Fast Idle and High Fuel Consumption

Question

I just bought a Manual 97 Mazda Lantis 323 1.8L DOHC 16 Valve BP and I was amazed that it went through 1/4 tank gas ( about 12.5 liters or 3.3 gallons ) in one day after about 60 kilometers / 37 miles of city driving. That seems extremely bad. According to Fuelly it should be getting around 26 mpg.

I noticed that the idle seems excessively fast, about typically between 1700 to 2000 rmp when I would expect it to be closer to 800 to 1k rpm. It's like I'm cruising down the highway while sitting at a stop light in neutral. It's a manual transmission and the previous owner said they had recently had the throttle replaced and it was replaced with a second hand throttle from an automatic.

Is the idle the likely culprit or could it be some other problem?

Answer 1

I think you need to do a throttle position sensor recalibration (reset).

(NOTE: I will post several different adjustment versions. I believe the top one is what you need (for a K8 engine), but will include another four depending on your engine.)

To do this for your vehicle you need to follow the steps below, depending on whether it is a stick/auto and which engine (SOHC or DOHC). I will include all the different descriptions:

1.8L (K8) Engine:

1. Detach the connector from the throttle position sensor.
2. Connect an ohmmeter between terminals IDL and GND.
3. Rotate the throttle linkage by hand. With the throttle valve fully closed, the ohmmeter should read 0.1-1.1 volts.
4. With the throttle valve fully open, the ohmmeter should read 3.1-4.4 volts.
5. If not as specified, adjust or replace the throttle position sensor.

SOHC Manual Transmission:

1. Detach the connector from the throttle position sensor.
2. Connect an ohmmeter between terminals IDL and E.
3. Insert a 0.004 in. (0.1mm) feeler gauge between the throttle stop screw and stop lever.
4. Verify there is continuity between terminals IDL and E.
5. Then replace the feeler gauge with a 0.02 in. (1.0mm) feeler gauge, verify there is no continuity between terminals IDL and E .
6. Then open the throttle wide and verify there is no continuity again between terminals IDL and E.
7. Next, connect the ohmmeter between terminals POW and E.
8. Insert a 0.004 in. (0.1mm) feeler gauge between the throttle stop screw and stop lever.
9. Verify there is no continuity between terminals POW and E.
10. Then replace the feeler gauge with 0.02 in. (1.0mm), verify there is no continuity between terminals POW andE.
11. Then open the throttle wide and verify there is continuity between terminals POW and E.
12. If not as specified, adjust or replace the throttle sensor.

SOHC Automatic Transmission:

1. Detach the connector from the throttle position sensor.
2. Connect an ohmmeter between terminals IDL and E.
3. Insert a 0.004 in. (0.1mm) feeler gauge between the throttle stop screw and stop lever.
4. Verify there is continuity between terminals IDL and E.
5. Insert a 0.024 in. (0.6mm) feeler gauge between the throttle stop screw and stop lever.
6. Verify there is continuity no between terminals IDL and E.
7. Connect an ohmmeter to the throttle sensor terminals Vt and E.
8. Verify that resistance increases as throttle valve opening increase.
9. With throttle valve fully closed the resistance should be below 1 kilohm and as throttle valve is fully opened resistance should increase to approximately 5 kohms.
10. If not as specified, adjust or replace the throttle sensor.

DOHC Manual Transmission:

1. Detach the connector from the throttle position sensor.
2. Connect an ohmmeter between terminals IDL and E.
3. Insert a 0.004 in. (0.1mm) feeler gauge between the throttle stop screw and stop lever.
4. Verify there is continuity between terminals IDL and E.
5. Then replace the feeler gauge with a 0.027 in. (0.7mm) feeler gauge, verify there is no continuity between terminals IDL and E
6. Then open the throttle wide and verify there is no continuity again between terminals IDL and E.
7. Next, connect the ohmmeter between terminals POW and E.
8. Insert a 0.004 in. (0.1mm) feeler gauge between the throttle stop screw and stop lever.
9. Verify there is no continuity between terminals POW and E.
10. Then replace the feeler gauge with 0.027 in (0.7mm), verify there is no continuity between terminals POW and E.
11. Then open the throttle wide and verify there is continuity between terminals POW and E.
12. If not as specified, adjust or replace the throttle sensor.

DOHC Automatic Transmission:

1. Detach the connector from the throttle position sensor.
2. Connect an ohmmeter between the terminals IDL and E.
3. Insert a 0.004 in. (0.1mm) feeler gauge between the throttle stop screw and stop lever.
4. Verify there is continuity between terminals IDL and E.
5. Insert a 0.024 in. (0.6mm) feeler gauge between the throttle stop screw and stop lever.
6. Verify there is no continuity between terminals IDL and E.
7. Connect an ohmmeter to the throttle sensor terminals Vt and E.
8. Verify that resistance increases as throttle valve opening increase.
9. With throttle valve fully closed the resistance should be below 1 kilohm and as throttle valve is fully opened resistance should increase to approximately 5 kohms.
10. If not as specified, adjust or replace the throttle sensor.

NOTE: These procedures were pulled from AutoZone.com and are specified for the Protege which should be the same for the Lantis 323, as these are supposed to be the same platform and engines.

The TPS should look something like this:

The connector face should look something like this:

Other things to do if this doesn't work would be to clean the throttle body area (inside/out) completely. Also, consider changing out the before cat O2 (lambda) sensors. If these are lazy, you would see a large increase in gas usage.

EDIT: One other thing to check is to ensure you don't have any vacuum leaks ... completely forgot to mention this. I'd specifically check around areas where the work was done, like at the throttle body. This might account for the higher than normal RPM as well as the high gas usage. When the O2 sensor "sees" the higher than normal oxygen level in the exhaust stream, it dumps more fuel into the mix and your gas mileage suffers. Seems like it would be the other way around, but it happens.

Answer 2

I had the same issue. I just managed to soft it. There may be a few causes to this, but for my case, I found that the idling air intake hose connecting the lower part of the throttle body was missing the hose clamp. Due to this, during idling, additional air were being sucked in via the small gap between the hose and the throttle body. I think the guy who did the maintenance last time didn't put it back. I've installed a hose clamp and it seems to be back to normal idling RPM now. Hope this help.

Answer 3

This particular problem was solved by replacing the coolant temperature sensor and installing a thermostat.

Apparently the coolant temp sensor was telling the engine it was -20C and so it was running extremely rich and was artificially raising the idle. After replacing that the fuel mileage got much better, but it was still running a bit high, and when I hooked it to a scanner I saw the engine temp was never getting above about 65C. Upon inspection I discovered that the previous owner had removed the thermostat. After installing a 82C thermostat the average operating temp got up to about 85C - 90C, but surprisingly installing the thermostat had no effect on the gas mileage.

After that it ran fine during the winter, and then started having a rough idle / stalling at stop lights problem once the ambient temperature outside got above about 15 - 20 C. I've posted a separate question about that here.