MX-3.com

Ok, so, about a year ago, I rebuilt the motor completely, and replaced tons of parts. Before the rebuild, the motor was burning oil like crazy, leaking oil, etc... however I was getting around 36mpg highway.

Ever since I put it all back together, I have not been able to get more than 24.3 MPG. It doesn't matter how I drive or whether it is highway or city. Just a constant 24 mpg.

It is timed properly using a timing light, the plugs and wires are all good, the fuel filter is newer, and I used a bottle of fuel injector cleaner.

It occasionally stumbles slightly.

Does this sound like a sensor that went bad, or does anyone have any other ideas?

I also replaced the O2 Sensor about 2 years ago with one from ebay.

Stumbling could indicate mixture problems, or burning lag. Both of which point to the plugs.

You could also have a vacuum leak. How does your intake hose look? On my wife's 95 we had to replace the TPS as well as the IAC along with the injectors (opted to just replace them all if one was going out). Try switching gas brands too... for some reason Exxon in North Carolina sucks in my mx3s.

Hmm... The plugs are less than a year old, and I re-ringed the new pistons, but I will check them.

About the intake, I replaced the hose with a custom Ram air.
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But, it does make a sound at low RPMs that sounds like it's taking in way to much air through the filter... I don't know what that is about.

that is a clean engine.
you might wanna check your plugs. if the car is getting bad MPG, then it could be the plugs...
also, i know this sounds off, but check the cat.
my miata used to get bad milage when it was completelly stock...
checked everything and nothing added up, till someone told me to check the cat.
it was clogged.
got a new one, and it not only gave me better MPG, but more power as well.
just things to look for.

OK, here are some more ideas to check off. lol

Now it runs bad when warmed up.

I just replaced the Plugs and the wires.
Maybe slight increase in mileage. (so far, the best is 26.7mpg)

Gutted Cat. Now it is a "custom" high-flow cat. lol
Maybe more power, but no better mileage.

Replaced temperature sensor.
No better.

I check compression yesterday, and here are the results.
#1 (155psi) #2 (156psi) #3 (150psi) #4 (150psi)

Something weird happened. After plugging the gauge into the #1 cylinder, I started it, and it ran for a few seconds, then died. I tested the compression on the other 3 just cranking it. After test, I made sure all of them were in tightly, and now it won't start. It's getting fuel, just seems like it's not getting spark.

Before I tested the compression, the car would run fine when cold, then after warmed up, the rpms would jump all over. It would run terribly.

Does this sound like the disty finally died? My other idea is the mass air sensor.

I ran into the same issue while doing a compression test. Somehow managed to fry the disty.

Wow. ok, well fortunately I have an extra. lol

I'll install it in the next few days.

Does ~ 150 psi sound pretty good?

I think the book said anywhere from 135 - 192

So, next question is why it runs terrible after warmed up. Mass Air Sensor? Injectors? I think that is all I haven't replaced. I'm running out of ideas.

You mentioned that you gutted your cat. You do realize that your car has 2 cats? A warm-up cat and a main cat. On top of that, gutting your catalytic converter will help neither your mileage nor your performance.

Correct me if im wrong but arnt you suppost to remove all of the plugs and disconnect the disty before doing a compression test.

I'm sure you have already done this but it doesent hurt to ask. Did you pull the codes? heres the how to: http://www.mx-3.com/phpBB3/viewtopic.php?f=24&t=65390

If you are not showing any codes you could have the injectors flow tested.

Did you gut both cats? We gutted my 95 first cat and replaced the second and that solved it for thee most part. Maybe a faulty alternator is causing a voltage issue creating the stumble? Kind of a long shot.

How's your fuel pump?

From what I can tell, it's good. When I turn the key, I hear it run. and, I smell it in the bay when I prime it.