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

[dville_gt]

It is no secret that the IMRC's and poorly designed PCV setup on the 96-98 Cobra's cause a ton of carbon to build up on the secondary intake valve (the one behind the IMRC plate. This is especially bad in cars like mine since it is very rarely rev'd high enough to open the IMRC butterfly's, it is daily driven, and has been running the notoriously bad fram PCV valve.

I did a low pressure leakdown test last night and these were my results:

1- 29 %
3- 37 %
7- 35 %
2- 60 %
6- 28 %
5- 20 %
4- 28 %
8- 27 %

Pretty high all around, especially cylinders 3, 7, and 2. Air seems to be mostly escaping into the intake when testing all cylinders. Didn't see any bubbles in my coolant, and didn't hear much through the exhaust or dipstick tube.

This leads me to believe the issue is mostly caused by the intake valves poorly sealing. Since I've heard that these cars can have terrible carbon build up in as low as 40k miles (even with fairly aggressive driving), it doesn't seem to far fetched to believe that a motor with 60k of very very mild driving (and a poor pcv) could cause the secondary intake valve to build up so much carbon that it is causing a lot of leakdown.

If this is the case I've seen where people have successfully cleaned the intake valves by just pulling the intake/imrc's, then making sure the intake valves are closed and pouring in very strong carb cleaner, brushing the valves, then vacuuming out the excess liquid. Could this method clean them well enough to allow them to seal once again?

[turbostang]

How was the leakdown done? Hot engine? Every cylinder on TDC?

[dville_gt]

How was the leakdown done? Hot engine? Every cylinder on TDC?

no, it was on a cold engine on tdc of power stroke on each cylinder.

from what i've read the hot engine thing isn't 100% necessary and i was a little worried about pulling plugs out of a hot aluminum headed engine anyway.

[turbostang]

There's HOT and there's WARM, I wouldnt do it on a hot engine just because it's no fun - but it has to be warm at the very least. Alumium expands and contracts a LOT with minimal heat. I've pulled thousands of plugs out of HOT engines, it's not pulling the plugs that causes the problems, it's how they are put in, in the first place (I.e. Anti-Sieze etc) that makes them come out successfully.

The carbon on the intake valves should have a negligble affect - 99% of it is on TOP of the valve, not between the seating surfaces. I'm sure de-gunking it is a great idea, but I don't think it's the route of the problem. 60% leakdown is a LOT of leakdown, that thing should be blowing oil from every seal, gasket and bolt in the engine if that's the case.

Again, personally, I'd proceed as follows: Tuneup, PCV, degunk, oil flush a couple times and go with it - the next step is pulling the heads at teh very least, or possibly the engine. There's no working around it.

[dville_gt]

There's HOT and there's WARM, I wouldnt do it on a hot engine just because it's no fun - but it has to be warm at the very least. Alumium expands and contracts a LOT with minimal heat. I've pulled thousands of plugs out of HOT engines, it's not pulling the plugs that causes the problems, it's how they are put in, in the first place (I.e. Anti-Sieze etc) that makes them come out successfully.

The carbon on the intake valves should have a negligble affect - 99% of it is on TOP of the valve, not between the seating surfaces. I'm sure de-gunking it is a great idea, but I don't think it's the route of the problem. 60% leakdown is a LOT of leakdown, that thing should be blowing oil from every seal, gasket and bolt in the engine if that's the case.

Again, personally, I'd proceed as follows: Tuneup, PCV, degunk, oil flush a couple times and go with it - the next step is pulling the heads at teh very least, or possibly the engine. There's no working around it.

Ya, trust me, when I saw 60% on that cylinder I about shat, especially since besides the stutter that the car recently developed during acceleration the motor ran smooth, quiet, no smoke, and seemed to have as much power as it should. As a matter of fact I'd have never even thought to check leakdown if it were not for the number 1 cylinder having fluid in it when I pulled the plugs (and come to find out cylinder 1 is among the better of the cylinders tested).

I still do not know why cylinder 1 had fluid in it, my only guess is a bad plug/plug wire/or possibly coil causing the plug not to fire and loading that cylinder up with fuel over and over. Since this car uses a waste spark system that fires cylinders in pairs it seems unlikely that the coil could be the culprit since it would seem that 2 cylinders would be affected if the coil was weak/not firing. The fuel injector doesn't seem to be leaking at all, but I presume it is possible that it is dirty and hanging open sometimes(?)

I guess I'll pull the lower intake off tonight, start working on cleaning the intake valves. I need to decide if I am going to delete the IMRC's or just leave them be. Since the car is a daily driver I am not sure I want to sacrifice the low end torque that the plates help with, but I also think the design is flawed so if anyone has any input on that lemme know. Then I'll bolt it all back together, new plugs, wires, pcv valve (or even just 2 breathers which is what a lot of cobra folks seem to go with in place of the poorly designed pcv). Then warm it up and re-test the leak-down.

[turbostang]

Ya, trust me, when I saw 60% on that cylinder I about shat, especially since besides the stutter that the car recently developed during acceleration the motor ran smooth, quiet, no smoke, and seemed to have as much power as it should. As a matter of fact I'd have never even thought to check leakdown if it were not for the number 1 cylinder having fluid in it when I pulled the plugs (and come to find out cylinder 1 is among the better of the cylinders tested).

I still do not know why cylinder 1 had fluid in it, my only guess is a bad plug/plug wire/or possibly coil causing the plug not to fire and loading that cylinder up with fuel over and over. Since this car uses a waste spark system that fires cylinders in pairs it seems unlikely that the coil could be the culprit since it would seem that 2 cylinders would be affected if the coil was weak/not firing. The fuel injector doesn't seem to be leaking at all, but I presume it is possible that it is dirty and hanging open sometimes(?)

I guess I'll pull the lower intake off tonight, start working on cleaning the intake valves. I need to decide if I am going to delete the IMRC's or just leave them be. Since the car is a daily driver I am not sure I want to sacrifice the low end torque that the plates help with, but I also think the design is flawed so if anyone has any input on that lemme know. Then I'll bolt it all back together, new plugs, wires, pcv valve (or even just 2 breathers which is what a lot of cobra folks seem to go with in place of the poorly designed pcv). Then warm it up and re-test the leak-down.

I'd say you are close to bieng spot on, if not 100% on your diagnosis - It's a missfire that's developed from a breaking down wire or plug, or both. mayyyyybe a weak coil, speaking of, have you checked the primary/secondary resistance on them yet?

Wasted spark is of no consequence, it simply means that the coil sparks to the target cylinder as it should, but also sparks on a different cylinder (on the same pack) out of time, where it woudln't matter (as long as it's not on fuel delivery, or compression, it won't hurt anything) - so long story short, if it were misfiring on that extra cylinder, I HIGHLY doubt it would do anything, it's like a wasted spark that REALLy got wasted.

[dville_gt]

I'd say you are close to bieng spot on, if not 100% on your diagnosis - It's a missfire that's developed from a breaking down wire or plug, or both. mayyyyybe a weak coil, speaking of, have you checked the primary/secondary resistance on them yet?

Wasted spark is of no consequence, it simply means that the coil sparks to the target cylinder as it should, but also sparks on a different cylinder (on the same pack) out of time, where it woudln't matter (as long as it's not on fuel delivery, or compression, it won't hurt anything) - so long story short, if it were misfiring on that extra cylinder, I HIGHLY doubt it would do anything, it's like a wasted spark that REALLy got wasted.

I have not check the primary/secondary resistance, I'll put a multi on them and see what they look like.

As far as the wasted spark I guess my logic is flawed, I was under the impression that the same 2 cylinders were grouped together each time, ie 1-6, 5-3, 4-7, and 8-2, thus if there is an isse with cylinder #1 not firing due to the coil the same problem should occur in cylinder 6 since they are fired in pairs. When cylinder 1 fires it also fires cylinder 6 (wasted) and vice versa, cylinder 6 fires so does cylinder 1 (wasted). In my little mind I have trouble figuring out how it could fire off cylinder 6 okay and not cylinder 1 if they are fired off as a pair. (I am in no way doubting what you are saying, just having trouble conceptualizing it)

[turbostang]

I have not check the primary/secondary resistance, I'll put a multi on them and see what they look like.

As far as the wasted spark I guess my logic is flawed, I was under the impression that the same 2 cylinders were grouped together each time, ie 1-6, 5-3, 4-7, and 8-2, thus if there is an isse with cylinder #1 not firing due to the coil the same problem should occur in cylinder 6 since they are fired in pairs. When cylinder 1 fires it also fires cylinder 6 (wasted) and vice versa, cylinder 6 fires so does cylinder 1 (wasted). In my little mind I have trouble figuring out how it could fire off cylinder 6 okay and not cylinder 1 if they are fired off as a pair. (I am in no way doubting what you are saying, just having trouble conceptualizing it)

Your could be weak, it's worth checking.


Negative. It's called wasted spark, because it's... wasted. As in, not used for combustion regardless. With a 90* firing order, the adjacent cylinder would have to be on the exhaust stroke and therfor that's when the 'wasted' spark happens. So, if #1 is firing, the adjacent cylinder that's on it's exhaus stroke is getting the extra spark.

http://en.wikipedia.org/wiki/Wasted_spark

[dville_gt]

I get that for each fire one cylinder gets a useful spark and one gets a wasteful spark, but I just don't get how the cylinder 1 could be not firing and cylinder 6 could be firing if they are fired by the same coil. I assume in a v8 wasted spark system there are essentially 4 coils (each with the duty of firing 2 cylinders at a time (1 for power/1 wasted), so if the 1 coil that fires those two cylinders is bad wouldn't it be bad for both?

[Broncojohnny]

If you suspect one of the valves isn't closing because of carbon or because it may be bent, then wait until it is dark and shine a light down the spark plug hole, then look down the intake port. If you can see light around the edge of the closed valve then you have a problem.

Another old trick for diagnosing using leak down tests is to squirt some motor oil in the cylinder and turn the engine over a few times by hand. If the leak down number picks up substantially then you may have a ring sealing problem, the oil in the cylinder will help the rings seal temporarily.

[turbostang]

I get that for each fire one cylinder gets a useful spark and one gets a wasteful spark, but I just don't get how the cylinder 1 could be not firing and cylinder 6 could be firing if they are fired by the same coil. I assume in a v8 wasted spark system there are essentially 4 coils (each with the duty of firing 2 cylinders at a time (1 for power/1 wasted), so if the 1 coil that fires those two cylinders is bad wouldn't it be bad for both?

lol.

V8 wasted spark doesn't have to have pairs of coils, your car only has 2 total.

For your example, #1 would be, say, TDC - it's sparking with the purpose of making power as it should. #6 would be sparking something like 180* (or 270*?)later at the bottom of the exhaust stroke. Nothing happens.

Edit, had to re-read your statement. In theory, I suppose that could be the case, but theory doens't always prove correct.

[dville_gt]

I see, I knew there were physically only two coil packs, but how the internals were separated I didn't know if each pack was actually 2 coils. I understood the wasted spark part the whole time, I know that one of the 2 plugs firing is not going towards making power, I was just confused by how it could have a problem on just one cylinder if 2 cylinders are always fired together. Take the Northstar Cadillacs, they had a coil pack that contained 4 coils that had 2 plugs on each, each coil always fired off both plugs, if one coil goes bad you have 2 cylinders that are not going to be firing correctly.

[dville_gt]

If you suspect one of the valves isn't closing because of carbon or because it may be bent, then wait until it is dark and shine a light down the spark plug hole, then look down the intake port. If you can see light around the edge of the closed valve then you have a problem.

Another old trick for diagnosing using leak down tests is to squirt some motor oil in the cylinder and turn the engine over a few times by hand. If the leak down number picks up substantially then you may have a ring sealing problem, the oil in the cylinder will help the rings seal temporarily.

Good tips, I'd heard the oil in the cylinder, but the light in the plug hole makes sense as well.

[garycrist]

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

[BP]

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.