First off. Merry Christmas too everyone. Last night I came home with a 2000 sport 4.0. For the wife and as a second truck. My first wheels after the navy in 79 was a 72 commando, 77 renegade 5 with a 304 and a 79 J10. So after almost 40 years I have a jeep again.

I have a couple of restored vintage gmc trucks and know the gmc 228,248and 270's inside and out quite well so this 4.0 as they call it is easy to get around once I studied it all today. It has something wrong since the check engine light came on. Tried the turn key method it went through its paces on the dash and when completed it never displayed any codes? I got to Monday to sort it out so when I the truck back I know what's up before they do and they do know that I'm coming. Other than that it was taken care of from the surface. Barely off road if any. Oil all over the left front of engine compartment. The pcv line to intake has a hole in it and wrapped with electrical tape. Oil pressure lower than I like but haven't narrowed down for reason other than. 157000 miles of wear and tear nothing a set of bearings won't fix. I used the search feature to find some things with limited success. How to read codes with the key would really help and where I can hook up a vacuum Gage to see what the condition of this engine is and where to find a factory maintenance book.

My wife loves the upgrade and has been kissy kissy all day.

 

Welcome to the forum. I believe that the factory manual says that those 4.0's need minimum 7 or 14 psi of oil pressure.

Since you have an 2000 I would suggest to look into those heads on it. 2000/2001 models are well known for heads to crack.

 

I read about the low oil pressures and understand the swing in pressure readings based on a tight engine and engine speed. All my other trucks do that and they all have been rebuilt. People said gmc 6 heads were prone to cracking and they were and I understand why and how to prevent it. What is the reason these heads fail and how to prevent it? I change the oil and filter today and inspected the cooling system and neither show any signs of a cracked head. I go over this like I did with my antiques and with the help from the people of this site the jeep will be up to snuff one system at a time.

 

The factory changed the casting design in 1999 to adhere with stricter EPA laws or something. Somehow the design had the area between cylinder 3 and 4 utterly weak, and pretty often the heat would deflect it enough to cause a crack.

Best method of solution is to swap the heads to an aftermarket one or 2002-2006 TUPY heads.

 

Welcome from England, may you continue to get kissy kissy from the other half,
Merry Christmas and happy new year to you.

 

they stopped the dash code key trick in 98 i believe.
i have a 97tj and i can get the codes this way, but my 99tj and xj wont. neither will my 2000xj.
so you will need an obdII code reader to pull codes.

i wouldn't worry too much on that head cracking. not all do, just the ones that are neglected, so with some preventative measures, like, don't run it while it's overheating and don't let it overheat too many times without letting it cool in between. heck, maybe just don't let it get to overheating at all. lol

 

I tried the key method today. I got it to count up to 999999 and back down with the dash and gages and lighting up like a Christmas tree. When finished the only thing it did was to light up the check engine light and leave the oil pressure Gage reading 10 psi. What I have seen on here by searching the site is what I'd call mis imformation since I seen some b.s. replies. What I really liked was that the more experienced members called them on it. I have to look for a book today. Without it it's just like when I got my 51 gmc all I could do was look at it and wonder "where do I go from here"? When I got my 40 gmc I already knew the one and outs. One thing for sure the learning curve isn't as steep but can be just as painful.

 

Ran diagnostics heres what it displayed.
0300,random misfire
0301 #1 cyl. misfire
0442 Evap emission sys leak (small)
0445 evap emission sys leak large leak/no flow
0303 #3 cylinder misfire
0304 #4 cylinder misfire
I read somewhere you can see the crack through the filler hole. I couldn't see a crack when looking in the hole. What I did see which was barely visible was something green. I stopped there and called it a day because I know it's antifreeze and the head is cracked.

 

Welcome to CF!
Pull the valve cover.