Last post on Jan 17, 2013 at 11:44 PM
You are in the Chevrolet Metro/Geo Metro
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Chevrolet Metro, Geo Metro, Hatchback
#65 of 67 1990 Geo Metro only 25 mpg why?
Jan 13, 2010 (2:35 pm)
I have 1990 geo metro where head is recently changed and recent full tune up but mileage is only 25 mpg.
What could be the main reason why fuel economy is bad?
One mechanic said timing belt other said timing needs adjustment...am confused.
What are the main possible reasons?
is it a easy fix or it is just because it is old?
#66 of 67 Re: 1990 Geo Metro only 25 mpg why? [vikasintl]
Feb 01, 2010 (8:50 pm)
The answer to the last point you asked about is that there is no such thing as an engine getting bad mileage because it is too old. ANY engine, regardless of its age, will get good mileage if the internal parts are not worn excessively, the compression is good, and the fuel and ignition systems are adjusted right.
On the other hand; even a nearly new engine will get bad mileage if it has been damaged, or improperly adjusted, or from wrong connections of the vacuum controls; or by installation of parts which do not match the specifications of the parts that originally came on that motor.
To fix a problem like yours may be easy, or it may be very difficult; this all depends on what is wrong. If someone installed the wrong model head on it; that would be very hard to fix. But if they reversed the connections of the vacuum hoses; it could be easy to fix. But if the mechanic who works on the car does not understand how to troubleshoot this problem, or doesn't have a service manual for this car model (or is too arrogant, or too illiterate, or too sure of themselves to bother to read the manual); they may never be able to fix it. Metros are different than many of the cars that mechanics usually work on; and it takes the willingness to admit the limits to one's knowledge and then learn the unique and important details about this engine design, before a mechanic can even know where to look for the real problem.
There were at least two different cylinder head designs used in the 3 cylinder Metro. The base Metro and the LSI both used one head; while the XFI model used a different head. The camshafts are different in the two heads, and I believe the porting and possibly the valve sizes are also different. If a different model head has been installed; the computer on your car can not work properly with a change like that. So my guess is that the reason you are now getting such poor mileage is either that the replacement head came from a different engine model; or that the replacement head is in bad mechanical condition.
You can find out whether the head is in good condition by running a compression check on the motor. The compression should ideally be 195psi in all cylinders. If it is less than 155 in any one or more cylinders; the timing belt may have not been installed in the right position. That is a surprisingly common mistake that mechanics make. But if there is instead more than 20 pounds difference between any two cylinders; then the head and/or the rest of the motor is worn internally to the point where it becomes impossible to tune it properly. In that case; you'll need to get a remanufactured motor.
But if the compression is within the allowable limits in all cylinders; then it would be worth finding out whether the cylinder head is the right model for the year and model of your motor. Just because the head could be bolted on to the block does not mean it is the same model head as the old one. It may be very difficult to determine the model of the head that is now on the motor. If you can find a casting number on the head; you might be able to phone a Chevy dealer's parts department and ask them if they can tell you the engine model that head belongs on.
The other thing you can do is to find a vacuum hose diagram for that motor (which originally was printed on a label fastened to the underside of the hood; and is also found in the service manual for that year and model Metro) Make ABSOLUTELY certain that every hose goes exactly where it is supposed to (on BOTH ends). Also check the ignition timing with a strobe light; and be sure to disconnect BOTH vacuum hoses from the distributor advance units while doing so. The timing should be set to 6 degrees BTDC when the engine is idling. If you find the timing more than 5 degrees different than that, the timing belt should be rechecked to see if it is installed in the right position. (The timing belt connects the crankshaft to the camshaft, but the distributor is driven by the camshaft; so if the timing belt is not installed right, BOTH the cam timing and the ignition timing will be off).
If you check and adjust all the things mentioned above, and still can't find the problem, one other thing you can do is to check the exhaust emissions with an emission analyzer (at both idle and 2500 RPM). If the CO or HC levels are too high, that could come from the throttle position sensor being set too rich, or from a clogged EGR valve, or from the wrong spark plug model or wrong plug gap setting, or bad plug wires or a bad distributor cap, or from a clogged fuel filter or clogged air filter, or a defective vacuum or mechanical advance in the distributor. And all of those could cause bad mileage.
#67 of 67 Re: poor mpg's [4_banger]
Jan 17, 2013 (11:44 pm)
you have the automatic, the 5 speed gets tremendously better gas mileage however, the automatic gets "standard" to "good" gas mileage. For instance if you had the xfi model you would be getting 45-60mpg.