94 Metro problems (codes 13 and 51)

My first post! You guys have a great forum. This might be a bit long, but I'm really confused :)

Ok, so a few days ago there was a really cold night (on the brink of freezing), whereas typically it has been around 45-55. I started up my Metro that night and the 'Check Engine Light' comes on, and has stayed on for 3 days.

On the freeway, I found that the amount of gas it normally needs to get to 55mph only brought me to 40mph. I had noticed a gradual decrease in power over the last two months. This was the first time I'd ever seen the CEL, and the power drop was more obvious (maybe due to the cold). Also, the day before the engine light came on, I noticed something strange with the temp gauge. I started it up cold and it got to full temp in about 1/4 the time it normally does, but it stayed at the proper temp after that. I checked it while hot and found no bubbles, though I did find that the coolant was pretty dirty (little particles floating around).

- it has 118,000 miles
- I got it recently (105k) without much info on past maintenance.
- Everything looks original in the engine bay
- slight hesitation from stop when cold
- needs more gas pedal to get to highway speeds than it used to
- gradual drop in mpg over past 2 months
- started consuming oil about 6 months ago(~1 quart per 1000mi)
- no visible smoke from exhaust, but gasoline smell
- Occasionally the headlights will be dim for a few seconds after starting, but I haven't seen a consistent pattern

So thanks to a helpful thread on this forum, I got the engine codes today: 13 and 51, which are the oxygen sensor and the EGR I believe.

I don't know too much about engines, and I've never had to do anything to my Metro so I'm not familiar with the layout. I'm also not exactly sure what an oxygen sensor looks like. I think I found where the EGR is (below air filter, connected to intake, disc shaped thing closest to firewall). Anyway, I learned from this forum that carbon builds up in the passages of the EGR. However, I didn't have time today to tear anything apart, so I haven't eliminated that as a cause.

Considering the slight electrical problem (dim headlights), here is what I did do:
- checked battery water (fine)
- checked alternator tension/connection (fine)
- checked for burnt wires (none)
- Then I checked the battery terminals and associated wires. They are the frayed copper type. They were pretty gunked up with this strange brownish goo. So I broke out the toothbrush and solvent and cleaned them up pretty well and reconnected everything.

Upon starting up, the engine light was gone. Idle was smooth, and also a tad faster. I didn't touch the EGR. All I did was pull a few vaccum hoses to check for blockages (didn't find any).

I have a hard time believing that gunked up battery terminals could have solved my problem. As I recall, disconnecting the battery resets the error codes, so why wouldn't the error codes just reappear if I actually had a clogged EGR or a faulty O2 sensor?

If the light doesn't come back, would it still be a good idea to replace the sensor and clean out the EGR?

What about the strange temp guage behavior?

How does the increased fuel consumption factor in?

Could be a number of things causing the symptoms you mentioned. If you plan on keeping the Geo and plan to drive it on a regular basis, I would suggest that you do a tune-up on it to bring things up to a known state. Change the plugs, plug wires, distributor cap, rotor, air filter, PCV value and replace any vacuum hoses that are loose, has cracks or are brittle, to start. This is pretty cheap to do on a Geo and will "fix" a lot of problems.

Next in line would be to replace the fuel filter (by the left rear wheel), run some injector cleaner in the fuel tank, spray some carb cleaner in the throttle body, and clean the EGR value.

Change oil and filter every 3K miles without fail.

Thought of a couple of other items. Keep battery connectors clean and replace cables if needed. Change radiator cap and check coolant level and condition. Flush and refill if dirty - good time to change the thermostat also.

Reading the symptoms, you may have more than one problem. One thing is for sure, your are burning way too rich and the engine is heating up very quickly, probably due to the increase in the amount of fuel being burned.

The fuel burn problem may be related to your DTC 13 (O2 sensor voltage not changing). If I'm not mistaken, when the ECM gets a DTC 13 it automatically goes to a full rich injection. But I doubt if this would cause your fuel economy to drop this drastically, however.

Along with the suggestions that GeoJim gave you I would suggest you get an overall health check of your engine by doing a compression test. When in engine has a weak cylinder they can act in strange ways.

Also verify the exhaust gas ports are clear by removing the EGR valve and injecting compressed air in both directions. Blocked EGR ports will cause a DTC 51.

Try Jim's suggestions first and perhaps these ideas and let us know what happens. Good luck!

Tude123 makes a good point about the EGR. If you take off the EGR value to clean it, it's important to make sure that both passages into the block at that point are clear. I had to use stiff wire and a lot of carb cleaner to get my passages clear. The passages being blocked did not seem to affect the overall mpg, but I understand that it may raise the internal operating temperature of the engine. It would also increase pollutants, but we here in WV don't check for such things...