Hey guys i got a 98 xj with a 4.0 and an ax-15 with 125,000 miles it started knocking when i would crank the starter but it only does it if the outside temp is below about 40 degrees. any ideas???? im stumped....  

Does it fade in a few minutes after engine starts?  

Nope it happens immediatly as the engine fires and usualy goes away after at idle ..... also how often should the spark plugs need replacing?....  

Kinda sounds like rod knock. Rod bearing are getting worn and oil drains out of them when engine is hot and it sits for a while and it takes a few seconds to float them again with oil pressure. What grade oil are you using and how often are you changing it?  

mobile 1 full synthetic 10w30  

Did you just switch to it recently and how often is it changed?  

ive run full synthetis since 2005. i change it about every five months  

SYN oil is generally not good for a older engine unless it has been run since new. It is thinner and can result in lower oil pressure and SYN oil still gets dirty (more often in a older engine) and needs to be changed. Many use SYN oil in a older engine hoping to renew it but it is not wise. At most maybe use a SYN blend oil or better still a good conventional oil and change it every 3000 miles or less in a older motor and you will get a very long service life. At this stage stop using SYN and give it a fresh change with conv oil and new filter and see what happens.  

4.0L's a notorious for piston slap, during cold starts and at idle you hear a slight knock, sometimes it is louder than others. I have had several of these and they all do it.