OK, #1303 has been sitting for quite a while. I am just now starting to get it ready for the autocross season, so yesterday I fired it up for the 1st time in many months. It fired right off (with a jump start) & ran great for a about 10 minutes as I let it idle. Then, I went to move it & every time I gave it any more than say 20% throttle it would bog down & run rough. It kinda sounded like a 2 step set at 2000 RPM. If I backed out of the throttle slowly it would settle back to a normal idle. It was getting dark, so I just idled it back into the garage, shut it off & thought I would look into it more today. Well, this morning I go to try & start it (pretty cold out, 29-30 degrees), & it cranks over quite a bit, coughs a couple times during cranking, but won't fire. I try it again, only this time (& every time after) it won't even cough while cranking, but as soon as it stops cranking I get this LOUD backfire out the exhaust. The backfire actually happens AFTER the engine stops cranking. Any ideas guys? I took a quick look under the hood, but everything seems in place. Just thought I would post here before I start digging into the car with no real direction. Thanks in advance.

Too rich?

I checked the timing belt & all looks OK....it ran well yesterday in my driveway so I was pretty sure it didn't jump teeth or anything. I also went & checked all the electrical connections under the hood as well, but everything seems in order. Once again, the car has been sitting for 5 or 6 months, & before I parked it then car ran great. Does this sound like a possible computer issue? I put a used but rebuilt Eprom ECU in it about a year & a half ago after my original one failed....after being rebuilt & capped can they go bad again? This problem has me very concerned, I have never had a car backfire like this....after the engine has stopped cranking. It is LOUD, like a gun shot, I'm afraid it's going to hurt something. Logically thinking it seems like fuel is building up in the combustion chamber but the plugs are firing at the wrong time??? If that's correct, then maybe ECU, CAS, TPS??? I'm guessing now, just not sure where to go from here. Thanks again in advance for any help.

Since it's night time & freezing outside, I'm not going to mess with it until tomorrow, but I keep thinking it over in my mind. It's as if there is no spark while the starter is engaged, or maybe when the ignition switch is turned all the way to "start". But then as the key is released, it gets a quick spark since the engine is still slightly turning (& the ignition is still "on") & then BANG....it lights off the fuel in the chamber. I will confirm if there is spark while the engine is turning over tomorrow, but does that sound like something you checked on your car? The other thing I have been thinking is bad fuel. I run 100 octane unleaded through it, but I think it's like a year or more old. The fuel will obviously fire, the backfire proves that, but maybe not in smaller amounts....I don't know, I'm guessing at this point. This has me so puzzled, this car has always ran sooooo well.....I remember reading threads on here in years past thinking, "man I must have a good car because I don't seem to have all the problems other's seem to have". Guess I shot myself in the foot thinking that lol Well, I sure hope my problem isn't what you have, I don't have the time to rip this car apart & put it back together again chasing a gremlin. Anybody else have any ideas??

Is the car throwing a cel at all?
I'm wondering if the ecu temp sender may have chosen this exact moment to die on you.
That would explain the cranking problem but may not explain the backfire?

check ur cas and ignition timing

OK, I figured a couple things out & I'm making some headway, but still can't get it to start. It's got spark while the engine is cranking, & it appears to be getting plenty of fuel. I pulled all the plugs & not only were they pretty black, but they were wet with fuel. I must admit though, I have not heard the pump humming with the key in the run position & the engine not running, should it be? I can't remember, I know I've heard it when the engine is running. There definitely seems to be fuel in the engine, so I would think it is working.... Yes Steve, the backfire does seem to be caused by a quick spark AFTER the key is released from the start position. I think you are right, the injectors quit but there is still a slight rotation of the engine, enough to fire the plugs becuase the ignition is still "on". I'm guessing it pops because the engine has stopped turning so the pressure is forced out the exhaust of whatever valve is open. Just my theory, but I definitely noticed a spark after the key was released as I was checking it. So, now as I try to start it, I make sure to rotate the ignition switch quickly to the off position rather than the run position. Since I have been doing that, no backfire. I did try spraying some starting fluid into the intake, & with the throttle wide open during cranking I did get a couple coughs & sputters like it was trying to fire. So, at this point I am really leaning toward bad fuel, so I think tomorrow I am going to try & drain the tank & put some fresh 100 unleaded in there & see what happens. But before I do, I am definitely going to replace the CAS as I have a spare. No, throughout this whole ordeal, no CEL, but in all honesty I can count the amount of times this car has triggered a CEL on one hand. Considering it has a stock ECU & I run a lot of fuel & boost through it, you would think I would see them all the time. Aspod, what is a "blacktop"? As for the MAF, I am running a GM unit with the translator. I checked the connector on it & it seems OK, could it be the connection where it's tapped into the CAS wire? Anyone know how to test the GM MAF to see if it's working properly? Sorry guys for all the questions & trouble, once I get this thing going I plan to document the entire autocross buildup for the benefit of everyone....just need it to run first Thanks again for the help.

Quoting race2win:

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Aspod, what is a "blacktop"?

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CAS from 93-94 dsm (maybe more?). Its a hall-effect sensor that gives a better quality signal. 2g guys with 6-bolt swaps use them to help combat the "random misfire."

had the same problem with my galant not too long ago the coolant temp switch had a bad connection in turn the car would not fire and fouled out the plugs. the switch fails and reads -30 something and the car is a real bitch to get started

check the connections there, its possible the switch could be bad... if the car has spark fuel and injector pulse it should start.

OK, here's an update on this frickin car...... I read another post on here where someone else was having similar issues, & the coolant temp sensor was recommended to him as well. That seemed to cure his problem, so today I checked it. Sure enough, the wires going into the plug were all cracked & exposed. So, I put another sensor in from #733/1000, & I also cut the plug off & used nice fresh small female spaid connectors & shrink wrapped it up all nice. Still no start!! Backing up a little, this week I have also changed the CAS, coil pack, & power TR unit. When going to change the CAS, I noticed the nut holding the side where the brace is was loose (the brace that goes to the throttle body). The other nut was tight, & the CAS didn't appear to have been moving at all, but I thought I had found the problem. NOPE! My problem now is that I have had the 2 different CAS on it a couple times, & since it won't start I have no idea if it is on correct or 180* backwards. What sucks is now with everything else I try, I have to pull the CAS & rotate it just to make sure I don't rule anything out. Now, these parts I am using are from my other GVR4, #733/1000. This car has been sitting for many years & is pretty much a parts car at this point, but I figured what's the chances of it's parts being bad too....I don't know, maybe one of them is, I just have no idea which one. So, I'm not sure where to go from here. I do want to try a new coolant temp sensor, but my local auto parts stores don't have any in stock. Plus, the picture they show for it doesn't match what the OEM one looks like. Any ideas on this? Should I wait & get it from the dealer? Problem is the nearest Mitsu dealer is like 2 hours from here.... I also want to try another ECU just in case. I know that when my original ECU took a dump it had similar symptoms. The ECU that is in the car now is also from #733, but it has a sticker on it saying it was rebuilt in 2000. I just took it out of the car & took a look inside....no trademark fishey smell or obvious board damage, but who knows. This car has me soooo pissed off right now.....it sure makes me miss the Miata autocross car I had. I was so excited to get this project going this week & do things like changing the brakes, re-routing the intercooler & pipes, change the turbo, etc.....all to get it ready to win autocrosses....but now I have wasted a week chasing a gremlin with no end in sight, I could have almost been done with everything else by now This car is about to get parted out & I'm going to buy another Miata.......

This might help..

Yes, I completely checked all the wiring for the MAF, MAFT, & the SAFC wiring at the computer. No, I do not have a logger, but according to the SAFC the KAR values range in the 25-32 Hz area while the engine is cranking. It's not zero like yours was, but I don't know if the numbers I am seeing are correct. I will check the CASs again to make sure they are installed correctly, when you guys say "clean" them what do you mean? I cleaned the outside up, are you talking about cleaning the inside in some way?

I've had similar symptoms just recently after it sitting for a couple months after finally replacing a dead battery. It started right up, but ran rough and lopey, probably on 2 or 3 cylinders. While it ran, I pulled each plug wire and each were arcing away. I then started pulling fuel injector plugs and found one that caused no change to idle. At first, I thought "oh great, ECU damage killed an injector driver". But strange, though unlike yours, by the time it warmed up the faulty injector fixed itself and it's been running great since.

If this doesn't sound like you, then I vouch for coolant temp sensor and/or wiring. To avoid confusion, there are three temp sensors on the thermo housing. The important one is the two-prong sensor on the underside. The top one is for A/C, and the single prong one is for instrument cluster. I had faulty wiring and caused similar extreme rich conditions so bad that attempts to accelerate actually lose power.