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You may know that a lean mixture causes reduced power, preignition, and detonation (the latter two especially in the presence of excessive heat). It also causes increased combustion temperature which consumes oil.

You may also know that coolant in the exhaust from an internal engine leak will destroy your O2 sensor and catalytic converter.

It is common belief that the only problem a rich mixture causes is reduced gas mileage and increased pollution. So when a car starts to get bad mileage and a rich smelling exhaust, it is usually ignored.

What you may not know is that a rich mixture causes several problems as well.

• A rich mixture will send more unburned HC into the catalytic converter to be burned, which overheats and destroys the converter over time.
• A rich mixture will foul spark plugs, reducing mileage even more and exaggerating the effect on the converter as more unburned fuel enters it.
• A rich mixture causes carbon build-up in the cylinder, reducing the life of the piston rings and possibly causing them to stick.
• While a rich mixture does lower combustion temperature, a rich mixture will wash oil from the cylinder walls, reducing lubrication and causing the oil that is washed off to be consumed.

Some things that can cause a rich mixture:

• Bad fuel injectors (spraying a stream instead of a fog)
• Bad O2 sensor (reading lean all the time, so ECM richens mixture unnecessarily)
• Bad ECM, or running in open loop due to failure of some sensor needed for correct closed loop operation
• Insufficient coolant (ECM does not go into closed loop)

Advanced timing also causes higher ignition temperatures and therefore greater oil consumption (in addition to detonation and a ruined engine if a knock sensor is not present).

This is also a good reason NOT to “warm up” a modern fuel-injected engine with a modern motor oil in it by idling it. Doing so simply prolongs the period when the engine is cold and running rich, and as such leads to oil consumption and contamination.

So if your engine is using oil and it’s not leaking externally, check the PCV valve, ignition timing, and rule out a rich mixture caused by bad fuel injectors, sensors or a clogged air filter, before tearing into the motor.

One way to check the mixture if no ECM code is present is to install a new O2 sensor and monitor its voltage after it is warmed up. The voltage should be around 0.7 volts. If you remove a vacuum hose, the voltage should dip to 0.3 volts or so. The injectors can be removed and serviced for $100-150 by mail order.

Oil Consumption and Bearings

It’s a well known fact that worn bearings lead to low oil pressure, wearing the rings which then allow oil consumption and allow more contamination into the crankcase, destroying the engine in a vicious cycle. Excessive bearing clearance is also partially responsible for that oil consumption. Worn bearings throw more oil up into the cylinder than the oil ring can dispense with, and the rest of the oil is burned. Then you have not only low oil pressure, but also dirty oil AND a low oil level to deal with.

So if you have a rod knock you’ve been ignoring, time to drop the oil pan and fix it right — before the bearing spins and repair of the engine becomes more difficult (requiring removal and cleaning of the engine, machining of the crank, and replacement of all bearings), the rod is thrown and repair of the engine becomes impossible, or the oil consumption and low oil pressure ruins the rings – replacing rings is an inexact science and best avoided when possible. Buy the correct size bearings (according to the stamps on the old bearings) and new rod bolts, then install the new bearings by cleaning the crankshaft, “clipping” the bearing into the interference fit grooves on the rod, applying oil, loctite, assembly lube, or nothing at all to the bolt threads as called for in the factory repair manual, and tightening the rod bolts to the correct final torque.

Sticking Rings

When people talk about “sticking rings” or “stuck rings”, be careful not to left the terminology confuse you. Stuck rings can actually refer to two events. The first is when an engine has been stored for a long time, and rust has set into the cylinders. The rings are “sticking” to the cylinder walls in this case, and the engine is seized. The best way to get this kind of engine loose is to soak with WD-40 or a penetrating oil, and attempt to turn the crankshaft by hand. When the rings come loose, they may or may not sustain damage, the only way to tell is to do a compression check. To prevent this kind of sticking rings, remove the spark plugs and spray WD-40 into the cylinders before storing.

The more common kind of stuck rings that happens in a motor that has been used even recently is that the rings themselves become stuck in the piston groove. The rings no longer seal against the cylinder wall because their “spring” is not allowed to expand against the cylinder wall. This happens when hard carbon and varnish build up on the rings. Once it has happened, there are several ways to address it. First, rule out all other sources of oil leaks, or oil burning in the head such as valve stem seals, worn valve guides, spark plug tube seals/o-rings, etc.

If the rate of oil consumption is relatively slow, try an ester motor cleaner like Auto RX. This is put into your oil and left in it for 1500 miles, then the oil is changed. It is a slow cleaner and may take two applications to demonstrate a difference.

If Auto RX did not help, or the rate of oil consumption is so fast that using Auto RX at $20 a bottle is uneconomical, then it is time for more drastic measures to get the rings unstuck. The motor cleaner used can be Marvel Mystery Oil, Seafoam, or even Automatic transmission fluid. Don’t use motor flush solvent (Kerosene) or fuel injector cleaner in the following procedures.

• Remove spark plugs and add a small amount (teaspoons) of MMO, Seafoam, ATF, or Berryman B-12 to the cylinders. It is important that the engine is warm when you do this. Allow to sit for 30 minutes to two days. Crank the motor to spit cleaner out of cylinders, soaking it up with rags, then install spark plugs, allow the smoking to stop, and change the oil and filter. Repeat as necessary.
• Alternately, the cleaner can be introduced through the brake booster vacuum hose with the engine warmed up, adding just enough through the hose for the engine to die. Wait 30 minutes, then start the engine, run it until the carbon burns off, and change the oil.
• Change your oil every 1000 miles with 15W-40 diesel oil until the problem is solved. Before each oil change, add 1 qt MMO or ATF to oil, being careful not to overfill crankcase. Drive no more than 100 miles, allowing the motor to warm up completely for as long as possible. Change the oil and filter immediately. You can use a higher cleaner to oil ratio, but go easy on the engine if you do this. The oil in MMO is 3W and ATF is 7W, which does not provide much protection to the engine bearings, and if the filter is clogged, any suddenly loosened particles could clog the oil pickup.
• Before each oil change, add several quarts of MMO, ATF, or Seafoam to a tank of fuel. Change oil when it becomes dark.

You can combine any of the above strategies, i.e. if you want to add MMO to the crankcase and Seafoam to the intake before changing oil, that is a good idea.

In order of solvent concentration (and reverse order of oil protection): Motor flush, Seafoam, MMO, ATF. When using in a 75% oil/25% cleaner concentration in the crankcase, you want to be very careful not to load the engine when using flush, be “nice” to the engine when using Seafoam, and you can drive the engine as normal when using MMO or ATF. Some people fill and run the entire engine with MMO/ATF immediately before an oil change, do not under any circumstances rev or load the engine if you do this.
If you choose to use a kerosene motor flush product, it is important that you not allow the engine RPM to increase above idle or place a load on the engine, due to the risk of running the bearings dry. Solvents such as those contained in MMO, top end cleaners, and motor flush will cut through the oil film on bearings, so any oil starvation will then lead to a bearing failure.

To prevent rings from sticking in the future, use a good detergent motor oil such as Mobil 1, or even 15W-40 diesel oil — since it has more detergents to prevent coking in diesel engines — obey the oil and filter change intervals, fix any other sources of oil burning (such as worn valve stem seals and excessive rod bearing clearance) as soon as possible, and don’t allow your motor to run rich. Many motor oils claim that their detergent packages will gradually unstick rings, but if this is true at all, it is a much slower process than the above techniques.

This entry was posted on Thursday, March 1st, 2007 at 12:30 pm and is filed under Auto Repair / Engineering. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.