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Discussion Starter #1 (Edited)
My 93 Altima (stock) is having a strange problem. First of all, it is hard to start, it will turn over for a good 5-6 seconds every time before it fires up. But it runs fine once its going. However, once the car has warmed up, the idle becomes very low and choppy, dropping as low as 500 rpm at a red light and the engine struggles to stay running. The strangest part is once I shift out of gear (it's an automatic) into park or neutral, the engine will drop even lower and occasionally stall completely unless I give it some gas. When I come home from work at night, which is only a short 10 minute drive down city roads, I have to turn off the headlights, rear defrost, and the fan or the rpm's just fall to zero when I shift into park. When I let off the gas in this situation, the engine speed falls to the point where it almost stalls, then speeds up again to 7-800 rpm, then goes back down. This usually continues until it falls low enough to stall on one of the cycles. After some highway driving the issue becomes even worse, the engine will very nearly stall even while still in gear, and stalls when not in gear 100% of the time unless I give it some gas. When I try to start the engine when it's warm already (like if I just stopped to go into the store) it will turn over but not actually start unless I give it a bit of gas.

I did recently take the car to my mechanic because one of my cylinders wasn't firing. He replaced a bad fuel injector and told me to run some injector cleaner through, which I did, and that's approximately when this problem started. At the same time all the spark plugs were replaced so that probably isn't the issue.

Any ideas what the problem could be?
 

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sounds like the coils in your injectors are bad. to test them remove the electrical connectors from your injectors and take an ohm meter and touch the two leads to the prongs on your injectors, it wont matter which is positive or negative because there isnt a ground, the injectors should ohm 11-12 at your engines running temp. if you need injectors i recconmend motor man fuel injection, thats where i bought mine they were $34.99 each ask for Howard
as far as the injector cleaner, that is the worst chemical you can run through that particular injector because they have saturated coils meaning the fuel passes through the coil winding of the injector. the coil is coated in varnish which is what the injector cleaner is made to break down which in turn kills the injectors.
 

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Discussion Starter #3
Well I checked the injectors and they all come in the 11-12 ohm range. So I guess that isn't the problem. Any other ideas?

For now I've made the cord that goes from the gas pedal to the engine a little bit more taught, so even when my foot isn't on the gas it idles a bit faster, around 900rpm. It seems to be working for now but I know it's not a permanent fix. Braking feels different and the engine still seems a bit rough even at 900 rpm.
 

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I had a very similar problem. I ended up taking my car to a local shop after three weeks of trying to self diagnosis the problem. They hooked up the computer and gave me a full diagnostics. Cost $75 for a full diagnostic. My Mass Airflow Sensor was bad. The sensor never threw a code, but the readings where 3 to 4 times higher than they should be. I replaced it myself with a rebuilt one and my problems went away. Cost $217.99 and it took all of 10 minutes.

After you check all the basic Engine tune up stuff, (air filter, spark plugs, fuel filter, timing) here are some other things to check

Check Battery voltage while car is running and not running (should be over 14 volts when running and around 12.5 volts when not running)
Clean the Throttle body.
Clean the IACV assembly. Difficult to get to but worth a cleaning. I cleaned mine and it definetly helped. Also check it for proper operation of the IACV assembly. It helps to remove the oil filter first, you need to get at the assembly from under the car and from above the car.

Check the distributor for oil leaks.
Check the Throttle position sensor.
Check the coolant temperature sensor
Check the air intake temperature sensor.
Check your fuel pressure.
Knock sensor.

Most likely, from your description, the problem is a "closed loop" issue. When the car is cold, it operates under "open loop" which is a set of predefined conditions and ignores feedback from the MAF, and other sensors. After the engine reaches operating temperature, the ECM switches over to "closed-loop" control and adjusts the Engine based on sensor input. The "closed-loop" condition is where your problems are so look closely at the sensors (TPS, MAF, Air intake temperature, Engine Temperature) or one of the system the ECM controls like the IACV. Problem is not likely to be with the Vacuum lines or the O2 sensors.
 
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