fuel economy loss

Lain · #1 (**permalink**) 02-15-08, 09:37 AM

I own a 1993 Eldorado touring coupe northstar 4.6L.

Ever since I have replaced the spark plugs and wires/ignition coils I have noticed a dramatic loss of mpg. It went from getting 19-21 mpg to 10-12 mpg. I also smell something burning when I cut it off after it idles up to temp. What could this be?

**Ranger** · #2 (**permalink**) 02-15-08, 11:36 AM

Recheck your work. Make sure you did not leave a sensor unplugged.

raymondk · #3 (**permalink**) 02-15-08, 12:22 PM

Does it run smoothly, or miss and or pop-backfire? Is it running in closed loop? The smell is probably raw fuel burning in the converter. This engine fires 2 cylinders at the same time in series so ignition problems usually occur in pairs. Try putting the old stuff back on. If it did 21mpg as mine does, and doesn't miss under load, it didn't need anything. If it misses idling test each cylinder idling by using the manual injector cut out. That will indicate which cylinder if any is missing at idle. Two missing 180 deg. appart in the firing order would indicate a coil or bad or crossed plug wire(s) check each wire with an ohm meter. A good wire will be in the 10-50K range depending on length, a bad one will be over 100 K. Check coil primary and secondary resistances . They should be within 10% of each other. A bad one will be off by a factor of 5 or so. I have seen instances where converter fires have destroyed the pellets in the converter and they piled up and blocked the exhaust flow . The O2 sensor readings should be way off.

chubbyranger · #4 (**permalink**) 02-15-08, 01:52 PM

Pull codes:

http://www.cadillacforums.com/forums...s-warning.html

And if you used anything but AC Delco plugs, go back and take them out.

RENO99DEVILLE · #5 (**permalink**) 02-16-08, 05:11 AM

I am having that same problem in my 94 but my check engine light is not on so does that mean I have no codes? The spark plug wires & plugs were changed and now I'm getting 12mpg but the sad thing is I didn't do the work so what can I check/look out for/check sensors (which sensor) or anything else I can check to resolve this?

submariner409 · #6 (**permalink**) 02-16-08, 04:35 PM

Both vehicles in question have a waste spark 4-coil/8-wire system. I'll bet one or more plug wires are on the wrong plugs. If you or someone changes wires in the future, do it one at a time.

("Waste spark" refers to the system in which one coil fires two plugs at the same time: One plug is at or near the top of the compression>power stroke, the other (waste) plug is at the top of the exhaust stroke, so no fuel to ignite......)

Your ignition timing may be way out on one or more cylinders. Use a FSM to solve the puzzle.

chubbyranger · #7 (**permalink**) 02-16-08, 04:45 PM

Recheck the wires. The coils are numbered, you have to stick your head in the hood toward the back to see the numbers. The wires should also be numbered. Check to ensure the number on the wire matches the number on the coil. If they installed cheapo non-numbered wires it will be more work. The cylinder numbers are stamped in the valve cover.

On your 94 if the CEL is on you should definitely have codes.

RENO99DEVILLE · #8 (**permalink**) 02-16-08, 04:51 PM

Yea mine doesn't have codes, so should I still check the wire numbers to match the coil numbers? How are the coils marked?

submariner409 · #9 (**permalink**) 02-16-08, 05:12 PM

Coil tower 1 goes to wire 1 which goes to plug 1. Anything else is wrong.

RENO99DEVILLE · #10 (**permalink**) 02-16-08, 05:16 PM

Quote:

Originally Posted by submariner409

Coil tower 1 goes to wire 1 which goes to plug 1. Anything else is wrong.

Does the coil tower have a number stamped on it? I will check all this right now and will let you know if I had any wrong, thanks submariner409!!!

submariner409 · #11 (**permalink**) 02-16-08, 05:24 PM

I'm doing this in the dark....Chubbyranger or one of our techs should have the coil ID info. Look back a couple of posts....Chubbyranger gave you the answer......

RENO99DEVILLE · #12 (**permalink**) 02-17-08, 07:37 PM

PROBLEM SOLVED. I guess I just needed to reset the system after the fuel injection service, tune-up, wire & spark plug change and new battery. Now I'm back in the 20mpg range. Thanks to everyone who helped!

dirtyguido · #13 (**permalink**) 02-18-08, 03:39 PM

My 2000 SLS did the same thing... Was giving erroneous readings on the display, and had me worried. Besides plugs, (mine has boots not wires with the coil packs atop the plugs) I also did air and fuel filters. It was ranging 6.5 - 10 mpg. I reset it on the dash, but that didn't seem to help. BUT what clued me in was the amount of fuel it claimed to use...

I had reset it because I was alarmed, and then at 1/2 tank the dash claimed I had used 13.4 gallons of fuel. I filled the car up, and it took about 8.6 gallons. I really don't know what caused the dash to uncalibrate itself, but who knows. I figured it was a sender or something not reading 100%.

What cured it actually surprised the heck out of me.... I went to install my subwoofer, and as part of the installation, I had to disconnect and then reconnect the battery. When the battery reconnected, somehow the dash magically recalibrated itself and shows normal mileage and fuel consumption readings. Go Figure.... So your fuel economy didn't drop, but somehow when you do the service on these cars, it messes up the computer....

So that may be a quick fix.... I really don't know why, but it works.

Take care,

George