99 getting 9.5 MPG
Grand Champion
Joined: Aug 2009
Posts: 5,971
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From: MAINE
my plenum never looked blown/never burned oil. i always looked through the TB, and never saw pools of oil, but a coating on the inside. Then when i took the TB off all together thats when i found my plenum WAS in fact leaking. If you do change the Plenum, get the manifold ported and polished (cost me 40 bucks), and do the Kegger Mod, get a Hughes Plenum Plate, or APS Plate, and new Intake gaskets WITH bolts.
Then you'll never have to worry about the plenum again
all together it cost me like 300 for those parts. (got a spare manifold for free though)
and that with Plugs, wires, Dist. cap/rotor, coil, cleaned fuel injectors, seafoamed, and now i get 13-14 mpg city...i was getting 10 mpgs
So all together, maybe 600-700 bucks if you do it all yourself
and i still need to change CAT and 02 sensor
Then you'll never have to worry about the plenum again
all together it cost me like 300 for those parts. (got a spare manifold for free though)
and that with Plugs, wires, Dist. cap/rotor, coil, cleaned fuel injectors, seafoamed, and now i get 13-14 mpg city...i was getting 10 mpgs
So all together, maybe 600-700 bucks if you do it all yourself
and i still need to change CAT and 02 sensor
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Professional
Joined: Dec 2009
Posts: 113
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From: Alton, Illinois
Here are some shots into the intake. There is older oil pooled up in the intake closer to the firewall. I just changed the oil and you can see the new oil in the shots that I took straight down the intake. I don't know why it's not showing vacuum when I test for a plenum leak, but it is obviously leaking in there.
Last edited by Chadlow21; Dec 28, 2009 at 12:53 AM.
Professional
Joined: Nov 2009
Posts: 106
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First I'd like to say hello. This is a great forum.
Here is my problem, I have a 99 quadcab with the 5.9L, all stock 93xxx miles. I am averaging 9.5mpg and the truck just doesn't seem right. It doesn't seem like it has very much power, it has a hickup in the idle that I don't like, and I think I can hear spark knock when I accellerate. All af this sounded like the Plenum was bad. I tried a vacuum check and it read zero vac in the crankcase, so I guess it's not the Plenum gasket(which was already replaced once under warranty). It just had a complete tune-up, new catalytic converter put on, a new o2 sensor, and the TB was cleaned. Is there anything else that I should check out? There are no check engine lights on. This would be so much easier if there was. Thanks in advance.
Here is my problem, I have a 99 quadcab with the 5.9L, all stock 93xxx miles. I am averaging 9.5mpg and the truck just doesn't seem right. It doesn't seem like it has very much power, it has a hickup in the idle that I don't like, and I think I can hear spark knock when I accellerate. All af this sounded like the Plenum was bad. I tried a vacuum check and it read zero vac in the crankcase, so I guess it's not the Plenum gasket(which was already replaced once under warranty). It just had a complete tune-up, new catalytic converter put on, a new o2 sensor, and the TB was cleaned. Is there anything else that I should check out? There are no check engine lights on. This would be so much easier if there was. Thanks in advance.
Grand Champion
Joined: Aug 2009
Posts: 5,971
Likes: 45
From: MAINE
^ same with me....i dont tow though, but get great MPG's considering most get 10...
and by the looks of it, it sems like the plenum is leakin again...i know there old residue, but on the plate itself, that looks like fresh oil. doesnt have to be a big leak to cause problems...
and by the looks of it, it sems like the plenum is leakin again...i know there old residue, but on the plate itself, that looks like fresh oil. doesnt have to be a big leak to cause problems...
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Professional
Joined: Dec 2009
Posts: 113
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From: Alton, Illinois
Yea, the plenum has to be leaking. I just cleaned the motor out with seafoam through the brake booster line yesterday right before I changed the oil. The older oil that is just around the edge and that new oil should not be there. I doubt there is a compression issue, I am thinking it just needs a new plenum gasket.
Last edited by Chadlow21; Dec 13, 2009 at 10:25 PM.
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Joined: Jan 2008
Posts: 8,374
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From: Sioux Falls, SD
It needs more than a new plenum gasket.
It was replaced once before, under warranty, and it blew. Replacing it again, it will just blow again. You are fixing a symptom, not the problem.
You need the Hughes or APS plenum kit. New gasket, new "belly" plate made of aluminum, and new intake bolts. Problem solved, should never have to touch the plenum/gasket every again.
It was replaced once before, under warranty, and it blew. Replacing it again, it will just blow again. You are fixing a symptom, not the problem.
You need the Hughes or APS plenum kit. New gasket, new "belly" plate made of aluminum, and new intake bolts. Problem solved, should never have to touch the plenum/gasket every again.