92 YJ Running Rough at High RPM - JeepForum.com
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post #1 of 23 Old 06-25-2020, 08:12 PM Thread Starter
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92 YJ Running Rough at High RPM

I have a 1992 YJ, 4.0l 5 speed. Started with high idle issues. I replaced the IAC, TPS, and CPS. No luck. Replaced the injectors next, not new but tested, then a new MAP and intake temp sensor. Somewhere during this process it went from idle issue to spitting and sputtering at high RPM. I've replaced the pick up in the distributor, plugs, wires, cap, coil, and button. I even replaced the speed sensor. Changed the throttle body, O2 sensor and fuel pump. Changed the fuel filter and the fuel pressure regulator. Changed all the vacuum lines and the fittings. New PCV valve too. The only two things that are different is I completely removed the non-working A/C. I removed the Cat from the new exhaust. At 2000 RPM and above the literally misses so bad it wont accelerate. It misses almost in sync with the shift light. Y'all have any suggestions I would appreciate it.

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post #2 of 23 Old 06-27-2020, 05:28 AM
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Before I bought all those parts I’d a) put a can of isopropyl drygas in and more importantly b) find the vacuum leak at the manifold or brake booster.

If you are going to throw money at it ignition wires would be my first purchase

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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post #3 of 23 Old 07-01-2020, 10:00 PM Thread Starter
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I appreciate the response. I found my problem! Had a piece of the manifold gasket gone behind the intake. Also the manifold bolts were getting loose. I'll post again when I get it back together.
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post #4 of 23 Old 07-02-2020, 03:31 AM
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That’s great!
It just sounded like an air leak to me. Glad for you it was that easy.

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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post #5 of 23 Old 07-04-2020, 12:13 AM Thread Starter
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Well...not so fast my friend! Back to square one. Replaced the manifold gasket. Also put the stock exhaust manifold back on. Hooked everything back up and still runs very rough at higher RPM. even backfired a few times. I'm at a loss. PCM maybe?? Any help will be appreciated.
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post #6 of 23 Old 07-04-2020, 06:30 AM
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You need to check the fuel pressure, confirm cam timing, and use the unlit propane torch method to verify no vacuum leaks. First.

Is the idle normal?

BTW when I replaced a piston in mine- when I restarted it it did NOT like it at all. I got it stay running enough to gently drive it a few miles. And warm up. At second start it was still wonky at idle but once temps came up it straightened right out. Sometimes the computer needs a little bit to reprogram itself to run properly.

When you say backfire: up the intake, or exhaust banging? Both indicate different things. Unburned fuel in the exhaust in mpfi can indicate an air leak. Sticky valves or even timing chain issues can let fire up the intake

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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post #7 of 23 Old 07-04-2020, 02:20 PM Thread Starter
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It idles high on a cold start. Takes several minutes for it to settle down. I've done the propane test never changed a bit. Once its warms up it idles perfectly. Its very consistent 2000 RPM it starts acting up. RPM gauge bounces violently and the motor shakes bad. I put a universal cat back in to rule that out. I've not checked the timing with a light, don't have one. It has had a clatter in it since I've had it. I'm almost sure its lifter noise. Could the cam be worn enough to cause the valves to not be opening properly? Also I changed the rear main seal a couple of months ago and noticed the timing chain had what I thought was a lot of slack. Researched it and read where a 1/4 to 3/8 inch slack is not unusual. How do you adjust the timing since the distributor doesn't turn in its hole? I'm wondering if this is fixin to turn in to a rebuild. Would pulling the A/C have any affect? I get the 33 code. Open A/C circuit. I wouldn't think that would cause this problem.
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post #8 of 23 Old 07-04-2020, 02:22 PM Thread Starter
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Also fuel pressure is normal. Checked after new pump and filter.
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post #9 of 23 Old 07-04-2020, 02:27 PM Thread Starter
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Backfire seems to be through the intake.
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post #10 of 23 Old 07-05-2020, 04:49 AM
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Consistently acting up 2000 rpm is an indicator that to me to explore a couple things. Ignition issues, poor connection of wires to the coil and distributor, bad coil, timing chain issues, poor air/fuel...

Having said that, I still suspect an air leak somewhere

While it would be good to rule those out I would do a compression test on each cylinder and confirm that the valves are closing first.

Also, a bottle of isopropyl drygas won’t hurt anything; rpm misfire and odd irregular backfires and exhaust popping can result from moisture/water in the fuel tank. A quart of MMO in the oil can help loosen sticky valves- it also might clean bearing surfaces on an old motor increasing clearance (lessens oil pressure sometimes) so just don’t run it longterm.

The point is to check all the basic functions. Write them down, eliminating them one at a time as causes. What you can’t eliminate is what you need to explore further. Skipping basics is when you start throwing parts at it.
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[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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post #11 of 23 Old 07-05-2020, 10:35 AM Thread Starter
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Thanks, I'll keep you posted!
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post #12 of 23 Old 07-05-2020, 08:29 PM Thread Starter
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OK so here's where I'm at...I found today that I had a loose coil wire. Changed the wire, same problem. I removed the coil and tested. Resistance was a little on the high side. So I exchanged it. Lifetime warranty. New coil had a different number so I'm thinking...Ok I had the wrong coil. Cool maybe this will be the fix I've been hunting for. NOPE!!! I'm wondering now if maybe the distributor gear, or the gear on the cam, is worn to the point that when I rev the motor it gets just enough out of time to spudder like it does. I don't know. I can say one thing with confidence... there is no vacuum leak. I checked it from the intake to the transfer case! With the propane. I'm about ready to see if it'll run full of 5.56 rounds!! Really though I feel like this is something fairly simple...
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post #13 of 23 Old 07-05-2020, 08:32 PM Thread Starter
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I am going to do the compression check and check the timing with a light this week.
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post #14 of 23 Old 07-08-2020, 06:26 AM Thread Starter
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I read an interesting article last night. Exhaust valve springs?? Exhaust valves not closing fast enough causing missing at high RPMs... What do yall think??
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post #15 of 23 Old 07-08-2020, 07:18 PM
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Imho 2000 isn’t high rpm. In my opinion I’d have trouble believing valves would float at 2K with a motor in otherwise reasonable shape.

Compression test...

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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