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
Last edited by candymancan; 05-27-2013 at 09:44 AM.