Okay, so let me start by saying what I did to my truck....

New plugs, wires, coil, cap, rotor, thermostat 180*?, 50mm TB (plenum fix and kegger mod and new Cat done 2 years ago)

internal.. had head surfaces machined, installed new rockers.

(didn't change cam yet)

New piston rings, new front and rear seals, new freeze plugs, honed cylinders, new oil pump, and rebuilt transmission.

(My friend is about to sell me a wrecked Dakota w/ a 5.9, so I may try and build that up and drop it in...)

Anyway, lets get to the problems...


Okay. After 200+ miles of highway driving. I had filled my tank, drove to Baton Rouge and back (140 miles) and topped my tank and it took $32 dollars, or 11.6 gallons.

So on th interstate and keeping it under 2,000rpm (around 60 the whole time so 1,600) I got around 12mpg.

Thats lower than what I used to get in city driving.

Also my engine is running under 160* leading me to believe my thermostat is defective or I was sold a 160* stat. Then my computer tells me my O2 sensors both read .9 volts. Now isn't that the highest voltage (richest setting).

I used a stethoscope to check my fuel injectors and they sound like they are opening and closing.

But I really believe my truck is running way to rich.
I did have the timing retarded before, but now I have it back to stock. I don't think that has anything to do with it.

I know a cold engine runs rich.. So could a bad thermostat that is causing my engine to run cold, be causing the problem? (I run on the very lowest point on the normal range barely above 130)

Could I have bad 02 sensors, (but I thought if they showed voltage they were ok) mine both show .9 on the computer.

MY TROUBLESHOOTING STEPS...


1. Check voltage @ the o2 sensors via multimeter.

2. replace thermostat, cause something is wrong with that


I want to buy a tuner so I can flash my PCM with an economy setting.

I normally drive a 30+ mpg car, but want to get around 15-17 mpg hwy..

I'm taking it to BR again tomorrow. I'll come back to this thread with more info.

 

I'm not sure at what temp the engine goes into closed loop at; but 160 is pretty cold.

If it's not getting to closed loop (which I *believe* happens at ~175-180) then the o2 input is null--the pcm isn't listening to them. What it is doing is running off of preset tables; which would drop mileage considerably over a long trip like that.

Take the stat out and test it, it may be stuck open. Also, it should be stamped somewhere on the bottom with the opening temp.

You're not getting the "engine cold too long" (p1281, I think) code?.... strange

 

no I'm not getting that CEL...

I am familiar with it, because a couple of years ago I had an overheating problem, and I ran with no thermostat and got that code.


Going through my garage I found a few thermostats lying around, I'm sure at least two are for this dodge. Now if the thermostat was in a normal spot, I would change it right now, but its a PITA on this truck so I have to do it after work tomorrow.
I can't believe how cold its running, with Max AC on and a long drive it still ran very cold. It never gets warmer than say 150*-160* even after two hours of continuous driving with max A/C.

I thought rich due to the amount of fuel burn and the high voltage on the leading o2 sensor.

I guess too cold is better than too hot, but I've never seen it this cold.
It could be a bad engine coolant temp sensor, but I changed that when I did the pump 2 years ago.

I did a lot of work to this truck in the last few weeks.

I don't want this problem, and now I see I also new new upper ball joints (boots dry-rotted from sitting in sun for over a year) and I have a weird noise coming from the right front wheel (sounds like a warped rotor).

So the work is never done... Not to mention broken cupholder and after market alarm isn't working anymore, causing my door locks to not work.. :banghead:

oh yeah, and my ignition lock doesn't work, don't need keys at all, and I live in the most crime ridden city in the US....

But hopefully I'll get this thing going good again. (though using a plastic bumpered truck to push cars around the other night didn't help the paint job)

 

Damn man, it never ends does it? lol, I feel the same way (though not quite as bad).

I was going to mention the ECT, but if it was the culprit, you'd be having a/f trim issues. If the ECT is saying 160 (but you're really at, say 190) the PCM would trim to make it run hotter because it thinks its below operating temp, when really it's as hot as it needs to be.

In short, your truck would run, but it would run horribly because the PCM cant figger out what the ideal a/f ratio is. That's what happened to me when my ECT shorted out.

 

Are you sure its running colder, or is the thermostat just saying it is? I know that on some of these trucks that the Gauge sending unit and the PCM temp sending unit are the same. But they're all electric and measure temp by the resistance right? I am wondering if the resistance isn't getting changed somehow? Or am I completely off base here?

 

The easiest troubleshoot will be to start with the T-stat.

I expected my computer to go through a "re-learn" process, as I did change a lot of stuff, and it sat for so long.

(I also didn't mention I also need an alignment and I have play in the wheel)
Honestly I'm not looking at much money to fix just time....

Anyway, gonna take the stat out, test it, and make sure I have a 180* in place (or would I be better going to stock 195*I think)

 

Do you have a scan tool? If so can you read coolant temps with it? If so then use it to check coolant temps to make sure your gauge sending unit is correct.

Also you could try to block off SOME of the radiator on your next trip with cardboard to see if higher temps help fuel mileage.

Did you put in colder plugs when you changed them?

Could the head work be hurting your fuel mileage?

 

Colder plugs have little to no impact on engine temp. The heat range refers to how hot the ground strap gets. Cooler heat range spark plugs just move the ground strap further from combustion (where it is colder) to reduce predetonation and spark knock caused by a hot spark plug.

The head work would improve efficiency if anything.

 

The 99 engine only has one temp sensor for the PCM, and the gauge. So, if it is lying to the PCM, that will be reflected on the gauge.

If the PCM believes the engine is still "cold", it will richen the mixture.

Get an infrared thermometer. (the little gun thing you point at stuff, and it tells you the temp.) Point it at the thermostat housing when the engine is "fully warmed....", and see what it says. This will tell you if the ECT sensor is lying. If the gauge is accurate, replace the thermostat, if it isn't, replace the ECT.

If the O2 sensors are showing a fixed voltage, they are not working the way they should. This may be due to other issues..... Or, they might just be bad.

I am not sure what temp these engines go into closed loop.... and I am unable to come up with any information on it either... might call dodge customer service on this one.... But, on the GM engines, closed loop would occur at temps as low as 106 degrees, and that was WITHOUT heated O2 sensors......

 

Scan tool (in degrees C*) confirms dash engine temp. Since I have a 180* thermostat handy as well as the working old stock one, I am going to change that.

I just drove the truck into my work area, letting it cool down a little, and am about to pull the stat.

Though I do think there is an 02 sensor problem as well.
I know these things don't get great mileage, but I was hoping for an improvement, this mileage is worse than when my plenum was leaking..