Cavu2u

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

zwede

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.

MrJlr

If those numbers are correct, your enging is WORE OUT

OMF

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.

mr303

Standard L48 when cold will give you 160+ psi.

63split63

Did the same guy that checked your fuel milage also take the compression readings .

Bill

Cavu2u

Thanks. Will retake with the throttle open (or will it make that much difference anyway? )

Cavu2u

Thanks for the input.

Cavu2u

Yeah.

Me.

Cavu2u

Cavu2u

Know that I know, I agree with you on that.

This project sat from '98.

tfi racing

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.

Cavu2u

Something's screwy here.

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

scrappy76

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.

Cavu2u

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.

CaseyJones

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.

REELAV8R

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?

Cavu2u

Cam could have been changed, as after the balancer came off, I took a peek at the timing chain by lifting the cover away a little and saw a double row gear/chain set.

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

joewill

way way low compression numbers. either your gauge is not right, your methodology is not right or you have something very basically wrong with your engine.

mr303

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.