Catalytic converter or oxygen sensor?

[rick390]

My 97" Geo Metro "service engine soon" light has been on intermittently for about six months. The code says something like catalytic efficiency out of range. Is replacing the cat a reasonable diy project. Should I try replacing the oxygen sensors first, or at the same time as the cat?
The light has been out for about a week now. So I'm confused if the problem is indeed the cat or maybe the o2 sensors. My mileage has gone down from the 50's to the mid-40's per gallon. Any suggestions would be appreciated.

[Ryland]

pull the sensor and test it with a propane torch, or if it's a single wire sensor just replace it all together, the cat is held in by 5 very rusty bolts, a nut cracker and some cursing is required.

[Johnny Mullet]

If the code was a (P0420) Cat efficiency below threshhold, then I bet my left nut the cat is bad. You can replace the cat, gut the cat, or bypass it, but legally, you need a new one.

If you chose to gut or bypass, then the rear O2 sensor needs not to go in. Leave it plugged in and tie it to the strut tower to keep the check engine light out.

Also, there is usually a reason the cat melted down, whether it's age or the car could have low compression or running rich. I would begin with a compression test on the cylinders before wasting money on a new cat to have it also melt down.

[Mogal]

Hi all, I have a few questions that I would like to get answered...

What is the open/closed loop that everyone refers to?

How do you drive by a vacuum Gage, what are your limits your looking for?
Some say they drive solely by the vacuum Gage, but others say they will use 75% throtle... If I put the throttle to even 50% my vacuum dives to 0 hg.
Others say to regulate the throttle to 5-7 hg while accelerating.
My idle (while warm) is 18-19hg
Is this what everyone strives for?

Accel - least amount of vac without going into open loop
Cruise - highest amount of vacuum @ efficient rpm

I've changed my plugs, check the cap/rotor/wires, all is fine. Tires at 44psi.
I just changed my o2 sensor tonight, not sure of age, but probably original to the engine('93) - the old o2 gave no error code
Hoping this will boost my mpg by some... (its a heated 4 wire o2)

I Checked the compression and get 120-125psi on each cylinder(kinda low?)
I was told about the water trick... rev to 4-5k and slowly dump in about 1 liter of water, not letting it stall. Should steam out allot of built up carbon. - Should I do this?

I have adjusted my timing to 8deg BTDC thinking of maybe adjusting to 10 deg... using the cheap (LOL) gas at 87 octane. No engine knocking at 8deg.

When one 'glides' do you just clutch-in, or do you pull the gears to neutral?

I made a small bezel to cover the front grill ports... I had to add in the breather hole, as the fan started to kick in... (its 12c here today)
I made it from the plastic cardboard from home depot...
Mornings are still kinda cold, I don't have a block heater... hoping this will help warm up in the mornings, and help with aero.

So far with my current tank(first eco tank) I have just rolled 500km and I would normally be hunting for a refill now... I still have a 1/4 tank left!!!!

Any other 'adjustments' I can make to help improve things... I've read over the 100 tips... great list!

TIA
Chris

[TestDrive]

Welcome to ecomodder, Mogal.

With so many questions about anything but Catalytic converters and/or 02 sensors - you should have started a new thread. Maybe some kind moderator will do it for us?

Closed loop refers to times when the ECU readings from the 02 sensor(s) into account in order to maintain a stoichiometric air/fuel ratio. Open loop refers to times when the ECU ignores O2 sensor readings and provides a richer fuel mixture eg. cold engine, wide open throttle.

As to how to drive by vacuum gage - if you're trying to maintain a rate constant rate of cruise, you want the highest vacuum reading you can obtain and still maintain the cruise rate. Arguments begin when you start talking about best reading while accelerating. If you've got a scan gage or other gage that will show when the ECU goes into open loop, the lowest vacuum reading where the ECU doesn't go into open loop is probably close. If you don't have that instrumentation - 5-7hg is probably a reasonable guess.

With the engine warm, you want the highest possible hg at the lowest, smooth idle.

I doubt changing the O2 sensor will make a difference as there was no code. Keep the old one as a spare.

120-125 psi is low. Not sure of the specifications for a metro of unspecified engine size, but probably ~180-200 psi. Was the correct test procedure used to get that reading? Was the engine fully warmed up? Did you do both a wet and a dry tests. Were all plugs removed? Battery fully charged? Starter in good condition (not cranking slow)? Throttle wide open?

The water trick won't cure the low compression (assuming you've got low compression). I'm not a fan of the water trick in any case.

As a general rule, you should shift into neutral. If you just push the clutch in you risk premature failure of the clutch throw-out bearing (may have a different name on metro).

[Mogal]

My apologies with the thread, I actually thought I started a new one... not sure what happened there...?
(sometimes I just do too many things at once on the comp... lol)

When I did the compression test, the gage was installed in one cylinder, and the other two plugs were loose. I did not give any gas, I just clutched in and rolled the engine 5-6 times... engine was warm, but not running temp.
Should I redo the tests, with the engine at operating temps, and the gas peddle depressed? I found it odd that they seemed low, but they were all equally low...

Thanks for the vacuum info... I will try to maintain a 5-7in vac and see what happens as I do not have a SG.

Mods: Pls move this thread as required, thanks (and sorry)

[TestDrive]

Engine should be fully warmed up.
All plugs should be removed.
Ignition should be disabled. (Procedure is different on different cars. Will let Johnny Mullet or some other metro owner supply that info.)
Throttle plate and choke plate should be wide open.
Use remote starter switch to (or have assistant) crank engine while you note compression guage reading on 1st and 5th stroke. (Should have ~2/3 specified compression on 1st stroke. Should have full specified compression on 5th stroke.)

Here is a more complete write up - Popular Mechanics Complete Car Care ... - Google Book Search

[some_other_dave]

Another reason to go into neutral when coasting: With the clutch in, your wheels are still spinning the output shaft inside the transmission, and also the input shaft and the clutch disk. You have disconnected the clutch disk from the flywheel by pushing in the clutch pedal, but the transmission bits are all still moving. This takes (a little) energy to do.

When you put the transmission in neutral, the input shaft is no longer connected to the output shaft inside the transmission. So you're only spinning the output shaft and its associated stuff, not the input shaft and clutch disk. This takes (slightly) less energy to do than the above, so you will have (slightly) longer glides.

You'll find that a fully-warmed-up engine will usually give higher compression ratings than a cold one, and that removing all of the plugs will also raise the numbers. Make sure your battery and starter are in good shape, too, as those also affect compression pressures. As does your alttiude...

-soD

[mikeross]

Hi there I just want to know what are the symptoms of a bad catalytic converter? How can I know if my cat conv. needs replacement already? Thanks again!