Bad Seal? All cylinder misfire codes!

I have a 4.0 '96 XJ. Check engine light has been on since about october. I do not have an OBD scanner so I take it to autozone. the first time I took it there were 4 codes: Bad O2 sensor, random misfires, and 2 specific cylinder misfires. I didn't have the time to put the new sensor in so i just ran it for about 2 months

I just got it checked again and this time there was a misfire code for every cylinder. I put the O2 sensor in yesterday and I haven't noticed any change. The light is still on and every once in a while it flashes... A friend told me the head might be cracked, or there is a bad seal.

My dad told me that his car once had a bad seal where coolant leaked into the oil. I'm thinking this might be what is going on but I do not know much about the 4.0 and if that is even possible with it. I have to refill the coolant about once a month and whats funny is that there is no puddle or even any signs of a leak anywhere. Its been 2,000 miles since I changed the oil and I don't remember it being particularly runny.

Can anyone diagnose this? Thank you. This is my first post, I've been reading this board for a while now and I think it's a really great community!

*edit:
I forgot to mention how its running. It runs real rough at idle but the rpms are normal i think (under 1k?). Not that it has anything to do with this but there is a vibration at 45mph and i believe it is my u-joints which I will be replacing as soon as i have the time.

Sounds like you've got an internal coolant leak, such as a blown head gasket and/or a cracked or warped head. Does it over heat?

well when i first got it it was overheating but stupid me, i never checked the coolant. it was completely dry! So probably every month since september of this year i've had to add about a quarter of a gallon of coolant-water mix.

Well I think your right i just read online that a leak in the head gasket could ultimately result in coolant coming out of the exhaust and ruining an oxygen sensor, i think i should check that out before the new sensor goes bad if it hasn't already.

How's your oil look? Look like a chocolate milkshake? How about the tranny fluid, like a strawberry milkshake? If either of them do, that confirms (among other things) where the coolant is going, if you're not seeing any obvious external leaks. However, if it is mixing with the oil, it might be at just a small enough rate, that it's not obvious by a visual inspection of the oil (a sample would need to be sent off for testing in this case).

How many miles on the XJ? Have you tried pulling the plugs (especially #2 and #4) to see what condition they're in? You can tell a lot about whats happening by the condition of the plugs (running too rich/too lean, coolant leaks...etc etc).

Almost 160,000 miles. when u say check plugs 2 and 4 from where do u count? from the front backward? or the back froward? I will do this tomorrow probably, and yes the oil is kind of chocolatey i guess you can say.

What is the probability of this being just a bad head gasket to a cracked head?

So far all signs point to YES there is an internal leak.

If i'm not mistaken it's from front to back. The 96's have a pretty solid head on them, and unless it really got cooked I don't think it would be cracked....probably just a blown head gasket. However a it is hard to say for sure over the internet. One of those "have to see it" deals.

At 160K, I would go ahead and replace the head gasket. While you've got everything apart, it might not be a bad idea to send the head off and have it resurfaced, provided it's still in good enough shape to be reused.

Edit: Also, when was the last time a good tune up was done on it? Might be time for a set of good plugs, wires, and a new cap and rotor. I would first get your coolant issues taken care of, and then move onto a tune up.

How much? and is replacing the gasket a DIY job for the novice 17 yr old car enthusiast? I have some basic knowledge... Sorry for all the questions

Musjunk22 said:

How much? and is replacing the gasket a DIY job for the novice 17 yr old car enthusiast? I have some basic knowledge... Sorry for all the questions

Click to expand...

To be honest, I've never had to do any head work on an engine (only 19 yrs old here), but on the 4.0, from what i've read, it doesn't seem to be that complicated. Do some browsing around on here, I know its been discussed numerous times.

not to be devils advocate, but have you checked your fuel pressure? symptems in the 1st post kinda sound like low fuel pressure.... just a simple check to rule that out.
i never like doing some huge job then finding out it didn't fix it...