Help with 4.7 jeep valve problem

-
aarong5

aarong5

Aaron Grimes
Joined
Nov 7, 2009
Messages
343
Reaction score
9
Location
London Ky
I bought a 2000 jeep grand Cherokee with a missfire cyl 8 put coil, fuel injector,and plug in it. Still missing. Someone told me it could be a valve sticking, found out on the internet that you can put a piece of paper up to the tailpipe and if it tries to suck it up the pipe then your valve stuck. Tried that and sure enough and try to suck the paper up. I bought some valve doctor stuff, and put it in the crankcase also-ran seafoam, through the crankcase and intake. At this point when you first started the engine runs great and passes my paper check. Continues driving great until you shut the engine off for about 10 minutes then it misses again until the engine completely cools down, once cool when you start it runs fine.
Another issue is having is the oil pressure gauge drops to 0 after the engine run for a minute. I'm not sure if the gauges messed up or if it's really dropping, but I've drove it for 20 30 miles with no issues with the gauge reading zero. Any tips to getting a valve permanently freed up?
 
4.7's are known for valve seats dropping, and rockers falling off, also they have very small oil passages so if maintenance was neglecting you could have a slugged up motor. I would pull the valve covers and take a Peak might be a burned exh valve.
If all the valve stem heights look good along with the rockers might want to try some motor flush as a last resort before tear down.
 
The oil psi gauge on my 03 dakota is not even a true gauge it's an idiot light made to look like a gauge it's never moved off the exact spot in the 230k I have driven it! Not sure on your jeep.
 
If the valve was burned, they wouldn't run right ever would it? Also the oil pressure gauge is either at 40 pounds or 0 so I figured that the gauges probably not accurate but it does make me feel better about the oil pressure
 
Terrible oiling design, passages too small, oil pump in a horrible place, no cam bearings so if you are running low oil pressure the cam could wear out the head. The rockers are bad about slipping off too. You may have sludge build up.
 
Good deal on the paper test; works great with a dollar bill BTW.

As said, the seats fall out in some 4.7's (NOT on all of them; one local machinist here says if you get to 150-220k miles and they're good, then they are good for life), but if it did that, you would have biiiig problems. The lifters are also kinda easy to fail; if one pumped out some, it could hold a valve open a bit.

You may have a bad head gasket. That will explain the #8 misfire indications, and running poorly when hot, and maybe the paper flutter. Is your temp gauge showing hot after running for a while? IN that, case, it is certainly the head gasket. We had the same thing on #8 and it was the head gasket. The rads tend to clog up rather easily, and if you overheat once or twice, then the head warps and the head gasket goes.

Does the oil pressure gauge just suddenly drop to zero and stay there? If so, it is likely the sender or ECU. The ECU has current sensors on some of the sensor lines (inside the ECU), and if they sense the current spiking up on that line, it decides there is a short on that line and shuts off the associated gauge circuit. I know this is the case for the fuel gauges, and it sounds like your oil pressure gauge circuit is doing the same. If you drove it with 0 oil pressure, you would not go more than a minute or so before the thing started coming apart.
 
Apply air to that cylinder with both valves closed. Where the air exits from is where your problem is.
 
I'm not convinced that old trick works right. Yrs ago I had a guy call me and said he had just bought a car and it was running rough and was sucking a paper up to the exhaust and was always told it was due to burned/stuck valves. I went over with my compression gauge to check it for him and when I pulled out the plugs they were worn so bad some had an .080 gap. I tried it before pulling the plugs and it did in fact suck up a paper to the exhaust easily when idling. I checked compression and all cylinders were plenty good to fire so we stuck a new set of plugs in it and it ran great and no longer sucked the paper up to the exhaust.

As for the oil pressure gauge oddities I have a 02 Dakota with the 4.7 and my oil pressure gauge works like it should, not like and idiot light meant to look like a gauge. If your engine truly has zero oil pressure there's no way it'd make it 20 miles without blowing up. My guess is the oil pressure sending unit is bad.
 
fishy68, I am 50% with you on that.... I have observed mild suck-up with the paper for misfires but I have also seen it be dramatic for burned exhausts valve or poor exhaust valve seating (worn guides). And it never happens (for me at least) on a smooth idling engine. So it does indicate something it not exhausting evenly; when done right, you look for a very sharp 'smack' of the paper up against the exhaust pipe. .......Maybe I just do it because it is fun....LOL
 
nm9stheham said:
fishy68, I am 50% with you on that.... I have observed mild suck-up with the paper for misfires but I have also seen it be dramatic for burned exhausts valve or poor exhaust valve seating (worn guides). And it never happens (for me at least) on a smooth idling engine. So it does indicate something it not exhausting evenly; when done right, you look for a very sharp 'smack' of the paper up against the exhaust pipe. .......Maybe I just do it because it is fun....LOL
Click to expand...

Your right that it is worse if you do have a valve problem. I just meant it doesn't have to be a valve issue for it to suck the paper up. Just a misfire can cause it. Now that I look back I should have more clear in how I stated it
 
I have been working on cars and engines of almost all kinds most of my life and I have never used the paper trick on the exhaust ONCE.

The reason is that it is too unreliable to really tell anything so what RRR told you to do ALWAYS points out the problem.
 
Well, I guess I am the backwards type, but I usually don't haul an air compressor with me a few hundred miles when looking at a car...LOL
 
nm9stheham said:
Well, I guess I am the backwards type, but I usually don't haul an air compressor with me a few hundred miles when looking at a car...LOL
Click to expand...

They do make portable air tanks.
 
RustyRatRod said:
They do make portable air tanks.
Click to expand...
Never tried one for that....I do have them, and have used one for field work. Easy to run out of adequate air pressure. And most people don't have them... so what do they do for such a test? (But never fear, RRR, I do like your advocacy of the pressure testing;; IMO it is generally a shop based test.)

The paper-on-the-exhaust test is like many other tests: you have to be careful with results that give more than one possibility. I figured that out out a couple of decades ago. But we all continue to use all sorts of tests with results that point to a multitude of possible casues: compression tests, voltage readings on charging systems, listening for engine taps or knocks; there are gazillion posts from very good diagnosticians on FABO that say things like "well, based on that symptom, it could be x, y, or z". We don't stop using any of those tests just because they provide ambiguous results, we use them to indicate if there is SOME issue that needs further investigation and to narrow it down..... or that there is not an issue.

Well I hope the OP is making some progress.....
 
You must log in or register to reply here.
-

Back
Top