terrible gas mileage 3.4L v6

Great info...

 

changed the plugs this morning, they were the stock 2 prong plugs and I only had to remove the one bracket for the middle plug on drivers side. the plugs still looked new with maybe a few thousand miles on them, no sign of any wear, so I doubt they had any issues but they were lose, I only needed mild screwdriver turning effort to unscrew them so no "breaking loose" was needed. that said there was no sign or indication they had any leakage and they were "barely" snug.

found a hard pcv hose but didn't appear to be leaking so I replaced it.

the air duct and valve body is dirty (but not excessively) so that's probably the rough idle thing and I will clean that after lunch, right now I am fiber glassing my wiper reservior neck that is cracked so waiting on it to dry before I mess with anything else.

I did remove the fender duct next to the air box and cut off the 90 in the fender duct then I reinstalled it so it will get better airflow, its my own version of the deck plate mod which I think is a much better way to do it and it completely retains the stock look of everything. it works just as well as the deck plate mod but its completely covered so you never have to worry about anything getting mud splashed up in there like you do with the deck plate mod. course I don't go off road anyways.

as of right now i cant see any reason for the lower range mpg but maybe that's just the way I drive but I maintain the speed of traffic on the highway and off the highway I drive like grampa has no where to go so I doubt that's it.

I haven't tested the mileage on the last two tanks (I kept doing things like dash light mod and pulling fuses looking for shifter light fuse so that stuff reset the trip meter lol) but without doing anything it did seam like it was suddenly getting better mileage just based on the amount the gas gauge is moving to do my regular 140 mile round trips

 

ok done, and the valve body was pretty dirty where the butterfly comes close to contacting the body. the IAC holes were filthy as well but without the EGR system it wasn't nearly as bad as my 99 was when I cleaned its TB.

its all clean now and I reset the ECM so hopefully that did it. its cold idling at 1800 rpms now lol, but im sure that will correct itself in short order.

too soon to tell about mpg till I put a few tanks through it to let it reprogram but it seams to be much smoother running and idling.

I must say holy shit the 3.4 is easy and simple to work on, my 2.4 has like 6 hoses connected to the throttle body and 3 of em are underneath where you cant even reach or get to them without special long pliers at just the right certain angles then there are plugs under there also equally impossible to reach and the throttle body bottom bolts cant be seen or reached easily. this 3.4 is a dream to work on, there is literally nothing to it.

 

I just got the timing belt, all other belts and water pump done @ 112,600. The engine is running a lot better (smoother, quieter), and the fuel gauge does not seem to be moving as fast as it did previously.

 

Good to hear, I get 18 mpg highway with a lot of heavy stuff bolted on and 33s, so you should definitely be getting more than me. I religiously use chevron gas and run a bottle of Techron concentrate through the fuel system every oil change. I just replaced my plugs, wires and PCV valve+grommet at 150k. I also use Mobil 1 0w-30 oil and just did one dose of water decarbonization through the PCV. I try to change the air filter yearly.

FWIW my truck idles around 750-900 rpm when warm and in neutral. 600 seems a little low to me.

 

well cleaning the throttle body made a but difference because it wasn't cold idling on warm up at 1800 rpms before lol, it was not something I took note of but it wasn't anywhere near 1800 before that's for sure. im thinking the cold idle screw (if it has one) might be out of adjustment, just never know whats been tampered with on used vehicles but the top screw seems to still be set from factory with the yellow paint mark

 

final update:

ok I just took a 220 mile highway trip and I got an average of 18.3 mpg now so im calling it fixed.

18 mpg is so much better then the 15 mpg average I was getting on the highway and I was doing 75 mph the whole way running at 2500 rpm so I think 18.3 mpg is pretty darn good considering so it would appear cleaning the TB is what did the trick. I bet if I had held it to just 70mph or even 65mph then maybe I would have gotten 19mpg out of it.

the rough idle was because the IAC holes were full of crud and partially blocked and the dirt buildup in the TB around the butterfly must have altered the fuel air mix to cause the poor mileages I was getting. I know the plugs I took out still looked new so im sure they were not the problem at all.

I will run several cans of fuel system cleaner through it now and see if it improves any more but I think im on par for the dealer listed mileages now, I think they claim its 17/19

 

Just returned from a 2k mile trip to IA (I took the scenic route through NE). Highest mileage per tank was 21.9 and lowest was 17.3 (there was a really strong head wind) with a mean of 20.5.

I have a 96 3.4 V6 XC, 4WD, manual (hubs too). Transmission, transfer case and diffs have synthetic gear oil. Don't know the condition of the plugs, injectors, MAF cleanliness. I drive like the old man I am

I bought this truck in Jan '14 with 163k miles (post trip 170k). Since purchase and before trip I have replaced the rear springs (after market stock), front & rear shocks with new coils on the front, upper and lower ball joints, drive shaft carrier bearing, speedometer cable and the AC condenser.

 

I should probably clean my IAC as well. I've never done that before. My idle is pretty smooth but not perfect.

 

on the v6 its supper easy, well for mine it was for sure.

just take that small long top plate off (4 screws) and there is nothing in it, literally, its just a bunch of holes, so you use a tooth pick dipped or sprayed with a little carb cleaner to slide in and out to clean the holes out real good and your done, replace the cover and that's it, you don't even take the TB off or unhook anything.

but that said, if the IAC needs cleaning why not go ahead and clean the TB as well

 

My TB is probably pretty clean, at least it was when I changed my plugs in the fall. But the IAC I have no idea. It's probably pretty dirty.

 

Before this was done, I'd be lucky to get 145 miles per half tank. I have over 220 on a tank, and still have more than 1/4 tank. This is no placebo, maybe is time y'all do your belts and ancillary service!

 

proper maintenance is there for a reason. the manufacturer states the lifespan of "wearable" parts and replacing them at the proper interval will keep the vehicle running at optimum performance. this will help maintain the EPA and manufacturer's stated specs.

driving habits will help but to each their own as long as you know and accept how you drive

 

On my previous tank, I got 292 miles for the tank, from the same gas station. I divided the miles per tank, vs the refuel and it yielded an average of 16.75 miles per gallon. Last night, I refilled the tank after 276 miles. I still had 3 gallons left, and when divided by the gallons, I got 18 miles to the gallon.

I should say, that I use synthetic oil for the engine, and rear diff. 1.25 additional miles per gallon may not seem like much, but to me it is. That coupled with a more responsive engine who does not feels like running out of air, is a win in my book!

 

I do mostly interstate driving at 65mph I get 19-20. I also run Cf5 injector cleaner every oil change and 44k every other oil change.

 

looks like im staying at a steady 17 mpg average

 

I recommend you check the O2 sensors. My buddies 03 V6 was getting like 10mpg and it turned out it was due to a bad O2 sensor. cost him about $80

 

yep, it started running a little rough again at times until it fully warms up so I think I may need either a new upstream o2 sensor or new MAF sensor

 

I used to get about 9 mpg in my 1st gen... Then again I was lifted, Locked, and had full armor running on 35's lol

 

Have you tried running a fuel system cleaner like Lucas or seafoam, Might help. If you YouTube seafoam, there's a bunch of good videos how to use it properly.