Cannot get motor to run smooth

JODY BRAND

MEMBER
1993 MAZDA MX3
1.6L
4 CYL
2WD
MANUAL
210,000 MILES

Car ran great until a week ago. It started missing, sputtering and became harder to start. No check engine light on during all this. It cuts out going up hills, but smooths out after cresting the hill. If I am on the highway on a hot day it starts to sputter and loose power around seventy mph. When I get off the highway it runs rough at idle, sputtering and coughing. If I shut it off it is near impossible to start again. Still no check engine light. If it is cold out it will crank over but no start, I usually park on an incline and it will start by popping the clutch but not by turning it over. Mechanic friend said change coolant sensor so I did. Now check engine light is on, took that sensor back got another, same thing. Check engine light is on and car runs really bad. Trouble code flashes 9, coolant sensor. I have changed all three coolant sensors, the one screwed into the head on front of motor, the fan sensor screwed into the thermostat housing and the on in the intake manifold. The plug was bad on the intake sensor so I replaced it also. The sensor in the intake was the one I replaced twice, check engine light went out when I changed the connector plug, motor still rough running. I removed the throttle body and cleaned the butterfly, put it back together and now the idle is up and down between 900 and 2,000 rpm's and motor is still sputtering. I disconnected the coolant sensor in the intake and the motor does not hesitate but still has high idle. I reconnect it and the motor falls on its face. I am at wits end, can you steer me in the right direction?

1 Reply

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Hi and thanks for using 2CarPros.com.

The looping up and down in idle speed is most likely a problem with the idle air control valve (IAC). The idea that the coolant temperature sensor was an issue is not too far off. It tells the computer the coolant temperature, and from that, the computer determines how rich or lean to make the fuel mixture. With that in mind, I would suggest getting a live data scanner. Scan through to the coolant temperature sensor and see if the reading is correct. The temperature indicated should be very close to the ambient temperatures outside (if the car was cool and not used).

If that turns out to be good, the next thing I would check is fuel pump pressure. The idea that runs worse when under a load may indicate weak pressure from the pump. Here is a link that shows in general how to check pressure:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Here are the directions specific to your vehicle for checking the fuel pump. At the end, it indicates what the manufacturer's specifications are to be.
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LINE PRESSURE
WARNING: Before disconnecting any fuel line, release fuel pressure. When replacing fuel system components, keep sparks, cigarettes and open flames away from fuel. Plug hoses while they are disconnected.

1. Disconnect negative battery cable.
2. Connect fuel pressure gauge between filter and main fuel hose.
3. Connect battery cable.
4. Start engine and run at idle.
5. Line pressure should be 30 - 38 psi (206 - 265 kPa).
6. Disconnect vacuum hose at pressure regulator. Line pressure should now be 37 - 46 psi (255 - 314 kPa).

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For the idle issues, here is a general overview of the IAC and directions on replacement.

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

The IAC or idle speed control is located under the throttle body. Take a look at pic 1. Make sure it's connected and no damage was done to the wiring. Here are the directions to test the IAC / ISC. The remaining pic correlate with these directions.

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COMPONENT TESTS AND GENERAL DIAGNOSTICS
AIR VALVE

1. Connect jumper wire between diagnosis connector TEN and GND terminals.
2. Connect a tachometer.
3. Start engine.
4. Idle speed should gradually decrease as engine warms up and air valve closes.

IDLE SPEED CONTROL (ISC) VALVE
Idle Speed Control (ISC) Valve Testing
imageZoom/Print
1. Unplug ISC valve connector.
2. Measure resistance across terminals. It should be 10.7 - 12.3 ohms @ 68°F (20°C).
3. If not as specified, replace valve.
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Let me know if this helps or if you have other questions.

Take care,
Joe

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