Is a low compression engine going to seize up?

No. In fact it may be less likely to fail. The first problem is you must identify the cause of the low compression. We used to do that with a cylinder leakage tester. I suspect by now you're familiar with a compression tester gauge. It looks at how much pressure a piston pushes out of its cylinder. A cylinder leakage tester works the opposite way. There's no check valve in its hose. You inject compressed air into the cylinder, through the spark plug hole, and through the tester which has a pressure regulator built in. The gauge shows the percentage of leakage. As I recall, anything under ten percent is good, but much more importantly, you can check four places to identify where that leakage is occurring.

For each cylinder tested, its piston must be at top dead center on the compression stroke so both valves are closed. It's best to do this after the engine has been warmed up so parts fit better. Pistons in particular are slightly oval-shaped when cold. They expand non-linearly, meaning they become closer to round when they expand from engine heat.

The first place to check for leakage is at the tail pipe. Expect to hear a little hissing, but it shouldn't be excessive. That's a sign of leakage past the exhaust valve.

Next, listen by the intake manifold or throttle body with it opened up. Hissing there is caused by leakage past the intake valve.

Most import engines are of the "interference" design, meaning, if the timing belt breaks or jumps a few teeth, the open valves will be hit and bent by the pistons as they coast to a stop. Replacing bent valves is a rather expensive repair. They usually don't cause slightly low compression. Bent valves result in no compression at all. The hissing at the tail pipe or throttle body will be unmistakenly loud.

Next, we listen by the Oil fill cap. If you hear the hissing there, it's caused by worn piston rings. That can be caused by low oil pressure due to other causes, but it results in less oil being squirted onto the cylinder walls to lubricate the pistons and rings. Beyond that, metals have come so far, those piston rings can last three and four hundred thousand miles.

The last place to check is for a stream of tiny bubbles in the radiator. Many are designed today to make that nearly impossible, but if you do see that, it's a sign combustion gases are leaking into the cooling system, usually through a corroded cylinder head gasket, but possibly through a crack in a cylinder head. or cylinder wall. These leaks are almost always much too small to show up with compression readings.

Engines with low compression typically run uncommonly smoothly. Lower cylinder pressures are built up. Think of gently pushing your friend sideways to make him lose his balance, vs. punching him in the shoulder to make him fall over. The light push gets the job done, but with less reaction and smoother action.

Badly worn piston skirts from prolonged lack of lubrication can lead to piston slap. I have no way to describe that unless you've already heard it at some point. Engines can run like that for years. It won't cause seizing on its own, but if a lot of metal particles build up and manage to make it through the oil filter, they could collect in the crankshaft and connecting rod bearings. That will cause their extremely soft first layer of metal to tear apart. Once that starts, the damage accelerates very quickly. The resulting knocking can start within a few miles.

These are a lot of "ifs". I've owned low-compression engines in cars with well over 400,000 miles. Seizing or other damage was the last thing on my mind. My biggest concern was the slightly lower power, and would I be able to pass that freight train of cars when it became absolutely "necessary".

Be aware too how differently you have to treat high-compression, high-performance engines. They don't idle as smoothly. They require a higher octane gas that's harder to ignite, but it prevents spark knock. (Higher octane gas does not produce more power on its own). Stresses on many parts are much higher so those are more prone to failure. To prevent a list of other problems, turbocharged engines start out with a lower compression ratio than normal, typically in the order of 7.2:1 instead of 8:1. Diesel engines have compression ratios in the order of 22:1. You know it when you're standing next to one of them. Noisy and rough, but lots of power waiting to be unleashed. Also very high cost of maintenance and repairs compared to their lower compression gas counterparts.

Also consider the timing of spark and valve opening is critical to engine performance. If a timing belt jumped one tooth, or more likely was accidentally installed one tooth off, compression and power will be greatly reduced. None of that will lead to total engine failure, but the vehicle will not be very fun to drive.

Here's links to some related articles you might find helpful:

https://www.2carpros.com/articles/the-reasons-for-low-compression

https://www.2carpros.com/articles/symptoms-of-low-compression

https://www.2carpros.com/articles/how-to-test-engine-compression

Also consider that we expect to find different types of defects or failures depending on whether all cylinders are equally low, or just one cylinder is low. Mechanical problems like a broken rocker arm can restrict incoming air flow and reduce how much it can be compressed. That will only apply to one cylinder. The rest will be normal.

It sounds like you're getting "wrapped around the axle" with way too much information and misinterpretations. You're going to give yourself ulcers worrying about things no one else does.

For your oil changes, your attitude should be, "well, I don't have time to get it done today. I'll try to get at it next week.

What the oil is packaged in is irrelevant. There can be dozens of different size and shape containers coming down the conveyor belt, including some labeled for discount store brands, and they all get filled from the same tank. The cost per quart varies only because of the different costs of those containers and how they have to be shipped. My dealership used to get bulk oil delivered from a tank truck, but it was the same that was packaged in plastic quart and gallon jugs for customers who wanted to take them home.

For the engine noise, the place most of us would start is by listening next to the pulleys with a stethoscope while the engine is idling. If the bearings in one is causing a grinding or buzzing sound, it will be obvious. Another approach is to remove the serpentine belt and run the engine long enough to see if the noise is gone. If it's still there, then it's due to something internal to the engine. The biggest cause of buzzing noises are the idler and tensioner pulleys.

You'll need a live person to see if the engine is running properly. The "putt putt" at the tail pipe is normal. It's when that turns into a smooth, steady hiss that we are concerned. That is caused by a plugged catalytic converter. Engines do vibrate when running, and four-cylinder engines do that more noticeably. That's why all engines and transmissions are mounted on rubber mounts. That prevents the vibration from transmitting into the passenger compartment.

You're bouncing off in too many unrelated directions. Computer modules don't become damaged from disconnecting the battery. If that were the case, millions would be failing every day when the cars' batteries have to be replaced. You may have read warnings about some modules locking up from simply disconnecting the battery. That is designed in to force owners to trailer their cars to the dealer to have them unlocked, often for over $1000.00. BMW, VW, Audi, and GM are the worst examples of this customer-unfriendly business practice. Any time that is a potential concern, there are inexpensive "memory savers" to keep the computer's memories alive while the battery is disconnected. The problem is those are for when the battery must be replaced. When the service instructions say to disconnect the battery during the service, you don't want to use a memory saver as that takes the place of the battery. These memory savers usually plug into the diagnostic connector under the steering column. They have a very small internal battery which is more than sufficient to do the job. Years ago, when replacing batteries at the dealership, I connected a small battery charger to the battery cables, but not because it was needed. It was doing so was easier and faster than resetting the clock and the customer's radio presets. It was just a time saver that saved me perhaps two minutes per vehicle.

Regardless, the battery doesn't have to be disconnected to remove the serpentine belt. On some engines all that is required is to pull the spring-loaded tensioner pulley to the released position so the engine won't run the belt. The belt just has to be loose on the crankshaft pulley. Run the engine long enough to tell if the noise is gone. That can be one to two minutes; far longer than necessary, without causing a problem.

Some serpentine belts can be really difficult to remove, and even harder to figure out how to route them when reinstalling them. For those, just pull it off the crankshaft pulley and leave it routed around all the other pulleys. This is a common procedure we do all the time when trying to locate the cause of a buzzing noise. If the noise is still there, you'll know it the instant you start the engine, but you can cross off things like generator bearings, power steering pump bearings, and tensioner and idler pulley bearings as the cause of the noise.

If the noise is gone with the belt not running, it's those same items to look at. Most of the time you can figure out which one is responsible by spinning the pulley by hand. You can feel the roughness, or there may be a lot of play in the shaft. Often the noisy one just sounds dry, like there's no grease in the bearings.

Nope. Keep 'em coming. That's what we're here for. I actually run into the opposite of what you said, about removing battery cables, from many uninformed people. I apologize that I missed it that you meant while the engine was running. You are exactly correct about that. I've never heard this about Nissans, but once GM redesigned their generators for 1987 models, those develop huge voltage spikes. It is the battery that dampens and absorbs those harmful spikes, but as they age, the lead naturally flakes off the plates, then they lose their ability to do that. It has been really common now for a GM vehicle to go through four to six replacement generators in the life of the vehicle. The voltage spikes they develop are what takes out the internal diodes and / or voltage regulator. Those spikes can interfere with delicate engine sensor signals too and cause strange running problems. To reduce the number of these repeat failures, replace the battery at the same time, unless it is less than about two years old. The old battery can work fine in an '86 or older model.

This trick of removing a battery cable to tell if the generator is working goes back to when mechanics didn't understand how these simple systems work or how to diagnose them. Back in the '60s and '70s there were no delicate electronics to damage, but the trick still had a major flaw. It revolves around the fact that all "AC generators", ("alternator" is technically a Chrysler term), have at least six "diodes". Those are one-way valves for electrical current flow. If one of them fails, the most current that can be developed is exactly one third of its rated output current. That is a generator in need of replacement, but it can keep an older engine running, so the test gives a false result. In some other car brands, the voltage regulator watches the slight dips in voltage that are part of the normal output voltage in three-phase systems, which these are. The regulator will shut down a perfectly fine generator when the battery cable is removed. In this case, people replace what they think was a bad generator, then still have the same problem they were chasing originally.

Today that trick of removing a battery cable has much more serious consequences. Even to the point a mechanic caught doing it is likely to be fired. An engine might stay running with a bad generator, or it might shut off with a good one, so the test is pointless, but it's those voltage spikes that start the problem. Every year I did a demonstration for my students with a generator test bench. When I removed one battery cable, it was possible to get the generator's output voltage to exceed 30 volts. Computers do not tolerate that, and most will be damaged. Besides damping out those voltage spikes, the battery is a part of the voltage regulator's circuit, and it is needed to allow it to maintain safe system voltage. The only thing that saves some competent do-it-yourselfers is all generators are very inefficient at lower speeds. System voltage won't run away if the engine stays at idle speed. Normal testing requires us to raise engine speed to 2,000 rpm.

I should stop here a minute and explain, to generate a current mechanically, you need three things. You need a wire, (we use coils of wire because their more efficient), you need a magnet, (we use an electromagnet because it's easy to adjust its strength), and most importantly, you need movement between them. That's why we have to spin the electromagnet with a belt and pulley. It's not practical to adjust current output by adjusting engine speed while you're driving. That's why we do it with the voltage regulator adjusting how much current it sends through the field winding. That's the spinning electromagnet.

It's that movement between the coils of wire and the electromagnet that is required, and that gets to be too low at idle speed. At that point the generator's efficiency is so low, damage to computers is often avoided when a battery cable is removed. It's if engine speed is increased that the damage starts to occur.

Again, a lot of my story refers to GM vehicles. All other manufacturers use similar circuits to run their generators, but to my knowledge, none of them have the same problems with voltage spikes. It's only on GM vehicles I recommend replacing the battery when they have to replace the generator.

Nissan was one of the first to use the electric steering gear you mentioned. I attended a class built around that design. I have a similar system in my Ram truck. They're perfectly suited for an electric car where there's nothing to run a hydraulic pump, but the design has been incorporated into other models too. The cost to replace the rack and pinion assembly has come down a lot the last few years, but it is still about ten times higher than for the old, standard hydraulic assemblies. They are no easier or more difficult to replace, but there's no pump or leaking hoses to worry about. The biggest advantage is it's easy to build in other optional add-on systems like stability control. The biggest disadvantage is it greatly adds to the complexity of wheel alignments with needing to reset sensors, and it adds more computers to a system that never needed these touchy computers before. The old systems were so reliable, trouble-free, and easy to diagnose and repair. Those days are gone.

"but if the battery won't hold a charge and is being discharged but is still connected to the generator will that also damage the computer modules?"

No. Now you're describing system voltage that is too low, not too high. When it's too low, at some point each computer will start to do weird things or shut down entirely.

A different problem that hasn't been mentioned yet is blowing fuses. All computers have capacitors in them that store electrical charges. Think of them as extremely small batteries. Those will recharge after power has been removed, then restored. Simply reconnecting a battery cable can result in a rush of current sufficient to blow a fuse for one or multiple computers. After that, all that is required is to locate any blown fuse and replace it. There is no other damage.

The same thing can happen when connecting jumper cables. Most of the time, since the dead battery is still providing some voltage to keep the capacitors charged, there is no current surge when the jumper cables are connected, however, if some system does go dead after the jump start, it will be due to a blown fuse and nothing more. To avoid that possibility, I like to connect jumper cables while the other car's engine is stopped. Depending on how dead the dead battery is, that engine may start without having to start the jumping engine.

These are all precautions to prevent what doesn't happen very often.

Most people don't pick up on that. The issue has to do with the importance of using correct terminology in the classroom. DC generators do indeed put out direct current. The biggest drawback was the field windings were the stationary coils, that had the typical three amps flowing through them, and the output current was taken off the rotating armature. All that current had to pass through the brushes, so 30 amps was about the best you could get.

AC generators pass, at maximum, three amps through the brushes into the rotating field winding. That's real easy to do, and it has no effect on the output current rating of the unit. Where the confusion comes in, is the AC generator was developed by Chrysler for their 1960 models, and they copyrighted the term, "alternator". GM had their version in 1964, and Ford had theirs a year or two later, but they used the term "generator" in their service manuals. Regardless, everyone knows what we mean when we ask for an alternator. All of them put out three-phase output which is very stable and efficient, but it is three alternating current circuits. Only direct current can be stored chemically in a battery. It's the diodes that reroute the currents so it all goes the same way into the battery. Diodes are used in all electronic equipment that's plugged into house outlets, as far back as since we had electrical power in homes.

Chrysler was the world leader in innovations that benefitted car owners. That includes the first electronic voltage regulator in 1970, the first fully electronic ignition system in 1972 on Dodges and 1973 on Plymouths and Chryslers, the first computer-controlled ignition system for 1977, first anti-lock brakes in 1969, lockup torque converter in 1977, first to use air bags, first computer-controlled transmission in 1989.

Not all of those innovations were perfected right away. By the time all the bugs were worked out, other manufacturers had their versions. Today GM is one of the leaders in innovations, but theirs are designed to make more money after the sale. That includes computer modules that have to be purchased through the dealer and the dealer has to program them to your car. In a recent case, my friend already had the new module, then the dealer charged him $600.00 for the few minutes it took to install the software and program it to the truck. As with everything else, it gets copied by everyone later, so today we're all paying for these customer-unfriendly business practices.

Most import generators have the voltage regulator built in. The advantage is there's nothing to diagnose; just replace the assembly. The advantage to having the regulator be a separate module is you only have to replace the defective part, not everything. Chrysler domestic models have never used an internal regulator. GM was the only one that had theirs built in starting somewhere around 1972. In my opinion, it was the world's second best design. It was easy to diagnose, and it was easy to repair once the aftermarket industry made replacement parts available. GM dropped that nice design and went to, by far, the world's worst design starting with 1987 models. Ford also used to have a separate regulator, but around the early '90s, they went to a nice unit with the regulator bolted right on the back where it was easy to get to, diagnose, and replace. Later they added an extra cover to the back to make it very difficult to diagnose.

As for the fan, I think you'll find one in every generator, but some are inside and some are set on the shaft before the pulley is pressed on. All have a bearing in front and on back. In that respect, all of the designs work the same way.

Very often there's two, three, or four different current ratings for a generator for a given model. It's important to note that AC generators will only develop exactly the amount of current needed by the electrical system, and to recharge the battery, and no more. The original generator size was chosen to meet the highest demands of the car's electrical system. There's no advantage to switching to one with a higher current rating. It won't develop any more current. It simply has the capacity to develop a higher maximum current. All that's needed to achieve that higher capacity is to add a few inches of wire to each stator coil. You can switch to a generator with a different current rating, but there is one time that can cause a problem. On older cars, a fuse link wire was spliced into the wire going from the alternator's output terminal to the battery. Those are a small section of smaller diameter wire with insulation that won't melt or burn. On later models there's a very large fuse in the fuse box, and it's usually bolted in. Those fuse devices are sized at the factory based on the current rating of the generator being installed. If you switch to the larger generator, it still will only develop only as much current that's needed, except during the "full-load output current test". That is a professional test that is done to verify it can develop as much current as it's supposed to be capable of, and it will help identify when one diode has failed. This test just takes a few seconds, but a larger generator might be able to develop more current than the fuse is rated for. Fuse link wires used by Chrysler and older GMs, act like slow-blow fuses, so they might survive this test. Regular fuses that are bolted in will blow, but there's no actual defect. Just replace the fuse.

Remember when I said one of the things needed to generate a current is to have movement between the magnet and the piece of wire? And that the slower that movement is, the less efficient the generator will be? This relates to the pole pieces you asked about. The field winding is one long piece of wound-up wire, with a north and a south pole. The strength of the magnet is seriously increased by adding an iron core to concentrate the magnetic lines of force. They cut those pole pieces into a group of fingers that each has a north or south pole around them. Instead of switching polarity twice per revolution, you get about a dozen north / south transitions per revolution. In effect, the speed of the movement of the magnetic field is sped up a lot. That adds to its efficiency. Without those fingers, you'd need the engine to run at around 5,000 rpm before substantial current would start to be generated.

Happy I could help. Please come back to see us with your next problem. I'll be here.