Cylinder Isolation

HuskieMobileMan · March 7, 2005

I was wondering if there was some way to isolate a cylinder so that it is inoperable. The engine is a 350 with TBI on a 89 GMC van. I failed emissions with the van and one of the cylinders is not functioning properly. It is not the ignition system. I have not checked the compression, but I believe there is something internal (valve, cam, piston) that is not functioning properly.

Is there someway to remove the cylinder from operation?

Would it be bad to restrict the exhaust from that cylinder and remove the spark plug wire? That way nothing would pass into the exhaust from that cylinder. Could I do the same thing but leave the plug wire on? This way it would fire, but exhaust would probably reenter the intake and exit through another cylinder.

Would doing this be safe?

Any other suggestions?

Thanks in advance.

GM-Tech · March 7, 2005

Take both rocker arms off (and pushrods out). This is basically how they shut down cylinders on the new Displacement on Demand engines. Obviously it doesn't remove those parts, but they do disable the two valves on 4 cylinders.

Whether or not this will help your emissions is something I can't answer.

HuskieMobileMan · March 7, 2005

Take both rocker arms off (and pushrods out). This is basically how they shut down cylinders on the new Displacement on Demand engines. Obviously it doesn't remove those parts, but they do disable the two valves on 4 cylinders.

Whether or not this will help your emissions is something I can't answer.

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What does this require? Can it be done by just removing the valve covers on the one side of the engine? Sorry if I am ignorant on the subject. Thanks.

GM-Tech · March 7, 2005

What does this require? Can it be done by just removing the valve covers on the one side of the engine? Sorry if I am ignorant on the subject. Thanks.

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Yep. Remove the valve cover, remove the nuts holding the rocker arms on the cylinder you want to disable, pull the arms and pivot balls off, slip the push rods out and put the valve cover back on.

HuskieMobileMan · March 7, 2005

What does this require? Can it be done by just removing the valve covers on the one side of the engine? Sorry if I am ignorant on the subject. Thanks.

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Yep. Remove the valve cover, remove the nuts holding the rocker arms on the cylinder you want to disable, pull the arms and pivot balls off, slip the push rods out and put the valve cover back on.

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Thanks so much!!! I hope this works!

redvett · March 7, 2005

If you are going to a testing station i doubt if it will pass. For one when you disable 1 cylinder the exhaust will be louder and may raise questions. If the check the system at the ALDL it will show long & short term trim values way off.

Your best bet is to fix it right.

HuskieMobileMan · March 7, 2005

If you are going to a testing station i doubt if it will pass. For one when you disable 1 cylinder the exhaust will be louder and may raise questions. If the check the system at the ALDL it will show long & short term trim values way off.
Your best bet is to fix it right.

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Why would the exhaust be louder?

Butch 02 Denali · March 7, 2005

I think I'd leave the exhaust rocker in place and just remove the intake. It's bad enough the engine has to dead cycle through one compression stroke, with the exhaust valve rocker off it would have a second compression stroke per cycle (4 cycle engine)

GM-Tech · March 7, 2005

I think I'd leave the exhaust rocker in place and just remove the intake. It's bad enough the engine has to dead cycle through one compression stroke, with the exhaust valve rocker off it would have a second compression stroke per cycle (4 cycle engine)

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Actually, with both valves closed, the piston will not be compressing anything. Since the intake is closed, it can't draw air in, and if it can't draw anything in, there is nothing to compress. This is easily seen when you do a compression test on a cylinder that has a rocker arm off.

As mentioned, GM uses this very method for their displacement on demand engine to shut down 4 of the 8 cylinders. They do it with the lifters, but the same effect.

redvett · March 7, 2005

If you are going to a testing station i doubt if it will pass. For one when you disable 1 cylinder the exhaust will be louder and may raise questions. If the check the system at the ALDL it will show long & short term trim values way off.
Your best bet is to fix it right.

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Why would the exhaust be louder?

Because you are missing a pulse cycle and the muffler is designed for 6 cylinders and you now have 5. The DoD engines have a special muffler to work on both the cycles.

Have you ever listened to a vehicle missing going down the road is very noticeable.

HuskieMobileMan · March 7, 2005

If you are going to a testing station i doubt if it will pass. For one when you disable 1 cylinder the exhaust will be louder and may raise questions. If the check the system at the ALDL it will show long & short term trim values way off.
Your best bet is to fix it right.

<{POST_SNAPBACK}>

Why would the exhaust be louder?

Because you are missing a pulse cycle and the muffler is designed for 6 cylinders and you now have 5. The DoD engines have a special muffler to work on both the cycles.

Have you ever listened to a vehicle missing going down the road is very noticeable.

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It is actually an 8 cylinder engine. Right now the cylinder in question isn't doing much. I pull the plug from the cap and it doesn't change the performance of the engine. I am getting a spark at the other end (plug) and I have replaced the plug as well (and I gapped it). I will let you know what happens after I take a look under the valve cover.

71GMC_3/4T · March 8, 2005

Yep, your problems might get real obvious real fast once you have the valve cover off. I hope it's something simple like a bent pushrod or broken rocker, and not something real pricey...

HuskieMobileMan · March 9, 2005

Here are my results. I did a compression test on the cylinder that was bad and one that was functioning properly, and they both read 120 psi. I did the test when the engine was cold. The directions said to do it when the engine was at operating temp, but I didn't feel like burning up my hands.

I took the valve cover off the engine and didn't see anything unusual. The only thing of note was the valve closest to the transmission was pushed down. This was on the cylinder that is not functioning properly. All the others were up and the rocker moved freely (a little bit each way). I am guessing that the one valve was down due to its position in the cycle.

Is it possible to briefly crank the engine (ignition disabled) to see the movement of the valves? Would I get sprayed by oil by doing this?

Does anyone else have any more suggestions?

I really appreciate the help.

GM-Tech · March 9, 2005

Is it possible to briefly crank the engine (ignition disabled) to see the movement of the valves? Would I get sprayed by oil by doing this?

Just turning over the motor to watch the valve action will not get you sprayed. You could even run it for a short time without making too much of a mess. Yeah, the valve could have been down because it just happened to stop in that position of the cycle.