I have a 2005 4.0 that has a P0301 code. Runs nice and steady when warming up but when it reaches temp it misses. I read many posts and checked the plugs, coil, and fuel injector all checking out good. I did a compression check with a warm engine cylinders 2-6 between 142-150, cylinder #1 30 psi, did it twice with same results. I squirted some oil in the cylinder and the compression only went up maybe 2 psi. The question is, is there any thing else I should check before I start removing the head and get it checked out? There is no coolant in the oil or signs of exhaust in the cooling system. Starts right up and no fog out of exhaust and it is a Tupy head. Any suggestions on what to look for and check? In the mean time I am going to remove the valve cover and see if I see anything out of the ordinary.
Low compression in cylinder #1 and P0301 code
- Thread starter Jeepbener
- Start date
Sounds like you have a bent, burnt valve, or a broken, collapsed spring. If it was rings the oil would have showed an increase in compression. You can run it to TDC on that cylinder and put compressed air in, listen for it exiting the intake or exhaust, it'll tell you what valve is giving you problems. You can also pull the valve cover and look for a damaged spring. You might get lucky and not have to pull the head but the odds are pretty slim. I had a 4.3 in a Blazer lose a cylinder, it was a collapsed spring and I was able to change it with the head on using compressed air.
The cylinder leak down test is a good tattle tale. How many miles on it? Some repairs are cheap and easy, some border on the cost of a re-manufactured engine. Know when to hold em and know when to fold em. Assuming by location you have a rust free Jeep, plan for the long haul.
The engine has 84000 on it , I wanted to ask before I set my mind to removing the head. I will bring the piston up to TDC and do the compressed air, maybe it will tell me which valve, I hope it’s a spring. Then I probably need to remove the valve cover so I can see the Valve springs. Motor runs good so even if I have to replace the head still worth it! Are these interference engines? I noticed there are two sizes head gaskets, one thinner than the other? Thanks for the advice!
It’s not an interference engine but does have this ability to become one if it drops a valve. I’d pull the valve cover first. You’re going to need to do it either way. Examine the springs on #1 closely. If the spring is broken or collapsed you can use compressed air to hold the valve up while you use a spring compressor to replace the bad spring. More than likely you’ll be pulling the head, but good luck either way.
You might look into the valve rotation fix. I think there were issues with the valves sticking and no longer rotating the way they should. I think you can manually rotate the valve without removing the head and it sometimes fixes a smaller leak. 30psi don't sound small but it might be worth a try before pulling the head.
I definitely have a valve problem, a crack in the head would have to be extreme to lose 120 psi out and antifreeze would be going somewhere which is not the case. It’s either stuck open, bad spring or bent. I read a post where the guy found a burnt spot on a valve causing it to give the P0301code, says he replaced the valve and it was fixed. lm losing to much compression in that cylinder. I will do as you suggest and remove the valve cover and check it out.
If you do have a burnt valve, I would drop the head at a machine shop and have them do a valve job. When a valve burns typically the seat also sees some damage. It should only run a few hundred bucks to have the head completely gone through and checked for cracks.
I did the the compressed air in cylinder # 1 lined up the timing marks and the air came out of the throttle body, just to check I rotated the timing one more time an lined up the timing mark with the same results, nothing out of the radiator . I took the valve cover off and I didnt see anything unusual . The push rods were easy to turn but no up and down movement.
Intake valve? Anything from the exhaust? Probably time to remove the head. Should it be just one burnt valve, I'd be inclined to have an entire head job done and I'd consider replacing the cam, lifters, timing chain and gears. The parts aren't expensive and it wouldn't hurt. Your mileage is low and perhaps just the lifters would be okay. I love excess however. Moderation is for monks. You should not have anything coming out of your radiator unless the cylinder was cracked through to the water jacket. If the air came from the block you'd suspect rings or piston. These are doughty engines, but millions have been made over the decades and occasionally a material flaw crops up. Price it out and also price out a re-manufactured engine with a warranty swap. It is good to consider options from every angle.
I will have to remove the head and I suspect a warped valve. The machine shop will probably be the ticket if everything checks out and price is right or get a complete head from Clearwater. New lifters on a used cam is not good, have to replace with new cam. I believe the engine is in great shape and I suspect it may have been overheated at one time. has a new water pump and the last owner gave up on why it wouldn’t run right, he did all kind of things to remedy, that’s why I got it for 3k, everything works! it even ran well except for that code. But I agree that now is the time to reseal the engine and replace those items that go out around 100k. I just want a good running Jeep that I can trust going into the mountains looking for dinner! Keep you all posted.
Sounds like you got a deal for $3k if this is the only thing wrong with it! You have a good opportunity to lay out a little cash & time and have yourself a real nice running TJ.
It's really rare for an Intake valve to fail before and exhaust valve due to the hot gases escaping past an open exhaust valve unless it's a spring. If it were mine, I would replace both springs on #1. And since you have a leak, compressed air won't hold them up to replace them, but here's how I did mine:
I placed the piston at the bottom of the compression stroke. Much easier to do with all the plugs removed. I then stuffed 1/4" cotton rope in the cylinder. I just pushed it in with a screwdriver. I probably put 10' (feet) or so in. I then turned the crank until the roped was compressed against the valves and holding them in place. I replaced both valve springs, turned the crank back a little in the opposite direction and pulled the rope out.
I placed the piston at the bottom of the compression stroke. Much easier to do with all the plugs removed. I then stuffed 1/4" cotton rope in the cylinder. I just pushed it in with a screwdriver. I probably put 10' (feet) or so in. I then turned the crank until the roped was compressed against the valves and holding them in place. I replaced both valve springs, turned the crank back a little in the opposite direction and pulled the rope out.
Last edited:
Hmmm, what or how did you come to the conclusion the springs were bad? I like your Idea, I’m about to remove the exhaust and intake to remove the head.
Similar threads
