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Discussion Starter · #1 ·
Hi Everyone. I am new to this forum and am thankful to find it. I have an 07 Odyssey EX-L and the gas mileage has just taken a huge drop to 15 mpg from 20 mpg. After reading and searching these threads for many, many hours, I reached the conclusion based on insight of several posts that I should start by check my spark plugs. As other members have noted, the center front plug was barely finger tight and the tip was covered with oil/gas. Just as an FYI, the passenger side front and rear plugs looked fairly normal, but had a very slight pinkish coloration on the ceramic part before the tip. The center rear plug looked ok, but slightly worn, and the driver side front and rear plugs looked normal.

After having replaced the plugs, I now want to verify the ignition timing. So my question is can I set the timing using an inductive timing light that was made to slip around a traditional plug wire? Furthermore, the online manual that I am using says to check the idle speed and jump the SCS line with the Honda Diagnostic System (HDS). Do I need to purchase an HDS or another scan gauge to perform these tasks? Or can I use my timing light and tach gauge that I bought years ago?

Thanks for all of the useful information on this forum!!

295 Posts

The place to start looking for fuel mileage drop off is in the sensors supplying data to the ECU. The two primary sensors that come to mind are the A/F sensor (primary O2 sensor, front O2 sensor) and the MAF or MAP sensor. Both these sensors provide feedback to the ECU which can cause changes the the fuel mileage your vehicle will get.

In today's vehicles one can not check ignition timing as in the old classical sense (using an inductive timing light). The same holds true for the engine idle adjust, computer controlled based off of sensor input and other control inputs. With fairly exotic computer programs and interfacing to the OBDII port the timing could be checked BUT there is no feature that would allow for adjustment. Any adjustment would require re-flashing the computer E PROMS, something which is basically against the law (you would be playing with vehicle emissions and that is not legal).

The A/F sensor supplies information on the ratio of oxygen in the fuel mixture, if the mixture is too lean then the feedback system causes the EFI to increase the supply of fuel, if the mixture is too rich the the feedback from the A/F sensor leans out the mixture delivered to fuel injectors. The system is in constant feedback and undergoes many adjustments while one drives the vehicle.

The manifold absolute pressure sensor, or MAP sensor, is used to measure the amount of air flowing through the intake system and into the engine. The sensor sends airflow rate data to the engine control unit, or ECU, which can then adjust the air-to-fuel ratio for optimum fuel combustion. Over time, the MAP sensor can become dirty with contaminates when this happens it could mean the sensor needs top be cleaned.

If either of these sensors malfunction to the point of not working at all then the ECU goes into closed circuit function and you would set a DAC and the CEL would light up. BUT if either of these sensors becomes marginal in function THEN the DAC and CEL would not be set and one possible outcome of the marginal sensors would be a drop off in fuel mileage.

Please understand that there are other issues that could cause drop off in fuel economy (radical change in driving habits, dramatic increase in traffic flow) but these are two of the first places I'd start with when no CEL or DAC is set and I'm sure that I'm seeing a non self induced drop off of fuel economy. Lots of luck, Russ.

2 Posts
Discussion Starter · #4 ·
Thanks for your help with this issue. I have verbiage from a manual that I purchased.

In the section about ignition timing:


1. Connect the Honda Diagnostic System (HDS) to the data link connector (DLC) (see step 2

2. Turn the ignition switch to ON (II).
3. Make sure the HDS communicates with the vehicle and the powertrain control module
(PCM). If it doesn't communicate, troubleshoot the DLC circuit (see DLC CIRCUIT
If a DTC is present, diagnose and repair the cause before continuing with this test.
5. Start the engine. Hold the engine speed at 3,000 rpm with no load (in N or P) until the
radiator fan comes on, then let it idle.
6. Check the idle speed (see IDLE SPEED INSPECTION ).
7. Jump the SCS line with the HDS.
8. Connect the timing light to the No. 1 ignition coil harness.
Fig. 3: Connecting Timing Light To No. 1 Ignition Coil Harness
9. Aim the light toward the pointer (A) on the timing belt cover. Check the ignition timing
under a no load condition (headlights, blower fan, rear window defogger, and air
conditioner are turned off).
Ignition Timing: 10° ± 2° BTDC (RED mark (B)) at idle in N or P

10. If the ignition timing differs from the specification, check the cam timing. If the cam timing
is OK, update the PCM if it does not have the latest software (see PCM UPDATE ), or
substitute a known-good PCM (see SUBSTITUTING THE PCM ), then recheck. If the
system works properly, and the PCM was substituted, replace the original PCM (see PCM
11. Disconnect the HDS and the timing light.

As I said in my original post, I experienced a significant drop in gas mileage. I replaced the plugs with OEM NGK and replaced the air filter with a K&N. The first tank of gas used in all stop and go traffic gave me 17.3 mpg, up from 15. However, I was averaging 19 mpg in stop and go traffic.

I understand that the timing cannot be "set" by the me. However, these procedures seem to give me a starting point for further analysis. If the timing is off, it may point to a problem with the PCM. At this point I would take it to the dealer for replacement.

Otherwise, based on input from rberman999 (thanks again) I would move on to checking the MAF and 02 sensors.

I think I can get back to my original mileage. I have spent hours in the past fooling with the sensors on my Vette. I have a complete assortment of jumper connections for the sensors on my vette that allow me to take digital voltage readings without penetrating the wire with a needle to read the voltage. I realize that Honda Engineering is different, but is there a similar way to check the performance of or troubleshoot these sensors by reading voltage without replacing them with new ones?

2,227 Posts
Given that you can do nothing with the ignition timing, I suspect you are barking at the wrong tree.

Buy a cheap (enough) scanner that has live data capability. You would be able to get so much information as to spin your head.

If you shop wisely, $100 would get you such a scanner.

Another option would be scangauge or ultra-gauge. Both have ability to get all the engine parameters supported by your vehicles.

- Vikas
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