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Old 01-30-2007, 07:15 PM   #1
sd88supra
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Question hesitation, high cold idle and poor gas mileage

After getting 12 mpg, I took my 87 n/a to my mechanic to look for fuel leaks. He didn't find any, and told me it was most likely a bad o2 sensor. I replaced it, and it improved to 15 mpg. Still, its getting expensive as gas is a daily driver.

I thought that perhaps my mechanic missed a fuel leak, but after putting in a new o2 sensor the fuel odor went away. Also, whenever I go to fill my tank I hear the pressure come out. He said that given the high pressure in the fuel lines, even the smallest leak would be easily detectable.

Last time I changed my plugs and plug wires (in fact full tune up) was right around a year ago, 9k miles ago. I always keep my tires inflated correctly. And I've noticed that as my fuel economy has gone down the drain, my car idles around 1300-1500 when cold. When its starting to warm up, it idles around 1000 but a little unstable. Until its at ideal running temperature it idles fine. I also get hesitation when I accelerate (when cold, from 1500 rpm to about 2400 rpm), and whenever I keep it constant at 2000 rpm.

Plugs gone bad? I did a diagnostic and got code 71 (which I've had for a long time), but I had it even when I had good gas mileage.
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Old 01-30-2007, 11:24 PM   #2
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i have the same problem what's a code 71?
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Old 01-30-2007, 11:43 PM   #3
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code 71 is EGR malfunction. its not very specific, and i havent been able to figure out what exactly is wrong.
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Old 01-31-2007, 04:32 AM   #4
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Well, if the EGR is stuck open a tiny bit, your idle can increase; cause cylinder misfires (lowered MPG); and can give you some running troubles.

The EGR passages on the 7m usually will plug up with miles, and then you don't get any EGR gasses thru. This problem is usually not detected (nor cause troubles)until you have to go in for an emissions test. It causes high NOx readings.

The main running trouble this plugged or inop EGR condition can cause is pinging under hard acceleration. This is due to extremely high combustion temps, which lowering it is why the EGR is there.
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Old 01-31-2007, 07:08 AM   #5
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Quote:
Originally Posted by mrnickleye
Well, if the EGR is stuck open a tiny bit, your idle can increase; cause cylinder misfires (lowered MPG); and can give you some running troubles.

The EGR passages on the 7m usually will plug up with miles, and then you don't get any EGR gasses thru. This problem is usually not detected (nor cause troubles)until you have to go in for an emissions test. It causes high NOx readings.

The main running trouble this plugged or inop EGR condition can cause is pinging under hard acceleration. This is due to extremely high combustion temps, which lowering it is why the EGR is there.
I used to get good gas mileage even with the error code 71. It wasn't until recently that my cold idle went up (could outside temperature have anything to do with it?) The hesitation I've always had (around 2000 rpm), but it has gotten worse. Could it be that the EGR has worsened? In fact, the harder I accelerate the smoother it runs. No pinging. I'm gonna check my timing belt. I fact, I think I'm gonna go for a rebuild soon. I'm starting to show signs of a well worn engine.
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Old 01-31-2007, 07:55 AM   #6
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Try cleaning your egr valve first before anything else, mrnickleye is giving good advice there. Id clean mine for my crappy mileage but I don't have one installed and my bad mileage can be attributed to my recent installation of pace setter headers. the minute increase in performance doesn't compensate for the more recent trips to the gas station. I lost over 50 miles a tank and made sure the O2 sensor was flush when they installed it. Its only back by about 2 feet.
Maybe the performance increase is due to my wallet getting lighter and we all want lighter supras.
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Old 01-31-2007, 04:11 PM   #7
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Before you go ripping it apart to stare at the timing belt why don't you run a cylinder compression test? A very easy, cheap and accurate way of determining if the engine needs a rebuild. Low or uneven compression will rob you signifcantly of mpg.

My EGR valve passages were half restricted by carbon build up, could not even stick my pinky in the passage. I still saw 18.7mpg on a turbocharged motor.
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Old 01-31-2007, 11:59 PM   #8
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There is also a cold temp sensor that helps with the cold start injector timing and richens the fuel when cold. If it always reads cold then it will always see the need to run rich. Try checking that sensor.

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Old 02-01-2007, 03:24 AM   #9
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where would this sensor be located? the high idle only occurs when cold. But it warms up fine, and when it does it idles normally. I'm also starting to suspect that my cat is plugged up.
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Old 02-01-2007, 05:34 AM   #10
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Wink See info on this in FAQs section.

Quote:
Originally Posted by rnoswal
There is also a cold temp sensor that helps with the cold start injector timing and richens the fuel when cold. If it always reads cold then it will always see the need to run rich. Try checking that sensor.

Russ
Not correct.

This sensor is called the Cold Start Injector Time Switch, pg FI-112 in TSRM. It only operates during cranking and engine that is below 56*. Never after the engine is running.

When active (only while cranking the motor), it supplies 12v to the Cold Start Injector, which will open and squirt a bunch of fuel into the intake manifold.

To prevent flooding, this switch will stop working automatically (heater element inside it)if you crank the engine over for more than 12 seconds, and it hasn't started.

Quoting myself: The EGR passages on the 7m usually will plug up with miles, and then you don't get any EGR gasses thru. This problem is usually not detected (nor cause troubles)until you have to go in for an emissions test. It causes high NOx readings.
So......
You won't have idling or running troubles with plugged EGR passages, or inop valve.
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