yet:
check timing
check compression
check for proper spark plugs (zfr4f-11), and that they are in good shape
which ecu do you have? cali or federal?
check your o2 sensor readings Link: https://www.gassavers.org/showthread.php?t=2427

and here's my thread from having mileage problems, it may offer you some more ideas: https://www.gassavers.org/showthread.php?t=6072

You can use a DMM to monitor the Oxygen sensor. It does bounce around a lot, but you can tell what range it is in and you can tell if it goes into open loop because of the voltage.

With regard to your IACV valve, you need to get that replaced before you worry about fiddeling with anything else. The rpm bouncing around when it's idling is because the IACV valve is leaking. You can pull the intake hose to the IACV valve off of the air filter and plug it with something, even your thumb. If the IACV is bad, the idle will settle down, but go erratic when you remove the plug.

The IACV valve should not allow any air to pass through, if their is no vacuum on the valve. Since you probably will have a shop do the work, you will probably end up with a new valve. However, it will probably be your last.

Gary, thanks for your comment about the DMM. That's helpful. And I think that IACV info will also come in handy for me, sooner or later. Thanks.

Thank you all for some very helpful and informative info.

I've just ordered a new IACV. I'll probably get a manual and take a look and do some of the stuff myself, both because I've had such lousy luck with mechanics and because it might actually be fun.

I'll do a compression check and timing check, take a look at the plugs (perhaps post pics here) and figure out which ECU I have, then report back.

Thanks much!

I think a new IACV will help, also check the hose/tube just after the air filter. I don't know you car very well but on my mazda it was the cause of poor mpg. I could only get 37. After it was replaced, tune up and timing help me get 47 mpg that i am getting now.

my california vx also gets mileage under 40mpg
 

i am the orginal owner,
i replaced plugs, o2 sensor, and had the valves adjusted,
with no big help to my mileage
i am running on "delta essex" tires at 41 psi

NO !!!! The PCV valve on the VX does not rattle. In fact if it does, you've got the wrong one on there. "The OEM PCV (Positive Crank Vent/Valve) is no more than an emissions elbow with no internals inside it" -TomO
https://www.gassavers.org/showthread.php?t=5559