Sudden MPG loss!!!
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Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
Sudden MPG loss!!!
Has anyone experienced a noticeable drop in MPG? I have a 98 and 99 (both coupes)... around 75,000 to 80,000 miles, both cars went from averaging 26-30 MPG on the highway to now struggling to get 24 MPG. I have stainless exhaust cat-backs on both cars along with bigger/cold air intakes, but the significant drop happened long after that work was done. PLEASE HELP!!!!.... my green, efficient American 8-cylinder is turning into a gas hog!!!
Melting Slicks
Joined: Mar 2004
Posts: 2,404
Likes: 408
From: Horseshoe Bend ID
Has anyone experienced a noticeable drop in MPG? I have a 98 and 99 (both coupes)... around 75,000 to 80,000 miles, both cars went from averaging 26-30 MPG on the highway to now struggling to get 24 MPG. I have stainless exhaust cat-backs on both cars along with bigger/cold air intakes, but the significant drop happened long after that work was done. PLEASE HELP!!!!.... my green, efficient American 8-cylinder is turning into a gas hog!!!
Thread Starter
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Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
I can't speak to the '99's spark plugs. Based on one person's opinion, I swapped out my delco plugs in the 98 with NGK plugs (don't know the P/N off the top of my head, but pretty sure correct heat range) gapped down to .040". As for the O2 sensors, I would safely bet both cars have the original O2 sensors.
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Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
I did put in the higher performance wires from Summit just before the NGK plugs... no difference. Gas is almost always 93 octane BP fuel.... maybe a stray tank here and there. I had considered MAF sensor...... and even the O2 sensors.... but whatever has caused this isn't throwing any kind of code in the computer.
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Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
Is it possible something is getting fouled up and I'm burning it off?
Le Mans Master
Joined: Feb 2012
Posts: 6,644
Likes: 479
From: OK
What air filter system you running? Catch Can?
Just throwing some things to think about...this is why I mentioned the Catch Can
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover) #01-06-01-023A - (Jun 19, 2002)
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover)
1999-2001 Chevrolet Camaro
1999-2002 Chevrolet Corvette
1999-2001 Pontiac Firebird
with 5.7L Engine (VINs G, S -- RPOs LS1, LS6)
This bulletin is being revised to add model years for the Corvette, information on the LS6 engine and parts information. Please discard Corporate Bulletin Number 01-06-01-023 (Section 6 - Engine).
Condition
Some owners may comment on higher than expected oil consumption. When checked, the oil consumption could be in the range of 400-600 miles per quart (700-1000 km/L).
On the LS6 engine only, the technician may find oil behind the engine throttle plate and in the intake manifold.
Cause
The cause for this condition may be an interaction between the piston rings and the cylinder bore in vehicles that are operated at higher RPMs -- typically manual transmission vehicles driven in a manner where the engine is frequently or consistently operating at greater than 3200 RPM.
On the LS6 engine only, the engine has a unique aluminum valley cover that has composite oil separating baffles and PCV plumbing incorporated. In some cases, the PCV baffle may not be properly sealed to the valley cover, causing oil to enter the PCV system.
Correction
On the LS6 engine only, replace the engine valley cover if oil is found behind the throttle body or in the intake manifold before replacing the piston ring. Refer to Engine Valley Cover Replacement in the Engine Mechanical - 5.7L subsection of the Corvette Service Manual.
A new set of piston rings is currently available through GMSPO. The new rings are part of a complete piston ring kit. Install only the number 2 compression ring and the oil expander ring from the piston ring kit. All other rings in the piston ring kit should be discarded. The original number 1 compression ring and the oil ring rails should be re-used in their original positions on the piston. All pistons should be used in the same cylinder bore.
Important
Do not dress or hone the cylinder bore. Nothing should be done to change the bore finish for this condition. Changing the bore finish may aggravate the condition.
Refer to the Unit Repair Manual for appropriate ring removal and replacement procedure.
The number 1 compression ring and the upper and lower oil expander rails are re-used because they are already broken in for the bore that they are in. The new number 2 compression ring is made with a very sharp edge that will break in quickly.
Changing only the piston rings noted, with no change in driving style, should change oil consumption to an acceptable level. Changes in driving style that reduce the amount of time spent at higher RPMs will also positively affect oil consumption.
In addition to the standard size piston ring kit listed below, a 0.25 mm oversized piston ring kit is available. The oversized piston ring kit should only be used in those rare instances where the cylinder bore size has been machined larger to accommodate the 0.25 mm oversized piston rings.
Just throwing some things to think about...this is why I mentioned the Catch Can
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover) #01-06-01-023A - (Jun 19, 2002)
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover)
1999-2001 Chevrolet Camaro
1999-2002 Chevrolet Corvette
1999-2001 Pontiac Firebird
with 5.7L Engine (VINs G, S -- RPOs LS1, LS6)
This bulletin is being revised to add model years for the Corvette, information on the LS6 engine and parts information. Please discard Corporate Bulletin Number 01-06-01-023 (Section 6 - Engine).
Condition
Some owners may comment on higher than expected oil consumption. When checked, the oil consumption could be in the range of 400-600 miles per quart (700-1000 km/L).
On the LS6 engine only, the technician may find oil behind the engine throttle plate and in the intake manifold.
Cause
The cause for this condition may be an interaction between the piston rings and the cylinder bore in vehicles that are operated at higher RPMs -- typically manual transmission vehicles driven in a manner where the engine is frequently or consistently operating at greater than 3200 RPM.
On the LS6 engine only, the engine has a unique aluminum valley cover that has composite oil separating baffles and PCV plumbing incorporated. In some cases, the PCV baffle may not be properly sealed to the valley cover, causing oil to enter the PCV system.
Correction
On the LS6 engine only, replace the engine valley cover if oil is found behind the throttle body or in the intake manifold before replacing the piston ring. Refer to Engine Valley Cover Replacement in the Engine Mechanical - 5.7L subsection of the Corvette Service Manual.
A new set of piston rings is currently available through GMSPO. The new rings are part of a complete piston ring kit. Install only the number 2 compression ring and the oil expander ring from the piston ring kit. All other rings in the piston ring kit should be discarded. The original number 1 compression ring and the oil ring rails should be re-used in their original positions on the piston. All pistons should be used in the same cylinder bore.
Important
Do not dress or hone the cylinder bore. Nothing should be done to change the bore finish for this condition. Changing the bore finish may aggravate the condition.
Refer to the Unit Repair Manual for appropriate ring removal and replacement procedure.
The number 1 compression ring and the upper and lower oil expander rails are re-used because they are already broken in for the bore that they are in. The new number 2 compression ring is made with a very sharp edge that will break in quickly.
Changing only the piston rings noted, with no change in driving style, should change oil consumption to an acceptable level. Changes in driving style that reduce the amount of time spent at higher RPMs will also positively affect oil consumption.
In addition to the standard size piston ring kit listed below, a 0.25 mm oversized piston ring kit is available. The oversized piston ring kit should only be used in those rare instances where the cylinder bore size has been machined larger to accommodate the 0.25 mm oversized piston rings.
Last edited by 73Corvette; May 7, 2016 at 05:33 PM.
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Pro
Joined: Jan 2016
Posts: 550
Likes: 53
From: Fort Myers Florida
I can't speak to the '99's spark plugs. Based on one person's opinion, I swapped out my delco plugs in the 98 with NGK plugs (don't know the P/N off the top of my head, but pretty sure correct heat range) gapped down to .040". As for the O2 sensors, I would safely bet both cars have the original O2 sensors.
Thread Starter
Advanced
Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
Fuel mileage suffered before which was my reasoning for the new plugs and shorter gap. Lately I have been seriously contemplating returning to the OEM Delco plugs at spec gap.
Thread Starter
Advanced
Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
Thread Starter
Advanced
Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
What air filter system you running? Catch Can?
Just throwing some things to think about...this is why I mentioned the Catch Can
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover) #01-06-01-023A - (Jun 19, 2002)
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover)
1999-2001 Chevrolet Camaro
1999-2002 Chevrolet Corvette
1999-2001 Pontiac Firebird
with 5.7L Engine (VINs G, S -- RPOs LS1, LS6)
This bulletin is being revised to add model years for the Corvette, information on the LS6 engine and parts information. Please discard Corporate Bulletin Number 01-06-01-023 (Section 6 - Engine).
Condition
Some owners may comment on higher than expected oil consumption. When checked, the oil consumption could be in the range of 400-600 miles per quart (700-1000 km/L).
On the LS6 engine only, the technician may find oil behind the engine throttle plate and in the intake manifold.
Cause
The cause for this condition may be an interaction between the piston rings and the cylinder bore in vehicles that are operated at higher RPMs -- typically manual transmission vehicles driven in a manner where the engine is frequently or consistently operating at greater than 3200 RPM.
On the LS6 engine only, the engine has a unique aluminum valley cover that has composite oil separating baffles and PCV plumbing incorporated. In some cases, the PCV baffle may not be properly sealed to the valley cover, causing oil to enter the PCV system.
Correction
On the LS6 engine only, replace the engine valley cover if oil is found behind the throttle body or in the intake manifold before replacing the piston ring. Refer to Engine Valley Cover Replacement in the Engine Mechanical - 5.7L subsection of the Corvette Service Manual.
A new set of piston rings is currently available through GMSPO. The new rings are part of a complete piston ring kit. Install only the number 2 compression ring and the oil expander ring from the piston ring kit. All other rings in the piston ring kit should be discarded. The original number 1 compression ring and the oil ring rails should be re-used in their original positions on the piston. All pistons should be used in the same cylinder bore.
Important
Do not dress or hone the cylinder bore. Nothing should be done to change the bore finish for this condition. Changing the bore finish may aggravate the condition.
Refer to the Unit Repair Manual for appropriate ring removal and replacement procedure.
The number 1 compression ring and the upper and lower oil expander rails are re-used because they are already broken in for the bore that they are in. The new number 2 compression ring is made with a very sharp edge that will break in quickly.
Changing only the piston rings noted, with no change in driving style, should change oil consumption to an acceptable level. Changes in driving style that reduce the amount of time spent at higher RPMs will also positively affect oil consumption.
In addition to the standard size piston ring kit listed below, a 0.25 mm oversized piston ring kit is available. The oversized piston ring kit should only be used in those rare instances where the cylinder bore size has been machined larger to accommodate the 0.25 mm oversized piston rings.
Just throwing some things to think about...this is why I mentioned the Catch Can
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover) #01-06-01-023A - (Jun 19, 2002)
Higher Than Expected Oil Consumption (Replace Rings and Engine Valley Cover)
1999-2001 Chevrolet Camaro
1999-2002 Chevrolet Corvette
1999-2001 Pontiac Firebird
with 5.7L Engine (VINs G, S -- RPOs LS1, LS6)
This bulletin is being revised to add model years for the Corvette, information on the LS6 engine and parts information. Please discard Corporate Bulletin Number 01-06-01-023 (Section 6 - Engine).
Condition
Some owners may comment on higher than expected oil consumption. When checked, the oil consumption could be in the range of 400-600 miles per quart (700-1000 km/L).
On the LS6 engine only, the technician may find oil behind the engine throttle plate and in the intake manifold.
Cause
The cause for this condition may be an interaction between the piston rings and the cylinder bore in vehicles that are operated at higher RPMs -- typically manual transmission vehicles driven in a manner where the engine is frequently or consistently operating at greater than 3200 RPM.
On the LS6 engine only, the engine has a unique aluminum valley cover that has composite oil separating baffles and PCV plumbing incorporated. In some cases, the PCV baffle may not be properly sealed to the valley cover, causing oil to enter the PCV system.
Correction
On the LS6 engine only, replace the engine valley cover if oil is found behind the throttle body or in the intake manifold before replacing the piston ring. Refer to Engine Valley Cover Replacement in the Engine Mechanical - 5.7L subsection of the Corvette Service Manual.
A new set of piston rings is currently available through GMSPO. The new rings are part of a complete piston ring kit. Install only the number 2 compression ring and the oil expander ring from the piston ring kit. All other rings in the piston ring kit should be discarded. The original number 1 compression ring and the oil ring rails should be re-used in their original positions on the piston. All pistons should be used in the same cylinder bore.
Important
Do not dress or hone the cylinder bore. Nothing should be done to change the bore finish for this condition. Changing the bore finish may aggravate the condition.
Refer to the Unit Repair Manual for appropriate ring removal and replacement procedure.
The number 1 compression ring and the upper and lower oil expander rails are re-used because they are already broken in for the bore that they are in. The new number 2 compression ring is made with a very sharp edge that will break in quickly.
Changing only the piston rings noted, with no change in driving style, should change oil consumption to an acceptable level. Changes in driving style that reduce the amount of time spent at higher RPMs will also positively affect oil consumption.
In addition to the standard size piston ring kit listed below, a 0.25 mm oversized piston ring kit is available. The oversized piston ring kit should only be used in those rare instances where the cylinder bore size has been machined larger to accommodate the 0.25 mm oversized piston rings.
In the 98, I have a Y-shaped intake with two cone K&N style filters. Yes, I have recently cleaned and re-oiled the filters
Original air bridge and MAF sensor in place. Also, throttle body/coolant bypass kit in an effort to help alleviate the problem.The 99 has a true CAI installed by the previous owner.
Le Mans Master
Joined: Jun 2009
Posts: 6,528
Likes: 752
From: South Dakota
I had something similar happen on a '95 C4.
About halfway on a 350 mile highway trip the mpg dropped. It didn't return to normal in about a week with normal driving. I had a suspicion that I might have gotten some bad gas, but don't know if it was or not.
I was doing some unrelated work on it and disconnected the battery. When I restarted it after connecting the battery it ran good again. The mpg went up about 5 mpg.
I suspect that the computer learned from a bad situation and it did not clear itself. (possibly long term fuel trims, or a spark retard from bad gas? I don't know.) Once the battery was disconnected it lost the "learned" information and started over from the base program.
I would at least try disconnecting the battery and see if it helps. It is probably something else like O2 sensors, but it is easy and free to try losing any learned programming to see if it helps.
Good luck.
About halfway on a 350 mile highway trip the mpg dropped. It didn't return to normal in about a week with normal driving. I had a suspicion that I might have gotten some bad gas, but don't know if it was or not.
I was doing some unrelated work on it and disconnected the battery. When I restarted it after connecting the battery it ran good again. The mpg went up about 5 mpg.
I suspect that the computer learned from a bad situation and it did not clear itself. (possibly long term fuel trims, or a spark retard from bad gas? I don't know.) Once the battery was disconnected it lost the "learned" information and started over from the base program.
I would at least try disconnecting the battery and see if it helps. It is probably something else like O2 sensors, but it is easy and free to try losing any learned programming to see if it helps.
Good luck.
Le Mans Master
Joined: Feb 2012
Posts: 6,644
Likes: 479
From: OK
I had something similar happen on a '95 C4.
About halfway on a 350 mile highway trip the mpg dropped. It didn't return to normal in about a week with normal driving. I had a suspicion that I might have gotten some bad gas, but don't know if it was or not.
I was doing some unrelated work on it and disconnected the battery. When I restarted it after connecting the battery it ran good again. The mpg went up about 5 mpg.
I suspect that the computer learned from a bad situation and it did not clear itself. (possibly long term fuel trims, or a spark retard from bad gas? I don't know.) Once the battery was disconnected it lost the "learned" information and started over from the base program.
I would at least try disconnecting the battery and see if it helps. It is probably something else like O2 sensors, but it is easy and free to try losing any learned programming to see if it helps.
Good luck.
About halfway on a 350 mile highway trip the mpg dropped. It didn't return to normal in about a week with normal driving. I had a suspicion that I might have gotten some bad gas, but don't know if it was or not.
I was doing some unrelated work on it and disconnected the battery. When I restarted it after connecting the battery it ran good again. The mpg went up about 5 mpg.
I suspect that the computer learned from a bad situation and it did not clear itself. (possibly long term fuel trims, or a spark retard from bad gas? I don't know.) Once the battery was disconnected it lost the "learned" information and started over from the base program.
I would at least try disconnecting the battery and see if it helps. It is probably something else like O2 sensors, but it is easy and free to try losing any learned programming to see if it helps.
Good luck.
Safety Car
Joined: Mar 2015
Posts: 3,638
Likes: 245
Has anyone experienced a noticeable drop in MPG? I have a 98 and 99 (both coupes)... around 75,000 to 80,000 miles, both cars went from averaging 26-30 MPG on the highway to now struggling to get 24 MPG. I have stainless exhaust cat-backs on both cars along with bigger/cold air intakes, but the significant drop happened long after that work was done. PLEASE HELP!!!!.... my green, efficient American 8-cylinder is turning into a gas hog!!!
Thread Starter
Advanced
Joined: Jun 2007
Posts: 63
Likes: 12
From: Altamonte Springs FL
I haven't pulled plugs just yet.... I will also give QCVette's battery idea a try. Usually I always fill up at Costco because the fuel is a little cheaper. I asked several attendants there and they all say their fuel is BP fuel. I also saw a BP truck filling their tanks so I am pretty confident. Just the same though, I have gone to a proper BP station and didn't notice any change.
Also, I should clarify that both cars didn't start this at the same time.... only the same mileage. The '98 is 20,000 ahead of the '99. Both cars exhibited this behavior at approx. 75,000-80,000.
As to the suggestions about O2 sensors.... can an O2 sensor be going bad without giving a code? Like perhaps the readings aren't quite out of the acceptable range, but their really close?
I would like to thank everyone so far who has been responding.... it's nice to be able to bounce problems like this off other owners who have had similar issues
. I really hope we can figure this out!!!
Also, I should clarify that both cars didn't start this at the same time.... only the same mileage. The '98 is 20,000 ahead of the '99. Both cars exhibited this behavior at approx. 75,000-80,000.
As to the suggestions about O2 sensors.... can an O2 sensor be going bad without giving a code? Like perhaps the readings aren't quite out of the acceptable range, but their really close?
I would like to thank everyone so far who has been responding.... it's nice to be able to bounce problems like this off other owners who have had similar issues
. I really hope we can figure this out!!!