Help with Compression Test
Thread Starter
Instructor
Joined: Jul 2009
Posts: 223
Likes: 0
Help with Compression Test
Hello All,
I am in the process of doing a compression test to my 1989 Corvette.
I was reading about it and I need to disable the fuel injection system and spark.
I am going to unplug the white and black connector into the distributor to disable the spark.
What would be the best way to disable the fuel injection system. The fuse for the fuel pump will also disable the ECM and oil pressure so I don't know if that would be good to remove.
If anyone has insturctions for a Corvette engine compression test please let me know! Thank you!
I am in the process of doing a compression test to my 1989 Corvette.
I was reading about it and I need to disable the fuel injection system and spark.
I am going to unplug the white and black connector into the distributor to disable the spark.
What would be the best way to disable the fuel injection system. The fuse for the fuel pump will also disable the ECM and oil pressure so I don't know if that would be good to remove.
If anyone has insturctions for a Corvette engine compression test please let me know! Thank you!
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Pull the injector OR fuel pump fuse(s), remove the pink wire going to the dist. cap, chock the throttle open, pull all the plugs and test. Crank about 5 compression strokes for each cylinder. Write down the results and look for less than ~10% diff between cylinders.
Thread Starter
Instructor
Joined: Jul 2009
Posts: 223
Likes: 0
That is not true. Oil pump is mechanically driven; any time the engien rotates, it pumps oil regardless of what is going on in your fuse box.
Pull the injector OR fuel pump fuse(s), remove the pink wire going to the dist. cap, chock the throttle open, pull all the plugs and test. Crank about 5 compression strokes for each cylinder. Write down the results and look for less than ~10% diff between cylinders.
Pull the injector OR fuel pump fuse(s), remove the pink wire going to the dist. cap, chock the throttle open, pull all the plugs and test. Crank about 5 compression strokes for each cylinder. Write down the results and look for less than ~10% diff between cylinders.
I replaced all the spark plugs about 2 months ago and when I pulled one to start testing each cylinder for compression I noticed the fairly new spark plug is very black... not dirty,... more like just black in color. I've heard that is a issue with the spark plug not getting spark or possibly the injector is flooding the spark plug. Anyone heard something like that? How do you test to see if a plug is getting spark?
Thread Starter
Instructor
Joined: Jul 2009
Posts: 223
Likes: 0
However, I stuck a ohm meter on the plug wires and they are like 1.5 and online it says they should be 6.0 or higher. I replaced the rotor and plugs but I guess I need to replace the spark plug wires aswell.
Safety Car
Joined: Jun 2009
Posts: 4,738
Likes: 15
From: OC Calif
St. Jude Donor '10
Hhmm I think I jumped the gun. The white center part of the spark plugs is still fairly white but the top face of the threads is pretty black. So I don't think they have a problem.
However, I stuck a ohm meter on the plug wires and they are like 1.5 and online it says they should be 6.0 or higher. I replaced the rotor and plugs but I guess I need to replace the spark plug wires aswell.
However, I stuck a ohm meter on the plug wires and they are like 1.5 and online it says they should be 6.0 or higher. I replaced the rotor and plugs but I guess I need to replace the spark plug wires aswell.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
Measure the length of the spark plug wire in inches. Divide the resistance reading by the length of the spark plug wire to determine ohms per inch.
You need to take the reading on the individual wire and convert it to the ohms per inch. MSD 8.5 super conductor wires are rated 40-50 ohms per foot as an example.
You need to take the reading on the individual wire and convert it to the ohms per inch. MSD 8.5 super conductor wires are rated 40-50 ohms per foot as an example.
Le Mans Master
Joined: Oct 2004
Posts: 6,314
Likes: 508
From: South-central Missouri
Getting back to the compression testing...
I find this procedure to be the most consistent (for me, at least):
Pull ALL the spark plugs: Less drain on battery, less wear on bearings, more consistent results...
you can pull the fuse for the injectors, if ya want, OR just hold it at WOT as you crank - which is the "clearing flooded engine" mode which shuts off the injectors when RPM is low (starter speed).
put a dab of anti seize on the threads of the compression probe to help seal and later to keep the plugs from seizing.
Start with a fully charged battery, and put a battery charger on the battery while doing the testing. The point is to keep the starter speed as uniform throughout the testing as possible.
Install the test probe and crank it for at least 5 strokes. All cylinders should be within a range of 15# or less (less than 10 for a new or freshly rebuilt motor is OK.)
As for oil pump, it is mechanically coupled to the distributor. You want oil flowing to the bearings while cranking, so ya do not want to pull the distributor while doing a compression test. If you have the distributor out for some reason, stick it back in before doing the test. Oil pressure while cranking...is a GOOD thing!
That #8 plug is one SOB to get to from above. I found it easier drop the exhaust piping from the (stock) header and access the plug from below. (Or, I suppose you could take a hole saw and punch a hole from the fender all the way through to the plug and us a long extension to get to the plug...JK but frustrating enough to tempt me at times!!
) I've not done a compression test on a Corvette with an LTx or L98 with LT headers, so I don't know what to tell ya there.
BTW, my thoughts would be to scan that puppy and checking the resistance of the injectors (hot) and or doing a fuel pressure leak down test before I did a compression test. If your getting some blue smoke from the exhaust and it's using oil...maybe. But, I'm going to guess that your problem is not related to compression at this point.
P.
I find this procedure to be the most consistent (for me, at least):
Pull ALL the spark plugs: Less drain on battery, less wear on bearings, more consistent results...
you can pull the fuse for the injectors, if ya want, OR just hold it at WOT as you crank - which is the "clearing flooded engine" mode which shuts off the injectors when RPM is low (starter speed).
put a dab of anti seize on the threads of the compression probe to help seal and later to keep the plugs from seizing.
Start with a fully charged battery, and put a battery charger on the battery while doing the testing. The point is to keep the starter speed as uniform throughout the testing as possible.
Install the test probe and crank it for at least 5 strokes. All cylinders should be within a range of 15# or less (less than 10 for a new or freshly rebuilt motor is OK.)
As for oil pump, it is mechanically coupled to the distributor. You want oil flowing to the bearings while cranking, so ya do not want to pull the distributor while doing a compression test. If you have the distributor out for some reason, stick it back in before doing the test. Oil pressure while cranking...is a GOOD thing!
That #8 plug is one SOB to get to from above. I found it easier drop the exhaust piping from the (stock) header and access the plug from below. (Or, I suppose you could take a hole saw and punch a hole from the fender all the way through to the plug and us a long extension to get to the plug...JK but frustrating enough to tempt me at times!!
) I've not done a compression test on a Corvette with an LTx or L98 with LT headers, so I don't know what to tell ya there.BTW, my thoughts would be to scan that puppy and checking the resistance of the injectors (hot) and or doing a fuel pressure leak down test before I did a compression test. If your getting some blue smoke from the exhaust and it's using oil...maybe. But, I'm going to guess that your problem is not related to compression at this point.
P.
Thread Starter
Instructor
Joined: Jul 2009
Posts: 223
Likes: 0
Getting back to the compression testing...
I find this procedure to be the most consistent (for me, at least):
Pull ALL the spark plugs: Less drain on battery, less wear on bearings, more consistent results...
you can pull the fuse for the injectors, if ya want, OR just hold it at WOT as you crank - which is the "clearing flooded engine" mode which shuts off the injectors when RPM is low (starter speed).
put a dab of anti seize on the threads of the compression probe to help seal and later to keep the plugs from seizing.
Start with a fully charged battery, and put a battery charger on the battery while doing the testing. The point is to keep the starter speed as uniform throughout the testing as possible.
Install the test probe and crank it for at least 5 strokes. All cylinders should be within a range of 15# or less (less than 10 for a new or freshly rebuilt motor is OK.)
As for oil pump, it is mechanically coupled to the distributor. You want oil flowing to the bearings while cranking, so ya do not want to pull the distributor while doing a compression test. If you have the distributor out for some reason, stick it back in before doing the test. Oil pressure while cranking...is a GOOD thing!
That #8 plug is one SOB to get to from above. I found it easier drop the exhaust piping from the (stock) header and access the plug from below. (Or, I suppose you could take a hole saw and punch a hole from the fender all the way through to the plug and us a long extension to get to the plug...JK but frustrating enough to tempt me at times!!) I've not done a compression test on a Corvette with an LTx or L98 with LT headers, so I don't know what to tell ya there.
BTW, my thoughts would be to scan that puppy and checking the resistance of the injectors (hot) and or doing a fuel pressure leak down test before I did a compression test. If your getting some blue smoke from the exhaust and it's using oil...maybe. But, I'm going to guess that your problem is not related to compression at this point.
P.
I find this procedure to be the most consistent (for me, at least):
Pull ALL the spark plugs: Less drain on battery, less wear on bearings, more consistent results...
you can pull the fuse for the injectors, if ya want, OR just hold it at WOT as you crank - which is the "clearing flooded engine" mode which shuts off the injectors when RPM is low (starter speed).
put a dab of anti seize on the threads of the compression probe to help seal and later to keep the plugs from seizing.
Start with a fully charged battery, and put a battery charger on the battery while doing the testing. The point is to keep the starter speed as uniform throughout the testing as possible.
Install the test probe and crank it for at least 5 strokes. All cylinders should be within a range of 15# or less (less than 10 for a new or freshly rebuilt motor is OK.)
As for oil pump, it is mechanically coupled to the distributor. You want oil flowing to the bearings while cranking, so ya do not want to pull the distributor while doing a compression test. If you have the distributor out for some reason, stick it back in before doing the test. Oil pressure while cranking...is a GOOD thing!
That #8 plug is one SOB to get to from above. I found it easier drop the exhaust piping from the (stock) header and access the plug from below. (Or, I suppose you could take a hole saw and punch a hole from the fender all the way through to the plug and us a long extension to get to the plug...JK but frustrating enough to tempt me at times!!
) I've not done a compression test on a Corvette with an LTx or L98 with LT headers, so I don't know what to tell ya there.BTW, my thoughts would be to scan that puppy and checking the resistance of the injectors (hot) and or doing a fuel pressure leak down test before I did a compression test. If your getting some blue smoke from the exhaust and it's using oil...maybe. But, I'm going to guess that your problem is not related to compression at this point.
P.
Great thanks! Yes, I am mainly doing the compression test just to rule it out. I have already ohm'd the injectors and done a fuel pressure test. They were fine. I do not get any smoke out of my exhaust.
I have my meter set at 20k ohm and its still reading 1.5 on the first. The second wire jumps back and fourth from 1.5 to 4.5 so I am definetly replacing all my wires just to rule them out as well.
