[Vilpo]

So i've been getting a little bit bad mileage with the car for a long time now and after checking just about everything else i desided to check the o2 sensor again, though i replaced it like 6 months ago.
On idle, it bounces over and below 0,5 volts, for taking a bit over a second to go over and back. So that sounds a bit slow right?
Well, i tested it around 2000rpm and it was some faster, it went some under a second to go over and back.
So is the sensor bad and giving me low mileage?

 

[NAJ]

The O2 only reacts to air in the exhaust and then the controller makes small fuel corrections and the O2 will react again, that is why it crosses above and below .450 volts, the idea being that the fuel corrections will keep the voltage as close to .450 as posssible.
It sounds like you have the symptoms of a lazy O2.
It is hard to tell if yours is lazy especially since it is new as you do not know how quickly the signal is being sent, they make testers that send a signal and check time to respond.
The O2 is usually slower at idle than at higher rpm.
Usually a lazy O2 will reach the top and bottom of the range before crossing, what voltage are you seeing? Is the voltage staying in a .3-.7 range or is it in a .1-.9 range?
Is it staying on the lean (below.450) or rich(above .450) range longer on one than the other?
The O2 can become contaminated by certain fuel additives and certain RTV's.

 

[Vilpo]

I pulled out the sensor today to check it. It was all black on its surface. The signal is about 0.3-0.9 when its crossing. Is it normal, or should it give a bit lower readings, as you mentioned NAJ?
Also i noticed that if i give it some throttle and let off, it drops down to about 0.2 for a second or two..

 

[NAJ]

It appears to be hanging on the rich side too long(.9 volts) but is this caused by a lazy sensor or something else causing the vehicle to be slightly rich and it is taking longer to make the fuel correction?
Be sure the engine is reaching operating temp(a 160 stat may cause this problem), you have 16-20"hg to the baro and map, ignition timing is correct, there are no restrictions in the air intake system,fuel pressure is correct, FPR is not leaking and the injectors are not leaking down.
If all of that is correct replace the O2, that is the only way you will know.
.2 volts on decel is probably normal as the mixture is leaned on decel.

 

[Vilpo]

Well, my guess is that it could be something else to make it too rich since when i replaced to this o2 6 months ago, i noticed it wasn't still making that good mileage, as it was like last summer (33mpg on highway). Even though it was a bit better than now (26mpg highway). Even though i think the city conspumtion has gone to the worst (15mpg). And it still pulls well on small throttle.
I think timing should be ok, since i just checked it during timing belt replacement. The two things i'm not so sure about are tps and map. The prob with tps is that the signal range is 0,8-3,7v. When many sources say it should be 0-3v or 0-5v. And the weird thing about map is that the signal is quite normal (reacts well to throttle), but sometimes signal is 1,6v at idle and sometimes 1,1v at idle (when warm), which seems more normal to me. Maybe thats just the idle air valve opening more when cold?
I might be getting to use a scanner near days, so i can confirm those sensor readings then..

 

[Vilpo]

Ok, i tried to clean the sensor by heating up with a welding torch and i think i got most of the carbon off. It didn't get much quicker on idle but now it's mostly between 0,3-0,7 on idle. Though, if i give it a touch of pedal, it goes quicker but between 0,3-0,85. So does this mean something's enrichening the mixture too much above idle? NAJ, do you have an idea about this?

 

[NAJ]

It sounds as if the O2 is functioning normally.
What are you considering poor fuel mileage?

 

[Vilpo]

Ok, thanks for the help. Well, right now i get like 9-11 mpg average. And things i have noticed is i used to get about 30 mpg on the trip computer, when driving 30 mph in city, now i get 15 mpg at the same speed. Also i used to get to 35 mpg when driving 50 mph, now i get 26 mpg at that speed.

 

[NAJ]

Have you checked the mileage by fuel used vs. mileage driven?
What work if any has been done to the vehicle since the poor MPG condition appeared?
Don't forget your MPG will be lower in cold weather.

 

[Vilpo]

Yeah, i've been resetting and checking the trip meter too, everytime i fill gas so i can confirm the consumption and i get even a little bit worse mileage calculating it from there...
No work really done to it until the mileage started going worse.. After that i even changed to o2 one time and it made consumption a little bit better, but not where it originally was.
I finally got to use a scanner yesterday, and i checked the tps/map/coolant sensor values seemed all normal. Only thing i was wondering about was the "long term fuel trim" -value that was -19,5%. That means it has been running too rich, if i understood correctly?

 

[bhazard]

Well going by how much gas you're using Id say you can afford to try another o2. Maybe your FPR is leaking or something?

 

[NAJ]

I have never found long and short term adaptive fuel trim to be effective in diagnosing any driveability condition.
Long term fuel trim is going to change as an engine gets older and loses its efficency.
There are also a lot of variables in fuel efficency.
Driving habits, weather conditions, additives in the gasoline and changes in the refinery process in your area, engine mechanical condition,fuel system efficency, ignition system efficency, cooling system efficency, tire pressure,etc.
If you want to eliminate all possibilities you have to check all systems. Do a compression test, cylinder leak down test,check cam and ignition timing, fuel pressure, injector balance test, check ignition system efficency on a scope, be sure the clutch is not slipping.
Also on our cars a vacuum leak regardless of size will create lower MPG without causing a rich indication. A central vacuum leak allows additional air thru the entire system, the MAP sensor sees the additional air and the controller compensates with a longer injector pulse width. A large leak will create a high idle condition, a smaller leak may not give any indication other than your MPG concern.
Once you have verified all basics are correct take a good look at baro readings and MAP voltage/vacuum.The lower the vacuum to the MAP the longer the injector pulse width will be. Engine mechanical affects vacuum.
The O2 sensor is only a trimmer and reacts to air in the exhaust to keep the fuel in tight parameters, if anything else is not correct the O2 cannot do its job.

 

[Vilpo]

Well yeah, i guess i could try another o2 too, maybe a genuine mopar sensor this time. No, i don't believe in leaking fpr, since i just put in a new adjustable fpr to keep fuel pressure in correct specs (something i've talked about under another topic here).
Compression should be ok, since i did an engine rebuild about 8 months or so ago replacing all top end seals and piston rings. I just did a cambelt replacement, during which i checked cam timing and ignition timing to be correct. Scanner showed ignition timing to be about 24 degrees btdc at idle, does that sound correct also?
Fuel pressure/pump delivery rates have been checked. Injectors delivery rates were checked the same time. They were in spec. Spray patterns seemed good. No clutch slipping, since it's an automatic. I don't really suspect an air leak, since i replaced all the seals in top end, and have tested it with some spray solvent, also idle seems very good and steady.
I read somewhere that the long term fuel trip shouldn't exceed 10 degrees either way, you wouldn't agree with that?
What's a normal baro reading? Just an estimate would do, so i could check the baro reading on the scanner..

 

[bhazard]

Double check all that stuff anyway, fuel pressure, timing, all that stuff.

 

[NAJ]

I have never it long and short term fuel useful but a lot of techs do, I can tell what fuel system is doing by watching O2 volts and crosscounts.
Also I do not believe long and short term fuel were available until OBD2. Long term is a very slow fuel correction as the controller learns even after a repair has been made, short term chases the O2 sensor and is a quick learn fuel curve.
I am not really sure what long term should read, on brand new cars I see long term at
-7%.
Baro should read whatever the atmospheric condition is at your house when you read it, check the weather online for your zip(I have YaHoo weather on my toolbar) and go see what the controller is seeing, with the key on/engine off Map and Baro should be the same.
If you like I will post Chryslers diagnostics for code 52 "rich exhaust indication"

 

[Vilpo]

Ok, yeah the baro reading is just about correct 110kpa around here.
Thanks NAJ, that could be useful with this problem.

 

[Vilpo]

NAJ, can you post the "Rich exhaust condition" explanation here?
I replaced the o2 sensor and i put the car on scanner right away. The pulse looked much better right away, switched some quicker than the old one.
Economy was maybe 3 mpg better than before on highway. In city it still seems the same.. And long term fuel trim went to about -30% now. So i guess there is still something not right..

 

[NAJ]

Not exactly sure what you are looking for.If the O2 is crossing (twice in 8 seconds according to Chrysler) the system is operating normally.
Since you have a driveability concern (poor fuel mileage) without fault codes these are the tests Chrysler wants run in order to narrow down and isolate the cause.
Secondary Ignition and Timing,(including advance) (using a scope)
Fuel Pressure
Coolant and TPS Calibration
MAP Sensor Calibration
Idle Speed Motor Operation (Using the DRB2)
Park/Neutral Switch Input (Auto Trans)
Engine Controller Power and Ground Circuits
Engine Vacuum
Minimum Idle Air Flow
Engine Mechanical
I assume we are working on a 91 3.0L, what is a Saratoga?

 

[Vilpo]

Ok, thanks. The car is i think an european variation of Dodge Spirit, an AA-body?
One thing i was wondering about is that ignition timing, when i adjust it to 12 btdc, coolant sensor removed. When i reconnect the sensor, timing is about 23 degrees btdc at idle. I also checked it on scanner when i was driving, it went between 20 and 40 degrees btdc. Do those numbers sound normal?
I guess i will also check the powers and grounds for ecu, just incase.

 

[NAJ]

All I can find on spark advance is "does it advance with rpm?", if so it is ok.
If you follow Chryslers diagnostic sequence for a driveability concern without a fault code and find no problems they say replace the engine controller.
I would suggest that since you have a mpg concern and have been chasing this concern for quite a while that you follow Chryslers diagnostics in order so you do not miss anything.
Step one is to check ignition patterns using an oscilloscope. Basically they want to be sure the ignition system is capable of producing a minimum of 25 kv and that all cylinders are firing in the normal range 10-15 kv. A scope will give you indication of ignition problems(weak coil by looking at coil oscillations, open or shorted wires, resistance problems in the cap/rotor), rich/lean in a cylinder(s)(rich = lower than normal kv, lean = higher than normal kv) or high/low kv caused by a mechanical problem and can narrow down a problem thru the entire rpm range.
Here is the beginning of the "No Fault" Diagnostics. As you complete each let me know and i will post the next couple of tests.
I apologize for the poor image quality, my Son had this manual stored in his toolbox and it got damaged from moisture.