Bad Piston Rings Affecting Braking?

brakes

#1

My car is currently nearing 187,000 miles. Accelerates slower than it used to, exhaust reeks of oil, and to keep oil in operating range a quart of oil must be added every 200-400 miles. So I’m replacing the piston rings as soon as I have time for the work, and for the vehicle to be down.

Meanwhile, the brakes seem less powerful lately. Now, I recall reading about Jake brakes, that the reason gasoline vehicles didn’t use exhaust brakes was because they had automatic engine braking due to the way the engine functioned: when the throttle is closed all the way, the downstrokes generate a vacuum, which consumes energy as the piston moves down in spite of the vacuum it generates (whereas diesel Jake brakes waste the energy by expelling compressed air that would normally revert back into the engine).

So the question is [preferably experienced mechanics who have done a lot of work of this kind and can give a definitive answer] could a loss in braking power be attributed to bad piston rings (i.e. the vacuum that would normally be created and slow down the engine is not created because air comes in from the crankcase) and replacing the piston rings will solve my braking problem, or a. is the vacuum not that much of a contributor to stopping power or b. (if it is), if there was enough suction loss to noticeably affect braking should the engine be incapable of running - and therefore I must work on the brakes too?


#2

If the engine is buring a quart of oil in less than 400 miles due to weak piston compression it’s days are numbered. Why worry about any possible collateral issues?


#3

Probably a combo platter, your brakes may be failing due to loss of vacuum to booster due to your engine is toast. Now new rings may be a temporary save, but if the wear is due to lack of maintenance (ie oil changes) you should skip that band aid and replace the engine or car, imhop (for the internet impaired in my humble opinion)


#4

The worn piston rings with the resulting loss in compression would affect the engine vacuum. However, there would be less vacuum to the power brake booster. This may be why the brakes seem less effective. Unless the car is a manual transmission and you use the engine compression and brakes to slow down the car, I don’t think the lower compression from worn rings is causing the loss in braking effectiveness.
I suppose a ring job with expansion rings and a valve job might buy you another 30,000 miles of service with careful driving, but this is just a sloppy patch. A complete engine rebuild or a remanufactured engine is the best way to go if you are keeping the car. However, at this mileage, the transmission may need work, the brakes may need work if the problem isn’t low vacuum to the power brake booster. I think it is time to get another car.


#5

Yeah, very poor vacuum, which you’ll have if the compression rings are shot and the cylinders worn, won’t pull on the diaphragm in the brake booster as strongly. You may, however, want to check the vacuum before proceeding. You may also have other problems, like burned valves, that are contributing to the low vacuum, and if you do there’s no point in not knowing before proceeding with pulling the pistons.

IMHO if you’re planning on just putting new rings in you’re likely to be very disappointed in the results. Chances are your cylinders are also worn, which means they’ve gone conical, and there’ll be too much wear on the cylinder walls for the rings to properly seal. New cylinders have controlled cross scratches, called “honing” to hold the proper amount of oil on the wall surface after the oil wiper rings go by… and that’s what oil rings actually do, wipe the excess away. But they work WITH the honing. It’s a system. Once the honing is worn away, all bets are off for new rings. Cylinders go (wear) conical simply because the combustion pressure causes the rings to press against the walls harder, and there’s far more force against the cylinder walls at the top of the stroke than at the bottom.

You really should be planning on a rebuild to include having the valves done, which should include testing or better yet replacing the springs, as well as crankshaft work and new bearings both for the crankshaft and the rods… as well as new seals, etc. etc. … in other words, a rebuild. Or putting in a rebuilt engine from a crate. If the rest of the car isn’t worth putting a rebuilt motor in, or at least a boneyard motor, than perhaps it’s time to go car shopping.

In summary, I think you’re being overly optimistic in thinking that a “ring job” will help you… either with compression and it’s twin sister vacuum, or with excess oil usage.


#6

I’m in agreement with mountainbike that the end result may not be what you’re expecting. It is possible to obtain improvement with honing and new rings but there are ifs.

Even if everything is done as competently as possible longevity may be an issue because at near 200k miles I would wager the cylinders are all egged and tapered.
With the pistons and rods out that leads to crank bearing concerns. You may be installing new bearings onto crank journals that are also worn and/or tapered.

I think in a case like this you might be better off with a boneyard motor.


#7

OP could measure the intake manifold vacuum at idle. If the engine is working properly, should measure in the 18 to 20 inches of mercury range.