possibly running rich with poor gas mileage

[messiveian]

2000 grand am
2.4L
auto tranny
no CEL so no codes

i just recently have been tracking my mileage with the way rick has told everyone to. That is fill up drive fill back up and calculate how much fuel went into the tank and how many miles driven. Well after doing that a few times i have had average of 17,18,and 15 MPG. this is combined city and highway about 50/50. to me this seems very low

I drive normal and keep my shifts at or below 3000rpms

air filter is new, just had an oil change. one thing i do realize though is once in a while it will misfire but never set a code. it onlt does it when it idles and just a few times. this only happens when its wet outside.

about 3 months ago i drove though a rally deep puddle at night and water got sucked into the engine. the car didnt stall out but sputtered ALOT and thats when the CEL flashed for multiple missfires. that went away without doing anything i just let it dry out.

I just need to know where to look for options on what to do next. i assume plugs and wires but even before i went though the puddle my mileage was like this too. I had the EGR valve replaced when i got the car 4 months ago.

Also i have black soot on my exhaust tip but none on the bumper so im assuming that is pretty normal.

Thanks everyone

[rixGAphx]

'00, 8-yrs old; about 95k miles??

4-cyl, so there are no plug wires as such, just boots.
Do a complete tune-up, boots and plugs (ACDelco 'RapidFires').
New front O2 sensor, combined with plugs, will do wonders for the milage (assuming the current ones are OEM).

Deeeeepppp puddle, if water got thru the airbox all the way to the intake.
The water might have damaged the MAP sensor and/or the IAT sensor; enough so they give incorrect readings, yet not so damaged as to throw a trouble code.

If the engine got so much water that there was sputtering fo the engine (though not an outright stall), then you may have 'hydrolocked' one or two cylinders.
This occurs when there's a slug of liquid water sucked-in through the intake, then compressed on the 'compression stroke' by the upward-moving piston.
If the volume of water exceeds the volume of the compression chamber, then the piston is *trying* to compress an 'incompressible fluid.'
The result is generally that the piston's connecting rod bends :eek:
Forevermore, that piston never goes to the op of the cylinder, and so it never produces the 9.5:1 compression ratio that the other cylindes have.
If the con rod were only bent *a little*, then the compression might be down to 9.0:1 or some such; the cylinder will still fire most of the time (and no SES would be set), but the efficiency of the engine will be very low.

So to check for this, I would do a compression test at the same time as the spark plugs are removed for changing.
I would do this before spending money on O2, MAP, or IAT sensors, since a bent rod requires some serious pondering and $$ to fix.

Hope this helps,
-Rick

PS: That auto tranny also needs service (fluid change) at this milage.
Old, dirty fluid will also suck gas milage, as the TC must work harder to trasfer force.
And if the puddle were really deep, water *might* have gotten into the fluid depending on the condition of the vent on your tranny.

[messiveian]

sorry rick its a 3.4L V6 and it has 108,000 miles on it. its my friends car and i have it in my possession right now.
when he went through the puddle it would only sputter at below 2000rpms and anything above that would be fine. and as soon as it dried out it worked just fine. i am wondering if it may be a spark plug wire because it will only misfire when its wet out so im wondering if maybe the wire isnt insulating corrctly and moisture is making the misfire on the wire. you can barly feel the misfire and it happens only about twice at a red light and sometimes it never does it.

the car idles at about 550rpms or a little higher if the car is cold. and the plugs are OEM

i dont believe that any cylinders got hydrolocked. my friend had an old corsica and when it ran on 3 cylinders it sounded horrible. my friends car sounded pretty smooth when he went through the puddle. i was with him and my other friend when his corsica ran on 3 cylinders and it sounded nothing alike.

also the same gas mileage occurred before he went through the puddle so i have to believe that driving through the puddle didnt effect gas efficency at all.

[rixGAphx]

sorry rick its a 3.4L V6 and it has 108,000 miles on it.
it's my friends car and i have it in my possession right now.

the car idles at about 550rpms or a little higher if the car is cold.
and the plugs are OEMthen mebbe change your preamble in THIS THREAD from "2.4L" to read 3.4??? :justathought:

Certainly the plug boots of an 8-yr old V6 could be loose/cracked enough to allow 'current leakage'.
Definitely replace the plugs and plug wires.

And IMO, the V6's engine air intake is so highly-convoluted that it's virtually impossible for any dangerous amount of water to get to a single intake runner, unless you're knee-deep in standing water and running the engine at WOT.
So I too doubt that water-in-the-cylinders occured, and therefore I agree a compression test isn't necessary.

550 seems rather low IMO; the PCM should be doing everything it can, via the IAC and FI, to maintain ~650 (warm, AC 'off') to 900 (cold, AC 'on').
but if it's not throwing codes, and not stalling-out, then good is as good does.

The car will need a front O2 sensor soon, and milage will increase again when that is installed.
So far, it's just tired and slow and causing a slightly-rich mixture; eventually it will fail completely.

Hope this helps,
-Rick

[radarman012000]

I believe if any cylinder doesnt fire the CEL light would come on.

It will throw a PO300,1,2,3,4,5,6 if the CMP sensor senses a slow down on the rotation of the crankshaft, and depending on the position of the Camshaft position sensor it knows which cylinder didnt fire.

If it cant determine which cylinder it will throw a PO300 code.

Especially is the cylinder doesnt fire multiple times, such as with a bent rod or hydrolock

Just my 2c worth

[rixGAphx]

I believe if any cylinder doesnt fire the CEL light would come on.I *think* the PCM will record each misfire, but won't set a code/SES unless there is a succession of misfires within a short time.

I dunno; until you just stated it, I never knew how the damned PCM knew there was a misfire, much less on which cylinder (short of analyzing whether the spark 'returned' to the coil).

[radarman012000]

I *think* the PCM will record each misfire, but won't set a code/SES unless there is a succession of misfires within a short time.

I dunno; until you just stated it, I never knew how the damned PCM knew there was a misfire, much less on which cylinder (short of analyzing whether the spark 'returned' to the coil).

Yes the PCM records damn near everything.

That analysis happened to me on my 96 3.1

The CEL light would come on and I would check the codes....PO303= Cylinder 3 misfire..

Turned out to be the coil pack for that cylinder.

But yes the CMP and CKP work together so the PCM will know there is a misfire and which cylinder it comes from

It only takes one misfire to light up the CEL light...multiple misfires will set multiple codes, even multiples of the same code...so if it misfires on only one cyl 5 times...there will be 5 PO305's stored

[radarman012000]

Here is an overview of how that happens:

The CMP (crankshaft position sensor) reads the teeth on the gear on the end of the crank (reluctor ring) to determine crankshaft position along with the CKP (camshaft positions sensor) to determine camshaft position.

If a cylinder misfires for whatever reason, the crankshaft "slows down"(rotation) cus the cylinder didnt ignite, thus not providing any power to push the piston down. The CMP senses this "slowdown" and sends a signal to the PCM with then reads the CKP sensor to determine which cylinder it was. Then is sets the code, and illuminates the MIL

The PCM uses these two readings, along with others, to determine timing, fuel injectors (pulse width) to maintain optimum fuel/air ratio.

I learned all this from research over various weeks when I had my other problem

[messiveian]

well thank you both rick and radarman. i am going to checnge his spark plugs and wires soon and see if that not only stops the slight misfire but also increases his gas mileage. if the gas mileage remains the same then i will test the o2 sensor and replace that if needed

[radarman012000]

Keep in mind, mine is a 96. You stated yours is 2000. wholly different beast. Pre 99's wiring setup is totally different, but yours still has OBDII, so the misfire diagnosis should hold.

OBDII's are generally all the same, except for a few small differences here and there that are particular to a certain manufacturer, but overall, if you can diagnose one OBDII, then you can diagnose MOST other OBDII's....even if they are on Toyotas or Fords.

IE, on my 2000 Dodge Dakota, it has OBDII but a few extra sensors to monitor extraneous things that a 2000 Pontiac doesnt have, IE a battery temp sensor to regulate the charging coming from the alternator.

I havent seen that extra bit on any other vehicle I've owned, but looking at the codes for my 96 GA and the codes for my 2000 Dakota, they are nearly identical

Just my 2c worth

[rixGAphx]

OBDII's are generally all the same, except for a few small differences here and there that are particular to a certain manufacturer, but overall, if you can diagnose one OBDII, then you can diagnose MOST other OBDII's....even if they are on Toyotas or Fords.
This is completely true, as far as a DIY mechanic and hand-held scanners is concerned.

But there are 5 different signalling protocols currently in use with the OBD-II interface, and they vary significantly in how they work.
The 'commonality' is that each has a sort of internal translator that allows it to communicate to the outside world (specifically, to EPA emissions computers) according to the paramaters of SAE J1962; specifically:
* All must have a 16-pinout connector accessible from the driver seat;
* All must use the same P0xxx codes.

But the internal manner in which the software operates is as different as AM radio vs FM radio vs XM satellite radio.
Almost as different as Apple vs Windows vs Linux.

SAE J1850 PWM (pulse-width modulation - 41.6 kbaud, standard of the Ford Motor Company)
SAE J1850 VPW (variable pulse width - 10.4/41.6 kbaud, standard of General Motors)
ISO 9141-2. This protocol has a data rate of 10.4 kbaud, and is similar to RS-232. Primarily used in Chrysler, European, and Asian vehicles.

For J1850 PWM (Ford), 'High Voltage' is 5V.
For J1850 VPW (GM), 'High Voltage' is 7V.
These computers operate off their own internal transformers.
For ISO 9141-2 (Chryco, with its 'mother corp' Daimler), 'High Voltage' is defined as Battery Voltage (12V nominal, 13.2 actual for american passenger vehicles).

So the Dodge Dakota may NEED some battery heat detection/overcharge protection as a part of the operating system's substantially-different hardware.

Benchtop scanners, that look deeply inside all different types of 'puters, must be much more complex, in both hardware and software, to determine what's actually happening with any given sensor or subsystem.

According to this wikipedia article, http://en.wikipedia.org/wiki/OBD-II#OBD-II , a new protocol is on the way which still interface with OBD-II:

"ISO 15765 CAN (250 kbit/s or 500 kbit/s). The CAN protocol is a popular standard outside of the US automotive industry and is making significant in-roads into the OBD-II market share.
"By 2008, all vehicles sold in the US will be required to implement CAN, thus eliminating the ambiguity of the existing five signalling protocols."

Just what I've read, and am barely able to understand. :D :D
-Rick

[radarman012000]

We prolly should start a thread on just OBDII...:lol:

That would take up a lot of our otherwise idle...:eek: