I have a 1994 Jeep Grand Cherokee. I replaced the spark plugs, wires, distributor cap, and rotor. I don't think the Cap and Rotor have ever been changed before now, by the look. Anyway, I've checked and double checked the firing order, and all of the wires are on their connections firmly. It idles and accelerates smoothly, but backfires and runs rough once it's up to speed. Any suggestions?
Check your engine codes. Even though the light may not be on, there still may be a code that has popped up in the OBD. One way you can check yourself is go to your car and with out starting the vehilce, turn the key on and off five times in a row continusly. After the fifth time, the check engine light should flash. Watch and count as it flashes. Its like morse code. It will flash a certain number of times then pause for a split sec and flash again. Then the light will stay off for a few more seconds and if there is another code, it will repeat the flashing sequence mentioned above with the next code. If you have a digital odometer display, sometimes it will digitally display the code along with flashing the engine light.
Two codes most commonly seen everytime you test is 12 which means in the last 50 times the car has started the battery has been disconnected. The other code is 55 which means that all the codes that are active have been displayed and its done.
If any other codes besides these two come up, take your car to Autozone and have them scan the car with their scan tool. Then they will help pinpoint you in the right direction.
Out of curiosity, has the transmission been changed lately?
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You can put in your gasoline tank injector cleaner which cleans and unclogs the injectors first. If this does not help you then check one of the spark plugs to see if it is getting any spark to the tip of plug but have someone to check one of the plugs for you safely. Then you can also check inside the distributor cap and rotor to see the connections have not turned black which means no connections to each spark plug when the rotor goes around inside the distributor cap and if you see the connections inside the dist. cap that you need to replace them both. If you take the old distributor cap and rotor to make sure you put the new one the same way you took off the old one by using a permanent black or white marker and make a straight line from one of the outlets where one of the spark plug wire comes from to where the alignment will help you put the newer distributor cap. You can also take a photo of the distributor cap but on top view and then when you take if off to also take a photo view of the rotor as well.
If the wires are in the correct order, also make sure that you used the correct starting point, otherwise the base timing can be off by as much as 60 degrees. Turn the engine till the crank damper timing mark is pointing to 0 or tdc on the timing scale. If the rotor is pointing towards the engine, turn one more time. #1 is roughly when the rotor is pointing towards the passenger headlamp. Order is in the direction the rotor turns when cranking. If the engine was running with no problems before the change, only other thing could be that you inadvertently unplugged something or the security system armed. Other possibility is that if you did the change due to a running problem, whatever was causing that finally failed. If when you remove the cap again you find that the rotor is damaged, check the dist shaft for side play. Are you getting spark?
CHECK ASD RELAY THAT IS AUTOMATIC SHUTDOWN RELAY IN THE POWER DISTRIBUTION CENTER UNDER THE HOOD IN ENGINE COMPARTMENT.IF ASD RELAY GOOD PROBLEM COULD BE CAMSHAFT SENSOR IN THE DISTRIBUTOR.TO SAVE TIME AND MONEY HAVE JEEP SCANNED FOR CAMSHAFT SENSOR AND CRANKSHAFT SENSOR FAULTS.
Poor gas mileage can have many roots but the best way is always to start with the ignition system. Have a look at the sparkplugs, if you've got the I6 4.0 engine it is easy to check them. Spark plugs are cheap and easy to replace. Also check the spark plug wires, be sure the connections are tight and free from rust. Check the distributor cap and rotor and if they are rusty/greasy clean them up. After you've cleaned and checked the ignition items (spark plugs, wires, distributor cap and rotor) you can move on to the fuel injection system.
Replace the air filter, it is cheap and most people overlook that single item.
When was the last time you replaced the fuel filter? A clogged fuel filter reduces power drastically which in turn affects fuel mileage.