Hi All,

I've got a problem with my 98 Camry XLE: gas mileage dropped from 26 MPG to 14mpg. Smells like its running rich. WHen I first start the car I need to rev the engine to "clear it out" - brown/ grey smoke. Runs reasonably smooth with a bit of misfire every once in a while. If I drive hwy and it starts running rough I accelerate - cloud of brown/ grey smoke. Engine codes P0171, P0301, P0303, P0303. I changed teh MAF and that didn't help.
Brought it to my mechanic and he tuned it up - new Pt plugs, wires, 3 coils, PCV valve, fuel filter and fuel injection cleaner. He said "it runs great all set" and took $750. I drove it down the road and it was still running a bit rough and burning fuel at twice the rate it used to. Called him and he thought I should drive it for a while to "blow the carbon out". (What does that mean?).
I checked all the vacuum lines and checked that they were clear and not cracked, etc. Engine reves when I pull a vac line off intake manifold. All injectors make the clicking sound.

Anyone have an idea, or do I take it in to my mechanic and let him do the remove and replace until he bumps into the cause?

Thanks in advance for any advice!
Paul

 

The P0171 code may be causing the other codes. The code means the actual air/fuel ratio is lean, the computer is having to compensate for this by adding more fuel to get it back to normal. This could explain your lower mpg.

Things to check
-You changed the MAF.
-Vacuum line leaks
-Exhaust leaks
-Plugged injector
-Fuel pressure
-02-A/F sensor First sensor on exhaust pipe coming off each head.
-Coolant temp sensor for computer
-Leaking air intake hose for engine

If the engine idles normally the vacuum lines should be OK.

Check for exhaust leaks from about 1 foot after the emissions converter located under the car forward. Exhaust leaks in this area can effect 02-A/F senor operation.

How is engine power? The computer can not compensate for a plugged injector and this would cause a loss of power.

Fuel pressure needs to be checked with a gauge. There are no trouble codes for fuel pressure problems.

A bad 02-A/F sensor can cause MPG issues. Ditto for a bad coolant sensor.

The coolant sensor can be tested with an ohm meter.

A mechanic should be able to run diagnostics on the 02-A/F sensor using a real time ODBII scanner.

Blowing out the carbon means placing the engine under load. Such as full throttle operation and hwy driving. Still doubt this will solve the low MPG issue.

 

Thanks for your feedback - very helpful.

When I first start the car it runs choppy and I rev it to clear it out. Then it runs smooth. Cold idle at ~1100rpm, warmed up it idles at 750rpm.

No performance issues. Quick off the line, no stalling drives hwy fine with power to spare. Engine doesn't sound labored and the occasional missing and sputter hasn't happened recently. Would (low) fuel pressure problems cause performance issues?

Can't locate any exaust leaks. I don't hear any blow-by and can stall the engine by holding a rag over the exaust pipe. Exaust smells very rich.

Tested the temperatuer sensor as described in my Haynes manual - 2800ohms cold and 260 ohms warm. Normal.

I don't have O2 A/F sensor codes. I have a scanner, but am a novice with its use. I replaced the rear manifold and downstream O2 sensors this past summer. Is there anything I can look for in the vehicle data that would give a clue (O2S11 at 0.650; O2S12 fluxuates between 0.00 and 0.450 (mostly at 0.00 though) and 02S21 is at 0.630. FTRM21 is at 0.0 and when I rev the engine it goes to -4.5; FTRM11 is at 19.5 and steady).

Any further suggestions or feedback would be greatly appreciated.
Thanks.
Paul

 

The idle speed operation looks normal.

The ODBII info posted is confirming the P0171 code. The fuel/air ratio is somehow lean, the computer is needing to compensate by adding fuel. The rest of the codes can be the result of the lean condition.

It appears the 02 sensor are operating normally but sensing a rich condition. Above 0.45v is rich, below is lean. The computer is adding more fuel.

If your checking sensors make sure the engine is warm and revved up to 2500-3000 (or higher) rpm to heat them up from exhaust gases.

In theory the engine should run as close to 0% trim as possible.

As to the cause?

-Leaks in engine air intake hose between MAF and throttle body. Sometimes the hose cracks where it mounts to the throttle body.

-Vacuum leaks, check PCV system hoses, EVAP hoses, etc. Any hose that connects to the intake manifold.

-Low fuel pressure. Need a gauge to check this. Could be something with the fuel pressure regulator. Low fuel pressure as opposed to low fuel quantity from a plugged filter.

-Plugged injector. But if idling OK and has power then?

-MAF sensor.

You checked the computer coolant temp sensor, exhaust leaks and 02 sensor appear OK. Plus did a tune up and changed the fuel filter.

On this year of car you may still be able to reset the computer by disconnecting the battery or removing the EFI fuse for a couple of minutes. Might reset the codes and determine if this changes anything.

In theory the mechanic states the condition will work itself out. Perhaps a little help by resetting the codes may help.

Suggest cleaning the MAF sensor elements. These can be sprayed with a cleaner, use something safe for plastics. The internet should have posts on this, maybe even a YouTube video. Search on the subject plus the engine model.

Check the above. Guessing a MAF sensor issue but given these are expensive best to work all the issues until left with what remains. Unless you can find a low cost used one from a junk yard.

 

Thanks once again. I have an update:

Checked all hoses leading into the intake manifold and traced vacuum hoses where I could find them - didn't find a split or leak.

Reset the computer with my scanner; same codes came back and car still consuming fuel at teh rate of 15mpg. running a bit more rough. At times I can feel the engine miss fire (sort of like running out of gas). Stepping on the gas clears it up and results in black/brown smoke coming out of exaust.

I brought it to my mechanic - reluctantly - and explained all the symptoms and askd him to check the vacuum and exaust and fuel pressure before he replaced the MAF.

Fuel pressure was OK, no detectable leaks in exaust and vacuum hoses (confirmed), O2 sensors confirmed working with his diagnostics. Reluctantly agreed to replace the MAF (recall I had done that originally, but bought a rebuilt one). $200 later same issue as I originally posted. I've driven it 200 miles since changing the MAF and there's little change - seems to miss fire at a different rate now, but power is still there and the car accelerates well both from a stop, uphill and at a top end speed. Sometimes I think I hear a high pitched whistling (but it could be my imagination!).

So to recap - checked temp sensor, O2 sensors, fuel filter changed, fuel pressure OK, tuned it up (plugs wires, 3 coils), new air filter, MAF, PCV valve and put a bottle of fuel injector cleaner in the gas two tanks ago.

Running out of ideas - could it possibly be a blown intake manifold gasket? is there something that can check this? Does anyone have an idea what could be causing this? Clearly not a problem that the computer diagnostics can pick up. Still getting P0171 and P0300 codes and exaust smells like its burning rich and the 15 MPG makes travel expensive!

HELP!

 

An intake manifold would be unusual.

Both 02 sensors read about the same, a leak would need to be common to both cylinder banks.

Wonder if any injectors are leaking (not plugged but leaking). This could explain the smoke at start up as the injector may have leaked gas into the cylinders. Did the shop do a leak down test. Check fuel pressure engine off after 5 minutes for excessive pressure drop. Should be at least 21 psi.

Also test the TPS unit. Possible for the wiper arms to get dirty resulting inaccurate output voltages.