2000 Protege 1.6L Manual - Rough Idle and no power until reaches 170F

Hey fam!

This is somewhat of a humdinger, as I have been going nuts trying to find the cause.

The Car:
2000 Mazda Protege LX
1.6L 4-cyl
5 Speed Manual Transmission

The Issue:
The car runs and drives beautifully when first started. The issue is that once the coolant temperature reaches 170F (op temp) and the ECU goes into closed loop, the engine has a super rough idle, is misfiring, and has a serious loss of power.

Helpful information:
  • As I said previously, everything is great until the car goes into closed loop operating mode at 170F. I have a OBD2 logging software that shows the RPM instability as soon as it hits 170F.
  • Battery @ ~12.8v when engine off.
  • Voltage drops down into the 11's while running, but when hooked to another car while running, voltage is stable > 12v and issue remains.
  • Fuel trims are all over the place, I am not sure what they normally look like on this car but they bounce all over the place from -30 to 25.
  • For a few seconds after hitting temp, it will buck a few times if driving (Power, no power, then power again, etc)

What has been done so far:
  • Upstream O2 sensor replaced.
  • MAF sensor replaced.
  • New spark plugs
  • New coils
  • New plug wires
  • New intake pipe
  • New coolant temp sensor
Confirmed Good:
  • There are no vacuum leaks
  • MAF (New)
  • Upstream O2 (New)
  • Compression (Tested)
  • Plugs
  • Wires
  • Coils

Current Thoughts:
  • It is definitely a thought of mine that the ECU could be bad. Not only is it only acting up at temp, which is when the ECU actually does real work, but my OBD2 sensor is seeing updates at random intervals. Sometimes the sensors will update every second, sometimes it takes 15 seconds to update. When it starts taking 15 seconds, it usually does this consecutively for a while. Sometimes the data sticks for 60 seconds.
  • If not the ECU I am pretty positive that it is a sensor, as open loop before temp runs 100% fine.
  • It's possible the crank position or cam position sensors are failing, I will test those with a multimeter today.

My 2000 Mazda Protege operates great until it gets to exactly 170F (Closed Loop), at which time it idles super rough and has almost no power during acceleration. If driving when it hits this point, it bucks for a few seconds then goes to complete crap running.
Last edited:
I found that the MAP sensor is not reading much at all. It's reading -5 psi at idle and -3psi at WOT. The exhaust feels like a leaf blower so it is definitely running very rich. My guess is once it goes to closed loop, it begins running extremely rich from the incorrect MAP readings. I have a new one ordered and expected to arrive on Tuesday, so we will see!
More Updates:

Fuel Mixture Conditions

  • I drove the car about 30 miles today and noticed that the upstream O2 sensor was reading that the engine was running lean, then got a CEL code for O21 sensor fixed. I got out, popped the hood, and the exhaust was glowing bright red. A pretty good indicator that the engine is indeed running very lean.
Fuel Pressure Regulator
  • I have had in the back of my mind that the fuel pressure regulator could be bad and is not letting fuel through. Well, I went ahead and put some vice grips on the return line and the engine was still running lean, so it does not appear that is the issue. I also did a fuel pressure test from the pump directly and the pump is pushing about 90psi. I do not have a T to tap into the fuel line while connected to the engine to see what running pressures are, however, I speculate this is not the issue.
MAP sensor
  • This one makes me feel a little stupid, but I found that the vacuum line was actually off of the MAP sensor. I am not sure how it was still calculating pressures at all, assuming that the PCM/ECU was just calculating it based on other sensor information.
Hanes Manual
  • I got my hands on a Hanes Repair manual and am going to go around testing all of the sensor voltages for sensors that are related to the fuel map.
Have you cleaned out your EGR valve? Not saying that this is the smoking gun you're looking for but I remember my 1.6L running rough when the EGR was dirty.