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post #1 of (permalink) Old 06-28-2001, 06:07 PM
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Just Fixed Strange Vac Leak Problem

I just fixed a vacuum leak on my 258-I6. I had a hell of a time finding it. Here were my symptoms:
1. After about 2 blocks of driving on a cold engine, I would get a loud whistle from the engine. The whistle varied inversly with throttle...i.e., let off the gas and it was the loudest.
2. When I stopped, then engine ran typical of a bad vacuum leak.
3. Trying to find the leak proved impossible. It sounded like it was comming from the base of the carb, but there was no leak there. Compounding the problem, as I let it idle trying to find the leak, it would go away.
4. If I got on the freeway, the leak would come back after a mile or 2 of steady driving.
5. Additionally, my heater core blocked for some reason, hence, no cooling water would circulate.

My conclusion was a cracked intake manifold. My reasoning was that with the water blocked from going through the intake, it was not heating up to proper temp. When driving on the freeway, the cooling effect of the air/fuel mixture was enough to cool down the intake and open up the crack. When putt putting around town or just idleing, the intake warmed up enough to close the crack. However, it did not explain why the leak did not show up on initial cold startup.

Anyway, I replaced the manifold with a spare one I had laying around. Leak gone. It took me about an hour to find the cause....it was the electric heat riser below the carb. Apparently, the temp range of the heat riser was enough to open up or close the leak. I don't know yet if its just the gasket of the body. I found the leak by droping water into the manifold and seeing where it came out. (if your going to try this method while still on the engine, use rubbing alcohol or something like it).

I just thought you might like to add this to your check list the next time your chasing vacuum leaks.

John...southern CA
84CJ7, 3"lift, 32"BFG, 4.10's, ARB Rear, Detroit Front, Solid Axle's, Durabak, York Air

They say we learn by our mistakes......I guess thats why we are all so smart.
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post #2 of (permalink) Old 06-29-2001, 12:27 AM
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Re: Just Fixed Strange Vac Leak Problem

Damn good troubleshooting..

<font color=red>[img]/wwwthreads_images/icons/cool.gif[/img] 78 CJ7 "Issues" [img]/wwwthreads_images/icons/cool.gif[/img]</font color=red>
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