Bill
6.2 MPG Problem
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
6.2 MPG Problem
Decided to lay the car up for a while and get this outrageous MPG problem fixed.
Removed the sparkplugs for a compression check, and in doing so, found #4 plug loose. Was able to screw it out with my fingers. I doubt this would decrease MPG that much, but anyway, here's what they looked like:
I was guessing they would look worse (darker).
Compression (psi) was as follows, cylinders 1 through 8 respectively, cold engine;
60
59
60
60
60
57
56
59
There is about 67K on the engine.
My question to anyone here: How much can a loose plug affect MPG? They only have about 200 miles on them (AC Delco).
I'm planing right now to just clean the plugs, put them all back in, TIGHT, and see what happens to the mileage.
Thanks in advance.
Steve
Removed the sparkplugs for a compression check, and in doing so, found #4 plug loose. Was able to screw it out with my fingers. I doubt this would decrease MPG that much, but anyway, here's what they looked like:
I was guessing they would look worse (darker).
Compression (psi) was as follows, cylinders 1 through 8 respectively, cold engine;
60
59
60
60
60
57
56
59
There is about 67K on the engine.
My question to anyone here: How much can a loose plug affect MPG? They only have about 200 miles on them (AC Delco).
I'm planing right now to just clean the plugs, put them all back in, TIGHT, and see what happens to the mileage.
Thanks in advance.
Steve
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
There's at least 100 PSI missing from those numbers. Did you crank it with the throttle wide open with a fresh battery and let it pump at least 3 times?
For comparison, my 10:1 engine cranks 200 PSI.
For comparison, my 10:1 engine cranks 200 PSI.
Melting Slicks
Joined: Jul 2006
Posts: 2,291
Likes: 492
From: Salmon Arm, BC
One Loose plug won't affect your mileage very much.
I would be concerned about your compression if I was you. How did you take the readings? Did you hold the carb butterflys wide open when taking readings? They should be somewhere around 110-130 on a stock or near stock engine. What is the duration on you camshaft....that does affect the compression readings.
I would be concerned about your compression if I was you. How did you take the readings? Did you hold the carb butterflys wide open when taking readings? They should be somewhere around 110-130 on a stock or near stock engine. What is the duration on you camshaft....that does affect the compression readings.
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
)
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
One Loose plug won't affect your mileage very much.
I would be concerned about your compression if I was you. How did you take the readings? Did you hold the carb butterflys wide open when taking readings? They should be somewhere around 110-130 on a stock or near stock engine. What is the duration on you camshaft....that does affect the compression readings.
I do not know if the PO did anything with the camshaft. I assume it's a stock cam.
I would be concerned about your compression if I was you. How did you take the readings? Did you hold the carb butterflys wide open when taking readings? They should be somewhere around 110-130 on a stock or near stock engine. What is the duration on you camshaft....that does affect the compression readings.
I do not know if the PO did anything with the camshaft. I assume it's a stock cam.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Drifting
Joined: Jan 2006
Posts: 1,832
Likes: 38
From: Cedar,BC
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
As the others pointed out,you should check the compression on a warm engine with the throttle wide open,but your test was likely good enough,looks like all the cylinders are within a few points of being equal-which is what is important about a compression test,not so much the actual psi.Your poor mpg is likely an ignition/timing/advance issue,or just a too heavy right foot.
Driving with a lead foot? Didn't happen. Not even close to anything that might be considered aggressive.
But this car is very responsive. It WILL move if the pedal is depressed. And that amazed me: a 3300 lb car with a supposed 190 hp motor.
Plugs are dark, but only one (#2) could possibly be considered wet.
That gas is going somewhere, and the heavy soot on the exhaust pipes, along with no external fuel leaks visable anywhere, makes me believe it's going thru the carburetor.
And those compression numbers. I was pleased with the consistancy. However I didn't know about the WOT position during the test.
Or perhaps it's the cheap-*** Harbor Freight compression tester. I will test the tester now also.
Thanks to all that responded.
Steve
Drifting
Joined: Jan 2011
Posts: 1,853
Likes: 81
From: Hamilton Virginia
With lots of black sot in the exhaust is definately burning way too rich.
Make sure your butterfly is opening fully once warmed up. Run you air/fuel mixtures screws all the way in till you feel they touch, then back out about 11/2 turns, then tweak them as needed. check timing, if too retarded you will not burn comepletey. also check to make sure your plugs are correct. A weak coil can cause problems.
Make sure your butterfly is opening fully once warmed up. Run you air/fuel mixtures screws all the way in till you feel they touch, then back out about 11/2 turns, then tweak them as needed. check timing, if too retarded you will not burn comepletey. also check to make sure your plugs are correct. A weak coil can cause problems.
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
Checked compression gauge with a known air compressor output:
Gauge reads 90 psi @ 100 psi from the compressor.
Took another compression reading with WOT this time. All numbers were 1 (that's ONE) psi higher.
Carburetor coming off and going to the shop.
Gauge reads 90 psi @ 100 psi from the compressor.
Took another compression reading with WOT this time. All numbers were 1 (that's ONE) psi higher.
Carburetor coming off and going to the shop.
Melting Slicks
Joined: Jul 2010
Posts: 2,503
Likes: 33
From: McGrady NC
St. Jude Donor '15-'16
I think I would squirt a little oil into a cylinder and then run a compression test on that cylinder to see if the rings will seal up. Those are low numbers even on a cold engine, it seems to me. With the carb off and all the plugs out is the perfect time.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
The plugs look pretty good for such poor fuel economy.
Those compression numbers are way low. Maybe carbon deposits on the exhaust valves is preventing them from sealing while doing a compression test. I don't see how the engine would even run otherwise. Do you have lots of blowby when it's idling? If so then suspect rings if not then suspect valves.
I say fill it up again take it on the highway for 100 miles and refill to see what highway mileage is.
You carb could be a little rich, but I would expect a lot more soot on the plugs if it were way out of wack.
You sure the cam has not been changed?
Those compression numbers are way low. Maybe carbon deposits on the exhaust valves is preventing them from sealing while doing a compression test. I don't see how the engine would even run otherwise. Do you have lots of blowby when it's idling? If so then suspect rings if not then suspect valves.
I say fill it up again take it on the highway for 100 miles and refill to see what highway mileage is.
You carb could be a little rich, but I would expect a lot more soot on the plugs if it were way out of wack.
You sure the cam has not been changed?
Thread Starter
Melting Slicks
Joined: Oct 2013
Posts: 2,569
Likes: 126
From: San Diego California
The plugs look pretty good for such poor fuel economy.
Those compression numbers are way low. Maybe carbon deposits on the exhaust valves is preventing them from sealing while doing a compression test. I don't see how the engine would even run otherwise. Do you have lots of blowby when it's idling? If so then suspect rings if not then suspect valves.
I say fill it up again take it on the highway for 100 miles and refill to see what highway mileage is.
You carb could be a little rich, but I would expect a lot more soot on the plugs if it were way out of wack.
You sure the cam has not been changed?
Those compression numbers are way low. Maybe carbon deposits on the exhaust valves is preventing them from sealing while doing a compression test. I don't see how the engine would even run otherwise. Do you have lots of blowby when it's idling? If so then suspect rings if not then suspect valves.
I say fill it up again take it on the highway for 100 miles and refill to see what highway mileage is.
You carb could be a little rich, but I would expect a lot more soot on the plugs if it were way out of wack.
You sure the cam has not been changed?
And speaking of balancer:
Both balancer keyways are at 6 o'clock. But look at how far off the timing mark is on the old when compared to the new. The mech who got the car running told me of the problem, and said he timed it "by sound".
Steve
Racer
Joined: Aug 2006
Posts: 330
Likes: 3
From: Wellington, New Zealand
I agree, I did a carb removed, cold compression test on my 115,000mls L48 and got a spread of 155 to 168 psi with most being over 160. The figures were actually higher than I expected from an old smog motor.