Help please – misfire when hot

hobbypete · Post by **hobbypete** » April 16th, 2006, 8:04 pm

I could really use some help with a very frustrating misfiring problem. I call it a misfire, but I’m not sure that’s what’s happening. It’s a 93 MX-3 with the 1.6 L SOHC 16 valve engine.

The motor runs great when it’s cold, but after it warms up and runs above 3000 rpm for a few minutes, the problem starts. When I tip into the gas a bit, it hesitates and gasps. The car bucks violently unless I ease off the gas and feed it gently. Then, when it exceeds 3000 rpm, the engine is strong and smooth. When I park it after such a run, it usually stalls. It will then restart only by flooring the gas and even then requires rapid pumping of the gas to catch and stay running – usually only above 3000 rpm. At lower speeds its very rough, misfiring and hesitating – trying to stall.

I’ve modified it with a short, aluminum 3 inch intake and 6 inch K&N. There’s a 2-1/4 inch Pacesetter cat-back system. Everything else is stock. Partly to chase this problem and partly to bring components back into spec, I’ve replaced the Idle Speed Control Valve, the Purge Control Solenoid, the Fuel Pressure Regulator and related Solenoid Valve, the Oxygen sensor, the thermostat and the Coolant temperature sensor, as well as plugs, wires, distributor cap/rotor and ignition coil. The Throttle Position Sensor tests OK and looks good inside. The Air Flow sensor tests OK on all combinations of contacts except wide open E1 to FC, which is 0.3 ohms instead of zero.

I have also by-passed the Idle Speed Control Valve and Intake Manifold with the coolant pipes in an effort to solve the problem, but in vain. The spark plugs (NGK V-Power) turned black in the course of 2-3 test drives. The exhaust tip is also black. Compression is 180, 175, 135, 165 – ring job next winter! Check Engine light remained ON throughout all of this. I just read something on another forum about how to read codes with a paper clip so I’ll try. I can’t find a used or new Code Scout code reader.

By my reckoning, the only thing left would be the ECU or a bad wire somewhere. I have tested all the W/R wires to the solenoids, injectors, ISCV and main fuse and they’re all good. I am really stumped and would appreciate any suggestions or observations anyone might have.

hobbypete · Post by **hobbypete** » April 16th, 2006, 8:42 pm

I just read the codes by shorting the TEN and GND terminals at the Diagnostic (if those are the correct ones to short). I saw a 3 and a 9.

3 is no G-signal, which I don’t understand.

9 is temperature sensor, which is a new one. Actually, the new one is mounted on the thermostat housing. The other one on the cylinder head which drives the temperature gauge seems to work fine and has not been replaced. I have already tried disconnecting both of these and the misfire persists. Maybe I have a faulty wire on the new sensor?

Limegreen mx-3 · Post by **Limegreen mx-3** » April 16th, 2006, 9:44 pm

There is another temp sensor on the intake manifold for the sohc left side between 4th-3rd intake runners. The sensor on the thremostat housing is for ur fan and the small sensor on the head is for ur temperature guage on ur guage cluster inside the car. Normally when ur needle moves up and down its that little sensor on the head that goes bad. As for the code 3 i found this a (G) camshaft position sensor. Not to sure thats the right one (code) wired that it states its for DOHC when u have a sohc . I hope that help u a little.

ariesdude · Post by **ariesdude** » April 16th, 2006, 10:51 pm

Limegreen is right - There is a sensor next to te 1st&2nd fuel injector for SOHC

This is the one that is actually connected to the ECU -
http://www.mx-3.com/phpbb2/viewtopic.ph ... 4&start=15

You have to reset ecu after you replace the sensor.

hobbypete · Post by **hobbypete** » April 16th, 2006, 11:21 pm

Hey, thank you. You guys are the best. I saw that little sucker when I took off the injectors, but it didn't matter at the time. Then I nearly went blind reading the wiring diagram and didn't see that there were 3 temperature sensors. I'll bet that's it. I have to travel for a couple of days, but I'll check it out when I return. Much appreciated!

corbanbrook · Post by **corbanbrook** » April 17th, 2006, 5:49 pm

This is your problem.

replace that engine coolant sensor and all the problems you mentioned above will dissapear.. No more stalling, no more engine flooding, no more weezing.

Same thing happened to me and i spent a lot of money trying to figure out what it was.

jimishadow · Post by **jimishadow** » April 17th, 2006, 8:48 pm

it will never run rite with low compression in one cyl.......you need at least a head gasket.maybe a new head if yours is to warped or cracked...

hobbypete · Post by **hobbypete** » April 20th, 2006, 10:09 am

Problem solved, with the two-wire thermo sensor in the intake manifold. Thanks again guys! Now I’ll chase the code 3 by checking the distributor wiring. So far, it appears I’ve got a broken wire – whew!

The engine actually runs very well, in spite of the low compression in #3. As it turns out, 135 psi is exactly 75% of 180, which is the minimum spec. Nevertheless, I’ll do rings and head gasket one rainy day this summer or fall. Putting a few drops of oil in the plug hole improves #3, so I think we know what to do.

Thanks for all your suggestions and comments.