Van
02-03-2002, 11:39 PM
I've had a very small miss at idle, ever since I finished tuning in 22 psi on pump gas. The miss isn't all the time and the miss isn't consistent, like a fouled plug would be. When it misses, the timing advances slightly and then settles back down.
I finally got around to doing a compression test and its results led to a CLDT.
This is where it got interesting...
First the compression test, I did it up here at 8200 ft and got low numbers as expected. I redid it down in Denver and got better numbers, with the problem cylinder getting the least increase, as expected.
Strangely enough the problem cylinder is #3. Cylinders 1,2 and 4 are all good, with #'s 1+4 being tops and #2 being only slightly lower. The wet test saw 15 psi gains across the board, except cyl #3, which saw a bigger gain of almost 20 psi. The first compression test pulse of all cylinders is above 100 psi, except #3 which was 75 psi.
So I decide that a CLDT is in order, because after the miss started, I decided it was time to see what a 2:1 mix of 100 octane would do for me, hehe. 24 psi with zero knock sums, I didn't even bother to see what the limit would've been past 24 psi as my headgasket and bolts are stock. That lasted about a week and I believe that I lifted the head, further compounding the problem.
My car only uses a quart between every oil change, it doesn't smoke at all that I can see. I cruise at 80+ mph daily, I hit 20+ psi at every light and on-ramp and I downshift all the time. I've had no overheating, not even a little temp spike at high boost, coolant pushing and there is no oil in the water or vice versa.
The wet test alone should mean rings. So I build my own CLD tester. Easy enough. I only bothered to test cyl #3, as I wanted to verify what I was gonna do next, quickly.
At only 40 psi, #3 is pushing some air into the crankcase, because you can hear it coming out of the oil fill location. Also verified with smoke. But even weirder is that ALOT more air is coming out of spark plug #2's hole!
Pistons 2 and 3 run up and down together, except that they are on different strokes. When #3 was tested it was on TDC compression stroke, both valves closed. That means that #2 is up on top of the exhaust stroke. So even if the exhaust valve on #3 was leaking, it would seem that most of the air would want to run out of the exhaust pipe instead of going backwards into #2 and showing up as so much air flow.
This makes me think I need a HG asap. Any other opinions?
All of this long-windedness leads me to a couple of observations and another question or two. There is a dead spot right between #2 and #3 cylinders in the HG. A small blown spot right there would make my results entirely possible. But towards the intake and exhaust sides of the headgasket between #2 and #3, there are two small holes in the gasket. One is small and the other is slightly larger... Are these both water jacket holes?
Let's say that on #3 side the gasket has a decent sized hole that gets smaller towards #2. When normally running, as #3 comes down on the power stroke, #2 is coming down on the intake stroke, naturally siphoning out the charge. The same thing occurs when #2 is coming down on power stroke, #3 is on intake stroke. Kind of confining the leak between the two cylinders. Does this analogy make any sense to you guys?
Why would the wet comp test show such an improvement if the HG is toast? Maybe the oil "sealed" up the HG a little?
If I do rings now, it'll just be a cheap re-ring and hone. I was toying with the idea of putting in some stock bore 2g pistons at the same time. But the tech manual shows the 1g and 2g pistons as different sizes. Is that a typo or what? Is the OD really different? As I've heard of this being done and rather easily at that. Just have my rod's pin end done, hone and some new 2g rings. I can pick up some 2g stock bore pistons cheaply enough.
Any opinions or answers on these?
I finally got around to doing a compression test and its results led to a CLDT.
This is where it got interesting...
First the compression test, I did it up here at 8200 ft and got low numbers as expected. I redid it down in Denver and got better numbers, with the problem cylinder getting the least increase, as expected.
Strangely enough the problem cylinder is #3. Cylinders 1,2 and 4 are all good, with #'s 1+4 being tops and #2 being only slightly lower. The wet test saw 15 psi gains across the board, except cyl #3, which saw a bigger gain of almost 20 psi. The first compression test pulse of all cylinders is above 100 psi, except #3 which was 75 psi.
So I decide that a CLDT is in order, because after the miss started, I decided it was time to see what a 2:1 mix of 100 octane would do for me, hehe. 24 psi with zero knock sums, I didn't even bother to see what the limit would've been past 24 psi as my headgasket and bolts are stock. That lasted about a week and I believe that I lifted the head, further compounding the problem.
My car only uses a quart between every oil change, it doesn't smoke at all that I can see. I cruise at 80+ mph daily, I hit 20+ psi at every light and on-ramp and I downshift all the time. I've had no overheating, not even a little temp spike at high boost, coolant pushing and there is no oil in the water or vice versa.
The wet test alone should mean rings. So I build my own CLD tester. Easy enough. I only bothered to test cyl #3, as I wanted to verify what I was gonna do next, quickly.
At only 40 psi, #3 is pushing some air into the crankcase, because you can hear it coming out of the oil fill location. Also verified with smoke. But even weirder is that ALOT more air is coming out of spark plug #2's hole!
Pistons 2 and 3 run up and down together, except that they are on different strokes. When #3 was tested it was on TDC compression stroke, both valves closed. That means that #2 is up on top of the exhaust stroke. So even if the exhaust valve on #3 was leaking, it would seem that most of the air would want to run out of the exhaust pipe instead of going backwards into #2 and showing up as so much air flow.
This makes me think I need a HG asap. Any other opinions?
All of this long-windedness leads me to a couple of observations and another question or two. There is a dead spot right between #2 and #3 cylinders in the HG. A small blown spot right there would make my results entirely possible. But towards the intake and exhaust sides of the headgasket between #2 and #3, there are two small holes in the gasket. One is small and the other is slightly larger... Are these both water jacket holes?
Let's say that on #3 side the gasket has a decent sized hole that gets smaller towards #2. When normally running, as #3 comes down on the power stroke, #2 is coming down on the intake stroke, naturally siphoning out the charge. The same thing occurs when #2 is coming down on power stroke, #3 is on intake stroke. Kind of confining the leak between the two cylinders. Does this analogy make any sense to you guys?
Why would the wet comp test show such an improvement if the HG is toast? Maybe the oil "sealed" up the HG a little?
If I do rings now, it'll just be a cheap re-ring and hone. I was toying with the idea of putting in some stock bore 2g pistons at the same time. But the tech manual shows the 1g and 2g pistons as different sizes. Is that a typo or what? Is the OD really different? As I've heard of this being done and rather easily at that. Just have my rod's pin end done, hone and some new 2g rings. I can pick up some 2g stock bore pistons cheaply enough.
Any opinions or answers on these?