Here's the vehicle :
2000 Dodge Ram Van 1500 / 3.9l / 153,000 / 2wd

Latest prob.

My gas mileage went from 14.5-16mpg to just over 10mpg...

The mileage went quickly down hill a couple months ago (edit 1/22/15: actually it was early September) after some much needed repair work; more then just the waterpump... And by down hill it went from what I normally got for mpg, to every fill-up loosing a little more efficiency until it bottomed out around 10.9mpg. it's been here for the last three months or so.

Here's everything I did right before this started happening. Replaced Water pump, thermostat, radiator, air filter and serpentine belt. flushed the coolant system a bunch too of course.

The spark plugs, wires, PCV and distributor were changed about 6 months prior with no probs detected.

Also drained the rear differential, filled it with 77w90 and changed the engine oil out to full synthetic (based on the advice of a "master mechanic" who swore this would help me get more MPGs. I read a ton on it too and it didn't seem irreversible or like it would damage anything). had a little gasket leaking but Lucas seems to have fixed it.

last week I replaced the Idle air Control sensor with high hopes since the other was burned up. The engine sounds better and runs smoother but no MPG improvement...

not that it matters all that much but last week I also replaced all the ball joints, idler arms, and tie rod everything, front wheel bearing and brake pads. It is superbly aligned and tires are balanced and great. Was hoping this would also help with the diminished MPGs.

The next thing I might try is replacing the o2 sensors. Check engine light came on briefly and it was diagnosed as the rear one being faulty. But the light went off and never came back so i kinda forgot about it... plus the computer was reset (i think... questioning that now..) and it still didn't come on.

The van runs a little rich in my opinion. It also (slightly) bogs out when you step on it, feels a little mushy. I don't remember it always being like this. and there is a faint squeak from the front now. Like the fan belt or something... It's all new. Maybe the tensioner is going? I don't know.

I know it's a lot, probably gave you too much info, but any ideas will help guys.

 

Replace, or check the Ohms on the 2 wire coolant temperature sensor on the intake manifold. The one wire is a temp sender for the gauge

Make sure you used a 195f thermostat, not a 185 or a 165 t stat. The engine computer is looking for 195 and will make it run rich with 185.

O2 sensor, get NTK or Denso, not Bosch.

Make sure the catcon is not plugged or getting there.

Recently I had an issue with my radiator cap not allowing the radiator to draw from the overflow tank on cool down, and air got trapped in my intake manifold. My temp gauge started reading high, and mileage took a dump, down to the 10.5 range when I should have been getting 15 or better for the terrain I was driving.

Once I refilled and got a new radiator cap, no more issues and MPG back to normal.

Good luck.

 

thank you so much for taking the time-

the thermostat is a 195f and I have tried switching radiator caps to no avail. The vans temp stays even and low all the time. but I will check that wire.
I appreciate the o2 recommendations to. Just started my research.
very well could be exhaust probs too. I havent touched any of that since owning it.
Will report back after I dig in.

 

as you continue to chase those 'lost' MPGS, you may find yourself with diminishing returns on your investments. I'm sure this isn't anything you don't already know but sometimes we get tunnel visioned and lose sight of the bigger picture.

At any rate, losing any oil? If so, may be a plenum issue.

When you say temp stays low, does it at least come up to operating temp (lets say 1/3 up the temp gauge)?

Also, for the parts you replaced, particular parts are encouraged (as noted with the 02 sensor). For instance, you want brass contact rotor/cap.

Parking brake dragging? Feel the rear hubs after driving a bit. Abnormally hot?

Winter blend fuel can drop MPG. All of these things can compound...

 

I would be suspect of the gas you got...maybe it was tainted with water or lots of ethanol...


FWIW my 3.9 conversion gets 11-12 city 14 hwy...never more...never less

 

If the temp gauge is reading low, it is likely one of two things.

1. The temp sender is done. When mine failed it refused to go much above the first hatch mark, where normal is midway between H and C. It was under 6$ at AZ, but replacement was a bit tedious due to location under my AC compressor. Replacement returned the full range of the gauge.

2. It is actually running too cool due to a faulty thermostat, or perhaps it got stuck open. These fail safe t stat sold these days are known to go into fail safe mode, which locks the T stat fully open. Heat soak after shutting the engine down can be enough to trigger the fail safe, and I believe this has been an issue on Mopars. I know if I shut mine down after driving up a hill, then restart a minute later, the temp gauge shoots to the maximum, but then quickly drops as the water pump starts circulating coolant, and I think this type of heat soak event could be more than enough to trigger the fail safe locking arms.

3. the gauge itself can be inaccurate.

I'm not really sure what is still a good T stat these days. SuperStant used to be good, but then their manufacture was offshored with the expected rise in quality control complaints. I have a Motorad, and it appears to be lazy on the initial opening of the day after a cold start, and I would not recommend them.

Running to cool is also bad for the oil as it never really gets hot enough to burn off condensation, especially in short trip driving. Cloudy milky oil under the oil cap is a sign the oil is not getting upto temperature.

Vacuum leaks should also be considered as a detriment to MPG.

Have you performed a Key dance to pull the computer codes?

 

3. Fan Clutch is defective.

OP didnt clarify "low/even" as in overly low temps, i.e. - below operating temp

 

QUOTE - as you continue to chase those 'lost' MPGS, you may find yourself with diminishing returns on your investments. I'm sure this isn't anything you don't already know but sometimes we get tunnel visioned and lose sight of the bigger picture.

I have thought a lot about that and Im gambling for sure but this van is in such great shape. No rust and the first 120k miles were from one elderly owner from Florida who only used it for long distance road trips up the east coast. No winters. No rust. Transmission feels great. No burning or metal flakes in the reservoir. When it's running on point the engine feels so balanced and nice.

plus I put about $1500 into redoing the inside for touring. has a adjustable futon, aux power, fridge and cab, nice sound, heavy duty towing hitch. all new interior, floors and blinds. It's really sweet and comfy. The exhaust is in great shape too. New tires, new wind shield. If i can get over this hump so much of it will be new. Hoping to get another 200K out of it. its a pipe dream but ive talked to so many people with this same engine that have done just that

At any rate, losing any oil? If so, may be a plenum issue.

I'm not losing oil at all; I was after switching to synthetic but that stopped after some lucas; it's been three months with no difference. There is some leaking from the gaskets I can see and I plan on redoing all of that this summer when Im in the Northeast at my brothers shop. Banking on those leaks not being my problem since they were there before my MPGs tanked.

When you say temp stays low, does it at least come up to operating temp (lets say 1/3 up the temp gauge)?

The temp was high and kept blowing plastic couplers used around the AC/heater tubing. Changing the thermostat fixed all that. my guess is it's right around 185 or so. about 10:45-11:00 on the gauge - before t was way past 12:00. I'd say it sits pretty at 1/3.

Also, for the parts you replaced, particular parts are encouraged (as noted with the 02 sensor). For instance, you want brass contact rotor/cap.

I believe that' is what is in there... I'll have to check...

Parking brake dragging? Feel the rear hubs after driving a bit. Abnormally hot?

Parking brake is a year old and works great and the rear brakes were redone a year ago at the same time. I haven't checked the heat output. good idea. Would the alignment guys notice if the wheels turned hard?


Winter blend fuel can drop MPG. All of these things can compound...

Sort of doubt it's the fuel since this problem has been very consistent regardless of where I fill up. maybe all of Austin area has Sh*t gas and it came here right exactly after i did all my work but i doubt it. I have thought about changing the fuel filter though i don't think that's necessary just yet. Any fuel sysytem additive you would recommend? I'm weary of that stuff.. except the Water removers. That's all legit and I should add some soon...

 

1.) interesting. I will check it for sure. althought the needle is bewteen the two hash marks. it's never lower once its run for a little

2)haven't seen that happen... i hope not- what a pain installing a new thermostat is

3) before my repairs its was showing it running hot. and it was hot as it kept blowing hoses and fittings. Not gonna write it off though. / oil looks great, clear and not milky or too black. Changed it four months ago. / Vacuum leaks - this is something I need to check although nothing came up last time someone did a diagnostic. just ordered a http://www.ebay.com/itm/231458052177?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT - I've dabbled in the past with van repairs but Im going all in. Key dance?? hummm. nope. Another thing to figure out! i have a lot to learn I know... your input is amazing and gives me plenty to think about and research. thanks

 

hope not.. that was a bear to deal with. I will check into it...

also weather here sucks but I hope to dig in Saturday. of course I will report back with everything I learn. Also looking at the coolent sensor and MAP sensors... read a bit about them last night. The engine ran way hot for awhile. No doubt it burned these things up...