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Old 01-13-2014, 02:13 PM   #1
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Engine problems

I have a 87 YJ with a 4.2L. The engine idles good and accelerates well, but once I hit about 45mph it bogs down like a rev limiter. If I let off the throttle and press it again it will accelerate for a couple seconds then does the same thing. I have changed out the distributer cap, spark plugs, wires, rotor, fuel pump, and fuel filter. I have also punched out the catalytic converter. Can someone help please?

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Old 01-13-2014, 02:23 PM   #2
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Fuel pump or fuel pressure?

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Old 01-13-2014, 02:32 PM   #3
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I put a new fuel pump on it. But same problem.
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Old 01-13-2014, 04:29 PM   #4
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What about having a pressure test done on the system? Mine is in the shop right now for a very similar issue. I'll let you know what my mechanic tells me. My fuel pump heats up and it also whines all the time so I suspect my fuel pump is bad but the pressure test will also reveal the integrity of the delivery system.
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Old 01-13-2014, 04:47 PM   #5
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Sounds to me like maybe the vacuum advance isn't working correctly, could be a vacuum issue or a distributor issue. Depends on the miles on it. Might just need the timing bumped ahead a little.

There are a lot of variables that could be causing the problem. Carb. going bad. vacuum leaks, vac. advance on the distributor going bad.

Have you put a vacuum gauge on the motor? 18-20in. HHG is optimum for a 4.2L

That's how I time mine, I use a vac. gauge to get optimum vacuum, then adjust my idle to get a 750-850rpm.
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Old 01-13-2014, 05:08 PM   #6
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I haven't checked the vacuum on it. With the trans in park, I can rev the engine all the way up with no problems. It only does it when going down the road.
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Old 01-13-2014, 05:26 PM   #7
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When is the last time that the trans. fluid and filter was changed? could be as simple as trans is holding back the motor, if it revs without sputtering.
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Old 01-13-2014, 05:29 PM   #8
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I changed the trans fluid and filter about a month ago
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Old 01-13-2014, 07:35 PM   #9
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My mechanic called with the results of the pressure test on the fuel system. 15 lbs. when cold, dropped to 8 lbs. when warming up, 3 lbs. when at operating temp., and dropped below 3 lbs. and lost ability to drive after driving it for half an hour or so. Fuel pump tested bad and my heater core is leaking too. He'll be done tomorrow. Sure wish I would have had the time to do it myself because it cost a fortune for all the work he did. Oh well. It's only money.

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