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 Mazda MX-3 ECU Swap Help 
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Post Mazda MX-3 ECU Swap Help
Hey guys, I'm having a bit of an issue with my MX-3. Yes I still drive an MX-3, and I love it! :D For a while now when I first start it in the morning or after it's sat for a day or so, it runs like crap and the smell of gas is evident. So after getting sick of it I decided to bring it to my local trusted mechanics, and he came back and told me I need the bottom coolant hose that has 2 temperature sensors. This is where things get interesting. I did a KL-ZE swap last winter into this beast, and it's magnificent! The issue here, is that my MX-3 is a 1993, and from 1994+ they made a change to some wiring. In 1993 there was 2 temperature sensors within about 1-2 inches of each other, 1 controlled when the fan came on and the other I'll be honest when I say I don't know what it does. Well, since my wiring loom is from '93 and the engine is from 94'+, the engine only has 1 temperature sensor, therefore it's throwing a code once the engines warmed up about the missing signal, and as such my fan is always on when the engine warms up. My trusted mechanic is now telling me that that missing sensor also tells the engine how much gas to dump in the cylinders on start-up, thus causing the shitty idle and gassy smell. If that wasn't interesting enough, I'm about to blow some heads up in here. I THINK I have that pipe, but I MAAAYYY have thrown it out when I was making the old engine into a coffee table. My parts car I bought off a friend is a 1994 model, so that coolant pipe will only have 1 sensor as well. So now, our thoughts are if I just swap the 94 ECU over to the 93 body, it should take care of that issue right? Since the 94 ECU isn't looking for a signal from a second coolant temperature sensor it shouldn't have the idle problems and should take care of the fan issue as well. These are just our thoughts, can anybody maybe back this up? Has anyone had a similar issue? It seems to make sense to my friend and I, but that doesn't necessarily mean it will work :P

TLDR; will swapping a 94 Mazda MX3 ECU into a 93 Mazda MX3 fix a coolant temperature sensor problem and thus a crappy start-up idle as well

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April 09 2015, 9:43 AM
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Post Re: Mazda MX-3 ECU Swap Help
No you can't swap to the 94 ecu (and you don't want to) because of other considerations like your disty etc. Very simply drill then tap your existing coolant neck on the flat spot to accept the second coolant sensor. I used a shop vac to ensure no metal pieces got into the coolant system and I think I stuffed a rag into the neck as well. The sensor is not a pipe thread so make sure you use the correct tap to thread your hole. The process took me like 5 mins start to finish. Additionally there is no work around, you need that sensor. Good Luck

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2004 Subaru WRX Silver, stage 2, minty interior.
2002 Subaru WRX Blue, SOLD (best E test numbers I've ever seen)
94 MX-6. Sold
92 GS KLZE 5 Speed
96 GS 5 speed, KLZE, Sold
95 GS Minty Shape Sold
92 GS Sold
92 GS Parts Car scrapped.
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April 11 2015, 8:42 AM
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Post Re: Mazda MX-3 ECU Swap Help
I'd have to disagree with you there, cause I did the 93 to 94 ECU swap and it worked perfectly! The only issue I have now is a neglected exhaust swap. Due to running rough and super rich all the time from the 93 ECU the O2 sensors and EGR got all gummed up and it idles rough, sometimes will stall by itself when coming out of gear. It's throwing codes 15, 16, 17, and 23. I'll be driving it all week to get to work (don't have a choice) then probably this weekend I'll have to drive it the 3.5 hrs home so I can take the full custom exhaust off the 1994 parts MX-3 and that should solve all the issues :) Unless someone maybe knows of a temporary fix to make it run a little nicer? I know a ZE isn't meant to run an EGR but I just used the stock MX-3 K8 rear exhaust manifold so the EGR would still work. I've read driving with a bad O2 sensor and/or EGR may damage your cat too, but there's already a nice high-flow one on the other exhaust system so I don't really care about that.

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April 12 2015, 7:07 PM
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Post Re: Mazda MX-3 ECU Swap Help
Since the newer ECU seems to work fine, baggiespanther19, I'd say you should fix your error codes first.

15 LHO2S inactivation error
16 Exhaust gas recirculation (EGR) system
17 LHO2S inversion error
23 RHO2S inactivation error

PCME Fault Control Moding:

When any of codes 15, 17, 23 and 24 are set, the PCME falls out of closed loop mode and instead relies on calculations rather than feedback to control the A/F mixture. The effect will be lowered gas mileage, smelly exhaust, carbon accumulation etc.




Forget the EGR code, maybe think about buying a $5 block off plate for the head and a plug (large bolt) for your exhaust pipe. Buy two new O2 sensors from rockauto.com - Pretty sure they're around the $50 dollar mark each and they're the Denso OEM ones (exactly what mazda will give you except 3-4 times less the price). They will plug right in, hardly taking any time to fit them. THEN see how it runs before messing with exhaust for unknown issues. You're most likely running in limp mode with the ruined O2 sensors.

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April 28 2015, 4:52 PM
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Post Re: Mazda MX-3 ECU Swap Help
Just an update guys, my friend and I swapped the exhaust (which was a pain in the a--...had to dent in the oil pan a little to get the down pipe to clear...it's sitting tight against the oil pan now) and I discovered why it idled so poorly. I've attached pictures this time! In one of the spots on the stock exhaust where a hanger is welded on it had completed corroded, it was just held in place by rust and maybe 1/16" worth of steel. After taking the stock exhaust off I was able to just twist that hanger right off. The car is running beautifully at the moment, but still throwing those O2 and EGR codes. I don't remember how the EGR is attached to the engine, my friend connected it while I was under the car bolting the manifolds on, but he said it was pinched so it should still work. When I start the car, there's no codes, but when it warms up the CEL comes on but the engine still runs, drives, and accelerates like normal. I haven't checked the code, I'm just assuming it's still the same codes as before. Is it possible that the wiring from 93 to 94 for the O2 sensors to the ECU or how the ECU reads the O2 sensors was changed? I never had those codes with the old ECU and the old exhaust, so the next time I go home I'll be drilling and tapping a hole for the extra coolant temperature sensor, wiring it back up and switching from the 94 ECU back to the 93 ECU, and I'll see if that helps. RX8SE3P I'd rather not spend 100$ on new O2 sensors if it's not the issue! :P But then again they could be the issue and will need changing anyways...or maybe I need to switch back to the 93 ECU cause they made more changes than I thought they did...Stupid Mazda
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And the mod from the weekend, painted valve covers and intake manifold!! :D This was supposed to be an easy job....unfortunatley I also broke a fuel hose and fuel kept leaking from the banjo bolt you have to remove to take the IM off. Lost quite a bit of fuel there.....but it looks sweet!! :P "Powered by KLZE" oil cap is also in the mail, should get it in the next few days to top that mod off. Car is starting to look awesome :) This weekend: change rear brakes (one caliper pin is seized and pads are wearing unevenly and not grabbing properly), change rear strut mounts and adjust bottom spring seat to get rid of the noise from the rear suspension. Fun times! :P
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May 07 2015, 11:53 AM
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Post Re: Mazda MX-3 ECU Swap Help
Your O2 sensors won't just throw codes for no reason. They are most likely shot and in need of replacement. Had the exact same issue with my O2's. The ECU does not change sensor wiring. The only thing that can change from what I have seen is the distributor wiring.

You can switch back to the 93 ECU just to test but I'm sure your codes will show up again. I'd say leave it as the 94 ECU, change the O2 sensors and don't bother drilling the extra coolant hole. That'd be the least amount of work and best result in my opinion. :wink:

Oh and get a proper 2.5L curve neck manifold for your ZE lol :lol:

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May 08 2015, 8:23 AM
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Post Re: Mazda MX-3 ECU Swap Help
I'm interested to see how this turns out if you leave the 94 ecu in there. ZE has 22cc injectors while the K8 had 18cc injectors. New O2 sensors would help some what with fuel correction but you really need a KL31 or 36 jspec ECU or a probinator chipped ecu or go with a KL01 ecu with a KL02 vaf from a MX-6, 626 or Ford Probe.
Do you have to pass any kind of 'Clean Air' program where you live?

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2004 Subaru WRX Silver, stage 2, minty interior.
2002 Subaru WRX Blue, SOLD (best E test numbers I've ever seen)
94 MX-6. Sold
92 GS KLZE 5 Speed
96 GS 5 speed, KLZE, Sold
95 GS Minty Shape Sold
92 GS Sold
92 GS Parts Car scrapped.
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May 08 2015, 9:16 AM
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Post Re: Mazda MX-3 ECU Swap Help
RX8SE3P wrote:
Your O2 sensors won't just throw codes for no reason. They are most likely shot and in need of replacement. Had the exact same issue with my O2's. The ECU does not change sensor wiring. The only thing that can change from what I have seen is the distributor wiring.

You can switch back to the 93 ECU just to test but I'm sure your codes will show up again. I'd say leave it as the 94 ECU, change the O2 sensors and don't bother drilling the extra coolant hole. That'd be the least amount of work and best result in my opinion. :wink:

Oh and get a proper 2.5L curve neck manifold for your ZE lol :lol:


Hahaha, know anyone selling one? :P The only way I could manage to get one here would be a lucky find of an MX-6 or Probe at a junk yard :(
I should also point out something I noticed this weekend. The CEL does not come on when I'm just city driving, but as soon as I'm highway driving (you all know how it is....3,000+ rpm) it comes on, so maybe it's just EGR, I'm not sure. I'll pull the codes this afternoon after work. If the O2 codes are there well....I guess I gotta spend 100$ on O2 sensors!! :P

Daninski wrote:
I'm interested to see how this turns out if you leave the 94 ecu in there. ZE has 22cc injectors while the K8 had 18cc injectors. New O2 sensors would help some what with fuel correction but you really need a KL31 or 36 jspec ECU or a probinator chipped ecu or go with a KL01 ecu with a KL02 vaf from a MX-6, 626 or Ford Probe.
Do you have to pass any kind of 'Clean Air' program where you live?


No thank god there's no E-Test or any type of "Clean Air" program where I am. And even if they decided to start implementing it, I think my 93 MX-3 would be exempt due to it's age ;)
I'd LOVE to get me a Probinator ECU, or the ZE ECU from Japan. I actually tried to see if Mazda could order me a KL01 ECU....they can't, go figure! :P

Thanks for all the help guys! Much appreciated! Glad to know there's still a bunch of people that favor these fun little old cars :D
This long weekend it's the big one: stage 2 clutch and flywheel swap. I don't like how my ZE is running, just doesn't sound right some days, so I'm gonna steal the ZE from my parts car and put my tranny on it, cause the tranny on my parts car is shot...no 5th gear. As always stay tuned sports fans, I'll update tonight when I pull codes and this weekend with pictures and info on the swap! Can 2-3 guys get that job done in 3.5 days?! Only time will tell...

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May 11 2015, 12:33 PM
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Post Re: Mazda MX-3 ECU Swap Help
Small update for those that care :P It was raining like crazy Monday and Tuesday, so I pulled the codes last night when I got home...and she's throwing some really weird codes that don't really make any sense. The 2 codes that make sense are O2 sensor (LH side I think) and EGR valve, but then it's also throwing codes 02, 03, and 12, which are crankshaft, camshaft, and throttle position sensor. None of those make any sense though since the engine is running just fine, it's idling right where it should be, accelerates fine and everything. I'm not too worried though since we'll be putting a ZE in better condition in this weekend. Updates on that to follow next week! :)

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May 14 2015, 6:06 AM
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Post Re: Mazda MX-3 ECU Swap Help
Goooooodd morning sports fans! I can happily say the swap went well (for the most part) and the car is running again and behaving MUCH better! She's at a nearby Speedy getting an alignment as well as having the exhaust gaskets changed. When buttoning everything back up, we didn't have any exhaust manifold gaskets, or any gaskets to go on the exhaust spacers (right after the manifold there's a spacer so the downpipe can clear the oilpan...and even then it's juuustt touching the oilpan) so when it runs it sounds very restricted, almost like there's an exhaust leak. Doesn't sound like it's supposed to, I replaced the bolts connecting the spacer to the headers and the downpipe, and got a real nice shiner in the process thanks to incorrect use of a prybar on my part. After that I said f--- it, I'm done trying to fix that exhaust, I'll let a shop with proper tools and a lift take care of it. ANYWAYS! While doing the swap, we changed the timing belt on the donor ZE, as well as all the idler pulleys and the hydraulic tensioner. We were going to change the water pump too, but it was for a K8 and since neither of us were 100% certain it would fit/work well in a ZE we just decided to leave the current one in. It wasn't causing any problems anyways. We messed up the wiring for the reverse lights, so I don't have those at the moment (not a big deal), the shifter is too tight (my fault) so it's sometimes pretty hard to get it in gear, so I'll loosen those bolts a wee bit when I get the car back. I need to adjust the interior center console as well, as the e-brake lever doesn't like to go all the way down without some encouragement. But aside from all these niggly little issues, the car runs VERY well, the engine sounds healthier than I've ever heard one. And the best news, NO CHECK ENGINE LIGHT!!!! :D For the first time in the 2 years that I've had this car, I don't have a check engine light issue!! :D :D :D MAN is that f---ing awesome! For those wondering, yes I'm still using the 94 ECU, it's running the car well so I'm not changing it, specially since I'd have to drill and tap a hole for the 2nd temp sensor. The check engine light has only come on twice since the swap, once on the highway back home, but it went off when I had to stop for a light, and the other time was when it almost stalled cause I'm still trying to get used to the heavier clutch. It almost died, the lights came on as if the engine had been turned off, then it started back up on its own. I turned the car off and then back on and the light went away.
Now for those interested, PICTURES!! :D

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Green/Silver donor car, body, frame, and tranny no good. Engine, shift linkage, short throw shifter in good condition

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Just about ready to pull. My friend's first time pulling out an engine, he was pretty excited

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Engine out, he's pretty pumped :P

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my second time pulling one of these bad boys

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Timing belt and idler pulley work

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both engines out, swapping commencing

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working tranny on better engine, my local MX3 wizard helping me finish the job, the donor car was his before I bought it off him. Other friends Mazda 3 had its rear suspension collapse, he was on his way back to help us install the new engine, he never made it :(

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engine was in place and all mounted around 10pm :)

Don't have any pictures after the swap, I have a video of us starting it up with no exhaust connected. That ZE is LOUD with headers and no other exhaust connected :P If enough interest is show I can upload to YouTube and link it here. We also gutted the power steering system from my car, the donor didn't have power steering, the wizard had disconnected it and made it manual steering, as his power steering was either always making noise, leaking fluid, or just not working. Mine had just started leaking, power steering fluid with stop leak had stopped said leak, but I didn't want to deal with future issues. I had also use the reasoning at the time that "Race cars don't have power steering".....I miss power steering. The one issue that prevented us from starting the car right away after connecting everything: distributor. Don't EVER take your distributor apart and clean the contact points. Once we figured out that was the issue, we swapped the distributor over from my old ZE and it fired right up! :) So, I guess the original issue that caused the creation of this post has been fixed. It only took an engine and ECU swap to fix ;)
For all the future work, I'll be making a thread in the worklogs section. Not many more mods are planned for the rest of the summer, need to replace rear struts ASAP. Monroe's are dead already, only been in there about a month. Either got a shitty batch or they don't take well to lowering springs. Lifetime warranty though, so all it's gonna cost me to fix them is the price of a set of spring compressors. Aside from that, I'd like to take her to an autobody repair shop and have all the bubbling fixed up/painted. If at all possible, a Mazdaspeed bumper is something I'd like to entertain this year.

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May 22 2015, 11:24 AM
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