Bad Moon on the Rise?
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| I had quite the bad experience with this gauge, however I did obtain the first reading. The dark round gauge is kind of like a bad moon, especially if it delivers bad news. |
Nobody wants to receive bad news, but sometimes you just have to deal with it.
A compression check can tell you a lot about the condition of the hard parts inside your motor, the pistons, rings, valves, and even the head gasket. There is no other way to find this out, at least there hadn't been for a long time. Now there are leak down tests which can be performed after the compression test. These pressurize the cylinder and can help pin point the specific location that the cylinder is losing compression.
This traditional compression check can be performed at home with only a few tools and one piece of equipment. The compression pressure gauge. The gauge reads the amount of pressure within the cylinder as it completes the compression stroke of the cycle, just prior to ignition. If the compression is low, it is bad news. Especially if it is catastrophically low, like zero! That means that you may have a hole in the top of the piston, or a badly burnt or hung up valve. Usually it is just sort of low, that can mean worn cylinder and rings, worn/burnt valve seats, or a leaking head gasket.
A brand new, fully broken in engine will have the highest compression of it's mechanical life. It steadily decreases over the engine's life as the cylinders wear and the rings and valve seats lose their sealing. This is a natural process, just like human aging. Performance declines due this gradual wear and tear, its okay until it declines beyond a certain level. That is why an older engine uses more fuel and oil, loses power, and is not as smooth running as it once was. Since most vehicles have a surplus of power available they can lose some without the driver really noticing. He'll just step harder on the accelerator to achieve the performance that he desires.
Shop manuals are always kind of vague about how high cranking pressure should be, they seldom specify specific numbers for any engine. The primary thing is that all cylinders are within a close range, that will indicate normal wear, without any outliers that indicate real problems in a particular cylinder. After searching the Web I found that the acceptable minimum pressure is 90 psi, but the practical minimum is 100 psi. Ideal pressures would over 150 psi for an engine that has seen some usage and mileage. High pressures could be up to 200 psi in some cases. An engine can run with some pretty low pressures, these used to be the beat up, old smoking oil burners that we would see occasionally limping around town. That's pretty rare now.
Most modern cars have engines that will reach mileages of over 200,000 miles of service. That means the original, and even long term secondary owners, never see an occasion where the engine requires substantial repair to the block assembly. Failure of the automatic transmission or a blown head gasket usually spell the end of the road for most old cars. Then they end up at the wrecking yard. This doesn't mean that the problem cannot be fixed, any mechanical problem can be repaired, or the mechanism replaced, it's just that the repair doesn't always make economic sense. The repair can cost more that the vehicle is worth. A mechanic's labor is expensive. The cost of repairs has to be amortized over time and the future use of the vehicle. Most of the time the owner doesn't plan to keep the car long enough to make it worthwhile, they're usually ready for something different and newer.
Can you blame them? No, who doesn't like a new car?
So where am I with my Mustang?
The first compression test told me that there isn't a catastrophic problem within the #4 cylinder. My hunch is that the misfire doesn't have anything to do with the compression.
Could it be an ignition system problem? Maybe. This model of the 4.6 engine doesn't have a distributor, but it is also not an individual coil on plug system. With the two coils I'm guessing that it is a what's called a "waste spark" system where two cylinders receive a spark at the same time, but only one receives it at the end of the compression stroke, the companion cylinder receives an unneeded "waste spark. The misfire is indicated at only one cylinder.
It appears that the spark plug is firing, it sounds like the injector is working. It is possible that the injector is not working properly. At least that is my hope.
I am willing to invest a couple of hundred dollars in getting the injectors cleaned and inspected, that could be the solution. Though there are two things that I should check first.
I'm going to remove and check each spark plug wire for resistance. Then I'll also perform a compression check on all the cylinders. That can tell me about the relative condition of #4. I should have kept the old plug wires, like I did with the coils, then I could switch out the wires to try to isolate the problem. I could also switch a couple of wires and see if the problem follow that plug wire.
This was not the scenario that I had envisioned for my Mustang. I had either thought that I would keep it for a longer period, or that I would sell it while it was still in good running condition.
I had not anticipated any major problems with the engine.
No need to over react, I'll remove all the spark plug wires to inspect and test them. I tried to run the resistance check on each plug wire. I had a lot of trouble getting any reading on several wires. Especially #1! Not a good start. I had stuck a long screwdriver into the plug boot to make contact with the terminal. I tried it several times, moving it's position and wiggling it around. I still could not get a reading. I looked at the coil terminal which was clean and shiny. I looked into the plug boot and saw rust and corrosion!!!???
I used some spray terminal cleaner and q tips to try to clean the connection. After a lot of cleaning and scraping I was able to get a reading.
On several other wires I had a similar condition; #2, #6, and #7. I had to manipulate and pull on the leads to get a connection. Could the wires be the problem?
I recalled the time that we were driving back from San Luis Obispo in our '90 Dodge Caravan which stalled out and quit, driving back over the Cuesta Grade. We had it towed to the local Chrysler dealer who didn't find a problem at first, and released it back to us. It broke down again north of the grade, and they had to send their own tow. We had come down in two cars to pick it up, so we a least had a ride back home. But it was a wasted trip. The second time they "fixed" it, they replaced the plug wires. I was always dubious about that being the problem but it seemed okay after that. My regular mechanic told me that yes, a wire failure could cause a serious problem. I stored that info away for a later day, but I guess that I still really didn't believe it.
Well, I found four wires with corroded terminals. I pulled all the spark plugs and they were all clean. I don't think that I'm going to mess with the injectors... yet.
Next up will be a compression check and a new set of wires. I could skip the compression check, but I'd like to have a record of the motor's condition especially at it's high mileage. This would be a good record to maintain over the years and miles, however I don't remove the spark plugs unless I have to. Generally, I rely on the results of the smog test to give me an indication of the engine's condition. Obviously if it doesn't pass the test there must be something wrong with it! It's instructive to compare the readings every two years.
Time to move up to the next step in the process.
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