Hi,
Did you remove the cylinder 4 spark plug and inspect it? If so, what condition was it in? Since that cylinder is missing already, pull the spark plug wire and start it to see if the knock is gone.
As far as the vacuum, that is very low. It should be around 20. Are there any major vacuum leaks? Is there any evidence of a head gasket problem?
You mentioned you were planning to check compression. I feel that is a good idea. You could have a stuck valve in cyl 4 or just very low compression.
Here is a link to help:
https://www.2carpros.com/articles/how-to-test-engine-compression
Here are the directions specific to your vehicle, The compression requirements are listed in these directions.
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2005 Dodge Neon L4-2.0L VIN C
Cylinder Compression Pressure Test
Vehicle Engine, Cooling and Exhaust Engine Testing and Inspection Symptom Related Diagnostic Procedures Non-Trouble Code Procedures Cylinder Compression Pressure Test
CYLINDER COMPRESSION PRESSURE TEST
DIAGNOSIS AND TESTING - CYLINDER COMPRESSION PRESSURE TEST
The results of a cylinder compression pressure test can be utilized to diagnose several engine malfunctions.
Ensure the battery is completely charged and the engine starter motor is in good operating condition. Otherwise the indicated compression pressures may not be valid for diagnosis purposes.
1. Check engine oil level and add oil if necessary.
2. Drive the vehicle until engine reaches normal operating temperature. Select a route free from traffic and other forms of congestion, observe all traffic laws, and accelerate through the gears several times briskly.
3. Remove all spark plugs from engine. As spark plugs are being removed, check electrodes for abnormal firing indicators fouled, hot, oily, etc. Record cylinder number of spark plug for future reference.
4. Remove the Auto Shutdown (ASD) relay from the PDC.
5. Be sure throttle blade is fully open during the compression check.
6. Insert compression gauge adaptor Special Tool 8116 or the equivalent, into the #1 spark plug hole in cylinder head. Connect the 0-500 psi (Blue) pressure transducer (Special Tool CH7059) with cable adaptors to the DRBIII. For Special Tool identification, (Refer to - ENGINE - SPECIAL TOOLS). See: Engine > Electrical / Mechanical Repair
7. Crank engine until maximum pressure is reached on gauge. Record this pressure as #1 cylinder pressure.
8. Repeat the previous step for all remaining cylinders.
9. Compression should not be less than 689 kPa (100 psi) and not vary more than 25 percent from cylinder to cylinder.
10. If one or more cylinders have abnormally low compression pressures, repeat the compression test.
11. If the same cylinder or cylinders repeat an abnormally low reading on the second compression test, it could indicate the existence of a problem in the cylinder in question. The recommended compression pressures are to be used only as a guide to diagnosing engine problems. An engine should not be disassembled to determine the cause of low compression unless some malfunction is present.
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Let me know what you find or if you have other questions.
Take care and God Bless,
Joe
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Tuesday, March 2nd, 2021 AT 9:07 PM
