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4.7L with intermittent miss-fire, told blown head gasket?

14K views 13 replies 7 participants last post by  Hmurphy 
#1 ·
I have a 2001 Jeep Grand Cherokee Limited that started running rough and has an intermittent misfire and the sound of running water from the heater core (which was replaced just before this problem started. It has never overheated, it just started running wierd a week or so after the heater core was changed and the AC box rebuilt. Sometimes it runs fine but others it runs rough feeling like a series of misfires and a 40-50% power drop.

I took it to a shop which charged me a $100 to tell me it has a blown head gasket. There is no milky radiator fluid in the oil, nor is there any vapor or smoke coming from the exhaust. Occasionally the overflow tank is a little low but not by much and the radiator is always full when I check. There are no leaks under the vehicle either. I'm at a loss as the shop is telling me that Jeep has no procedures for pulling the heads and resurfacing them and that Jeep requires you to replace the whole engine. Something about if the head gasket is blown then the heads and block are both probably warped. That sounds like a load of stuff to me. Especially since Auto Zone has the head gaskets in stock.

Please advise? I'm really at the point now of just trying to replace the gaskets and seeing what happens, if I eventually have to replace the engine anyway, then what's it matter if I spend an extra hundred bucks trying this out.

Any ideas? Are blown head gaskets a common problem with the 4.7L V8?
 
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#2 ·
I would try replacing the gaskets, or maybe you want a second opinion before you do all that because I believe your shop is feeding you BS as it is.

Also I dont think its common for the 4.7
 
#3 · (Edited)
ask the shop that charged you 100 bucks if they used a combustion leak detector when they diagnosed your head gasket... would this be the same shop that did your A/C and dash removal ?

-Combustion Leak Detection Kit-


$25 bucks would have saved you the extra $75 and the big question mark as to weather your head gasket is bad. I bet you any money unless this was a dealership the tech who worked on your Jeep most likely did not have this proper test equipment.
 
#5 ·
actually my math is wrong here...

$25 would have saved you $125 because now you have to go out and buy one of these testers and if you spend precisely $25 then the total investment so far for the particular diagnosis problem would be $125 :D

and in the end when the tester tells you the head gasket is fine you would have saved thousands not to mention the gaping hole that particular repair shop is going to leave you walking out the door with...
 
#6 ·
Thanks for the quick replies. In answer to the questions posed, no it's not the same shop that did my AC "repair." After I get it running again I still need to have the HVAC box done again. Because now it makes a lot of noise, but doesn't blow half as much air as it did before. And I don't know about the compression leak tester. But I will find out. Because here in SoCal, a used engine is going for just under $2000 so anything that brings me in significantly under that amount seems to be worth a try.
 
#7 ·
I would suspect that the whoever did the heater core repair missed a vacuum line or electrical connector when they put everything back together. Was it running fine before the heater core issue?
 
#9 ·
Yes it ran fine before the heater core problem. I took it back to them to fix 3 seperate times, trying to give them an opportunity to make it right. They however chose to give me attitude instead so I contacted my attorney. His advice is to have it fixed again at another shop and then he will request payment by them. Or we can go to court. And thats all well and good except that I'm out of pocket the money to take it to another shop.
 
#10 ·
Ok since my last post I have pulled the spark plugs and all of them look good. None any cleaner or more corroded than the others. Ran a compression check on each cylinder and all were well within range. I replaced the ECM which cleared the check engine light. I flushed the cooling system till there were no more bubbles in in it. It seemed to run fine in the driveway for the 20 or so minutes we flushed the system. However as soon as I put it in gear it ran like crap again like it was having multiple misfires. And I can hear air/water flowing through the heater core. I drove it ten miles to my house and it runs like its got maybe half of the horsepower it used to. Any ideas? I'm at a loss.
 
#12 ·
It really does NOT sound like it has a blown head or anything of the sort. It sounds like whoever did the AC/heater box botched the repair and forgot/cracked/ripped/broke some vacuum lines. When you switch your HVAC switch from say, floor, to defrost, to vent does it make the switch or stay stuck in one mode? Does your cruise control work or does it not "take off" when you release the button and the gas pedal? These are signs of a vacuum leak.

You mentioned that you replaced the PCM and that cleared the check engine light, did you have the old computer scanned to see what the code was that caused the check engine light??
 
#13 ·
Run a test on the TPS also. Did you ever tap the throttle while it was running for 20 minutes while doing the flush, or was it just idling? TPS test is quick and simple, lots easier than tracing vacuum lines, and it'll eliminate one possibility.
 
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