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Unread 05-14-2012, 04:20 PM   #1
Randonexplosion
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258 problems under full load

Hey guys, I'm having problems with my 258. My girlfriend first noticed the problem while taking the Jeep to work on the highway. It will not do over 50-55. Driving around town it runs great (under partial throttle) and it revs without a hiccup while parked. The problem occurs when the rpms are around 2k and you give it heavy throttle while driving. It sounds like it is reving like crazy. I've never heard the motor sound like that before. Does anyone have any suggestions on what to check? i'm thinking it isn't getting enough spark/juice under load. Thanks for any help

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Unread 05-14-2012, 04:53 PM   #2
Randonexplosion
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A couple of other notes, my plugs look like it is burning a little rich. Also the fuel pump and filter arent very old. I dont put many miles on the Jeep.
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Unread 05-15-2012, 11:00 AM   #3
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Bump
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Unread 05-15-2012, 09:40 PM   #4
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You may be right, its hard to say without being there and looking at it. Look at the spark and see if its weak/strong, you should see a nice pinkish/purple spark and it should be able to jump around a half inch gap or so. Your problem sounds like it might be a fuel concern or weak ignition system but sounds like you have a good fuel supply. Maybe check the coil and spark quality, wires, cap and rotor and ignition timing.

Check the obvious stuff too like air filter, and throttle linkage to see if you are getting WOT, make sure the choke plate is not sticking closed. Also its possible that the fuel filter could be stopped up from debris in the tank or dirty/bad fuel.
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Unread 05-17-2012, 08:14 AM   #5
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Thanks for the suggestions. Everything with the carb seems fine, the choke plate is functioning normally. I will grab a fuel filter and switch that out. I played with the timing but that didn't make a difference. What is the best way to check the coil and spark quality? I wish I could explain it better. When I dump the throttle it sounds like I have a V8! Part of that could be the leak between the manifold and the down pipe though.
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Unread 05-17-2012, 05:56 PM   #6
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Try this vacuum test.
CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION

Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.
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Unread 05-18-2012, 05:59 AM   #7
james04si
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When going at 50 and you press the gas does it do more than just sound like its revving? If it is actually revving it could be your clutch slipping.
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Unread 05-18-2012, 11:14 AM   #8
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Quote:
Originally Posted by james04si View Post
When going at 50 and you press the gas does it do more than just sound like its revving? If it is actually revving it could be your clutch slipping.
No its not actually revving. The motor just sounds like a beast at WOT
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Unread 05-18-2012, 11:16 AM   #9
Randonexplosion
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Quote:
Originally Posted by cruiser54 View Post
Try this vacuum test.
CRUISERíS VACUUM TEST FOR EXHAUST RESTRICTION

Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.
I'm gonna go pick up a vacuum guage this afternoon. I am leaning towards exhaust restriction too. I'll let you know what I find!
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Unread 05-19-2012, 08:35 PM   #10
Randonexplosion
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It was the cat. I have an exhaust leak upstream from there that was causing the brutal sound.
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Unread 05-19-2012, 08:50 PM   #11
cruiser54
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Did you find the solution using the vacuum gauge?
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Unread 05-19-2012, 09:01 PM   #12
Randonexplosion
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No I didn't. Either I didn't follow the instructions well or there wasn't enough of a blockage unless it was under load. I just got curious and tapped it with a hammer and hear the loose honeycomb bouncing around.
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Unread 05-19-2012, 09:41 PM   #13
cruiser54
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Quote:
Originally Posted by Randonexplosion View Post
No I didn't. Either I didn't follow the instructions well or there wasn't enough of a blockage unless it was under load. I just got curious and tapped it with a hammer and hear the loose honeycomb bouncing around.


Lucky dog.........

Glad to hear you found it.
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Unread 05-19-2012, 09:52 PM   #14
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Thanks man. It was a lucky catch. Me and my friends were pretty puzzled.
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