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Since L and I have had our 1965 Clark Cortez motorhome, we’ve had a problem getting our RV to reliably start.
Because our engine struggles to start, one of the questions L and I get asked most is why we do not swap the engine.
I fully believe there is nothing wrong with our engine and that the carburetor is faulty, which is why I want electronic fuel injection (or EFI) — EFI would make our RV much better in terms of gas mileage and usability (no more adjusting the choke or carburetor, no more accidentally flooding the engine with gasoline –just key in and go). I’ve been trying to convince L to swap to EFI, but she is stubbornly against it because it is expensive.
“I’m not going to throw more money at our lass unless I know fundamental parts of her are strong,” she told me.
“What needs to be there so you change your mind?” I asked her.
“Let’s start with the engine,” she answered, “because we don’t even know if it is actually a good engine.”
She had a point — Our engine is not original, though it is also nearly 60 years old. The factory inline-6 was swapped at some point for a 5.9 liter Dodge 360 LA V8 so we did not technically know when it was installed, how many miles it had done, or what service it had.
Because of this, I immediately knew we needed to do a compression test.
For those who aren’t sure what a compression test is, allow me to expand your mind …
A compression test determines the overall health of an engine.
The test does this by measuring the amount of pressure created in each cylinder during the compression stroke. Let me explain further …
When an engine runs, there are four cycles as the cylinders inside the engine rise and fall: suck, squeeze, bang, and blow. This might sound rather crude, but it simplifies what happens …
Air and fuel are ‘sucked’ inside each cylinder through the intake valves. The air and fuel are ‘squeezed’ together as each piston rises in each cylinder. (Compression tests look at this part of the cycle — seeing how much the air and fuel mixture is squeezed together. Over time, compression can be lost through engine wear at the valves or piston rings, which can lead to the engine having bad fuel efficiency, burning oil, and having low power.) Each spark plug ignites the air and fuel, creating the ‘bang’. The exhaust gases are ‘blown’ out of each cylinder through the exhaust valve(s). This entire process is called a four-stroke combustion cycle.
Back to the compression test — If I could prove to L we had a healthy engine, she may be convinced to get EFI.
Plus, a compression test would help diagnose a problem we have noticed for a long time — Our engine kicks out large amount of smoke.
Spirits were high as L and I arrived at the farm, though there was a little apprehension in the air because we did knot know how our V8 engine would fair in its health check.
To keep the mood light, I tired singing to L … only to be told I was wasting time so I put my pretend mic down and tools up.
Back to the task at hand, we needed to start our engine only by the starter motor (not run on fuel) so I quickly set about disconnecting the fuel pump from the fuel tank. Once disconnected, we both moved inside to continue our work.
With L in the driver’s seat, we fired up our engine to get rid of remaining petrol in the fuel pump, fuel line, and carburetor. Remarkably, our V8 fired up and ran … and ran … and ran. Remember that this is the one time we wanted it to die, but it ran to the point where L burst into laughter and I doubted I had actually disconnected the fuel line.
Finally, the engine stopped.
The next jobs were to disconnect the power wire from the coil, remove the ignition leads (marking them with the cylinder number as we went) …
and remove all eight spark plugs.
L will go into the condition of our spark plugs in our next post as they were far from perfect, but the spark plugs (or any Cortez work that day) definitely will not be the stars of that post because something crazy happens which you will not want to miss so subscribe to get an email notification when that post is published!
Now it was time for the compression test — well, actually the first of two compression tests.
The first is known as a dry compression test.
This was the first time L had seen any compression test so we took our time as I explained what was happening (and we actually did the test twice at her request to learn more) …
First, I threaded a pressure gauge into the spark plug hole in Cylinder One (which is the front left cylinder if you are sitting in the driver’s seat).
Next, L turned the key in the ignition to allow the engine to spin three times exact so that the starter motor can build pressure and the gauge has a few compression strokes to give an accurate reading.
The great news is that Cylinder One’s pressure rose to a respectable 115 PSI (or pound-force per square inch, which is how pressure is measured).
That being said, here’s how the values are interpreted …
| Goal compression | 135 PSI or higher |
| Acceptable compression | Over 100 PSI |
| Troubling compression (engine work needed) | Below 85 PSI |
Now the real test was seeing if each cylinder had a similar reading …
Removing the pressure gauge from Cylinder One, I screwed it into Cylinder Three (the next cylinder on the left bank or side of the engine. Again, L turned the engine over using the starter motor three times. This time the gauge settled on 120 PSI — a great result!
This process was repeated six more times, finishing the left bank by testing Cylinders Five and Seven before working down the right bank testing Cylinders Two, Four, Six, and finally Eight.
Overall, our dry compression results ran from 115 to 145 so our engine had excellent compression, but the best news was that our results were very consistent.
The general rule with compression tests is that all values be within 20% of the average result. Our average was 130 PSI so 20% of that was 26, making our target range between 104 to 156 PSI — and all results sat tightly together within that. (Again, if one or more of our results had been significantly lower then it would have highlighted an issue within that cylinder.)
With these results, L and I were both very happy and confident in our engine — though L was more shocked that our engine was as strong as it was.
Now it was time for the wet compression test, which has the ‘wet’ name because a small amount of oil is added into each cylinder right before the pressure gauge is screwed into the spark plug hole. The oil is used to form a seal around the piston rings if they are worn, — If the values are higher than 20% of the dry test results (-10% or +10%), this would show that the oil is adding additional sealing to the pistons and that highlights wear on the piston rings. If there is no increase then it is likely that there is wear on the intake or exhaust valve/s.
Starting with Cylinder One again, a small amount of engine oil was poured into the cylinder, the pressure gauge was screwed into the spark plug hole, and the engine was turned over three times.
The gauge hit 145 PSI — That’s 30 PSI up from the dry test and within the -/+20%.
Identical to the dry test, we continued with the cylinders down each bank so Cylinders Three, Five, and Seven were tested before moving to the other bank for Cylinders Two, Four, Six, and Eight … and the results were in!
Our wet compression test results were between 145 to 175, giving us an average of 160. As per the dry test, all the results sat within the 20% tolerance range.
Altogether, our compression results were brilliant, especially for a near 60-year-old engine with an unknown service history. Also, these early LA engines are known for their ‘loose’ tolerances and abilities to keep running. The only semi-downside was that these results did not show conclusive evidence for worn piston rings, which would have been a likely cause of our smoke issue. That means our valve stem seals may be worn or cracked, but that job is for another time. Still, the compression test results proved that although our engine isn’t perfect, we have a healthy engine that has a lot of life left!
Therefore, did it convince L to get EFI?
…No, but it certainly helped her move closer to a “Yes” answer.
