2.5L misfire under load - JeepForum.com
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post #1 of 37 Old 06-27-2010, 09:04 AM Thread Starter
hevybluzer
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2.5L misfire under load

Hope someone out there can help! My '95 YJ (2.5L/5spd) has recently started to misfire, under load, between 1800 and 2800rpm's, and a new found propensity to backfire when letting off the gas to shift. The misfire feels and acts like one of the cylinders "drops" out. The only non-stock engine item is a 4.0L throttle body that I put on a couple of months ago (long before this problem started). I have changed the plugs (old ones were light tan/white and the 'trodes were eroded) as well as the cap and rotor. I have checked the vac lines. It's not throwing a check engine light. As this has now become my only mode of transport I would appreciate any help................
Thanks in advance......

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post #2 of 37 Old 06-27-2010, 09:05 AM Thread Starter
hevybluzer
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........and I put a new fuel filter on last year..............
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post #3 of 37 Old 06-27-2010, 10:55 AM
SDGBASSE
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Take a can of brake cleaner and make sure all the sensors around the throttle (IAC, TPS, ...) are clean.

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post #4 of 37 Old 06-27-2010, 12:16 PM
MJMPC
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What about plug wires?

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post #5 of 37 Old 06-27-2010, 12:32 PM
moonshinefuel
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X2 on the plug wires. Always check the simple and obvious possible problems first.

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post #6 of 37 Old 06-27-2010, 12:49 PM Thread Starter
hevybluzer
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the plug wires are only 2 years (and 4,000 miles) so I was kinda leaning away from that but hey, you never know! I am going to try the spray deal. How about the O2 sensor??? Could that do it??????? (only 2 years old as well)
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post #7 of 37 Old 06-27-2010, 02:41 PM
Roadyy
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I would highly doubt it is the O2 sensor.. I just replaced mine that was throwing a CEL and it never once backfired on me. If your plugs were tan/white and the rotor and cap were arcing offset then I would first look at the plug wires for good measure if they were not a high end brand.
Then check IAC and also check your fuel pressure at the rail just to make sure you are getting the correct amount of fuel..
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post #8 of 37 Old 06-27-2010, 03:37 PM Thread Starter
hevybluzer
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would a bad plug wire only act up under load? The idle and "unloaded" revs are OK.
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post #9 of 37 Old 06-27-2010, 05:50 PM
Roadyy
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lack of fuel will do the same thing.
Did you check your fuel pressure??
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post #10 of 37 Old 06-27-2010, 06:00 PM
moonshinefuel
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I always like to throw in the outside chance of an original failing coil too. Not sayin that is what it is, just like throwin it in.
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post #11 of 37 Old 06-27-2010, 06:06 PM
Roadyy
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That or the pickup coil in the dizzy.. since Que isn't here to throw it in.. lol
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post #12 of 37 Old 06-27-2010, 07:16 PM
Old4X
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Quote:
Originally Posted by hevybluzer View Post
would a bad plug wire only act up under load? The idle and "unloaded" revs are OK.
Yes.

Also, most other ignition problems will start out with this same type misfire.

418 miles on 19 gallons of gas. Can't beat the BBD !
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post #13 of 37 Old 06-27-2010, 10:20 PM Thread Starter
hevybluzer
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fuel pressure is 35 @ idle. needle bounces when engine is revved but doesn't drop below 35. I did a compression check and all cylinders are within 6 psi of each other and well within spec. (this motor only has 88k original miles on it) and has been well maintained. I am going to pick up a set of wires tomorrow and give that a try. There were signs of arcing on the inside of the dist cap and the rotor looked fried. maybe the wires. I am also going to pick up a piece of hose to extend the fuel pressure gage I have so I can watch it under load.
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post #14 of 37 Old 06-28-2010, 05:21 AM
EABiker
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Check the resistance of the IAT and Coolant temp sensors; If they are not between 1.5 and 3K ohms, they are giving the ECU false information and it will run too rich or too lean, and cause misfire under load.
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post #15 of 37 Old 06-28-2010, 07:47 AM
Que89YJ
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X2 with the engine coolant temp sensor and the Throttle position sensor. The engine controller responds to both for advance.
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