Valve issue affect compression test?
 

Hi all,

If I have issues with my valves not seating correctly, would this cause a
faulty compression reading on a test?

The reason for suspecting a valve issue is I'm getting a carbon trace on one
side of my spark plug from the electrode to the base and someone mentioned
that this could be due to a valve problem not seating correctly.

The interesting thing is though I'm getting this on all cylinders (the
carbon trace).

I'm readying a 4.0L head to go on my jeep and wanted to test the engine so
as not to put a good head on a suspect engine. The only test I know to
determine engine strength is a compression test but if this could be
in-accurate due to valve issues then I won't have a valid reading.

Thanks for any suggestions, comments.

Bill

Re: Valve issue affect compression test?
 

William Oliveri wrote:
> Hi all,
>
> If I have issues with my valves not seating correctly, would this
> cause a faulty compression reading on a test?
>
> The reason for suspecting a valve issue is I'm getting a carbon trace
> on one side of my spark plug from the electrode to the base and
> someone mentioned that this could be due to a valve problem not
> seating correctly.
>
> The interesting thing is though I'm getting this on all cylinders (the
> carbon trace).
>
> I'm readying a 4.0L head to go on my jeep and wanted to test the
> engine so as not to put a good head on a suspect engine. The only
> test I know to determine engine strength is a compression test but if
> this could be in-accurate due to valve issues then I won't have a
> valid reading.
>
> Thanks for any suggestions, comments.
>
> Bill

It's possible. The best engine test to tell the condition of an engine is a
cylinder leakdown test. This involves bringing the piston to TDC on the
compression stroke and then applying air pressure to the cylinder. The
leakdown tester will have 2 pressure gages on it. The first gage will read
whatever air pressure you apply to it, and th second gage will read a lower
number depending on the condition of the engine. A freshly rebuilt engine
will read about 5-8% leakdown due to ring end gaps and other imperfections.
I believe the maximum recommended leakdown number is about 15-20%, but don't
quote me on that. The nice thing is that when you use this test you can
determine where the leakage is occuring. If the intake valve is leaking you
will hear a hissing through the carb/throttle body, exhaust valve will be
out the tailpipe, rings can be heard through the PCV hose or dipstick tube,
and a cracked head or bad head gasket will bubble in the radiator. You
actually don't even need the gages, you can get an adapter that connects to
your air hose and screws into the spark plug hole. The gages are nice for
telling how much leakage is occuring however.

Here are a couple links:
http://www.tavia.com/cat8.html
http://www.racerpartswholesale.com/long29.htm

Also, if you search the net, you'll find plans to make your own tester if
you desire to do so.


Chris

Re: Valve issue affect compression test?
 

William Oliveri wrote:
> Hi all,
>
> If I have issues with my valves not seating correctly, would this
> cause a faulty compression reading on a test?
>
> The reason for suspecting a valve issue is I'm getting a carbon trace
> on one side of my spark plug from the electrode to the base and
> someone mentioned that this could be due to a valve problem not
> seating correctly.
>
> The interesting thing is though I'm getting this on all cylinders (the
> carbon trace).
>
> I'm readying a 4.0L head to go on my jeep and wanted to test the
> engine so as not to put a good head on a suspect engine. The only
> test I know to determine engine strength is a compression test but if
> this could be in-accurate due to valve issues then I won't have a
> valid reading.
>
> Thanks for any suggestions, comments.
>
> Bill

It's possible. The best engine test to tell the condition of an engine is a
cylinder leakdown test. This involves bringing the piston to TDC on the
compression stroke and then applying air pressure to the cylinder. The
leakdown tester will have 2 pressure gages on it. The first gage will read
whatever air pressure you apply to it, and th second gage will read a lower
number depending on the condition of the engine. A freshly rebuilt engine
will read about 5-8% leakdown due to ring end gaps and other imperfections.
I believe the maximum recommended leakdown number is about 15-20%, but don't
quote me on that. The nice thing is that when you use this test you can
determine where the leakage is occuring. If the intake valve is leaking you
will hear a hissing through the carb/throttle body, exhaust valve will be
out the tailpipe, rings can be heard through the PCV hose or dipstick tube,
and a cracked head or bad head gasket will bubble in the radiator. You
actually don't even need the gages, you can get an adapter that connects to
your air hose and screws into the spark plug hole. The gages are nice for
telling how much leakage is occuring however.

Here are a couple links:
http://www.tavia.com/cat8.html
http://www.racerpartswholesale.com/long29.htm

Also, if you search the net, you'll find plans to make your own tester if
you desire to do so.


Chris

Re: Valve issue affect compression test?
 

William Oliveri wrote:
> Hi all,
>
> If I have issues with my valves not seating correctly, would this
> cause a faulty compression reading on a test?
>
> The reason for suspecting a valve issue is I'm getting a carbon trace
> on one side of my spark plug from the electrode to the base and
> someone mentioned that this could be due to a valve problem not
> seating correctly.
>
> The interesting thing is though I'm getting this on all cylinders (the
> carbon trace).
>
> I'm readying a 4.0L head to go on my jeep and wanted to test the
> engine so as not to put a good head on a suspect engine. The only
> test I know to determine engine strength is a compression test but if
> this could be in-accurate due to valve issues then I won't have a
> valid reading.
>
> Thanks for any suggestions, comments.
>
> Bill

It's possible. The best engine test to tell the condition of an engine is a
cylinder leakdown test. This involves bringing the piston to TDC on the
compression stroke and then applying air pressure to the cylinder. The
leakdown tester will have 2 pressure gages on it. The first gage will read
whatever air pressure you apply to it, and th second gage will read a lower
number depending on the condition of the engine. A freshly rebuilt engine
will read about 5-8% leakdown due to ring end gaps and other imperfections.
I believe the maximum recommended leakdown number is about 15-20%, but don't
quote me on that. The nice thing is that when you use this test you can
determine where the leakage is occuring. If the intake valve is leaking you
will hear a hissing through the carb/throttle body, exhaust valve will be
out the tailpipe, rings can be heard through the PCV hose or dipstick tube,
and a cracked head or bad head gasket will bubble in the radiator. You
actually don't even need the gages, you can get an adapter that connects to
your air hose and screws into the spark plug hole. The gages are nice for
telling how much leakage is occuring however.

Here are a couple links:
http://www.tavia.com/cat8.html
http://www.racerpartswholesale.com/long29.htm

Also, if you search the net, you'll find plans to make your own tester if
you desire to do so.


Chris

Re: Valve issue affect compression test?
 

The 258's all seem to track the plugs the same as you describe.

If you do a compression test and all the cylinders come out the same, no
worries. If one is low, then you put a tablespoon of oil in the spark
plug hole and try again. If it then jumps up, you have ring issues. If
it stays low, suspect valve issues.

If two side by side are low and the wet test doesn't raise them, suspect
a blown headgasket between them.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

William Oliveri wrote:
>
> Hi all,
>
> If I have issues with my valves not seating correctly, would this cause a
> faulty compression reading on a test?
>
> The reason for suspecting a valve issue is I'm getting a carbon trace on one
> side of my spark plug from the electrode to the base and someone mentioned
> that this could be due to a valve problem not seating correctly.
>
> The interesting thing is though I'm getting this on all cylinders (the
> carbon trace).
>
> I'm readying a 4.0L head to go on my jeep and wanted to test the engine so
> as not to put a good head on a suspect engine. The only test I know to
> determine engine strength is a compression test but if this could be
> in-accurate due to valve issues then I won't have a valid reading.
>
> Thanks for any suggestions, comments.
>
> Bill

Re: Valve issue affect compression test?
 

The 258's all seem to track the plugs the same as you describe.

If you do a compression test and all the cylinders come out the same, no
worries. If one is low, then you put a tablespoon of oil in the spark
plug hole and try again. If it then jumps up, you have ring issues. If
it stays low, suspect valve issues.

If two side by side are low and the wet test doesn't raise them, suspect
a blown headgasket between them.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

William Oliveri wrote:
>
> Hi all,
>
> If I have issues with my valves not seating correctly, would this cause a
> faulty compression reading on a test?
>
> The reason for suspecting a valve issue is I'm getting a carbon trace on one
> side of my spark plug from the electrode to the base and someone mentioned
> that this could be due to a valve problem not seating correctly.
>
> The interesting thing is though I'm getting this on all cylinders (the
> carbon trace).
>
> I'm readying a 4.0L head to go on my jeep and wanted to test the engine so
> as not to put a good head on a suspect engine. The only test I know to
> determine engine strength is a compression test but if this could be
> in-accurate due to valve issues then I won't have a valid reading.
>
> Thanks for any suggestions, comments.
>
> Bill

Re: Valve issue affect compression test?
 

The 258's all seem to track the plugs the same as you describe.

If you do a compression test and all the cylinders come out the same, no
worries. If one is low, then you put a tablespoon of oil in the spark
plug hole and try again. If it then jumps up, you have ring issues. If
it stays low, suspect valve issues.

If two side by side are low and the wet test doesn't raise them, suspect
a blown headgasket between them.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

William Oliveri wrote:
>
> Hi all,
>
> If I have issues with my valves not seating correctly, would this cause a
> faulty compression reading on a test?
>
> The reason for suspecting a valve issue is I'm getting a carbon trace on one
> side of my spark plug from the electrode to the base and someone mentioned
> that this could be due to a valve problem not seating correctly.
>
> The interesting thing is though I'm getting this on all cylinders (the
> carbon trace).
>
> I'm readying a 4.0L head to go on my jeep and wanted to test the engine so
> as not to put a good head on a suspect engine. The only test I know to
> determine engine strength is a compression test but if this could be
> in-accurate due to valve issues then I won't have a valid reading.
>
> Thanks for any suggestions, comments.
>
> Bill