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My 2004 Mazda 3 (JDM version, 1.5L engine) stalls when I give it gas or try to accelerate. If it doesn't stall then the rpms would go down to almost 0 and then rise back up. Dash lights comes on when it almost reaches 0. Feels almost like it's running out of gas. Also, when I try to rev it at idle the rpm doesn't rise smoothly. It would stutter and rise, and die afterwards. Also happens when driving. RPM fluctuates on WOT. After stalling, car starts up immediately after cranking.

These issues only happens for 5-6 minutes after a cold startup. After that period the car runs and drives like normal. I don't live in a very cold environment. The temperature outside is around 32 degrees C.

I understand the car is in open loop on cold start. So I cleaned the MAF sensor. It made no difference. The ECT sensor readings are normal and the TPS also reacts to my gas pedal input.

The weird part is, there are no CELs other than a faulty O2 sensor heater control circuit. That code was there for the past 3-4 years and never caused any issues. One time when I was having these issues a MAP sensor code p0108 popped up but went away soon afterwards. MAP reads about 1.0 atm on cold start idle, revving it causes it to rise to 1.6 atm. Comes down to 0.5 atm within 5-6 minutes and revving it then causes it to jump to 1 atm. Not sure if these readings are normal. When the p0108 code was triggered the MAP reading was above 2 atm.

I should also mention that the wiring harness was repaired by a mechanic after the car blew an ignition coil in one of the cylinders. The problems started soon afterwards.

Is my problem related to the the wiring harness? Or the ECU itself? Would a faulty wiring or ECU only cause these issues on cold starts? Because she runs like a champ after the first 5-6 minutes.

I'm attaching pics of a graph that shows the rpm, MAF, MAP, TPS readings during a stall event. Your help would be greatly appreciated because I'm seriously lost.

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  • Welcome to Motor Vehicle Maintenance & Repair! I can't tell much from your images, but as I understand it, the green line shows the TPS. The TPS should be very linear when activated. In the graph it appears to spike, then drop out as the RPMs drop, then spike again. I'd test the TPS to see how it's functioning. You can do this without the engine running, but the ignition on. Like I said, it should be very linear and react just as your pedal is pressed. A bad TPS could very easily be causing your issues. – Pᴀᴜʟsᴛᴇʀ2 Jul 8 '18 at 13:00
  • What are the fuel trims when it is warm at idle and wot? Although you have what seems to be an open loop issue, fuel trims are probably correcting something and that may be a good clue. I'm not sure abut the FMEM strategy with the MAF disconnected but it is easy to see if anything changes or not.(You may have to try and start it a couple times before FMEM takes over fuel strategy.) Also what app are you using? – Milison Jul 8 '18 at 14:36
  • Yes, green line is TPS. I was pressing the gas pedal during the spikes. – Jmahmood Jul 9 '18 at 3:51
  • The long term fuel trim is around -12. Short term hovers around -5 to +7. The trims increase a bit in WOT. LTFT would go around -5 to -7. I've tried starting without the MAF but it dies instantly. Ordered a new MAF and should be with me in a week or so. I'm using the 'OBD Car Doctor PRO' app. – Jmahmood Jul 9 '18 at 3:56
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Replacing the Crank Position Sensor solved the problems. Turns out the CEL for it only comes up when the ECU doesn't receive a signal from the sensor for more than 4 seconds while the MAF reading is equal to or above 1.5g/s. Turns out the sensor was giving wrong crankshaft position values when cold and therefore resulting in incorrect timing for the engine.

Hope this helps someone in future!

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