P0300 and P0301

[William]

Ok. I own a 2003 Jeep Liberty with a 3.7l v6 that has about 113,000 miles on it. The other day I was driving the interstate and it started to shake really bad. I pulled off to the side of the road to see what was wrong. I put it in park and the shaking went away. As soon as I go to take off again, mind we are on the interstate, it begins shaking again. I pull over imediatally and shut the engine off to see what the problem is. Im thinking tranny. Well I turn the key over to the start position 3 times to pull the codes. But it ran perfectly after that. I got a P0300 and a P0301. I got these codes right after I changed the spark plugs. but i cleared them and they hadnt come back til now. Do you think that if I pull the ignition coil and switch it with another it will tell me if the coil is bad? What do you think that I should do?

Topic

[Wayne]

If it’s multiple, then it follows there is a common cause. You noted nothing odd when checking the plugs(coolant/oil/etc), yes?

To that end, the simplest answer is the usual cause. Is this just a recent thing shortly after filling up(Bad gas)? May wish to try running a bottle of “heet” through it.

[college man]

try doing a power balance test to narrow the problem.

[dreamer2355]

Watch the video’s that have already been posted for you and use some test equipment.

If you feel this is too difficult of a repair to diagnose, i would take it to a qualified repair facility.

Keep us posted.

[William]

I think that I have it figured out. ive checked everything i know but one thing. I believe it could be a vacuum leak. I dont know of any way to check for it tho. Can you all help me with this?

[Wayne]

Also as noted in another unrelated video of his which was more recent, a spray bottle with simple water works nearly as well without being as dangerous. You of course have to be cautious around ignition components, and more exacting with water being used.

[EricTheCarGuy]

Vacuum leaks actually have less effect the higher in the RPM range you go. Vacuum leaks will normally miss at idle. That said I’d be looking hard at the ignition system. I know you just did a tune up not long ago but lets just say not all parts are created equal. The reason I say ignition system is because if that fails it usually does under load and given that you’re driving at highway speed when this happens it’s possible that this might be the cause. In addition I don’t think a compression check is out of the question just to get an idea of what kind of health the engine is in. Here’s a video that can help you with that. If you have valves or rings that are starting to go no amount of parts you can throw at it will help so you might as well make sure the engine is good before you go much farther.

[William]

Hey Eric thanks for the input. I really do appreciate that you have taken the time to help me. I took the car to the lot where I bought it originally and they found a broken hose from my gas tank to the evap canister. We replaced the hose and all was dandy. The car was running better than the day I bought it. Now that its fixed the misfire still persists. This time with codes p0300, p0303 and p0306. Im really concerned. Do you still think it could be a compression leak at the cylinders? I mean what are the odds of all the cylinders having such bad compression?

[ScannerDanner]

So you didn’t have this problem until after changing the plugs?
This is a coil over plug system right? (6 different coils)
As someone else mentioned, start with the coil boots before spending money on the coils themselves.
It is actually common to cause a misfire problem after replacing plugs and not doing the plug wires or boots on COP systems.
I make it a rule that I do NOT replace spark plugs only. IF it is time for plugs, then it is also time for plug wires/coil boots

[William]

I changed the plugs because I had horrible gas mileage. I checked the gap on the plugs i pulled out and they were at .090 gap. I know for a fact they shouldnt be that big so I checked and they were supposed to be 0.040! Im no mechanic but Im not an idiot either and with an educated guess I would have to say that the plugs had never been changed. Where do I get plug boots from?

I also wanted to add that I only get the misfire when Ive been driving for a while. It seems to happen after I restart the engine while its hot. I cant even tell when it is misfiring. It just seems that the light wants to come on and say its a misfire. At least thats the code. I fixed an evap leak on a cracked hose and that fixed another engine code ive had for a while. now this code persists. My engine has 112,000 miles on it. Im also beginning to think that its low fuel pressure. I would geuss that since the original spark plugs were never changed that might mean the original fuel filter is still on there. Any Ideas?

Tests that I am going to perform:
Compression Testing
Fuel Pressure Testing
Battery Testing
Vacuum leak test

I plan on cleaning the throttle body

[William]

update: :S

Im starting to get a little pissed. I cant figure it out. I took a recommendation from a local mechanic and changed the coil packs around to see if the misfire would move. I still get a random misfire code and I only got a misfire code for cylinder number 3 this time. Well I left it with the mechanic today. Hopefully he will be able to find it. It really bothers me when the engine light starts flashing. The bad thing is that I have only had this car about 7 months.

[dreamer2355]

Ignition systems (particularly ignition coils)will fail under a load when there hot.

We had a vehicle in the shop that had a #1 misfire. It ran fine cold. Once the tech took the vehicle for a long test drive and pulled it back into the shop, it finally started misfiring.

Also stress testing coils can cause a misfire. Using a lab scope and looking at current ramps will be a big help.

Did you check fuel pressure as well as checking for vacuum leaks (sorry if you posted the answers to that question already).

[William]

As you can tell I am having a bit of a problem with this misfire condition. What you explained is exactly what is happening to me. When the engine has been driven for a while thats when it misfires. I dont drive more than 5 miles a day. So the engine doesnt have a chance to really get “hot” although by the time I reach my destination my temp gauge will register the normal temp. I took it for a “drive” today and drove about 20ish miles. The vehicle will do exactly what you stated but if I turn the vehicle off and back on while it is hot thats when the misfire light comes on. Im at my wits end trying to find this problem. How exactly do I do a load test on a coil pack? Is it possible to do it to all six with out the vehicle on? Also I never can feel the misfire… its just a code and a flashing engine light. Sometimes it will do it while climbing a hill. Today on my drive I got a p0303 on cylinder 3 before I got the p0300.:pinch: everyones input is very much appreciated.

[dreamer2355]

You would need to use a adjustable ignition tester or a lab scope for current ramps.

Here is a video from a quick Youtube search of a adjustable ignition tester for coils –

Also check your plug on the misfiring cylinder for carbon tracing.

I worked on a vehicle today that again only misfired when hot. I removed plug #4 (P0304) and there was evidence of carbon tracing on the spark plug porcelain.

You have a good idea in switching coils around to see if the misfire moves.

Keep us posted.

[William]

Today the mechanic I left the vehicle with said he was able to discover something strange. He said he was able to see the crank position sensor working part of the time. He said he was able to see this on his scan tool while he was driving. There is only one thing i dont understand. If the sensor is only working some of the time, why is it not showing a circuit problem? any ideas?

[jeff compton]

Im not going to go out on a limb and say that you may not have a crank sensor problem because I worked for almost ten years at a jeep dealer and changed lots of them but they normally didn’t fail like that in my experience! Most of the time if u saw it go in and out on the scan tool u felt the engine take a dip at the same time! It could be a slow refresh data rate on his scanner and nothing wrong with it! They can be a bugger to change and the aftermarket ones in my opinion are junk for some reason! I wouldn’t change it yet! For starters make sure u used the exact type heat range etc of sparkplugs that came out of the engine originally. don’t use Bosch or Autolite, use the champions or the NGK’s that came in it from factory! IT sounds more fom my expereince like the misfires that it has been experiencing for some time because of the wide plug gap has probably burnt the catalytic converter out! the 3.7’s and 4.7’s also had issues with valves burning! I’d suggest having a good diagnostics tech or the local jeep dealer take a peek at it!

[Author]

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