new cats & flex pipes 15-20% reduction in gas mileage
#1
12-05-2010, 01:34 AM
new cats & flex pipes 15-20% reduction in gas mileage
So a month or so ago, I developed an exhaust leak at the flex pipe where it connects to the cat. I made an appointment to get it fixed at a trusted custom exhaust shop I've been going to for years. During that time, my 1st ever check engine light came on. When I pulled the code it was for basically an exhaust leak (can't remember the code). Got 2 new aftermarket cats and flex pipes welded on ($530.00 vs $2200.00 OEM route), reset the CEL and all seemed well. My car seems to have a little more power if not the same amount. But my gas mileage has dropped 15-20% after the CEL and has stayed that way for the last 3 tanks (1000 miles, should have been 1200 miles). The CEL is NOT on and I have 0 codes stored.
Any ideas?
Any ideas?
Last edited by Jeffla; 12-05-2010 at 01:46 AM.
#3
12-05-2010, 12:49 PM
15-20 % increase in gas consumption is quite high.
Jeff,
Did you have the post-cat sensors cancelled? It appears (from what I read - it isn't my personal experience or "knowledge") that sometimes they do have some role in the trim adjustment.
In any event, it sounds like you're running richer, now. If it is related at all to the new cats' function, then you can make use of the Vag-com. I found something interesting, even though is written by a TT guy.
Take the following more as "food for thought", rather than my opinion as to an explanation for why your gas mileage increased right after you replaced the cats.
-
P.S. Oops, forgot to mention the Source.
Jeff,
Did you have the post-cat sensors cancelled? It appears (from what I read - it isn't my personal experience or "knowledge") that sometimes they do have some role in the trim adjustment.
In any event, it sounds like you're running richer, now. If it is related at all to the new cats' function, then you can make use of the Vag-com. I found something interesting, even though is written by a TT guy.
Take the following more as "food for thought", rather than my opinion as to an explanation for why your gas mileage increased right after you replaced the cats.
You can check your current state of trim by using VAG-COM or equivalent to look in Group 032 in your engine measuring blocks.
The first two fields will have percentages. The first field tells the fuel trim at idle (Additive). The second field tells the fuel trim at elevated engine
speeds (Multiplicative). Negative values indicate that the engine is running too rich and oxygen sensor control is therefore making it leaner by reducing the amount of time that the injectors are open. Positive values indicate that the engine is running too lean and oxygen sensor control is therefore making it richer by increasing the amount of time that the injectors are open.
It is totally normal for both the first and second fields to be something other than zero. In fact, zeros indicate either you just cleared codes (which will reset fuel trim values) or something isn't working properly. If values get too far away from zero, it will cause a DTC (fault code)
and can set off the MIL (commonly referred to as the Check Engine Light, or CEL).
Specifications for normal operation are usually somewhere near +/- 10%. In general, an out-of-spec value in the first field (Additive) indicates a vacuum leak since it is mostly present at idle, when vacuum is highest. An out-of-spec value in the second field (Multiplicative) indicates a fault at higher RPM, and may point to a faulty MAF.
..................................................
1.4 Basic VAG-COM Tests
In order to test your MAF, O2 sensors and catalytic converter, check out the following blocks in the "Engine" control module with your VAG-COM.
Blocks 002, 030, 032 and 033 are done in "Measuring Blocks" while blocks 036, 034 and 046 are done in "Basic Settings.
1.4.1 Block 002 - Load Registration
Check the MAF airflow reading at idle. Make sure the car is running and warmed up and the A/C is off. Go to Block 002 and look in Field 4. The value at idle should be between 2 and 4 g/s. If it's lower than 2 g/s, then there is probably a leak in the intake tubing between the MAF and manifold, or the MAF is faulty.
1.4.2 Block 030 - Oxygen Sensor Regulation
In Block 030, field 1 is a three-digit binary code which gives the status of the pre-cat oxygen sensor. Field 2 is the same thing but for the post-cat oxygen sensor. The digits indicate whether or not the sensor heater is working and whether or not the oxygen sensor control is operational and active. The value should fluctuate between 111 (heater on) and 011 (heater off). The last two digits can also fluctuate between '1' and '0', but should be predominantly '1'.
1.4.3 Block 032 - Oxygen Sensor Control Learned Values
In block 032, field 1 represents the fuel trim at idle (additive) and Field 2 represents the fuel trim at part load (multiplicative), i.e. while driving. The value should be between -10 and +10% (negative indicates the engine is running rich and positive indicates the engine is running lean). If the value is close to +25% (which is the upper limit), it usually means that the MAF is bad. If the value is somewhere betweeen +10 and +25%, it could mean that the pre-cat oxygen sensor is bad, there is a leak in the intake or that the MAF is on it's way out.
1.4.4 Block 033 - Pre-Cat Oxygen Sensor Control
In block 033, the value in Field 1 represents the pre-cat oxygen sensor control. The value should fluctuate at least 2% in the -10 to +10% range.
1.4.5 Block 036 - Post-Cat Oxygen Sensor Control
Test block 036 must be done in basic settings, not measuring blocks. Depress and hold brake pedal to run the automated test. The engine RPMs should rise to around 1400. Field 1 is post-cat sensor voltage. It must fluctuate slightly between 0 and 1V. Field 4 will say either "TEST OFF/ON" before/while the test is running and either 'B1-S2 OK' (sensor is good) or 'B1-S2 NOT OK' (sensor is bad) after the test is finished. Release the brake pedal after the
test finishes.
1.4.6 Block 034 - Aging of Pre-Cat Oxygen Sensor
Test block 034 must be done in basic settings, not in measuring blocks. Depress and hold brake pedal to run the automated test. The engine RPMs should raise to around 1400. Field 1 is the engine speed. Field 2 is the catalytic converter temp. Field 3 is the value which tells you how aged the sensor is (not sure what it's called). Field 4 will say either 'TEST OFF/ON'
before/while the test is running and either 'B1-S1 OK' or 'B1-S1 NOT OK' after the test is finished. The aging value must be above 0.80. The value for a new sensor is 1.99. The value will decrease as the sensor ages. Release the brake pedal after the test finishes.
1.4.7 Block 046 - Catalytic Converter
NOTE: The test in Block 034 must be done just before this test or it will NOT initiate!
Block 046 must be done in basic settings, not measuring blocks. Depress and hold brake pedal to start the automated CAT test (last approx. 100 secs - the cat needs to be warmed up above a certain threshold for an accurate reading - the threshold is usually 400°C). The engine RPMs (Field 1) should rise to around 1400. Field 2 is the CAT temp. This will also rise during the test. Field 3 is the cat conversion efficiency. If the cat is good, the value should be below 0.50 at the end of the test. Field 4 will indicate if the cat is good (CAT B1 - OK) or bad (CAT B1 - NOT OK). Release the brake pedal after the test finishes.
1.4.8 Block 060 – Throttle Body Reset
Throttle body reset (TBR) is a procedure used to calibrate the throttle body flap sensors. Sometimes it is confused with clearing the ECU adaptation tables. The only procedure to do the real throttle body reset is to enter VAG-COM basic setting block 060. However, the ECU usually does TBR after its power supply is cycled, i.e. the fuse has been pulled or the battery
has been disconnected, AND the engine is cold.
The reason for doing TBF is that some people have reported power lost after long period of unspirited driving, as if the ECU were adapting to the driving style. Presumably the ECU just looses its knowledge of the sensor values under WOT. However, if hesitation is caused by misguided adaptation, full reset is needed instead of TBR.
If you don’t have VAG-COM, or you actually want to reset the ECU for clearing the adaptation tables (often incorrectly referred as TBR), you can just pull the ECU fuse, or disconnect the battery. Leave the power out for 15min to ensure all the power has been drained. Then reconnect, and turn on ignition, but DO NOT start the engine. Listen out for the clicking under the bonnet, which is usually over after a minute. Turn off ignition.
Note. If you disconnect the battery, be prepared to enter the radio safety code.
The first two fields will have percentages. The first field tells the fuel trim at idle (Additive). The second field tells the fuel trim at elevated engine
speeds (Multiplicative). Negative values indicate that the engine is running too rich and oxygen sensor control is therefore making it leaner by reducing the amount of time that the injectors are open. Positive values indicate that the engine is running too lean and oxygen sensor control is therefore making it richer by increasing the amount of time that the injectors are open.
It is totally normal for both the first and second fields to be something other than zero. In fact, zeros indicate either you just cleared codes (which will reset fuel trim values) or something isn't working properly. If values get too far away from zero, it will cause a DTC (fault code)
and can set off the MIL (commonly referred to as the Check Engine Light, or CEL).
Specifications for normal operation are usually somewhere near +/- 10%. In general, an out-of-spec value in the first field (Additive) indicates a vacuum leak since it is mostly present at idle, when vacuum is highest. An out-of-spec value in the second field (Multiplicative) indicates a fault at higher RPM, and may point to a faulty MAF.
..................................................
1.4 Basic VAG-COM Tests
In order to test your MAF, O2 sensors and catalytic converter, check out the following blocks in the "Engine" control module with your VAG-COM.
Blocks 002, 030, 032 and 033 are done in "Measuring Blocks" while blocks 036, 034 and 046 are done in "Basic Settings.
1.4.1 Block 002 - Load Registration
Check the MAF airflow reading at idle. Make sure the car is running and warmed up and the A/C is off. Go to Block 002 and look in Field 4. The value at idle should be between 2 and 4 g/s. If it's lower than 2 g/s, then there is probably a leak in the intake tubing between the MAF and manifold, or the MAF is faulty.
1.4.2 Block 030 - Oxygen Sensor Regulation
In Block 030, field 1 is a three-digit binary code which gives the status of the pre-cat oxygen sensor. Field 2 is the same thing but for the post-cat oxygen sensor. The digits indicate whether or not the sensor heater is working and whether or not the oxygen sensor control is operational and active. The value should fluctuate between 111 (heater on) and 011 (heater off). The last two digits can also fluctuate between '1' and '0', but should be predominantly '1'.
1.4.3 Block 032 - Oxygen Sensor Control Learned Values
In block 032, field 1 represents the fuel trim at idle (additive) and Field 2 represents the fuel trim at part load (multiplicative), i.e. while driving. The value should be between -10 and +10% (negative indicates the engine is running rich and positive indicates the engine is running lean). If the value is close to +25% (which is the upper limit), it usually means that the MAF is bad. If the value is somewhere betweeen +10 and +25%, it could mean that the pre-cat oxygen sensor is bad, there is a leak in the intake or that the MAF is on it's way out.
1.4.4 Block 033 - Pre-Cat Oxygen Sensor Control
In block 033, the value in Field 1 represents the pre-cat oxygen sensor control. The value should fluctuate at least 2% in the -10 to +10% range.
1.4.5 Block 036 - Post-Cat Oxygen Sensor Control
Test block 036 must be done in basic settings, not measuring blocks. Depress and hold brake pedal to run the automated test. The engine RPMs should rise to around 1400. Field 1 is post-cat sensor voltage. It must fluctuate slightly between 0 and 1V. Field 4 will say either "TEST OFF/ON" before/while the test is running and either 'B1-S2 OK' (sensor is good) or 'B1-S2 NOT OK' (sensor is bad) after the test is finished. Release the brake pedal after the
test finishes.
1.4.6 Block 034 - Aging of Pre-Cat Oxygen Sensor
Test block 034 must be done in basic settings, not in measuring blocks. Depress and hold brake pedal to run the automated test. The engine RPMs should raise to around 1400. Field 1 is the engine speed. Field 2 is the catalytic converter temp. Field 3 is the value which tells you how aged the sensor is (not sure what it's called). Field 4 will say either 'TEST OFF/ON'
before/while the test is running and either 'B1-S1 OK' or 'B1-S1 NOT OK' after the test is finished. The aging value must be above 0.80. The value for a new sensor is 1.99. The value will decrease as the sensor ages. Release the brake pedal after the test finishes.
1.4.7 Block 046 - Catalytic Converter
NOTE: The test in Block 034 must be done just before this test or it will NOT initiate!
Block 046 must be done in basic settings, not measuring blocks. Depress and hold brake pedal to start the automated CAT test (last approx. 100 secs - the cat needs to be warmed up above a certain threshold for an accurate reading - the threshold is usually 400°C). The engine RPMs (Field 1) should rise to around 1400. Field 2 is the CAT temp. This will also rise during the test. Field 3 is the cat conversion efficiency. If the cat is good, the value should be below 0.50 at the end of the test. Field 4 will indicate if the cat is good (CAT B1 - OK) or bad (CAT B1 - NOT OK). Release the brake pedal after the test finishes.
1.4.8 Block 060 – Throttle Body Reset
Throttle body reset (TBR) is a procedure used to calibrate the throttle body flap sensors. Sometimes it is confused with clearing the ECU adaptation tables. The only procedure to do the real throttle body reset is to enter VAG-COM basic setting block 060. However, the ECU usually does TBR after its power supply is cycled, i.e. the fuse has been pulled or the battery
has been disconnected, AND the engine is cold.
The reason for doing TBF is that some people have reported power lost after long period of unspirited driving, as if the ECU were adapting to the driving style. Presumably the ECU just looses its knowledge of the sensor values under WOT. However, if hesitation is caused by misguided adaptation, full reset is needed instead of TBR.
If you don’t have VAG-COM, or you actually want to reset the ECU for clearing the adaptation tables (often incorrectly referred as TBR), you can just pull the ECU fuse, or disconnect the battery. Leave the power out for 15min to ensure all the power has been drained. Then reconnect, and turn on ignition, but DO NOT start the engine. Listen out for the clicking under the bonnet, which is usually over after a minute. Turn off ignition.
Note. If you disconnect the battery, be prepared to enter the radio safety code.
-
P.S. Oops, forgot to mention the Source.
#4
12-05-2010, 01:01 PM
Thanks Chefro I'll look into this. I did not eliminate any parts, all 4 o2 sensors are still there and functioning. We did replace the 2 pre-cat o2 sensors maybe 4 months ago just because they were old. Hoping it would bring my average mpg up from 21 but it made no difference. Now I'm getting anywhere from 16-18mpg average per tank. I've owned this car for 5 years and never seen the average mpg drop below 19.
#5
12-05-2010, 01:19 PM
You're more than welcome.
If you solve the mystery, please share with us as I'm (personally) very interested in your findings.
One more thing, just an idea:
maybe the gas consumption increase is just purely coincidental with the cats R&R, as bigdadi said - the weather changed. The colder temperatures will cause the mixture to be richer. Also, it takes a little more time to warm-up the car. (At least I try to warm-up the car close to normal temp before actually driving it - I'm an "old school" fool, I know).
You can also try to give your MAF sensor a clean and see if that makes a difference.
One last thing, if you don't already run it for the winter, switch the oil to a thinner grade. For example from 5w-30 to 0w-30.
I hope you find the problem.
If you solve the mystery, please share with us as I'm (personally) very interested in your findings.
One more thing, just an idea:
maybe the gas consumption increase is just purely coincidental with the cats R&R, as bigdadi said - the weather changed. The colder temperatures will cause the mixture to be richer. Also, it takes a little more time to warm-up the car. (At least I try to warm-up the car close to normal temp before actually driving it - I'm an "old school" fool, I know).
You can also try to give your MAF sensor a clean and see if that makes a difference.
One last thing, if you don't already run it for the winter, switch the oil to a thinner grade. For example from 5w-30 to 0w-30.
I hope you find the problem.
#6
12-06-2010, 12:11 PM
Chefro, FYI, the post o2 sensors are for operation of catalytic converter efficiency verification only. Fuel trim is done with the front sensors.
Jeffla, only thing that I can think of is that you may have restricted the exhaust flow with your new converters. I would look into the converters that you installed to see if they are properly sized. Here is a website that gives you general information about sizing.
Converter Sizing
See if your converters generally meet the requirement.
Jeffla, only thing that I can think of is that you may have restricted the exhaust flow with your new converters. I would look into the converters that you installed to see if they are properly sized. Here is a website that gives you general information about sizing.
Converter Sizing
See if your converters generally meet the requirement.
#7
12-06-2010, 12:21 PM
I called the shop and forgot to ask the brand they installed, but he explained that the cats he used were a high flow type with much greater flow then stock. I explained I was seeing a 20-25% reduction in MPG and he said that because of how many leaks I had in my exhaust prior to the fix I probably was getting better mileage because of it.
I did do a throttle body adaption yesterday after seeing it set an error for failed adaption earlier in the week. I ran the tests Chefro listed and was able to complete only a few of them. My maf sensor tested ok, the throttle body adapted properly, the o2 sensors all passed with an ok status, but I can't get the cat efficiency test to perform. It needs a cat temp of 400 or higher, at idle, my cars quickly cool to under 400 withing seconds.
I did do a throttle body adaption yesterday after seeing it set an error for failed adaption earlier in the week. I ran the tests Chefro listed and was able to complete only a few of them. My maf sensor tested ok, the throttle body adapted properly, the o2 sensors all passed with an ok status, but I can't get the cat efficiency test to perform. It needs a cat temp of 400 or higher, at idle, my cars quickly cool to under 400 withing seconds.
Last edited by Jeffla; 12-06-2010 at 12:38 PM.
#9
12-11-2010, 01:08 AM
Update:
Ok, I want to start this reply out with a sometimes people make dumb mistakes. Sometimes we overlook really obvious things even when others point such things out. But remember, no one is perfect, not even me.
Well, it turns out ...(I'm so ashamed to be admitting this) my tire pressure gauge which is attached to the air compressor has broken. Turns out my tire pressure was 22 in all 4 tires. After adding air my gas mileage has returned in full. I'm back to getting 21 average city/hwy.
Shakes head in shame
Ok, I want to start this reply out with a sometimes people make dumb mistakes. Sometimes we overlook really obvious things even when others point such things out. But remember, no one is perfect, not even me.
Well, it turns out ...(I'm so ashamed to be admitting this) my tire pressure gauge which is attached to the air compressor has broken. Turns out my tire pressure was 22 in all 4 tires. After adding air my gas mileage has returned in full. I'm back to getting 21 average city/hwy.
Shakes head in shame
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