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yater
12-05-2012, 09:10 PM
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

MLA
12-05-2012, 09:23 PM
What is yours or his idea of within range? I would fallow up the compression test with a cylinder leak down test. This will actually pinpoint the source of any compression loss.

yater
12-05-2012, 10:04 PM
The other cylinders were 185-195. What is involved with leak down test?

MLA
12-05-2012, 10:10 PM
120 psi is definitely out of spec with the others.

A lead-down involves placing the piston at TDC and filling the cylinder with regulated air while watching a pressure gauge to see of the pressure drops. If it drops, there's a leak. if its past the rings/piston, you will hear it coming out of the valve cover. if its and intake valve, you will hear it in the throttle body. if its an exhaust valve, you will hear it exiting the exhaust. If its a head-gasket, you will hear it exit another cylinder or gurgle in the water passages.

elliott
12-05-2012, 10:56 PM
120 is more than enough for the cylinder to hit on. so i dont think thats your prob. yea its lower than the rest, but it should hit. think you need to start looking else where.

elliott
12-05-2012, 11:01 PM
the poping your hearing its probably running lean, causes backfiring through the intake. might have a weak valve spring, slightly bent pushrod,

elliott
12-05-2012, 11:02 PM
plugged fuel filter or a weak pump

MLA
12-05-2012, 11:14 PM
IMHO, a cylinder thats running 40% below the others is worth investigating. This can result in a decrease of crankshaft speed when that suspect cylinder is on its power stroke. Im gathering the tech has isolated a miss at idle to a particular cylinder? If so, system fuel pressure would not effect a single cylinder, but all of them. An issue with the #7 injector would effect #7 cylinder itself. Have the tech take it to the next step and rule out a mechanical problem with that cylinder with low compression.

Kyle
12-05-2012, 11:19 PM
TxsRiverRat on here had the same problem.


I did the compression check and kept on diagnosing. We have the same year boats his is a 205 and mine is a 190 but both had the 351w. I changed the carbs out and found that his carb on my boat ran bad and mine ran bad on his boat but it ran better than his on his boat. Swapped them back and my boat ran great again. Ignition was great on both boats.

I pull the intake (cheap option) and inspected.


I found that 1 cylinder was not opening up all of the way. I pull valve covers off and loosen the rocker arms to remove push rods. They were straight. Pulled lifter and noticed wear.


Ended up being a bad cam lobe.


Replaced cam and lifters and she's been perfect ever since. He did replace carb as well but that was not his problem.

Kyle
12-05-2012, 11:29 PM
IMHO, a cylinder thats running 40% below the others is worth investigating. This can result in a decrease of crankshaft speed when that suspect cylinder is on its power stroke. Im gathering the tech has isolated a miss at idle to a particular cylinder? If so, system fuel pressure would not effect a single cylinder, but all of them. An issue with the #7 injector would effect #7 cylinder itself. Have the tech take it to the next step and rule out a mechanical problem with that cylinder with low compression.



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.

thatsmrmastercraft
12-05-2012, 11:29 PM
A flat cam lobe will make a popping noise at higher RPM.

Kyle
12-05-2012, 11:40 PM
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
12-05-2012, 11:47 PM
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
12-05-2012, 11:48 PM
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
12-06-2012, 12:20 AM
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
12-06-2012, 05:58 AM
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
12-06-2012, 06:01 AM
plugged fuel filter or a weak pump

Not for a compression problem.

MLA
12-06-2012, 08:49 AM
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
12-06-2012, 10:51 AM
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
12-06-2012, 11:08 AM
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.

yater
12-06-2012, 12:31 PM
The low compression cylinder plug looks fine, new according to mechanic.

MLA
12-06-2012, 12:43 PM
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.

The average cranking time per cylinder should be 5-6 seconds. So the total amount of time the engine will be spinning over during the entire test is less then 60 seconds on a V8. Any measurable amount of fuel entering the cylinder by a pulsing injector during the 5-6 crank time will exit the exhaust and open spark plug hole (on those cylinders not being tested) during the piston's exhaust stroke.

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.

I am 100% confident that a non-functioning injector on one cylinder would not create a 40% drop in compression as compared to the other 7 cylinders with a pulsing injector. At this point, it is only speculation on whether or not the injectors were pulsing during the compression test. A leak-down test on that cylinder would take 60 seconds to determine if there is a leak and a few more minutes to narrow down where the leak is. Far less man-hours then whats wrapped up here in speculation. :D

JimN
12-06-2012, 01:24 PM
The average cranking time per cylinder should be 5-6 seconds. So the total amount of time the engine will be spinning over during the entire test is less then 60 seconds on a V8. Any measurable amount of fuel entering the cylinder by a pulsing injector during the 5-6 crank time will exit the exhaust and open spark plug hole (on those cylinders not being tested) during the piston's exhaust stroke.

I am 100% confident that a non-functioning injector on one cylinder would not create a 40% drop in compression as compared to the other 7 cylinders with a pulsing injector. At this point, it is only speculation on whether or not the injectors were pulsing during the compression test. A leak-down test on that cylinder would take 60 seconds to determine if there is a leak and a few more minutes to narrow down where the leak is. Far less man-hours then whats wrapped up here in speculation. :D

It might not account for 40% but it still skews the test and the only way to see how much it does or doesn't affect the results is to test the compression both ways. Since this is dry intake, the fuel delivered to the cylinders is only going to the cylinders, not wetting the intake AND going to the cylinders.

At this point, I would remove all plugs (I would hope this was already done for the compression test), attach an air hose with 50 psi to the bad cylinder and crank it over manually, to listen for air escaping- if it's coming out the oil filler hole, we'll know it's probably a problem with one or more rings and if it comes out the intake or exhaust, it's probably a valve/gasket problem.

JimN
12-06-2012, 01:25 PM
The low compression cylinder plug looks fine, new according to mechanic.

Has this engine overheated recently, or did the problem start after anything specific occurred?

yater
12-06-2012, 07:57 PM
No known overheating incidents. Unfortunately, I have not owned the boat for long and the only records I have are service records from the past 3 years that do not indicate any red flags or major service.

Kyle
12-06-2012, 08:19 PM
Are you handy with tools?

Can you take something apart and put it back together again easily?



If so then you should be able to do the work yourself and save tons of cash.


I would be willing to bet that the engine does NOT have to be pulled. A simple cam swap or head issues can be done with the engine still in the boat.