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Significant gas mileage drop since plenum fix... Why ?

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Old 05-27-2013, 09:39 AM
candymancan candymancan is offline
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Default Significant gas mileage drop since plenum fix... Why ?

I have a 98 Jeep grand cherokee 5.9 v8, as some of you know i stop by these forums every now and then because you guys seem to know more about these engines and trannys then the Jeep guys do.

Anyway, I finally got around to removing my intake manifold and replacing the steel pan with a 1/4 thick allum pan and a new gasket. Everything went really good.. I used felpro gaskets for everything, and the intake gaskets i used smears of rtv on the water jackets per what alot of people said.. I didnt use those rubber gaskets for the front and the back tho i used a TON of the right stuff liquid gasket.. When setting the manifold down it went in place smoothly and I tighten it down per what the TSB said for the sequence up to 144 inch lbs (12 ft lbs).. I noticed the front of the manifold where the rubber gasket would go tho there was a gap between it and the block.. The rear didnt have this gap.. Not sure if that is normal or what... So I used extra the right stuff on that spot and when the manifold was on i smeard more in the gap to block it off. I think that gap is normal as another guy said he noticed that too.

Anyway.. Started the motor and it ran fine.. Oil level hasnt dropped one bit.. Before i would burn 1 quart or more every 3k miles.. Its been 1,000 miles now and it hasnt dropped one bit.. I noticed the intake was getting oily tho so i made myself an oil catch can using copper pot scrubbers and that stopped the oil from getting into the manifold.. It has caught a ton of oil so far.. The tailpipe is very clean too, prior to the manifold fix it was always black and sooty. now its clean...


My question tho is this.. I have noticed a big time drop in gas mileage since doing this... Like HUGE drop.. I used to be able to get 13.5mpg when i first got the Jeep 3 years ago.. it dropped to around 12mpg over the years. Now since this fix.. my gas mileage has dropped to barely 9-10mpg.. This is by measuring from the tank... The Jeep has a 23 Gal tank.. from full to half is about 12 gallons (yes i check by filling it up from half on the needle) and i can only drive 100 miles on that.. So something isnt right...

My idle hasnt changed its the same as it was prior to removing the manifold so i dont think I have any vacuum leaks.. But i tried checking for them using brake cleaner and all that did was catch fire and i had a engine fire.. why it caught fire on a cold running engine is beyond me.. But i am not using carb or brake cleaner ever again... So what do you guys think ?? Could it be the catch can i installed on the PCV line ?? Or did i install this manifold wrong ?? I really need those 2-3 mpg back.. I cant afford to be getting 9mpg anymore
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Last edited by candymancan; 05-27-2013 at 09:44 AM..
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Old 05-27-2013, 09:52 AM
dodge dude94 dodge dude94 is offline
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My truck had great mileage until the plenum blew out, then it tanked, then I got it mostly back. I'm thinking the vacuum leak was obviously making it run lean and thereby increasing your mileage, but you should be getting more than 9-10. If you think it's an issue, remove the catch can and reset your PCM.

Also, did you replace your plugs, cap/rotor, wires, pre-cat 02? How is your cat? My mileage was kinda starting to drop a bit after the plenum because my cat was going out. It's just a thought.
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Old 05-27-2013, 09:59 AM
candymancan candymancan is offline
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I never had any vacuum leaks i dont know what you mean by that ? What im saying is people suggested i might have a vacuum leak since taking my intake off.. but i spent 8-9 hours on intake manifold i did everything slowely and by the letter I dont see how i could cause a vacuum leak because it doesnt idle rough or idle high you know ? So pe ople told me to use brake or carb cleaner and spray around the manifold and i did that and POOF it caught fire and my engine was on fire for a few minutes before i could get the hose unkinked thankfully it didnt cause any dmg...

Do you think I could have a vacuum leak ? How do i check without flamable liquids ? I guess I'll unplug the catch can and hook it back up normally and reset the computer via battery and see if that fixes it..

I only replaced the spark plugs. I didnt replace my wires.. My cap and rotor are fairly new my plugs are cheapo 7mm wires probly 2 years old now. I didnt replace the o2 sensors or the cat. The cat is original i dont know how to check the condition of it.. banging on it it doesnt make any rattle noises so i think its ok in that respect.. but like i said prior to this i was burning oil yet suprisingly i was getting good gas mileage.. now im not burning oil and im getting ****ty gas mileage.. lol My pinging is gone now too btw.

Last edited by candymancan; 05-27-2013 at 10:01 AM..
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Old 05-27-2013, 10:03 AM
dodge dude94 dodge dude94 is offline
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The blown plenum IS a vacuum leak until you fix it. Which is why some folks report stellar MPG....until it blows out.

Since it sounds like you did everything correctly, I would seriously start watching that cat. Luckily in my case, mine started rattling before it started clogging. But if your cat is starting to clog, you won't notice the performance decrease now, but you might notice the slight mpg drop...
I would go ahead and replace (or gut, whichever) the cat and front 02 sensor (NTK or Denso only please). That stuff ain't cheap, but it needs to be replaced since they are both fouled up due to the plenum.
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Old 05-27-2013, 10:09 AM
candymancan candymancan is offline
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When I get the money I'll replace both O2 sensors and maybe look into getting a new cat but atm im broke.. Like completely broke lol.

So your saying the vacuum leak from the intak manifold plenum causes an increase in gas mileage ? Because prior to this i was always getting 12-13mpg... People were wondering how i was getting gas mileage like that in the city.. I have no idea i just do.. But i did have horrible pinging and oil loss.. Since this fix i did still have pinging compression check shows my cylinders all at around 180-188 psi... which is quiet high most likely from carbon.. but now i dont ping anymore.. and oil level hasnt gone down anymore.

Here are pics of my manifold.. FYI when you did you plenum did you notice when setting the manifold back down that there was a big gap between the block and the manifold in the front ?

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Old 05-27-2013, 10:11 AM
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crazzywolfie crazzywolfie is offline
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i don't think you need to replace both o2 sensors. you really just need to replace the one mounted on the y pipe before the cat. it is the one that gets fouled up and you can get a new ntk one from rock auto for about $30 or less plus shipping. you can even save 5% if you use a discount code.
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Old 05-27-2013, 10:13 AM
dodge dude94 dodge dude94 is offline
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It's usually only in the early stages of a blown plenum when it gets good mileage, before it starts sucking in oil, but in your case it apparently didn't matter. Have you seafoamed the engine yet? If not, go ahead and do so. That carbon can do some weird stuff...mine didn't ping yet made a huge smoke show. (smoked out the entire north end of my street. )

A gap between the manifold and....? I don't understand. lol And your pictures aren't there at all.
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Old 05-27-2013, 10:18 AM
candymancan candymancan is offline
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You know in the front on the engine block where the rubber gaskets from the manifold gasket kit are suppose to go ? There was a gap there when i sit the manifold down i noticed this gap between the manifold and the block.. the rear didnt have this gap as the liquid gasket i used got smushed down the front not so.. I did use seakfoam YEs i almost hydrolocked the engine doing it lol it make a loud CLANK noise when i was using.. scared me.. blue smoke was coming out my tailpipe when i was driving out of my neighborhood

But im wondering... If my cat was bad, and O2 sensors bad.. wouldnt i be getting 9-10mpg prior to this plenum fix as well.. ? Like litterally right after doing this i go from 12 to 9mpg
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Old 05-27-2013, 03:18 PM
candymancan candymancan is offline
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Ok i removed the catch can, hooked the pcv back up like normal.. reset the computer.. Ima drive it around some and see if this helps any.. till then anyone else have any idea's why my gas mileage dropped 2-3mpg since the plenum fix
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Old 05-27-2013, 05:37 PM
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none that are free. get a new o2 sensor and run premium gas. i run premium gas in my truck and ramcharge and i just changed out the o2 sensor and i was pretty surprised how clean it is.
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Old 05-27-2013, 05:37 PM
 
 
 
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