Could carbon build up on intake valves cause high leak-down numbers?

[dville_gt]

Are you sure you have enough volume in your air supply?

It's an 8 gallon 2hp that flows 4.5scfm @ 90psi, should be enough. Plus these were done with a low pressure leakdown gauge so it only uses around 15-20psi.

[dville_gt]

Yes carbon or other deposits building up can cause erratic leakdown numbers. Seafoam or GM's top end cleaner work great if used correctly and that is the problem. If a valve is bent you should be seeing 100% leakdown.

Would you use seafoam to manually clean them? I was just thinking about picking up 8-10 cans of carb cleaner and just working my arse off. Apparently the problem with doing the seafoam while the engine is running on these motors is that the secondary imrc plate doesn't open until ~4k rpm so sucking the can of seafoam into the motor at idle would do little to coat the secondary intake valve and I dont think it'd be a good idea to suck in the seafoam at 4k rpms

I have been digging around and it seems that some guys end up having what they say looks like over an 1/8th of an inch of carbon on the second intake valve while the other intake valve (the one not shrouded by the imrc plate) looks clean. I assume this is why they ditched the IMRC plates in 99.

Anyone know if it is worth it to ditch the plates? I see you can rig a relay to fool the computer, you can put springs on the actuator cables (too fool the computer), or you can get the computer tuned so you will not throw codes. I have heard though that you loose low end torque and that you might need to get it dyno tuned, if thats the case I'll just leave them.

[turbos66coupe]

Your thought of the wasted spark is right. Every coil is paired so if you had a dead coil on ay cyl1 then cyl6 would also be affected on its power stroke. If 6 is ok then you would look at plug wires/plugs.

When you say fluid what kind of fluid, was it gas oil or...

And on sea foam if your secondary intake valve can get carbon build up even though its not run hard enough to open the imrc then the sea foam should get there too.

[BP]

Put a bottle on the car. Nitrous = instant zero carbon

[dville_gt]

Your thought of the wasted spark is right. Every coil is paired so if you had a dead coil on ay cyl1 then cyl6 would also be affected on its power stroke. If 6 is ok then you would look at plug wires/plugs.

When you say fluid what kind of fluid, was it gas oil or...

And on sea foam if your secondary intake valve can get carbon build up even though its not run hard enough to open the imrc then the sea foam should get there too.

I have to believe it was gas, the viscosity was not consistent with oil, it literally dripped off the plug when I pulled it out, but the plug was dry as a bone the next day when I went to analyze it further. The coloring looked kind of like oil (very transparent, but a brownish hue to it), but I assume that could be due to being subjected to the carbon in the cylinder. When I turned the motor over via the starter with the plug out the fumes from that cylinder literally burned my eyes. Now that the car hasn't been run in a while the liquid in the cylinder seems to have disappeared (there was a puddle in there) further lending itself to the fuel theory. I had originally blamed it on a leaky injector but after pulling the rail and pressurizing the system I did not see even a drop (much less enough to flood out a cylinder).


So hopefully the issue with the stutter will be solved by the new plugs/wires. The high leak down will hopefully be solved by cleaning up the intake valves as best as I can and then re-testing everything on a warm motor.

[dville_gt]

Put a bottle on the car. Nitrous = instant zero carbon

On one site I was doing some research on the guy suggested hooking up a methane injection setup on it to clean them.