A discussion of engine function.

I would like to start a discussion about how these Metro Engines function.

Especially the 3 cylinder - 1.0 Liter engines and miles per gallon (Efficiency).

I read somewhere a long time ago that an engine is constructed to run efficiently.
It cannot do otherwise - unless something major is wrong with it.
Feed it air, gasoline and electricity and it must run - no choice!

How much does the ECM control the engines mpg efficiency.

After working on my '93 - rebuilt head and new rings, I saw quite a lot of
carbon on everything.

The compression was bad 60 - 95 - 155 in my cylinders. [1-2-3]

If compression is where it is supposed to be (what is that number?) and the
Fuel Injector is working well and the spark timing is correct, what else would
control proper combustion? Can the ECM screw it up? If so - how?

The air supply for the fuel/air mixture is controlled by the rotating air valve
and the throttle.

If compression, ignition and fuel are good, wouldn't the car have no choice but to
run efficiently?

What is happening to all these folks posting here when the mpg drops from
50 down to 30? What is the major reason that that is happening?

Is it bad valves? I would suspect so....

Where & why does all this carbon originate?

DoctorBill

Good Topic Dr Bill. I have the thread under fouled spark plug. My 92 Geo is drinking about 3 quarts of oil per week. I travel 90 miles per day round trip. I just did a compression test on mine and the compression is good. This is all on my post. However, this engine is great on gas mileage. I am getting 50+ miles per gallon. Makes me wonder if the oil is getting mixed in with the fuel thru the PCV valve and combusting it. Anyway, I am baffeled at the loss of oil, but the great mileage. Something in my engine is really right and really wrong.

As Mr. Spock would have said to Captain Kirk -
"There are two possibilities.
The oil is going out the exhaust.
The oil is leaking from the engine."

At three quarts per week [3/4 of a gallon!], I would think you should
see billowing clouds of smoke as you drive on the road and especially
when you accelerate.

If not, I would guess you have a bad leak somewhere.

I had engine leaks once in a car. The engine was a mess of oil and dirt.

Big mess on the driveway where I parked it....do you have that?

I cleaned the engine off super-dooper-pooper-scooper well with "Gunk" and
hot water (from a 3 gallon garden sprayer on solid stream out the tip).

Then you can see (maybe) where the oil is coming from.

Maybe have someone rev the engine when it is somewhat cold and watch
for leaks as you stand beside the car with the hood up....and a flashlight.

I can't imagine that much oil going thru the cylinders w/o clouds of smoke!

Is there oil in your coolant? Is the pan gasket leaking? Valve cover gasket?

Rear main seal!? Those are bad leaks! Lot of oil out only when running and gone
onto the street! Check that out! Would not affect engine performance!

Tear open a big cardboard box, place it under the car engine and run the
engine fast until the car gets too hot. Any fresh oil on the cardoard - if so
where in relation to the engine? Right under the rear main seal?

Sherlock Holmes it out! Eliminate one thing after another.

Be a detective. Experiment...

As a last ditch, final solution - go to a mechanic...($$$$$)

DoctorBill

Car does not drip any oil under it after driving 45 miles. I do see oil on the transaxle below the distributor and dripping from the vsv valve. I can't see any smoke comming out the tail pipe when idling or driving. But as you say, its going somewhere. I can try your suggeston and rev the motor and observe the engine.

Using some logic...

If it is not burning the oil, it has to be leaking it out.

If no oil is visible on the ground when you park it, the oil must
be essentially 'squirting out' as you drive and stops squirting
when you stop the engine.

If that be the case, the JUNK cleanup followed by the clean
cardboard under the non-moving car revved up might
reveal some type of high pressure - only while you are driving
leak.

If not - do you believe in magic? Did you piss off a shaman?
Maybe a gypsy curse....?

Just remembered - someone told me in a post a while back that
the distributor has 2 oil seal "O-Rings" and if one of them (which ?)
is bad, the distributor can leak a whole lot of oil....A Lot!

Suggestion - wrap several fresh, clean paper towels around your distributor
at the base where it goes into the head.
Tie them on with some string so they don't blow off and drive 100 miles.
Stop and take the paper towels off. Are they oil soaked? Could give you
the answer..... Experiment!

Mine leaked a good deal from there - hope my Head Rebuilt solves that!

Good luck...!

DoctorBill

Back to engines........and how they run. Somebody!

I had a 94 Saturn SL2 with similar problems. No oil on the engine anywhere. It smoked only very slightly when revved up high...but a close inpection revealed tiny droplets of oil being blown out the exhaust, even while idling. I never would have noticed it if I hadn't accidentally sprayed my house with it one day during a tune up (ouch!)
I'd just had a valve job done on it, so I don't think it was valves. It was a very high mileage engine...I sold it before I solved the mystery.
I hope you figure it out, thats a lotta oil....

Checked the distributor. It has a single o ring that i replaced a few months ago. Inside the distrbutor cap, their is a gasket/seal. At the bottom of the housing, you can see where oil is getting in. Is this supposed to happen? Seems like with the electronics in there that oil would not be good. Is there another o ring or seal that I am missing? I looked inside the block where I pulled the distrubutor and don't see any seals or o rings in there. Should there be one? The haynes manual I have does not mention any of this when describing the distributor. After driving the car today, I checked the distributor again, and only a slight oily feel under the distributor. None dripping. I did talk to someone who recommended I run 20W-50 on this as they had an older geo and this significantly slowed the oil usage. I was running 10w-40 which I read on some post was bad for this car. So maybe it is just burning it when I'm driving. When I stop and idle however, there is no smoke comming out the tail pipe. I'm gonna solve this or die trying.

there is a o-ring seal on the inside of the distributo body. to replace it you have to pull it completely apart and it takes a few special tools. if its leaking inside its better to get a rebuilt one or find a good used one. if just the outside of the dist is oily then you replace the outside one where it goes into the engine

An ECU is dumb. All it does is match up a signal from the TPS, air temp sensor, and the MAP sensor(RPM's are handled internally, no sensor necessary). It takes these numbers, and searches on a virtual grid(looks like an excel sheet) called a fuel map. It also has a spark timing map it uses these same sensors for. It finds the programmed match for the input, and dumps the apporpriate singal to the fuel injector, and the ingition coil. It monitors the combustion via the oxygen sensor, and adjusts the fuel/air ratio to be as close to stoich as possible(for a narrowband o2 sensor at least). Stoichiometric air/fuel mixture is approximately 14.7 times the mass of air to fuel. Now, this is NOT the peak efficiency of a motor. It is peak efficiency for the proper combustion of the fuel, not too lean(damages valves), and will pass emissions testing. Running an engine lean will have far better mileage, but damage engine components and make some nasty emissions. Carbon builds up in an engine because the soot is a by-product of the impurities in the gasoline and air burning. It breaks down to carbon from the extreme heat inside the cylinder. If an engine runs rich, you will have a far greater amount of soot inside the engine. This is the same soot that can ruin a cat.

A major reason people lose mileage is sensors failing. When the ECU can no longer keep a steady or proper reading from a sensor(faulty o2 sensor?), it will go into open loop mode. Open loop means it uses a factory designated fuel map that is generic, but will allow the engine to run with a failed sensor. It will run very rich, and the timing is usually very retarded. You'll notice a power loss, and you'll have to use more throttle to get her moving. This all adds up to using more gas. WAY more gas.

The only sensors the engine really needs to run are the TPS and the MAP. Any other sensors are for more efficient control of the AFR(Air-Fuel-Ratio).

As for all the other little bells and whistles on the motor.. The ECU is programmed to work with ALL of these devices in full function. If a PCV is clogged, or the EGR is plugged, the ECU doesn't know, and continues life as normal. This is VERY bad. An EGR valve returns a measured amount of exhaust gas to re-burn. It does this to somewhat choke the engine, and cool down the combustion chamber. Because of the EGR, and engine can run lean and still not burn up. Running lean will improve gas mileage a TON. But, if the engine can't get exhaust gas up into the intake manifold, it's like screwing without a condom :grinyes: . You're gonna get burnt! Combustion chamber temps will skyrocket, and it will warp and crack the exhaust valves. Again, the ECU doesn't know, so it doesn't care.

As for Fuel maps, our geo's use 2d fuel maps. Fairly dumb and not flexible at all, but they work. Here's an example of a 2d fuel map.
http://www.rbracing-rsr.com/turbo/FuelMapL.jpg

And here's a 3d fuel map. These require a much more powerful ECU. They are better in pretty much every way, but are a real mother to program and tune.
http://home.hetnet.nl/~speed4/bikes/955i/2002_fuel_map.gif

Now, when you start to modify a car, you start to screw with the ECU again. That's why there's devices like the Apexi SAFC-II. It modifies signals from your MAP/MAF sensor and converts them to signals your ECU will understand. The ECU spits the info back out, the SAFC modifies it again and sends it to the injectors. These are for highly modified engines that are beyond the ECU's control. Such as turbo'd motors or highly modded NA motors. If you wish, you can always put a piggyback ECU in your car, and see if you can pull some extra HP or MPG out of it. But beware, you won't pass emissions testing, and improper tuning can destroy a motor. Especially since ours don't have knock sensors.
Enough of my B.S. :) If ya want to know anything else, just lemme know :smokin:

I want to ask about how these 3 cyl Metro engines work because I am trying
to figure out why mine wouldn't work.

Ultimately my engine had bad valves and that was what zapped it.

Does that make sense? How do bad valves cause these problems
and what does the bad valve situation do the the ECM inputs ?

What is the sequence of events that lead to non-functionality with bad valves?

Even if all my MAP & TPS sensors were working fine, the burned valves
(#1 & #2) made the vacuum too low and I was getting a bad MAP message from my ECM.

Is that correct?

So there was no way the ECM could correct the problem....

Once I get these valves and rings corrected, the timing set right, then the
MAP should return a proper reading and the engine should work well.

No...?

Basically, it appears that there are only a couple of things that can go bad
with these 3 cyl systems. Main things - Valves, Injector, Timing - no?

You can check the timing. Invest in a Timing Light!
You can see if the Injector is working by looking down the TBI top.
You can test the compression for valves and rings. Invest in a Compression Gauge!

I kept putting off the compression test - too cheap to spend $20 on a gauge.
Had I done that test earlier, I would have known what was wrong long ago....

What else? Maybe Fuel Pressure via a cheap gauge at Harbor Freight?

Am I finally getting close to understanding this....?

As I tell my Chemistry students, explain it to me as if I were a moron and
then I will understand w/o having to assume anything.

DoctorBill

Doc;
Sounds to me like you are on track to understanding your engine. Just want to throw in a small (and admittedly simple) primer that might help. You are dealing with a 4 stroke engine: The 4 strokes are:

1) Intake - this is on the downstroke of the piston, intake valve open, exhaust valve closed. This creates a vacuum in the cylinder drawing in the fuel/air mix from the intake manifold that was put there by the fuel injector.
2) Compression - this is the upstroke of the piston, intake and exhaust valves closed compressing the fuel/air mix previously drawn into the cylinder.
3) Power - intake/exhaust valves still closed. This is where the sparkplug fires burning the fuel/air mixture in the cylinder forcing the piston down in the cylinder delivering the power to the crankshaft.
4) Exhaust - Another upstroke of the piston this time with the intake valve closed and the exhaust valve open, pushing the burnt fuel/air mixture out into the exhaust manifold and removing it from the engine.

So you can see that the engine is actually creating/delivering power every other time that the piston travels up/down in the cylinder. If any of the valves do not seal well you can see that you will either not draw in the proper air/fuel mix or not get rid of the burnt air/fuel dramatically affecting the performance.

So now - once I get the valves and rings all fixed, I should have a good vacuum
and compression.

Now - I would suppose "Timing" would be the next step.

I set the Timing Belt Pulley mark to the TOP while number one piston is at Top
Dead Center, then put on the Timing Belt.

Then I adjust the Distributor to around 5 degrees before TDC with my timing light.

Then, I set the TPS with my VOM to the settings in Chilton's Manual.

Then the Little Bastard ought to run like a charm....!

Yes?

DoctorBill

Doc,
As a professional mechanic I have had more experience with these little buggers than I'd like to admit. It is my job to maintain a fleet of 20 of these geo's. After a complete engine rebuild recently performed, a compression test revealed exactly 210psi in all three cylinders, not to mention a lot more pep than I ever thought a geo could have.
Anyhow, supposing you have sufficient vacuum (18-22in.kg, an indication of proper mechanical operation), it is important to set the timing to 5°tdc as you have done, of course being sure to ground the test terminal listed under the hood. However the TPS has a lot of influence on timing, and with aged components I have found that the specifications listed in the chiltons have, on some cars, produced less than satisfactory operation. On one car for example I had to set the TPS around 0.5ish volts, which is approx half a volt less than the spec listed in the repair manual.
It is imperative, especially with these little engines, to be sure that you haven't the slightest hint of a vacuum leak. Do not rely on a vacuum gauge to tell you whether or not you are leaking vacuum. The best method on these cars is the old TB cleaner in a can. Spraying all suspectable mating surfaces, namely the TB to intake manifold, and the intake manifold to head, listening for a drop in idle RPM. To check for leaks in hoses, clamp all hoses at both ends and listen for an IMMEDIATE difference in idle.
Another very important sensor under the hood of these vehicles is the Manifold absolute pressure (MAP) sensor, located at the passenger side firewall connected to the engine via a vacuum hose to the base of the TB. Be sure the tube is free of debris (a clog here is highly unlikely) and more importantly a leak in the tube itself. You can also backprobe the MAP and look for a difference in voltage from idle to wide open throttle. I have had some cars not respond to grounding the test plug under the hood, an alternative method of putting the car in test mode, is to simply unplug the MAP, and after a quick drop in idle (careful as this may result in a backfire) the ECM will cease to control ignition advance.
Make NO mistake about it. The ECM is absolutely nessecary to every aspect of the vehicle's performance. On rare occasions chaffing of wire harnesses leading to the ecm can cause shorts and major driveability problems that will not be resolved without the correction of bad wiring. On occasions even more rare the ECM itself can short internally, requiring a replacement computer. Most of the time however, it is not the ECM's fault for causing a poor running condition. 99% of the time the ECM will cause poor driveability due to having incorrect information fed to it by the sensors it uses to determine how much fuel and ignition advance the demands of the motor needs.
The air/fuel mixture on EFI engines is controlled solely by the ECM and CAN NOT be altered without altering ECM fuel MAPs. While the ECM is designed to adapt and compensate for normal wear and tear on engine components, it cannot compensate for mechanical failure such as those which may result in a significantly low vacuum reading (bad rings/cylinder bores, bad valves, blown head gasket, or vacuum leak).
A great deal of the carbon found in these engines is created by the poorly functioning PCV system which, especially on high mileage engines, fails entirely, often resulting in excessive oil consumption, fouled plugs, etc.

-MechanicMatt

First off Excellant Discusstion !
Matt and Alan, I really appreciate your insight.

Ive been wondering about the ECM's Fuel Map. Only question I have here is If the standard(2d) fuel map can be modified on a 1.0L. Reason I ask is that in another thread I saw where the Scanguage could actually send info to the system and I'm wondering if the info is in the form a a change to the fuel map.

Interesting reading about all the carbon. Insufficient combustion is obviously a big issue in these small engines. Doesn't take much it seems like to really wreek havok and progressivly get worse as it builds up and causes heat.

When I checked my 1.0L 2 months ago I found 165# in all 3 cylinders. I was told when I bought it 2 months ago that it recently had been rebuilt. I may have a bad guage and I'll probably replace it and re-test just for grins. But the car runs fine.

I drive major stop and go traffic (Houston) and the last 2 weeks I was able to get mpg up to 41.9 two weeks ago and 39.8 last week. This was after checking everything, changing air filter, cleaning EGR and passages(not really that bad), re-gapping plugs, re-setting timing(I too believe I had a problem getting the system to test with a jumper, took 3 times), inflating tires and adjusting my driving habits(slow starts, low throttle, early shift). I really think my traffice situation has alot to do with my mileage, wont know unitl I get a chance to get it back out on the highway.


RPG0

The 94 xFIs had a different ECM and cam, plus it was a lighter car that made them the MPG kings. There are only 3 cars allowed to park for free in downtown Salt Lake City - the green cars. Here's the list: http://www.ci.slc.ut.us/Transportation/Parking/mpg.htm I think that you can almost duplicate the MPG that the xFI got by changing the cam to an economy cam, and an advanced degree cam spocket. Check out 3Tech at teamswift.com. I think it is cool that the little Geo can get such great MPG and is recognized in SLC as such.