2001 Jeep Grand Cherokee 4.0 4x4 misfiring~?

[Savannah]

Yesterday, I drove all around town, everything was fine. This morning I wake up, start the jeep, and its fine in drive but as soon as I put it in reverse, its shaking, the check engine light is on and blinking. Now we have had our fair share of check engine lights, but it has never blinked before! So, I went to auto zone, they say its a cylinder misfire. I am not too vehicle savvy, so I ask what causes that. The guys said wires, spark plugs, or injector. So, I take it to my dad, ask him to check it out, can't find anything with the spark plugs. We start taking the injectors out one by one to see the difference in the idle. We get to the last one and there is no difference. My dad says he is pretty sure its that injector. Long story short, we replaced it and still having same problem. Its lunging, hesitating when I press accelerator, when stopped at a light, its shaking and vibrating horribly! Any suggestions? I don't know much about vehicles and my dad isn't too familiar with vehicles with computer systems...I feel like pulling my hair out!!!
Thanks for the answers so far. When we took over payments on the jeep, it sat for a year or so. We then did a full tune up, I believe in August of 09, replaced spark plugs, air filters, flushed everything, tires, we probably spent around 2K in repairs and maintenance. We had a vacuum leak then, got it fixed, but my dad did mention that there is a leak now...but I don't know where. I also noticed past few days, oil pressure at 80 when warm, 40 when cool...no overheating at all!
lol Theg8j, from what I am reading tonight on this issue, I should have never agreed to pay off this vehicle! I don't think I will buy another!

 

[TheGr8Jeff]

You can do all the things that Andrew R. said, right on track. But before you do all that, I would go to some place like Pep Boys and get a neon test light for the fuel injectors. They don't cost much and the instructions are on the box. Easy to do. All this is a process of elimination before you go through all that other stuff you want to make sure that you have power to the injector.There may not have been any thing wrong with the old injector, it just may not have been getting any power. Good Luck with Jeeps you need it.

 

[AndrewR]

Step 1 : Do a full tune up on the Jeep, its probably due for one anyway. Replace the wires, plugs, coil, cap and rotor, air filter, oil change.

Step 2: Check the fuel pressure going to the rail. Most jeeps have a tester valve right on the fuel rail. If its below like 35-40 psi, look into replacing the fuel pump/filter.

Step 3: Get a compression tester and test the compression in each cylinder. Take all the spark plugs out, disconnect the fuel injectors, disconnect the ignition coil, hold the throttle open, and crank the engine with the compression tester in each cylinder. If one of the cylinders has lower compression than the rest, you have found your problem. I dont know the mileage on your jeep but bad compression in a cylinder could be caused by many things: burnt/sticky valve, bad piston ring, head gasket, cracks in head/block...all which have significant additional symptoms.


Since you didn't mention any other symptoms like smoke or watery oil, and you didn't mention any overheating, there is most likely a simple reason for your misfire. When you unplugged that last injector and it made no difference in idle, it could be that the fuel is going in just find, but the spark plug isn't firing or there isn't enough compression for there to be a significant enough explosion. Start with the tune-up. Hopefully that will fix your problem. If it doesn't, you'll want to look into the compression, the fuel pump, or perhaps a vaccuum leak somewhere around that misfiring cylinder.

 

[g_insane]

Have a compression check done on each cylinder.