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Unread 05-25-2010, 06:18 AM   #1
johnson505
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1998 TJ Wrangler 
 
Join Date: Feb 2010
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Burnt valves

I have a '98 TJ with a 2.5 that has a rough idle. I have done several things listed here in the forums but it wasn't getting any better so I took it to the local dealer. They found low compression on cylinders #1 and #2. They think I have burnt valves. I took it to another mechanic and he confirmed low compression on cylinder #2. His reading on cylinder #2 was around 80 and cylinder #3 was reading 130. I wonder if you guys think it would be worth having the head reworked or would I be better off looking a new engine. My mechanic said he could fix it for around $600 as long as he does not find anything else wrong. The Jeep shows 130,000 but my mechanic thinks it is not the original engine so I do not know the actual milege. Any comments would be great.

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Unread 05-25-2010, 02:02 PM   #2
bruteboy
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what was the compression on cyl #1?
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Unread 05-25-2010, 07:22 PM   #3
johnson505
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According to the dealer it is 75 on #1. My mechanic was not able to check it cause the engine was still to hot.
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Unread 05-25-2010, 08:38 PM   #4
bruteboy
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if cyl# 1 and 2 have the same compression readings,it is possible that the head gasket is blown between them.
having 2 adjacent cyl's with the same comp reading,it is unlikley the valves are burnt.
I would expect to see much lower comp readings with burnt valves.
have your mech perform a leak down test with all spark plugs removed.
driving with this condition can cause cyl head and cyl block damage that is non repairable.
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Unread 05-26-2010, 05:54 AM   #5
johnson505
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Thanks for the info.
There is one more piece of information. When I bought the Jeep it had broken bolt on the exhaust manifold at cylinder #1. That is why I was not to surprised that it might have a burnt valve. Other than the rough idle it runs good and there is no smoke. Do you still think it could be the head gasket?
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Unread 05-26-2010, 05:59 AM   #6
lowicz96
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The broken bolt could be from tightening it down too much and as the manifold heats up it expands and breaks the bolt. Pretty common problem
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Unread 05-26-2010, 07:24 AM   #7
oteps
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Have a leakdown test done. That will tell you what is wrong, rings, valves or head gasket. If you are paying a shop to rebuild your engine, I would just get another one instead. most rebuilt engines have a 75,000-100,000 mile warranty. And can be installed in 1 day. I use jasper engines and trannys and havent had any problems.
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Unread 05-26-2010, 12:43 PM   #8
bruteboy
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OK,let's start at the beginning.a rough idle is the only problem?it runs fine while driving?
if yes, a scope test(incl exhaust gas analisys) and power balance test should be done first. followed by dry and then wet compression tests of all cylinders.a leakdown test is next.your mechanic should be able to confirm valve/ring or gasket issues after some or all of these tests.
intake manifold gasket leaks are common on these engines and can cause these symptoms.
egr leaks can also cause these symptoms.
did you have your mechanic diagnose the problem,or did he just do a comp test?
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Unread 05-27-2010, 06:17 AM   #9
johnson505
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When I bought the Jeep it was not running and had been sitting on a trailer in my friends backyard for 2 years. One of the problems was that a couple freeze plugs were pushed out. I had my mechanic replace all of the freeze plugs. After he replaced the freeze plugs he ran a pressure test on the coolant system. It past that pressure test. In order to replace some of the freeze plugs, he pulled the intake, ehaust manifold and transmission. He replace the clutch and also fixed the broken exhaust manifold bolt mentioned earlier. So I think I can rule out the intake manifold since it has a new gasket. I ended up getting a used computer and new fuel pump before we got it to run. I also replace the IAC and the TPS trying to fix the rough idle. The previous owner told me it had a rough idle before he sold it to me. He said that his mechanic thinks it has a cam lobe issue. I would think a worn cam would cause it to run bad all of the time and it runs fine other than the idle and has decent power for a 4 banger. Thanks for all of the comments. I am going to get my mechanic to run the test you mentioned (bruteboy) before I have him pull the head. What do you think about a reworked head on an old engine?
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Unread 05-27-2010, 11:25 AM   #10
bruteboy
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if the comp tests confirm the rings to be good,there should be no problem with a rebuilt head.
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