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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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  #2  
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 ?

Click the image to open in full size.

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Click the image to open in full size.
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Old 05-27-2013, 10:11 AM
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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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crazzywolfie crazzywolfie is offline
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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, 09:32 PM
candymancan candymancan is offline
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I have to run premium gas in thsi engine its got a more agressive ignition timing then the 5.9s in your trucks
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Old 05-27-2013, 09:33 PM
dodge dude94 dodge dude94 is offline
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Quote:
Originally Posted by candymancan View Post
I have to run premium gas in thsi engine its got a more agressive ignition timing then the 5.9s in your trucks
Please explain.




Serious, I'm not knocking you, I seriously want to know about this. Jeeps apparently have some interesting little stuff going on...
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Old 05-28-2013, 03:45 PM
candymancan candymancan is offline
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The Jeep grand cherokee's in 98 had one production year of the 5.9 engine only 14k ever made.. They had 2 versions.. One had a slightetly different camshaft and more agressive ignition timing and thus the HP/Torque are slightly higher then the numbers in your trucks this is the one i have.. Thats why on my gas cap, in the manual and the dash it says premium only more specificly 91 octane.. They also had a second version with the same cam.. but with a performance PCM that required 92 octane... The dodge trucks that did get this camshaft were the later model ones i believe the 2000-2001 models

I dont know the details 100% but this info i told you is true.. Look it up yourself.


Anyway I disconnected the catch can reset the pcm drove around town for 35-40 minutes and my overhead is now reading 12.7mpg and its only climbing higher... This is the gas mileage i remember.. prior to this it was struggling to pass 9.9mpg..

Why does my oil catch can lower my mpg so much ?? That doesnt make any sense to me anyway. Its cought a ton of oil since i installed it right after my intake.. but if my gas mileage was being hurt by it i guess those 1-2-3 tablespoons of oil in the intake every 1k miles wont hurt it.
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Old 05-28-2013, 05:45 PM
dodge dude94 dodge dude94 is offline
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Interesting.


How long are your hoses for your PCV? I'm using a total of 4 feet and so far I haven't noticed a drop in mpg.
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Old 05-28-2013, 06:11 PM
candymancan candymancan is offline
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They were about 2-3 feet. And im using this can its an air compressor filter from lowes.. I removed the stone filter and put the copper scrubby thing in instead.. and then just hooked it all up.. and it worked great too.. see below

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Old 05-28-2013, 06:14 PM
dodge dude94 dodge dude94 is offline
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That's the exact setup I have on mine.
However, I cut the copper scrubber in half and used teflon tape on the barbed ends. You DO have it pointed in the right direction, right?
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Old 05-28-2013, 06:58 PM
candymancan candymancan is offline
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Yes I made sure of it
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Old 05-29-2013, 09:57 AM
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That shouldn't make any difference at all..... Unless your PCV valve is the wrong flow rate? What brand valve ya got?
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Old 05-29-2013, 09:05 PM
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Maybe your overhead isn't all that accurate, also your other method of guessing by how many miles you go on half a tank could throw off true mpg. To get real mileage divide miles driven by number of gallons used.
Use a small propane torch, unlit of course,with small piece of flexible hose and direct it at suspected vacuum leak areas.
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Last edited by ReadRam; 05-29-2013 at 09:14 PM..
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Old 06-01-2013, 11:26 PM
candymancan candymancan is offline
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I did do the real method by going on how many gallons i filled up and how many miles i drove... I got 10.6mpg... and trust me from half a tank to full is always around 13 gallons of gas.. For Jeep its a known fact that half to empty always goes quicker then full to half because of this.. But i wasnt saying i used that to calculate mileage... I filled the tank up completetly.. drove about 120 miles and my tank was at half... then i filled it back up and divided that to the miles i drove and it came to 10.6

But anyway, i still seem to be getting low gas mileage.. I dunno, today i drove around. The car idles for a few minutes at home because i was filling my a/c up ect.. Drove around town and the overhead is now at 11.4mpg.. My PCV valve and grommet are newish and they are Mopar.

I dunno, i just dont see why it is getting such poor mileage after my plenum fix... My pinging is deff gone and i havent lost a drop of oil since but damn my mileage sucks... What the hell could I have done wrong installing the intake to cause this ?? I used a felpro gasket.. the intake went on fine bolted down to 144 inch lbs.. and drives great... Its just like i go from burning oil and pinging and getting 12-13mpg to not pinging and not burning oil and getting 10-11mpg. Im starting to not want to drive this thing anymore.. Go to my 4.0 Grand cherokee.. that thing has no power but at least it gets 16mpg in the city and 21 highway.

A propane torch isnt going to blow up my engine or set it on fire like the brake cleaner did will it ?

Last edited by candymancan; 06-01-2013 at 11:30 PM..
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Old 06-01-2013, 11:26 PM
 
 
 
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