| From | Message |
Mike98
Dodge Dakota
JOIN HERE
2/09/2008
16:46:00
|
 Subject: Gas mileage help
IP: Logged
Message:
My gas mileage has really taken a turn for the worse. Over the past 3 months, it's dropped steadily but rapidly from 22mpg highway to 13mpg highway, under the same driving habits and weather conditions. This is the 98 3.9L engine, in a 2WD/3.55/auto configuration.
I have replaced the O2 sensors and checked the ignition over, to no avail. Plugs look OK. The check engine light is NOT on, it runs smooth at all speeds, and still had acceptable power for the v6.
Does anybody have any advice for what to try next? I'm going to try new plugs/wires, but I'm doubtful that this will help.
Also, might there be engine codes in memory, even if the check engine light is off? Should I try to get autozone to read the codes anyway?
|
TuckerLdlow
Dodge Dakota
JOIN HERE
2/11/2008
10:07:16
| RE: Gas mileage help
IP: Logged
Message:
If you haven't already, try giving the throttle body a good cleaning with carb cleaner. Use a new gasket - just a couple bucks. Remove the throttle body, and then remove the MAP and IAC from it (don't need to remove the TPS). You need a TORX 25 screwdriver to get those sensors off. Use a toothbrush or parts brush to clean it all up. Exercise care with the plunger - damage it and you'll have to replace it. DON'T try to clean things with the TB on the engine - it'd make a big mess, and you don't want all that crap down in your engine. Put it all back together, fire it up and let us know what your results were.
|
brianj
Dodge Dakota
JOIN HERE
2/11/2008
11:13:14
| RE: Gas mileage help
IP: Logged
Message:
well something this drmatic could be something rely simple like a bad tank of gas. let it run bone dry and fill up at a quality station ex. bp etc....
|
Ross
Dodge Dakota
JOIN HERE
2/11/2008
12:49:46
|  RE: Gas mileage help
IP: Logged
Message:
Look at your air filter, make sure it is breathing almost the same as it did stock, Look under it and make sure that there is not anything in your air box. I can't say what is wrong, I will tell you this, if you get people at any auto parts store to check it for you, you will have a problem, a problem you can fix by buying something from them. The only other thing I could think of is clean your throttle body and check you vacuum lines!!! All this plays a drastic role. Good Luck
|
boomer8201
Dodge Dakota
JOIN HERE
2/11/2008
20:59:44
| RE: Gas mileage help
IP: Logged
Message:
I've been having some crappy gas milage myself but I figured it was the drastic drop in temp. but i'll take this advice myself and clean up everything and sensors. is there a way to test sensors to see if they are workin properly besides a scanner?
|
Dan M
Dodge Dakota
JOIN HERE
2/12/2008
20:43:05
| RE: Gas mileage help
IP: Logged
Message:
what kind of oil do you run? dino or synthetic?
In my old 95 3.9L i dropped to under 14 at best. turned out to be sludge. I did engine flush at pepboys, cleaned it out.
cheaper mans version is run marvel mystery oil (read about it, never used it) or quart of tranny fluid. change oil add qt of tranny fluid and new filter, run a few hundred miles. do oil change with all oil run a few hundred miles to flush. Then go back to reg oil changes. IF you do this always keep an eye on your oil pressure. If you see it start to rise, it may be clogged filter.
that's why engine flush is worth the money. drain oil run cleaner in at 42 psi in filter port and vaccuum out oil pan.
after I did that, normal milage was restored. the 3.9 is easy to get oil sludge/build up inside.
- Dan M
|
sport02
Dodge Dakota
JOIN HERE
2/14/2008
02:17:30
| RE: Gas mileage help
IP: Logged
Message:
yeah, about the sludge. a mechanic told me if i wanted better gas mileage to switch over to all synthetic fluids mostly the engine and differential. he said when i did it only put a quart of synthetic in and the rest conventional. he said the synthetic would act as a cleaner at first and would dislodge a lot of sludge. then next oil change switch over completely to synthetic and resume normal oil changes but 6000 miles for synthetic or sooner if you've got the money. is sludge normal for these engines? i have a 2002 3.9 and have also noticed a large drop im MPG but have a great running truck and at 82,000 when i put the first quart of synthetic in. I have about 1800 miles so far with the one quart added. the oil pressure is high when it's cold of course till it warms a little. can anyone expand on the whole sludge issue?
|
Bob Lincoln
Dodge Dakota
JOIN HERE
2/14/2008
09:04:09
| RE: Gas mileage help
IP: Logged
Message:
Get the codes from Autozone. Many can be stored without a Check Engine Light.
Look for the obvious vacuum hoses and wiring problems first, before buying sensors. Check also for FUEL LEAKS everywhere, especially if your power and performance are still OK.
|
Mike98
Dodge Dakota
JOIN HERE
3/03/2008
03:50:26
| RE: Gas mileage help
IP: Logged
Message:
Thanks everybody for all the help! Here's what I have been able to do over the past few weeks:
-New upstream O2 sens (I mentioned this already)
-Seafoam treatment through TB and crankcase
-Run a bottle of Techron through the gas tank, and buy only Chevron since (can't afford that much longer!)
-Change plugs to NGK FR-5 w/ new new no-name plug wires (replacing Bosch plugs that looked OK...)
-Change oil from dino (Castrol GTX) to Mobil 1 high mileage (synthetic) to try to work on sludge issues
-Remove and clean throttle body and related sensors, reinstall with new gaskets.
-Check air filter (K&N, looks fine)
600 miles later, here are the results:
-Much improved throttle response from stop
-seemingly more power at lower RPMs. I can hold certain grades in OD that used to require a downshift.
-Best tank so far: 17.0 MPG. An improvement -- but nowhere near the 20-22 range I was getting a year ago.
I'm worried that it might just be my engine getting old. When doing the plugs, I tested the compression, and it wasn't great. 160 psi on cyl. 1, but only 110 psi on cyl. 6. Adding oil raised it to 130 psi.
So next I will try the engine flush, it sounds like the best way to deal with sludge. I hope it will pay off with a few more years of service for this old truck.
I wasn't able to get AutoZone to read the codes for me (apparently they don't do that out here in SoCal... The guy looked at me like I was crazy when I asked.)
But there's a silver lining in everything... Checked the mail today, and I have to pass CA smog this month! So this tune-up will hopefully pay off with a clean bill of health for my truck.
I'll keep you posted, especially if I fail smog. But thanks again for the help!
|
daddio
Dodge Dakota
JOIN HERE
3/03/2008
08:33:01
| RE: Gas mileage help
IP: Logged
Message:
have you thought of proper tire alignment and inflation? also, if your area uses "winter blend" gas that could account for some of the mileage drop.
|
P2gee
Dodge Dakota
JOIN HERE
3/04/2008
10:32:18
| RE: Gas mileage help
IP: Logged
Message:
<<<<I'm worried that it might just be my engine getting old. When doing the plugs, I tested the compression, and it wasn't great. 160 psi on cyl. 1, but only 110 psi on cyl. 6. Adding oil raised it to 130 psi. >>>>>>>>>
50 psi difference between cylinders on compression test is unacceptable and probably the cause of your milage woes. Low compression usually translates into low manifold vaccum across the driving range. On MAP based fuel systems low manifold vaccum means more fuel is added. Low compression on MAP based systems is bad news because more fuel is falsely added and then there is not enough cylinder compression to deal with it. If you end up pulling the heads, i would go ahead and pull the front end off and replace the timing chain. Worn chains put MPG in the tank too!
|
Mike98
Dodge Dakota
JOIN HERE
3/04/2008
13:59:09
| RE: Gas mileage help
IP: Logged
Message:
Well the good news is that I passed the smog test.
But on the way home from the test I topped it off (at $3.70/gallon - holy sh!t) and found my latest average was 15.9 mpg. So probably my days of 22+ mpg are over...
I just checked the vacuum at idle it was a steady 18", which seems OK. But it makes sense that it's injecting too much fuel, at least in open loop - the exhaust smells WAY rich when the engine's cold.
Well, it could be worse... Truck still pulls well and the body and interior are in great shape. I think I'll hold off on an engine teardown and/or new truck purchase until something less avoidable comes up.
I just wish I had a V8 under the hood now, since I'm buying enough gas to feed one...
|
J and J Auto
GenII
3/05/2008
00:30:11
| RE: Gas mileage help
IP: Logged
Message:
fuel pressure 40 min
the higher the pressure the better atomization
temp reader sensors
IAT
Engine Temp
reading cold more fuel
Brake drag bad wheel bearing dry u joint all
will hit gas milage tire pressure also
good plugs and wires NGK Vpower run very well
$2.25 ea
synthetic fluids, tonno cover will help mileage
restricted exhaust or cat, vac leak
bad map or tps I recomend replacing these at
100,000
Before you replace any sensors they should be
looked at with a scanner threw there full range
from cold start to operating temp
a lazy sensor is bad news and will not trip any
codes
o2 sensors should go into closed loop within
2 to 3 min from cold start
all things to consider when gas mileage go bad
these trucks stock 17 to 19mpg with just a few
mods 20 and better
I as all my customers are all getting 20+ hiway
my 95 4x4 19 20 winter 23 to 25 summer my best
was 25.8
But most mechs see no codes so there is nothing
wrong not actually looking at each sensor
individualy to see how they are operating
most will not take the time to do this so many
times after being to a dealer getting crap
mileage and running slugish being told nothing is
wrong I find the problem by taking the time to
look closer
Vac 18.9 also baro reading if this is off your
mileage will be bad the baro is read when key
is turned on
good luck
Larry
J&J Auto
|
Bob Lincoln
Dodge Dakota
JOIN HERE
3/05/2008
08:09:41
| RE: Gas mileage help
IP: Logged
Message:
Check compression again. If one cylinder is really 110 and the others 160, that's the main source of your trouble, and won't go away.
However, more improvement can be gained with a new distributor cap and rotor. I see no mention of changing those. Use the ones from NAPA (Echlin brand) with brass-plated inserts.
|
boomer82
Dodge Dakota
JOIN HERE
3/05/2008
19:58:07
| RE: Gas mileage help
IP: Logged
Message:
Looks like I'll be lookin at a few things soon. Fuel pressure, IAT sensor, TEMP sensor, vacuum, MAP sensor, TPS sensor I actually have one new in the box, maybe I should put it on huh?
my buddy works at a dodge dealer and he can bring home the scanner if I ask him to. works out good for me! need to change trans filter and fluid. anyway to get the fluid out the torque converter? I heard take the a line of the trans cooler and run the truck till it stops? dont remember the other way
|
| | P 1 |
|
Post a reply to this message:
Username Registration: Optional
All visitors are allowed to post messages
|
