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With the coolant sensor plugged in the engine it sputters and does a big air gasp at half throttle, does not do it with coolant sensor.

Tiny
JODY BRAND
  • MEMBER
  • 1993 MAZDA MX3
  • 1.6L
  • 4 CYL
  • 2WD
  • MANUAL
  • 210,000 MILES
I was having trouble with a rough idle so I replaced the coolant sensor and its connecting plug. When I go to start it with the coolant sensor plugged in it sputters and does a big air gasp at half throttle. If I unplug that sensor the car runs quite a bit better but of course the CEL is on. Now the sensor I am referring to is the one located on the intake manifold. Is that the only coolant sensor that gives feedback to PCM? Or do the other two sensors meaning the single wire that screws into the head and the fan switch on the thermostat housing also give feedback to the PCM? I am at wits end, when the car is parked the throttle seems to work fine, I put it under a load it sputters and falls on its face. Going uphill it cuts out if I push it farther than half throttle. Give me a direction to go here. I am going to replace the fuel filter today and check the fuel pump pressure also. I have replaced every other sensor on the car. I am going to the salvage yard to get another ECM from another car like mine. If that does not fix it I have no clue. Help!
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Thursday, September 6th, 2018 AT 9:37 AM

20 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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Hello,

It sounds like you have a defective engine coolant sensor. We should run the codes to see what comes up. Here is a guide that can get us started:

https://www.2carpros.com/articles/diagnostic-trouble-codes-for-1995-and-earlier-mazda-cars-obd1

It sounds like you may have low fuel pressure.

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

Check out the diagrams (below). Please let us know what happens.

Cheers, Ken
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Friday, September 7th, 2018 AT 3:54 PM
Tiny
JODY BRAND
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The diagram of the car you posted is for a six cylinder car. Would you possibly have one for the 1993 1.6l 4 cylinder car?
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Saturday, September 8th, 2018 AT 9:58 PM
Tiny
KEN L
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Opps, here is the diagram for the four cylinder but I do not see the location. It should be near the thermostat housing.
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Sunday, September 9th, 2018 AT 12:21 PM
Tiny
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Okay, I checked the codes again, nine blinks, coolant sensor. I have changed the coolant sensor three times and changed the plug. Some other site said to cut the plug off and twist the wires on the harness side together, if the check engine light goes off then the problem Is the plug. Well that did not help, the check engine light is still on even with the wires twisted together. Still getting nine blinks after clearing codes. Motor still sputters and hesitates with coolant sensor plugged in and unplugged the motor has immediate throttle response but starts to fall on its face about 3/4 throttle. So what next?
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Wednesday, September 12th, 2018 AT 2:16 PM
Tiny
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And it is misfiring at idle.
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Wednesday, September 12th, 2018 AT 2:18 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi and welcome back. Ken is out of town so I am going to jump in.

The code you have indicates there is either a short or open circuit. Chances are there is either a broken wire (bad connection which you already checked) or the wiring to the PCM is damaged. I have attached the flow charts for testing this circuit. If you are unable to enlarge the pictures or cannot read them, let me know.

Picture 1 shows the code and its meaning, two is cut from the page which explains what could be the issue, and three is the flow chart and wiring schematic.

The misfire is a result of the fuel mixture.

I hope this helps.

Take care,
Joe
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Saturday, September 15th, 2018 AT 6:57 PM
Tiny
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Thank you for your help, both of you. I will let you know what I found out.
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Sunday, September 16th, 2018 AT 10:00 AM
Tiny
KEN L
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Please let us know what you find. We are interested to see what it is.
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Sunday, September 16th, 2018 AT 1:18 PM
Tiny
JODY BRAND
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I checked the resistance on the brand new coolant sensor out of the car and it was 1.6 right out of the box. It is supposed to be 2.7 I was told. I also checked for continuity in the plug and there was on one side of the plug. I am starting to think the ECU is bad maybe. As soon as I plug in the coolant sensor it runs rough and bogs out when I hit the accelerator. Unplug it and throttle has immediate response but car misfires. Any further thoughts?
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Thursday, September 20th, 2018 AT 2:21 PM
Tiny
KEN L
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Yep, the ECU sounds bad to me. I would get a used unit and plug it in to see what happens.
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Thursday, September 20th, 2018 AT 4:24 PM
Tiny
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Okay, I will let you know what happens.
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Thursday, September 20th, 2018 AT 4:29 PM
Tiny
KEN L
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Please do.
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Friday, September 21st, 2018 AT 10:06 AM
Tiny
JODY BRAND
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Well, I bought a new used ECU off eBay, plugged it in and plugged the coolant sensor back in after checking for continuity which it had. It starts fine but tries to stall out until it warms up a little. Checked the resistance on the coolant sensor and it was fine. When idling if I push the gas to the floor quickly it gasps for air and wants to shut off until I completely let off the pedal, then it idles normally with intermittent misfiring. I have looked for shorts in the wiring but to no avail. I just checked the air idle valve and when I pulled the plug off the rpm's did not change. Can I take it off and just clean the shaft going through it being it is a sealed unit? I will check the resistance on it when I pull it but need to know what the resistance is supposed to be. Thanks.
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Monday, October 8th, 2018 AT 4:52 PM
Tiny
JODY BRAND
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Oh, the coolant sensor wire has the correct voltage going to it. What resistance if any does the fuel pressure regulator have between the two terminals?
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Monday, October 8th, 2018 AT 4:59 PM
Tiny
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I forgot to mention that I am getting no codes at all.
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Monday, October 8th, 2018 AT 5:07 PM
Tiny
KEN L
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Good to hear, I would drive it for a bit so the computer has a chance to relearn. I would service the IAC and throttle bore. Here is a guide to help:

https://www.2carpros.com/articles/throttle-actuator-service

Make sure the tune up is current as well.

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Please let us know what happens.

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Tuesday, October 9th, 2018 AT 10:37 AM
Tiny
JODY BRAND
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Will a car start with the MAF sensor unplugged? Mine did and ran for about ten seconds then it shut off. The engine was at operational temperature. If it is not supposed to start I think I may have found my problem finally.
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Wednesday, October 17th, 2018 AT 2:47 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Welcome back:

If you unplug the MAF, the engine should continue running. The computer will resort to a default. Basically, it uses a fake signal or internal memory in place of the MAF. Since it stalls when you unplug the MAF, I feel the problem is elsewhere. If the sensor was bad, it would most likely stall when it was plugged in. I read back through this thread. You mentioned the coolant temperature sensor (the new one) showed the wrong resistance. Using a live data scanner, confirm that the temperature received by the computer is accurate. Also, have you checked for vacuum leaks?

Let me know,

Joe
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Wednesday, October 17th, 2018 AT 4:49 PM
Tiny
JODY BRAND
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I have checked three coolant sensors all out of the vehicle, two were used and the third was brand new out of the box. All three were the same temperature, all three had the same resistance. I have checked all the vacuum lines, no leaks. I do not think the brake booster is leaking either. I step on the brake and the RPM'S drop maybe just a little. I am about at wits end. I have never had a car out smart me until now. It shows no codes, yet does not run like it should. It will not start cold unless the coolant sensor is plugged in, it will not run half way decent unless it is up to operating temperature and coolant sensor is unplugged. Otherwise with it plugged in and motor is at operational temperature if you step on the gas the motor completely cuts out until you let off the gas completely, then it goes back to idle speed. When it is cold and you go to start it with the sensor plugged it will start on the second try, idle goes up to 1,500 rpm, idles smooth, then starts to drop after about ten seconds and shuts off unless I barely tap the accelerator. I have to tap it about five to seven times and then it will idle roughly. Once the temperature gets up I unplugged the sensor and I can drive it. It does not run great but well enough to get around. I tried plugging it in while it was warm and it acts intermittently like it wants to run like it used to but then it goes back to misfiring and cutting out. I took the throttle body off and cleaned the bore and took the air idle off and cleaned it good. Checked the resistance on the air idle it is within specs, checked the throttle position sensor its working fine. My next move is fuel pump and filter and fuel pressure regulator. There are two sensors on the manual transmission, one is on the passenger side of the transmission and has had the wires broke off would that be causing all this? My reverse lights work as well as the clutch safety switch. What is that sensor for?
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Thursday, October 18th, 2018 AT 3:25 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Welcome back:

The component you are referring to is the neutral switch service. I cannot see how it could be causing this issue. See the attached picture.

If the fuel pump pressure is within the manufacturer's specs, a new one wont make a difference.

You mentioned a code 9. Either you have a short in the ECT wiring or a wire is broken and no signal is being sent to the ECU. I hate to say it, but it is time to start tracing wires to see where that issue could be. Make sure the pins at the ECM are clean and not corroded, bent, or pushed into the plug. If it all is good, you need to check continuity for an open or a short.

I have attached a diagnostic flow chart for this problem. Follow it step by step. It is picture 3. Let me know what you find or if you have other questions.

Take care,
Joe
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Thursday, October 18th, 2018 AT 5:16 PM

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