Nissan Repair: 1995 Nissan Altima GXE, fuel octane rating, nissan altima gxe

Question
My 1995 Nissan Altima GXE has been surging when cranked.  While driving, the car will miss intermittently.  The major problem is after driving the car and making several stops and starts, and it gets hot (the car gets hot, not like the engine boiling over), it will go down to around 1000-1100 RPM's. You can push the accelerator completely to the floor but the rpm's still will not go over 1100.  After the car sets for a while and cools off then it will run again.  Going down the interstate at 20 mph because the car will not increase the rpms is not good.  The codes are showing a knock sensor code and a rear O2 sensor.  Are these the culprits or not.  Do not want to replace is not.  I have changed the spark plugs.  They appeared to be running lean. Thx.

Answer
Hi Sam, let's start with the knock sensor. When the knock sensor detects pinging, it sends a signal to the ECU and the ECU retards the timing. When the timing is retarded, the engine runs hot and has no power! But before we replace anything, lets see if it's actually the knock sensor's error. Is the fuel octane rating low? Does the engine have carbon build up on the pistons surface that increases the compression?  Is the ignition timing over advanced? Is the exhaust system partially clogged up due to a collapsed ex. manifold or a damaged catalytic converter? To check for clogged exhaust system which significantly reduces engine performance, do the following test: Connect a vacuum gauge to an intake manifold vacuum port. With the engine in operating temperature, increase the RPM to around 2,000. If the needle points to a 0 reading, the exhaust is blocked. If all of the above check out normal, then the knock sensor is faulty and it needs to be replaced.

Now the o2 sensor. The sensor may detect a lean or a rich exhaust gas and it sets a trouble code. Make sure that no adjustments have been made other than the factory adjustments. Check for vacuum leaks (lean signal) Check for clogged air intake system, such as dirty restricted air filter, dirt in the intake manifold components etc. (Rich signal) As a rule, I will replace both O2 sensors at 60,000 miles anyways as it is also recommended by the manufacturer. So, go ahead and replace it. This way you will not have to connect a digital volt/ohm meter set to 2 volts to check if the voltage on the sensor's signal wire varies when the engine goes into closed loop.