Yup. Also be aware worn piston rings are not real common and have to be real bad before they'll cause a misfire. To make matters worse, you can't expect your mechanic to identify worn rings when there is already a more common cause that has been diagnosed. You would scream that he is wasting your money by continuing to perform more tests after a cause had already been found. That would be like finding a nail in your tire that is causing it to go flat, but you continue to spend hours looking for additional causes for the same problem. Why would you do that? And would you fault anyone for not continuing to look for more leaks when you're paying that person by the hour? You've already found the overheating problem has been solved, so something had to have been done right.
There are at least five things that can cause low compression. A compression test only identifies that compression is low in a cylinder, not why. Compression tests take about a half hour, but if standard procedures aren't followed, the last cylinders tested will test lower than the first ones because the test requires cranking the engine while it's still warm. When the last cylinders are tested, the battery has become run down a little so the engine will crank slower. The engine has cooled down, so the pistons are designed to shrink to a slightly oval shape. That can reduce the rings' ability to seal. Your mechanic could have your best interest at heart, but the guy at the next shop might test the cylinders in a different order, might know to use a battery charger, and might complete the testing while the engine is still warm. His results could vary from the first mechanic's results, while nothing changed in the engine.
Once it was believed two cylinders were low in compression, a more thorough cylinder leakage test would identify why that was so. That test takes a considerable amount of time to set up for one cylinder, then it has to be done all over again for each additional cylinder. During the test, the percent of leakage is indicated on the tester, and the mechanic can identify the cause of excessive leakage by listening in various places. If an exhaust valve is leaking, there will be hissing heard at the tail pipe. If an intake valve is leaking, you'll hear the hissing at the throttle body. If the piston rings are leaking, the hissing will be heard at the oil cap or the dip stick tube. If the head gasket is leaking, you may see a stream of tiny bubbles in the radiator. The often-overlooked clue when two adjacent cylinders are leaking is you may hear hissing from the spark plug hole in the cylinder right next to the one being tested.
The problem with a leaking head gasket is they don't always leak all the time. Some only leak when the very high pressure shows up from the burning fuel while the engine is running. Some leaks only show up when the engine is stopped, but there's still some pressure in the cooling system.
I too would get a second opinion from a different shop, but I wouldn't tell them where the previous work was done. If the people at the two shops are friends, they may tend to look out for each other and minimize the seriousness of a mistake. If they're on unfriendly terms, the people at the second shop may go to great lengths to make it sound like the previous mechanic did shoddy work, and that could be the furthest thing from the truth.
If it is found the low compression is due to another leaking head gasket, you can expect the first shop to do that part of the job over at their expense. I was never angry with a customer if I had to do something over. I was embarrassed and apologetic, and I was twice as careful the second time. My service manager often gave the customer some freebies, like free oil changes, or a gift certificate to a nearby restaurant, to make up for their inconvenience of having to come back a second time.
I still take issue with the diagnosis for the misfires. Code P0300 suggests all the cylinders are misfiring at times. That can be caused by a weak ignition coil, (for engines that still use a single coil), low fuel pressure, or anything else that affects the fuel / air mixture for all the cylinders. If your misfires are to be contributed to the low compression, you'd have fault codes for cylinder number 1 misfire, (code P0301), and / or cylinder number 2 misfire, (code P302). When a code is set for a single cylinder, an easy trick is to swap places between that and one other cylinder for the spark plug, and that cylinder and yet a different one for the injector, erase the code, drive the car, then wait to see if a misfire code sets for the cylinder you moved one of the suspect parts to. That can be done rather easily too for the ignition coils on those engines that use individual coils for each cylinder.
The "lean" fault code can be the result of spark-related misfires because unburned fuel and oxygen are going into the exhaust system where the oxygen is detected by the oxygen sensor. On any car other than Chrysler products, a leak in the fresh air tube between the mass air flow sensor and the throttle body will allow air into the engine that is not seen in the measurements, so no fuel will be included in the calculations. That extra air will divide up evenly among all the cylinders and can result in random misfires and the "lean" code. The fix for that can be as simple as tightening a hose clamp.
This all boils down to getting that second opinion, and I have a strong suspicion the repair is going to be much less serious than new piston rings.
Friday, May 26th, 2017 AT 4:03 PM
