"They claim it's a VX block with an EX/Si head"

Did you keep your old motor? Maybe you could salvage the old head.

Here's another idea: swap your motor. Presumably there are tuner-types out there who have a D15Z1, and would much rather have a D15B.

Or sell your whole car to a tuner-type, and buy a more original VX. To you, the D15B is a bug; to someone else, it might be a feature.

I've confirmed that the engine is a D15Z1.

Tire size is slightly wider (165, I think, instead of 155) than stock, with stock rims.

The issue with the bent valve is related to the timing belt. I did have the timing belt replaced because of the age of the car and mileage, but the mechanic who did it was an "apprentice" at the shop, and the shop owner found the bent valve and thought it might have been related to the work on the timing belt, so they replaced the head, charging me only for the part without the labor. However, the mileage was still crappy before the timing belt and head were replaced, so it seems unlikely that's the culprit.

When I took it in to get the idle problem diagnosed, a *different* shop thought it was a leaky head gasket, and replaced it, but then discovered it was the IACV (so didn't charge for the head gasket replacement.) They (supposedly) did try taking the IACV off and cleaning it out and putting it back... and it's better than it was but it's still got an unsteady idle.

It does seem odd that the 02 sensor went bad (it was bad when I bought the car)

I don't think there's any issue with brakes dragging. They were recently replaced, and I've never had any problem coasting.

The car does have A/C (I bought it used off of ebay and added it after purchase, so it may not be VX air conditioning, but it works fine)

I did have issues with overheating which turned out to be the water pump. Since then, I've had no problems with warmup problems or overheating.

And I don't see any problems with the exhaust.

So it seems like the bad injector or something funky with the plugs might be a good place to look?

"Tire size is slightly wider (165, I think, instead of 155) than stock, with stock rims."

The stock size is 165/70-13. Do you really have 165? These days it's an unusual size, and lots of VXs are seen with something larger (like 175/70). Anyway, what's your tire pressure?

"it may not be VX air conditioning"

I don't think there's anything special about the VX a/c, so what you have is probably fine.

I notice some folks have worked on your car who weren't terribly skilled. One shop bent a valve, and then another shop gave you a head gasket you didn't need. Even though they tried to make it up to you, those are still fairly major errors. So I think you should do something it sounds like you haven't yet done: ask around and find out who in your town is really, really good at working on Hondas. And tell him what you've told us. Hopefully he'll be too polite to say 'you should have come to me a long time ago.'

I would think you would already have a good mechanic. Who took care of your prior VX, all those years?

Anyway, there are certain basic things that should be checked. Your head has been off twice, in a short period of time. Did anyone do a compression check? And I wonder if anyone has bothered to make sure your ignition timing is correct. Has anyone checked the air filter? Are you sure the plug wires are OK?

I'm sure a good Honda mechanic would think of other things to check, that I'm overlooking.

"So it seems like the bad injector or something funky with the plugs might be a good place to look?"

My hunch is that a bad injector would create worse symptoms, like hard starting, and bad hesitation. But looking at the plugs is easy, and the condition of the plugs can tell you something.

I had an AMAZING mechanic when I lived in the east bay area. He was that rare combination of being incredibly knowledgeable about Hondas specifically, very reasonably priced, finding the ways of doing the repair that represented the best value.

I've been looking for someone like him since I moved to the Sacramento area but haven't found anyone yet. Needless to say, I'm not going back to either of the two places I've tried so far (the timing belt place or the head gasket place). I'm reasonably knowledgeable about engines -- I used to own a Volkswagen shop years ago -- but these days, unless it's something relatively simple, I usually like to take it somewhere and have it repaired.

As far as I know, a compression check hasn't been done on the car since I've owned it. That does seem like a sensible idea. I didn't worry too much given 80K miles when I got it (my last one was going strong at 185K when it got squashed in an accident) but that seems like a sensible place to start looking.

If anyone knows of a particularly good Honda mechanic in the Sacramento/Folsom/Fair Oaks area, I'd love to know about him or her :)

I'll also see about checking the plugs, either myself or if I can find somebody that seems competent with Hondas.

Let's focus on the poor fuel economy. Again, this must relate to fuel sprayed into the engine if there are no fuel leaks. To get the best mpg, the ECU needs to precisely control the AFR (air fuel ratio) with engine input sensors instead of base maps and to do that it must monitor and control the LAF sensor. It seems to me that you will need to check the AFR which can only be checked indirectly with the dinosaur OBD1 system. The LAF sensor can tell you what your AFR is. The LAF is involved in AFR control only when the ECU is in closed loop operation. It's possible - although unlikely - that your VX is stuck in open loop. That's unlikely because engine temperature is the primary determinant of when the ECU switches from open to closed loop. That means your coolant temperature sensor input to the ECU would be faulty. Should get a CEL if that was the case.

It's also possible your engine has a problem with generating power due to low compression in one or more cylinders which would force the ECU to enrich the mixture to compensate for lost power. A compression check is easy enough to do and so is a leak down test. I am surprised / shocked that the mechanics who've worked on your car did not do these tests considering your fuel consumption problems.

Too bad you don't have the time or desire to work on your VX yourself. Professional mechanics treat your car like their second cousins while DIY mechanics treat their cars like their own children. I don't know how much money you've spent for repairs on your VX the past couple of years but it might be time to find something newer that is easier for mechanics to diagnose problems. And that my friends is OBD2.

I'd figure that if IAC valves are getting gummed up, that I'd check the crankcase breather and for oil in the intake... if it's oily, replace breather filter (sometimes you can cut kitchen scouring pads to shape) and replace the PCV valve. Just because it rattles doesn't mean it's works right, there's a biasing spring in there that gets weakened by long term exposure to moisture and slightly acidic vapors, the metered orifices get eaten away, end result is that pistons are operating against slight positive pressure instead of slight vacuum. That makes for about 5psi more effort needed in the cylinder to make the same output. You also get relatively more ring blowby, because the pressure differential seals them. Then because at low throttle openings the pressure in front of the airfilter/TB is slightly positive, if your PCV spring isn't keeping the valve open, the crankcase pressure will have to go a psi over positive to force it's way out. Another clue is if you open the oil filler at idle and get a faint pshhh of pressure and/or thin smoke. Anyway, the combination of raised crankcase pressure, poorer cylinder seal, and gumming up of TB sensors and actuators can make for quite a dump in fuel economy.

If it is not a VX head you won't see the mileage, regardless of the block.

regards
gary

Calchip: Sorry...I shouldn't have thrown my problem into this thread. Disregard the D15B stuff...I'm going to start a new thread on it. Found lots of material on it.

Sounds like you have a stock VX, right?

cal: "I used to own a Volkswagen shop"

Then you probably know a lot more about motors than I do.

"I usually like to take it somewhere and have it repaired"

I had been doing that with my other cars in recent years, but I find that the VX is fun to work on.

"I didn't worry too much given 80K miles when I got it"

I agree with you that I normally wouldn't bother doing a compression check on a Honda with so few miles, provided it was running well.

"I'll also see about checking the plugs"

You've got an option that didn't exist years ago. Yank the plugs, take a photo, and post it here. Hopefully someone will chime in with a helpful analysis of how they look. I'm not an expert at it myself, but I think plug appearance could potentially reveal a lot, in a situation like yours.
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jadz: "engine temperature is the primary determinant of when the ECU switches from open to closed loop"

And you'll also be in open-loop when the throttle opening is large, right? And O2-sensor voltage will indicate when this is happening, right?

This is something I'd like to understand better, so I appreciate the chance to learn something from you.
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road: "replace the PCV valve"

I think the VX doesn't have one. It has an elbow fitting that looks like one, but there's nothing inside.

At least that's my understanding. Someone please chime in if you know I'm wrong.

Yeah Monroe, opening the throttle to accelerate quickly will take the ECU out of closed loop. And the VX LAF sensor will be at negative voltage when read from the D14 and D16 pin outs when accelerating. This voltage will fluctuate though so you need a DSO to really see this output change with throttle angle.