Engine runs rough especially on idle

ALL SENSORS GO BACK TO THE ECM(COMPUTER), THE OXYGEN SENSOR IS EXTREMELY IMPORTANT, TELLS COMPUTER CHANGE OR ADJUST FUEL DELIVERY, AMONG OTHER THINGS, FIX THAT PROBLEM, PROPERLY, THEN U CAN MOVE ON. NEW AIR AND FUEL FILTERS FOR SURE ! IS BATTERY FULLY CHARGED ? THE GAS WE BUY IS GARBAGE, ETHENOL IS KILLING OUR ENGINES, ADD SOME PREMIUM FUEL, NEXT FILLUP, ADD STABIL 360 FUEL STABILIZER TO TANK, ARE YUR SPARK PLUGS AND WIRES IN GOOD CONDITION ? TUNE-UPS DO HELP. FUEL INJECTORS IF THATS WHAT U HAVE, GET DIRTY, CLOGGED, NEED CLEANER ADDED TO TANK, OIL CHANGE ALSO HELPS, GOTTA GET ENGINE RIGHT, AS U WOULD FOR INSPECTION. OR YOU'LL FAIL EMMISSIONS TEST, RIGHT ? GOOD LUCK MY FRIEND

tranny could be geared differenty from your old one which sensors where replaced o2? map? egr? maf? (mass air flow) could be smaller drive shaft as in circumference.

Possible causes of your issue:

1. Clogged air filter
2. Bad fuel pressure regulator (most likely)
3. Sticky idle air controller (also likely)
4. Burned or stuck EGR valve, or bad EGR solenoid
5. Bad throttle position sensor (especially if the "Check Engine" light is on or flashing)
6. Bad oxygen sensor (especially if the "Check Engine" light is on)
7. Water in fuel or bad fuel

What to do:

• Start by removing the air filter to see if the problem persists. If the air filter is dirty, replace it.
• Next, verify that the Check Engine light is operating by turning the key ON but not starting the vehicle. The Check Engine light should be illuminated. If not, replace the Check Engine light bulb.
• If the Check Engine light is ON when you start the car, try feathering the accelerator pedal to see if the base idle stabilizes or changes. If it does, replace the TPS (Throttle Position Sensor). This is a fairly inexpensive part.
• If the Check Engine light is FLASHING, or ON, next you will need to have a local mechanic or your local auto parts store attach an ODB scanner to determine what error codes are present. This will help you pinpoint the issue.
• Once you have pinpointed and corrected any issues present using the ODB codes, you will need to attach a FUEL PRESSURE GAUGE to the fuel rail test point to determine if the fuel pressure is correct. If it is too high, the FUEL PRESSURE REGULATOR has failed. Again, this is an inexpensive part but the fuel system pressure must be released before servicing this part. Best done by a qualified mechanic.
• HAS THE CAR SAT UNUSED FOR A PROTRACTED AMOUNT OF TIME WITHOUT FUEL BEING REMOVED OR ADDED? If so, the condition you are describing may be a result of bad fuel. Modern fuels contain ethanol, which causes moisture to condense in the tank when it is not agitated. This can cause water to build up in the fuel, and causes the fuel system to rust as well as an idle issue that resembles what you are describing. Try adding new fuel, and add a full bottle of FUEL SYSTEM WATER REMOVER. Remember: IF FUEL IS ALLOWED TO SIT IN YOUR VEHICLE'S TANK (without new fuel being added) FOR MORE THAN SIX MONTHS, IT WILL GO BAD.

So many options...oxygen sensor, egr valve, pcv valve, premium gas.

I notice my 94 pathfinder runs rough and cuts-off acting as if it is fuel starved or it has a bad coil (spark not hot enough). When I jump statrted the car with a 200 amp Battery charger /booster the engine smoothed out perfectly. So, I assumed it is the coil or the wires since I have just replaced the distributor cap and the rotor, replaced the fuel pump and fuel filter less than 200 miles ago, and the car has been sitting most of the summer without being driven. Im checking the vacuum lines and need to consider that the fuel pump, although a brand new Bosch unit could have a problem or somehow the filter is clogged. Miles on this vehicle are unknown but could be 200k judging from the wear and tear , but engine has good compression . It could be the catalytic convertor is clogged, as I have seen cars run this way before (car had a carburetor though) when the convertor clogged to the point of not running at all- could not start it and run for more than 10 seconds at a shot. The last couple of times I have started the car to diagnose the problem, the engine developed a rattle that is audible at low speed(idle and off idle) thru 1500 rpm. Thinking it could be the tensioner on the timiing chain has loosened-up and jumped timing. Lots of things to check- may as well run down every possibility and get it over with.

On the right side of your intake manifold,you will see a round mushroom looking thing with a vacuum hose running to it. You can check it by reaching under it and feel for a fairly large hole. inside the hole, there is a metal diaphragm . While the engine is idling, push up on the diaphragm and the engine should run very rough. If it doesn't, replace the EGR valve. Thats what it's called. Very easy to replace. two bolts. good luck

Hi, Try to replace fuel filter. It maybe clogged up and making engine run rough or poor. Also check and replace spark plugs. A bad spark plug will make it run rough too. Good luck.

On the lean area look for a vacuum leak anything there,like PCV hoses and pull the plug on the EGR is there a difference?Is the valve hot at idle? Now check the rich side pull the hose off the fuel pressure regulator is gas leaking out? MAF cant do this, IAC cant do this ,TPS, If everything checks out no vauum no EGR leak No fuel regulator leak you may have to find out if the 02 sensor is reading the correct bank .Bank one could be on bank two, and bank two can be one bank one,It could be mismatched

A code P0303 may mean that one or more of the following has happened:

• Faulty spark plug or wire
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector
• Burned exhaust valve
• Faulty catalytic converter(s)
• Running out of fuel
• Poor compression
• Defective computer

Possible Solutions If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

A. P0401 "Exhaust Gas Recirculation Flow Insufficient Detected" that there is not enough exhaust gas being admitted to the engine.
There are a number of things you can check, and others you need a scan tool to check.
What you can do is check for vacuum to the EGR valve. You can remove the EGR valve and see if it is stuck closed. Test it with a hand vacuum pump and see if it opens smoothly. Check the EGR valve orifice and make sure it is free and clear. Check and clean the EGR passages in the going to and from the EGR valve.
If this checks out, then the Differential Pressure Feedback EGR system will need to be scanned to check the EGR sensors and switches

A code P0301 may mean that one or more of the following has happened:

• Faulty spark plug or wire
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector
• Burned exhaust valve
• Faulty catalytic converter(s)
• Running out of fuel
• Poor compression
• Defective computer

You replace alot of things but you should replace the EGR valve and the throttle positioning valve also. I would start with the EGR valve