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Old 03-01-2011
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mpg<>gear question...

I'll try to make this as short and to the point as possible....95 reg cab 5spd 2.3, still gettin kinda crappy mpg's.....started out with 308 gears with 245/70/16.....truck orig had 205/55/14's? i believe stock.....put 373's in it and replaced DPFE, PFV vavle and solenoid, plugs, wires, cleaned upper intake, replaced front o2......just today got my chrome alcoa's back with 235/75/15 still with 373's....rpms are up a lil which i think may help since these motors like running in the higher RPM range but we will see.CEL is not on but has been runnin a P0420 for insuffecient cat...thats because there is NO cat....i know on fords when CEL light is on computer goes into safe mode where it will dump more fuel than air....but the light is not on...code is probably still in memory though...i just dont see how NOT having a cat would cause MPG's to suffer that much....yes i live in AL so i can get away with no cat they dont check,though i may get one on it.oh i do have a camper top on back but havent noticed any big difference between that and when i had my bed cover on..... Thanks!!!
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Old 03-02-2011
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i know on fords when CEL light is on computer goes into safe mode where it will dump more fuel than air....
Not true.
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Old 03-02-2011
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I would say since you know you have a cat "problem" that's probably your best bet. Couldn't hurt to have one and what are you REALLY gaining from not.
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Old 03-02-2011
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Not true.
partly true. Anything pre 97. becasue it was OBD.
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Old 03-02-2011
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if you didnt space your o2 sensor farther away from the exhaust so that it gets correct readings with no cat then of course thats why your mpg is terrible your motors confused as ****.
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Old 03-02-2011
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Originally Posted by StxDangerRanger View Post
partly true. Anything pre 97. becasue it was OBD.
Partly true again. Its pre 95 that is obd. 95 was the change over year and this truck is a obdII.

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Originally Posted by ES894x4 View Post
if you didnt space your o2 sensor farther away from the exhaust so that it gets correct readings with no cat then of course thats why your mpg is terrible your motors confused as ****.
All the exhaust work was done before I owned the truck. So there is no telling how it was done lol.
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Old 03-02-2011
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Originally Posted by StxDangerRanger View Post
partly true. Anything pre 97. becasue it was OBD.
pre-95 you mean. 95+ are OBDII.

EDIT: NVM mike got it

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Originally Posted by ES894x4 View Post
if you didnt space your o2 sensor farther away from the exhaust so that it gets correct readings with no cat then of course thats why your mpg is terrible your motors confused as ****.
I was thinking something along the same lines. I would imagine the computer would be confused from the readings of not having a cat. It would be receiving more carbon monoxide (or is it dioxide) and other stuff the cat absorbs so it would think the mixture is incorrect. That would cause you to run rich or lean and cause the bad gas mileage. I would go with a stock exhaust setup (at least until the muffler) and see if that fixes the issue.
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Old 03-02-2011
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Originally Posted by Sonic04Edge View Post
Partly true again. Its pre 95 that is obd. 95 was the change over year and this truck is a obdII.



All the exhaust work was done before I owned the truck. So there is no telling how it was done lol.
my bad. i hate when i forget crap like that.
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Old 03-02-2011
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Originally Posted by rwenzing View Post
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Originally Posted by hozII View Post
i know on fords when CEL light is on computer goes into safe mode where it will dump more fuel than air....
Not true.
partly true. Anything pre 97. becasue it was OBD.
The mixture control doesn't go into some overrich limp mode just because there is a CEL with a P0420. The cat monitor is there only to tell the driver/inspector that the upsteam cats are no longer doing their job for whatever the reason. And, OBTW, the P0420 code didn't exist in OBD.
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Old 03-02-2011
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I can't comment on the no cats thing but I have 4.10's and 225-70R14 tires (26.4 inch diameter) and with doing mostly highway driving, my mpg's don't really seem to suffer (mine's an ext cab btw). So 3.73's or 4.10's are probably giving you your best mpg. Was the cat code the only one that you've seen? (sorry I know I'm not too helpful on this, but curious)
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Old 03-02-2011
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all good plausable points..yes exhaust was that way when i bought it...just found out there was no cat...just headers then straight pipe then flowmaster (which i hate on 4cyl) i am plannin on gettin exhaust redone as soon as i get some more cash. i can get a magnaflow cat for $70 then for the exhaust shop to install it with a fact muffler and new pipe around $150 all together........i have seen the rear o2 spacers but not sure if they're worth the money or if i should just make a MIL eliminator for rear o2. thanks bob for the info on the safety mode thing....had heard that from a mech but prob was thinkin mine was a OBD not a II. im just puzzled and frustrated with this issue. aaand found out that the orig set up was 308's with 205/75/14's now im runnin 373's with 235/75/15's.
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Old 03-02-2011
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Originally Posted by red_rider View Post
I can't comment on the no cats thing but I have 4.10's and 225-70R14 tires (26.4 inch diameter) and with doing mostly highway driving, my mpg's don't really seem to suffer (mine's an ext cab btw). So 3.73's or 4.10's are probably giving you your best mpg. Was the cat code the only one that you've seen? (sorry I know I'm not too helpful on this, but curious)
sorry meant to quote you and got lost in my last reply...lol

with the 410's and your tire size you will get more topend where as with the 373's i get better tourque and low end....ummmm yeah i had several codes before but i have fixed and eliminated all of those now..i cant even remeber the codes....but like stated above i have done the DPFE, PFV and solenoid, fuel filter, plugs, wires, front o2, new radiator, hoses,belts, motor mounts, pulled and cleaned upper intake due to clogged EGR port, new battery, altanator, and terminals, and front crank seal. i guess the only thing i havent done is replace all the seals and actually get into the motor...lol she got a lil over 300k on her and going strong just tryin to figure out this annoying fuel consumtion issue......this lil 4 cly makes me feel like im back in my LeveL II with 4.0 410's and 33x12.5's that equals out to like 15-16mpgs.....lol
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Old 03-02-2011
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Originally Posted by hozII View Post
thanks bob for the info on the safety mode thing....had heard that from a mech but prob was thinkin mine was a OBD not a II.
If it had been OBD, then this would be a non issue because there is no rear O2 sensor, no cat monitor and no P0420 code in that system.
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Old 03-02-2011
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Originally Posted by rwenzing View Post
If it had been OBD, then this would be a non issue because there is no rear O2 sensor, no cat monitor and no P0420 code in that system.


LOL!! not much a mech then huh!? i learn something new everyday!! :-)
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Old 03-02-2011
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Originally Posted by rwenzing View Post
The mixture control doesn't go into some overrich limp mode just because there is a CEL with a P0420. The cat monitor is there only to tell the driver/inspector that the upsteam cats are no longer doing their job for whatever the reason. And, OBTW, the P0420 code didn't exist in OBD.
But the ECU still knows. you ever started your truck up and had it die real quick? thats the ECU going into "flood mode" which is the best i can explain it. where it dumps more gas than needed.
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Old 03-03-2011
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I'll try this just once more.....

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Originally Posted by hozII View Post
i know on fords when CEL light is on computer goes into safe mode where it will dump more fuel than air....
The OP ^^^ seemed to be under the impression that every CEL would cause the PCM to run an overrich mixture. That simply is not true and I thought he should know that.

Yes, certain conditions can force the PCM into open loop fuel or Failure Mode Effects Management or Hardware Limited Operating Strategy. No, a P0420 alone won't put the PCM into any of those.

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Originally Posted by StxDangerRanger View Post
But the ECU still knows. you ever started your truck up and had it die real quick? thats the ECU going into "flood mode" which is the best i can explain it. where it dumps more gas than needed.
The PCM always uses an overrich mixture during cold start to compensate for wall wetting and poor fuel atomization. It adds more fuel than is needed to use all of the oxygen but not more than is needed to make the engine start and run acceptably. After the engine starts, the mixture is brought back toward stoichiometric AFR as soon as possible to minimize emissions and wasted fuel.

The closest thing to "flood mode" I can think of is "clear flood mode". This is useful for an engine that is already flooded for some reason. Opening the throttle fully while cranking the starter turns all injectors off completely and allows the flooded condition to be cleared. This gives the customer the ability to overcome a flooded condition using a procedure similar to going to WOT to clear a flooded carbureted engine.
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Old 03-03-2011
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Originally Posted by rwenzing View Post
I'll try this just once more.....

The OP ^^^ seemed to be under the impression that every CEL would cause the PCM to run an overrich mixture. That simply is not true and I thought he should know that.

Yes, certain conditions can force the PCM into open loop fuel or Failure Mode Effects Management or Hardware Limited Operating Strategy. No, a P0420 alone won't put the PCM into any of those.

The PCM always uses an overrich mixture during cold start to compensate for wall wetting and poor fuel atomization. It adds more fuel than is needed to use all of the oxygen but not more than is needed to make the engine start and run acceptably. After the engine starts, the mixture is brought back toward stoichiometric AFR as soon as possible to minimize emissions and wasted fuel.

The closest thing to "flood mode" I can think of is "clear flood mode". This is useful for an engine that is already flooded for some reason. Opening the throttle fully while cranking the starter turns all injectors off completely and allows the flooded condition to be cleared. This gives the customer the ability to overcome a flooded condition using a procedure similar to going to WOT to clear a flooded carbureted engine.
LOL bob OWNED again!!
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