Rather than guess and throw parts, follow the diagnostic procedures from the service manual found here: www.charm.li
mystery misfire issue?
- Thread starter adventureboss
- Start date
Broken valve spring on #3.
Nick1994
$100 site donor 2024
Re-do the compression test yourself. Here’s my results on my ‘96 Cherokee 4.0L in 2017:
6-125
5-145
4–150
3-150
2-150
1-145
6-125
5-145
4–150
3-150
2-150
1-145
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That's quite a lot for a bottle! Wow! I tried the water thing, don't know if it made a difference in how well it runs. That being said, I still occasionally will have a slightly rough idle but no misfire codes and nowhere near as rough as it used to be and it's been fine for the past 600 miles almost with no codes. I don't know what to make of that.
We'll see when I take it in to another shop tomorrow morning.
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This is something a mechanic would be able to identify with a borescope right?
Nope. Pull the valve cover and look.
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The reason I'm skeptical is because all these compression testers seem to read something different. I'm not sure if it's something I'm doing wrong, but I got a loaner set from Autozone once and had numbers all over the place. Then went to Harbor Freight and bought a compression tester and had numbers within 10 psi of each other. I left that tester at my parent's place in another state.
So I'm going to need to justify the need to buy another compression tester out here, which seems like a waste. Instead I'm dropping it off at another shop tomorrow and will have them do it as well as a leakdown test and hopefully stick a camera in there.
Nick1994
$100 site donor 2024
Another new $30 tester from Harbor Freight is probably the best investment.
I rented one from Auto Zone that like you, was completely inaccurate. Had to return it and got one from another Auto Zone that was still new in the box.
Though doing a leak-down test might be worth it too.
Had a similar problem caused by a pushrod/rocker arm problem though it did tick a little hot. Noticed the rocker was tipped at an odd angle. If someone goes in the combustion chamber with a scope have them look at the back of the valves.
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Okay so I just ended up doing a compression test myself and here's what I got. Engine was hot, measured after 8 cranks for each one.
Cyl 1 (dry): 175 psi
Cyl 2 (dry): 180 psi
Cyl 3 (dry): 145 psi, wet 160 psi
Cyl 4 (dry): 170 psi
Cyl 5 (dry): 160 psi
Cyl 6 (dry): 180 psi
So it looks like the shop was right about the third cylinder being a little iffy, however a 15 psi increase does not seem like a significant amount which means piston rings are okay right? I probably have a valve not sealing correctly somewhere?
I also have not had the misfire come back ever since the seafoam and the water in intake cleaning and it has been 900 miles. I still will every so often have a slightly rough idle. What are my next steps?
Cyl 1 (dry): 175 psi
Cyl 2 (dry): 180 psi
Cyl 3 (dry): 145 psi, wet 160 psi
Cyl 4 (dry): 170 psi
Cyl 5 (dry): 160 psi
Cyl 6 (dry): 180 psi
So it looks like the shop was right about the third cylinder being a little iffy, however a 15 psi increase does not seem like a significant amount which means piston rings are okay right? I probably have a valve not sealing correctly somewhere?
I also have not had the misfire come back ever since the seafoam and the water in intake cleaning and it has been 900 miles. I still will every so often have a slightly rough idle. What are my next steps?
I had a 98 Cherokee 4.0 with a misfire on #1, and none of the usual suspects fixed it. Finally found a TSB about using Mopar Combustion Chamber Cleaner, which helped but didn’t eliminate the problem. I replaced both valve springs on #1 and it was fixed.
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So you think I probably have a top end problem?
I'm honestly not sure...just telling you my experience.
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So I had a shop do a leak down test today and it came in at just under 40% on cylinder 3. The shop says this "passes", but factory service manual says 25% or below is acceptable and above is not good. They could not pin point where it was coming from because they said some was coming from the piston rings, some from the valves, but it wasn't anything specifically.
They also think it could be a carbon build up issue because the water cleaning helped and suggested the GM top end cleaner, sold me a can. Thoughts?
They also think it could be a carbon build up issue because the water cleaning helped and suggested the GM top end cleaner, sold me a can. Thoughts?
Did you pull the valve cover off yet!
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Nope, this is my next step if compression does not improve after the piston soak and top end cleaner.
