98 GT Hesitiaion at WOT under load after intake manifold replacement

djdonte

New Member
Mar 15, 2010
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Hello all. I am posting on behalf of a friend of mine who isnt too car literate. I have been helping him work on a 98 Mustang GT with about 105K that he just picked up. It had a leak in the intake manifold coolant runner near the alternator, apparently a common problem. We went to pick a part and got one off of a crown vic and installed it. Also we dumped the exhaust at the flange before the rear mufflers. Now we have hesitation at WOT under a load. The car idles fine it seems and hasnt thrown any CELs. Also, If you race the engine at in neutral it doesnt misfire. Only when your moving and the engine is under load does it buck and misfire/hesitate.

I took it to a auto parts store to see if they could pull a code anyway, but no luck. He also told me that he thought the car is having problems because we pulled the intake off of a 4.6 crown vic and not a police interceptor or another GT. He claimed that the performance manifold on the GT and PIs flow better and the car was being bottlenecked. Upon inspection the manifold runners do look a bit different but we were comparing the all plastic part to the one with the aluminum coolant runner. In any case we went BACK to the junk yard and found an all plastic manifold from a PI and installed that but we're still having the same problem. Now I am thinking bad plug/wire/coil pack due to the massive coolant leak. Either that or a fuel problem but I dont see how we could have messed that up during the removal/install of the manifold.

Any thoughts and also any tips on diagnosing the problem. I am pretty mechanicaly inclined but I am a new to Ford.... I drive a 07 vette and 86 camaro so I am alot more knowledgeable when it comes to SBC and LSx than these 2v motors. Thanks in advance for reading! -Eric
 
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I had the exact same problem when I swapped on my PI intake manifold. I drove it for like 2 days before it finally threw a code, which was a misfire code. The problem for me was that coolant got down into the plug wells. Replace the plugs and I'll almost guarantee that the problem goes away.
 
Well we replaced the spark plugs and problem solved! Oddly, ONE of the plugs looked somewhat new, and the other 7 looked original, being ford motorcrafts with almost no electrode left. This intake has got to be the worst design I've ever seen. Thanks for the help guys! -Eric