Low Compression

[thatsmrmastercraft]

A flat cam lobe will make a popping noise at higher RPM.

[Kyle]

Originally Posted by thatsmrmastercraft

A flat cam lobe will make a popping noise at higher RPM.

Yup. TxsRiverRat had same symptoms.

7 cylinders out of the hole and started popping around 2000 and got worse the more you gave it.

[thatsmrmastercraft]

Originally Posted by Kyle

Yup. TxsRiverRat had same symptoms.

7 cylinders out of the hole and started popping around 2000 and got worse the more you gave it.

Had the same problem on an F150 many moons ago.

[MLA]

Originally Posted by Kyle

I wonder if the compression check was run with the safety lanyard pulled. When the lanyard is pulled then the fuel pump is shut off so that the injectors are not triggered durring the test. If there was a bad cylinder and injectors are spraying durring test (just pulling coil wire) then the test is not accurate and the bad cylinder would read low if the injector was bad.


You want all cylinders to be within 10% of eachother. Sounds like 7 of 8 are within that parameter. The bad cylinder is way low.


Check that plug. Is it oily?


Pull valve covers and check the torque on the rocker arms. If it has backed off then the spring will not function properly.

A faulty injector would certainly cause an engine miss, but should not effect the compression reading.

To add to the diagnosis. A cylinder with low compression and no leaks, would point to a valve-train problem like a lobe, lifter, push rod rocker or spring. Typically on the intake side.

[Kyle]

Originally Posted by MLA

A faulty injector would certainly cause an engine miss, but should not effect the compression reading.

To add to the diagnosis. A cylinder with low compression and no leaks, would point to a valve-train problem like a lobe, lifter, push rod rocker or spring. Typically on the intake side.

Agree with all but compression reading.

My disagreement would be the fact that if you add any liquid to the combustion chamber during the test then the test will read higher. Liquid being a few drops of oil, a full spray of the injector vs a 1/2 spray (possibly not that noticeable as a misfire at idle), etc.

DO NOT DO THIS OR TRY THIS but A good example to my madness is adding a few drops of water to a cylinder and you end up with bent valves due to the non compressible liquid in the combustion chamber. Now having less fuel in the chamber would read lower than a full spray and it could be an injector. Just justifying my slight disagreement


If the engine runs fine at idle doesn't 100% always rule out a bad injector. There is a big difference being 2000 in neutral vs underway or on load.


Personally I would bet cam or valve train but making sure the diagnosis was properly done is key.

[JimN]

Originally Posted by yater

Hi,
I have a Vortec 330 HP in my '99 Maristar. The motor is making an intermittent popping sound at high rpm and surges a bit just above idle. My mechanic says that its running on 7 cylinders at the idle. Unfortunately I haven't been able to replicate the popping at high rpm for him.
A compression test showed 120 in one cylinder. All others were within range.

Mechanic indicates that a valve job is the way to go. An comments, suggestions? Thx

Did your mechanic do a cylinder leak down test? That would be more definitive, as would including a vacuum test.

How rough is it running? Has anyone thought to remove the oil fill caps (if it only has one, remove the PCV) to listen for escaping exhaust gas or other sounds/smoke that might indicate blow-by?

The only engine I saw that was fairly new and had a compression problem was an LT-1 with a bent valve, apparently caused by the center electrode of a spark plug coming out- that was missing when I remove the plug from the bad cylinder. I don't know what caused it or what the boat owner was on, but he didn't even notice it running bad and it WAS bad.

[JimN]

Originally Posted by elliott

plugged fuel filter or a weak pump

Not for a compression problem.

[MLA]

Originally Posted by Kyle

Agree with all but compression reading.

My disagreement would be the fact that if you add any liquid to the combustion chamber during the test then the test will read higher. Liquid being a few drops of oil, a full spray of the injector vs a 1/2 spray (possibly not that noticeable as a misfire at idle), etc.

DO NOT DO THIS OR TRY THIS but A good example to my madness is adding a few drops of water to a cylinder and you end up with bent valves due to the non compressible liquid in the combustion chamber. Now having less fuel in the chamber would read lower than a full spray and it could be an injector. Just justifying my slight disagreement


If the engine runs fine at idle doesn't 100% always rule out a bad injector. There is a big difference being 2000 in neutral vs underway or on load.


Personally I would bet cam or valve train but making sure the diagnosis was properly done is key.

i agree with the theory, but the amount of atomized fuel introduced into the air-charge by any working injector at cranking speed will not be enough to wet the cylinder wall in the same manor as large droplets of oil squirted into the spark plug hole. Any fuel in the air charge will also be suspended.

[jfw432]

Originally Posted by MLA

i agree with the theory, but the amount of atomized fuel introduced into the air-charge by any working injector at cranking speed will not be enough to wet the cylinder wall in the same manor as large droplets of oil squirted into the spark plug hole. Any fuel in the air charge will also be suspended.

What about after cranking for a couple minutes over the course of testing all the cylinders? I'm sure some of that fuel will go out the exhaust valves but since it's an unburned liquid, it will start to build up over time. I don't have any idea...just throwing that into the mix.

[JimN]

Originally Posted by MLA

i agree with the theory, but the amount of atomized fuel introduced into the air-charge by any working injector at cranking speed will not be enough to wet the cylinder wall in the same manor as large droplets of oil squirted into the spark plug hole. Any fuel in the air charge will also be suspended.

But the standard procedure is to do the test, either by opening the throttle/crimping the fuel line on a carbureted engine or removing the fuel pump/injector fuse on an injected engine. Think about how many cycles are needed for the full test- at least two, with no combustion. Not much of the fuel delivered will leave through the exhaust port and that leads to an innacurate reading.