I have owned mazda 323's and 626's before and always found them to be great and reliable cars and fairly easy to work on. I currently have a 1990 323 with a 1.6 that I use as a daily driver to run errands about town, and drive to jobs when on the road for my photography business in order to save money on fuel expenses. I got the car in '03 with 106,000 on it, and modified it for best fuel economy. Changes I made were as follows:
Opened up the exhaust, removed converter & muffler, running straight pipe. Dropped engine operating temp to 130-140 degrees by installing larger radiator with no thermostat and larger fan. Used hotter series Bosch platinum 2 plugs & 8 mm wires. Hooked up inlet of air cleaner to opening in bumper with flex hose for true ram air.
With these modifications, and regular maintenance the car gets 40-42 mpg highway, and approx 35 mpg city. It has been a great car, and other than normal maintenance items (tune ups, oil changes, brakes) I have never had to really repair anything on the car til last fall. at around 220,000 I started getting pressure in the radiator, and was losing coolant, the head developed a crack between the water jacket and an exhaust port. With the help of a friend, I changed the head, and replaced the head gasket, and the car was back to running normally again EXCEPT that it would not idle correctly. It would stumble and lean misfire, then smooth out and surge up to around 1,100 rpms, then start to drop back to around 500 rpms when it would start to misfire again, and repeat the process endlessly. Other than at idle, the car always ran normally once you gave it gas and had no driveability issues, mileage was still good. The idle issue appeared to be the result of a vacuum leak, but try as I might, I never could track down the leak. I suppose the intake could have not sealed properly, however it's an old car, with 290,000 miles now, and not worth the expense of having a garage track down the idle problem.
A friend recently said why not disconnect the O2 sensor and force it to run in open loop all the time, and see if that makes a difference. Once I did this, the idle is silky smooth again, no misfire at all, there is no difference in driveability or power. Without the O2 sensor connected, the idle is perfect. It dosen't appear to be running abnormally rich either. The jury is still out on fuel economy, as i havent driven it enough yet to see if there is a difference, my question is, would it hurt anything to run it in open loop all the time without the O2 sensor?
I'm wondering if it would make much of a difference in fuel economy, since the misfiring and running rough at idle probably wasn't that good for the fuel economy. Plus I would think that the misfiring would be washing the cylinder walls with gasoline, and that wouldn't be too good for the engine. Now it runs perfect just like it always used to before replacing the head, so I wonder if it might just be best to leave it like this?
Any input would be appreciated. Dave

 

Wow. You made so many changes that I couldn't even try to figure out what's going on.
Increased air flow: could lead to lean condition.
Reduced back pressure: affects gas flows differently at different RPM's.

If everything were as Mazda designed than open loop would keep it running slightly rich to keep the heads from getting too hot.
With the changes, who knows.

 

True, I had made changes to several systems, basically applying race technology to a street driven vehicle. It's just a matter of making changes that work together... reduced backpressure allows better scavenging of exhaust gasses, more effectively emptying the cylinders on each exhaust stroke, allowing for intake of more fuel/air charge on each intake stroke. This results in more power, increased fuel economy, and reduced combustion chamber carbon deposits. Dropping the engine operating temperature and removing the EGR allows use of hotter spark plugs and an increase in combustion temperature, resulting in a more complete fuel burn, and therefore better fuel economy. Increasing the amount of cool air into the engine also means a slightly better fuel burn and more power, as cooler air contains more oxygen for combustion. And of course larger diameter ignition wires ensure a more robust spark.
The car had always idled and driven extremely well with these modifications, with a noticeable increase in both power and economy. Indeed I had driven it for many years before changing the head, and never had any problems. The lean misfire and surging idle had been present from the first time I started it AFTER putting on the rebuilt cylinder head. So obviously, something happened during the process of replacing the head. Vacuum leak seems the most likely scenario to me, the friend who suggested disconnecting the oxygen sensor said the surging was likely the result of the O2 sensor detecting a lean condition because of the unmetered air from a leak, and the ECM compensating by adding fuel, til the mixture was overly rich, then leaning it out again. With the O2 sensor disconnected, the system isn't able to fine tune as much, and appearantly the fuel mixture it's receiving at idle is quite to its liking, as evidenced by the glass smooth idle. I tried revving the engine hard and didn't see any black smoke that would indicate an overly rich condition, and the driveability is unchanged from that with the O2 sensor connected. So i guess I'm wondering if my mileage would suffer much to run it like that all the time?
Dave

 

You seem to know what you are doing. Thanks for providing the reasoning behind your modifications. I learned something.

"reduced backpressure allows better scavenging of exhaust gasses, more effectively emptying the cylinders on each exhaust stroke, allowing for intake of more fuel/air charge on each intake stroke."
I think this is true for very restricted exhaust systems. However, the back pressure waves are tuned to improve cylinder scavanging and modifications will affect where the optimal range is.

I can imagine that the exhaust gases are hotter than normal even though the lower temp thermostat keeps the engine from overheating. I mean it is possible that the HO2 sensors are on the way out.
Also you could try finding the vacuum leak using an enrichment tool (MAPP gas) or using a smoke machine (cigar!, don't make it a habit, though).

I would expect a slight loss of fuel economy, 5% maybe.

A question to you: did you have any trouble codes after the mods but before the head issue?

BTW some people de-power the power steering to significantly improve fuel economy. But I don't know the proper way to do it.

Keep us posted. It is interesting what you are doing.

 

I learned a lot from my uncle who was a master mechanic, and built a lot of custom racing engines for people. I often worked with him in his shop.
I never did have any codes from my car, no check engine light, after doing the mods. I bought it in '03 from a junkyard as a daily driver to save fuel when doing a lot of city driving or running errands for my business. Gave it a thorough tune up and modified it for best fuel economy. I brought the mileage from around 340 miles stock to a (10 US gallon) tank to around 420 miles to a tank after mods. However, even though I give it a tuneup and new plugs every year, I must say I haven't replaced the O2 sensor since I first put it on the road in '03 with 106,000 miles on it.
Yes, running hotter plugs with more cool, dense air does result in hotter combustion and exhaust temps, thus the reason I run the engines so cool. With these mods running at stock temp of 190 degrees, you would eventually burn valves or pistons. Interestingly enough, a couple of the benefits of cool running engine and increased combustion temps seem to be longer engine life (most of my engines last 350,000 miles or more before they start burning oil or need any kind of overhaul) and greatly reduced deposits. I imagine the greatly increased exhaust flow also aids in reducing deposits within the combustion chamber. Inside the combustion chambers there is very little carbon buildup, usually just a slight amount of grey ash. Indeed, if you had seen the head i removed from the 323 at 220,000 + miles, you would probably never have believed it came from an engine with so many miles. It looked nearly new, the valves and chambers were very clean. Also, the reduction in temperature seems to keep additives in oil from breaking down, and with regular oil changes (3,000 miles) my engines develop very little oil related deposits internally.
The idle problem has always stumped me though, from the day we put the head back on. Never did find a cracked vacuum hose or port, the only thing I could think of was the intake did not seal properly at some point. Would the increased compression from a new head cause any changes in the way an old engine would run? Anyhow, I never did want to pull the intake back off, because I am disabled, and it would be a big job unless I had help. (And help is getting harder and harder to come by these days)
I just wonder if I should try a new O2 sensor, but it'd be a shame to throw away 20 something dollars if it wouldnt make a difference.
Dave

 

Thanks for the info Parrothead!

O2 sensors can be bench tested. They need to reach a certain temperature to emit a voltage.
higher compression will cause some changes. Why else would you want higher compression? It's probably harder to keep a low idle. Higher compression promotes ping. I would try a tank of higher octane fuel just to see. Many European small car engines rely on very high compression and "super" gasoline. It's more economic that way.