I have the codes - the mechanic can't figure it out - can you diagnose Excalibur?

[sethdallob]

My kingdom for the person that can successfully diagnose Excalibur!

I've had the same problem with my 03 GX with 170000 miles on it for months. The dealer took 3 stabs and almost $1000 at it, and didn't fix the problem. An independent mechanic with Identafix helping out threw $700 at it, and no dice either.

I keep getting misfires in the 1800-2000 rpm range, mostly when cold, and seemingly worse at high altitude (I live at 5000 ft. and commute down the hill). Once warmed up, the car runs great. The dealer replaced all sorts of spark plugs, fuel injectors, coil packs, etc. and each time they "fixed" it, the car ran perfectly for about a week, then the same misfiring problem started again. The independent mechanic replaced the fuel injector driver unit, and that didn't fix it either. Today, the fuel pump started blinking and the gas level went to 0 (2nd time), but the car ran fine. The MIL is not on yet, but here are the pending codes and sensor readouts from my OBD-II scanner:

P300 (pending) - misfire
P301 (pending) - misfire
P302 (pending) - misfire
P303 (pending) - misfire

Emission Monitors:

Misfire
Fuel
Comp. (?)
Catalyst
O2 sensor
O2 heater

I'm open to suggestions - so far I've heard that my O2 sensor could be dying, but usually that would throw a code...

[Lakewood90712]

Re: I have the codes - the mechanic can't figure it out - can you diagnose Excalibur?

170K miles?. Fuel pressure regulator ? maybe ice forming inside until warmed up , real costly part (over $1,000). At 170K, Time for a new GX .I'm sure Curtis has a replacement ( 09GX ) on the lot.

[sethdallob]

Re: I have the codes - the mechanic can't figure it out - can you diagnose Excalibur?

I just spoke with Honda Bob, who might be The One. He says it sounds like a "TW sensor." Any thoughts on this? Aside from this little hiccup, the car runs great, and I plan on driving it into the ground before getting another one - hopefully at least another year!

[larrycng]

Re: I have the codes - the mechanic can't figure it out - can you diagnose Excalibur?

What kind of scantool are you using?

Usually a misfire code doesn not set a pending code, "pending" is for two trip codes (anything but fuel system. comprehensive component, and misfire).

In the list of monitors you gave, what was the status of the monitor (completed, pending and not run)? If there is a misfire code, some monitors will not run becuse there is a misfire code. The misfire will cause inaccurate results on some of the other monitors, (CAT, fuel system. O2) so the code keeps the other monitors from running and giving false information. The "comp" monitor is the comprehensive conponent monitor which monitors this like wiring, and some sensors

Assume the O2 sensor is zirconium type and not a wide band sensor, it would not effect the misfire monitor, but rather the cat and fuel system monitors. The misfire would affect the O2. Both types would show a lean reading during a misfire causing the fuel system to respond with a rich mixture. O2 sensors can be best checked with a lab scope. The zirconia type sensor you need to look for voltage swings (at least .2v to .8v) and the time between peeks (usually less than 100ms in general terms). The wide band sensors (A/F) sensor Honda will monitor current through the scantool. Voltage on A/F or lambda sensor will can tell you rich or lean. but is not generally used (you need to know what the voltage is on a normal CNG vehicle)

It sounds like its time to go back to the basics, check the mechnical condition, CNG fuel system (pressures and temperatures and sensors) Use the lab scope to check out components (both voltage and curent where applicable).

Hope this helps
Larrycng

[karlhafen]

Re: I have the codes - the mechanic can't figure it out - can you diagnose Excalibur?

The tw (coolant temp) sensor sounds like a possibility. It always pays to try the cheapest part last My first thought was that it could be an injector problem but it sounds like you have been there.

1. With a scan tool, check the engine coolant temperature while the problem is occurring. If it looks ok then:
2. Check fuel trims. If the trims are not between -15 to +15 unplug the air/fuel ratio sensor/front oxygen sensor, to see if it stops the misfire.

With regard to Identifix. I have used Identifix and like them in general , but I have found them to be weak on CNG and diesel vehicles. They list this guarantee:
"If you are dissatisfied with the information provided we will credit you in full for the hotline call. If our call records show that we recommended a part to you based on accurate information provided by you, that you put on and did not fix the vehicle, you can request a refund for the cost of the part. You will need to provide us with the new part that you purchased for this fix and the original sales receipt to qualify for a refund."
My real life experience is that guarantee is mostly a sales pitch and they will usually leave themselves an "out" when recommending an expensive part.