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Discussion Starter #1
Truck: 98 k3500

I've got a vacuum leak that comes and goes, it's only fairly consistent presence is under load at about 1200rpm, about 1/4 throttle. You can hear the whistle and watch the vacuum drop to 6.5-7psi, when it doesn't do it it's around 14-15psi. (Idle is 19-20psi) No codes have been thrown.

Of course, this fawks with the FPR and starts flooding the engine out because it thinks it's closer to WOT.

I've checked the EGR, each time it appears it should be operating normally. The plunger moves with some resistance, smoothly and it seats completely. It's electronic and expensive as hell for an experiment but it's the only thing I can think of.

Is there anything else?
 

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170 Posts
get some carb cleaner and start looking.....
 

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I know on the 7.4 I had that the vacuum leak that drove me nuts trying to find was on the back (firewall side) of the TB. It took a lot of digging and trying different things to locate it, but that was the most problematic one for me. On mine it would idle fine, and even drive "ok" just under casual driving, but anytime you tried to accelerate from low speed up to around 50 or so it would bog down and one time I pulled over to check things out one of the exhaust manifolds was starting to get cherry red...can't remember which side it was.

So...not sure if it's the same issue as yours, but thought it might be worth mentioning.

Good luck tracking yours down!
 

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Take off intake, and the front end and back end, they did not do a hell of a job.
Throw away there seals, and get a tube of 'black' stuff.
Let it set for about 15 min, then set it on, and snug it up.
/\ = factory pre-made seal.
/ \ = better, homemade seals.

This is/was a common problem.
 
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