Hi all, was trying to be a good brother today doing a major service and repairing the cracked exhaust manifold on my sisters 99XJ. Repairing the exhaust manifold obviously entails taking of the inlet manifold, unplugging all the wires, fuel line off the fuel rail etc. Everything went back together ok but when I fired it up, the engine raced to max revs instantly, pretty sure it would rev till blew up from the sound of it if I didn't switch it off. I can't understand how it could do this, particularly since the throttle butterfly does not open at all. I tried resetting the computer, no luck, I'm out of ideas at the moment other than towing it to the nearest Jeep dealer, Any idea's anyone???
XJ engine instantly revs out at start up
Oct 07, 2007
6 Replies
You have an air leak in the intake plenum. Either the gasket is not seated properly or you forgot a hose, especially the one that runs to the brake booster.
Did you use a new gasket and clean the block/intake plenum of any old bits that got stuck.
That usually means you forgot to hook up a vacuum line. You have an air leak into the intake someplace likely.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail > Hi all, was trying to be a good brother today doing a major service
It is going to take more than a small vacum line to do that. Maybe the line forvacuum power brake booster could cause that but you have a major air/acuum leak somewhere and this is what you should look for. not a simple small line.
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Your theory is good but you must rememebr that fuel flow is dictacted by MAP pressure and it would inject more fuel at high manifold pressures caused by leak. The TPS sensor play little roll in mixture as it is mainly used to smooth trasitions in power levels and to lean mixture out in WOT starts to clear flooding.
On a fuel injected engine the throttle vale is just a air flow control valve and engine does not really care where air comes from, valve or leak. It will make power unlike a carbed or most TBI engines.
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