ok guys , here it is. I bought an 03 jeep Cherokee 4.7 ho from an auction.
engine runs, but only on the back 4 cylinders.
with the engine running, I unplugged the coils one at a time.
the front 4 cylinders didn't drop the idle when unplugged, the back 4 did.
I then did a noid light test to ensure I was getting pulse signal.
all 8 cylinders had pulse.
next I check the ohms on all injectors. they were all good, and numbers very close.
next was a compression check. the back 4 showed #5-148,#7-150,#6-150, #8-146. the front 4 showed nothing. zero. the needle on the gauge barely bounced.
Guys, I am lost here. was thinking head gasket, but that wouldn't affect both bank heads, unless they both blew.
I could use some experienced advise here. thanks, Dan

 

Bent valves, piston rings, could be a number of things your gonna have to year it down to find out

 

That's where I was headed next, but figured it sure couldn't hurt to talk to folks who deal with these engines a lot more than I have.
appreciate the fast reply

 

That seems quite strange, and even interesting too.. Let us know what you find wrong.

 

Actually, this post needs to be moved to the Grand Cherokee section, but:

Did you hear the Jeep run before you bought it?

If you bought it with known engine problems, I guess you know what you have to do next.

 

OP said it came from an Auction so I would guess all he saw was it being pulled through for a few min.'s.

Strange the way its acting, I know very little about the 4.7HO except my "04" Overland Edition with 203k on it runs excellent zero problems with anything on it.

I would take it to someone who knows about them and have it checked out a total rebuild on them isn't much more than a 4.0 from what I've seen posted by a Jeep engine rebuilder in Tulsa, OK.

All I know is mine runs like a stripped azzed duck with the factory 3.73 gears and posi's. I know for a fact that it will outrun a late model GC with a Hemi in it. Something I should not have done but was one of those stop light things where I was kinda pushed into it.

 

Auction stuff is a crap shoot for sure. Some will let you start the vehicle during pre-inspection.

Zero compression on the front four, both banks, is weird. Is the 4.7 an interference engine?

I'm waiting to see if they bring out a Hellcat Grand Cherokee.

 

I would suspect failed timing chain with bent valves. The 4.7 has a primary chain from the crank to an idler (where a conventional camshaft would be) and 2 more chains from idler to each bank. Failed chain tensioner ...skipped chain...good luck

 

Its been a while, but I finally got around to taking the engine apart. this is what I found: the front 3 cylinders are damaged. #1 cylinder has a crack in the piston. new motor time.

 

the timing chain tensioner on the drivers side failed. I can push it in with my finger with ease. well dam, at least I know