High Compression in Cylinder 4

[SMHertz]

So I got my hands on 1969 2000 and am having an issue.

When I bought the car it had sat for 30 years in a garage and anything rubber was totally shot. It smoked like mad and the fuel pump was spitting gas.

Replaced the fuel pump diaphragm and all of the valve seals, cleaned the gas tank, ran some seafoam. Now I have an issue with cylinder 4, I ran a compression test and came up with these numbers 180,190,190,220. Cylinder 4 is a detonation machine and I cant figure out how to fix it, down low right off idle it detonates and smokes like mad. It also seems to be pulling oil from somewhere. The plug lasts about an hour before it fouls.

The fact that it has high compression makes thin the rings are fine but I have no idea.

Anyone had this issue, any help would be greatly appreciated.

Thanks

[Habitat.pat]

Several things come to mind but may not apply to a 2L.

If you have a head gasket allowing a lot of oil into #4, it could be causing high pressure. Same goes for a cracked head.

Since the pressure increases from front to back, has the head been cut incorrectly causing the combustion chamber to be smaller on #4?

Peace, Pat

[Gregs672000]

Agree with Pat... since it's burning oil it is clearly sealing better due to excessive oil being present. If the rest of the motor were showing low compression I'd suspect rings, but in this case I would examine the head side of things, specifically the guides and oil seals. It could be that the guides are somewhat worn and/or the seal(s) have failed despite your change, allowing oil into the chamber. However, it should not detonate as hard as you suggest from that compression unless your jumping on it and it's going lean ASSUMING the ignition timing is set right... What initial/idle timing are you using? Unless the dizzy was de-smogged/recurved, it should be 0, not 17 (in case you are unfamiliar).

Options: 1) try replacing the seals again in that cylinder, check the valves for excessive wobble as best you can by wiggling it.
2) pull the carb and look down the throat to see if you can see a wet valve stem. If dry, then it could be the exhaust guide/seal, so you could then remove the intake and exhaust to look at the exhaust. The second time I rebuilt my engine it was an oil burner and would not seal up. I was convinced it was the rings not sealing, but when I pulled the manifolds I could see oil running down the valve stems... for some reason the seals and shims used were allowing oil to sneak through (professionally built head, not me). All the shop did was swap the position of the shims to being inside the seal cup instead of being under it and that solved the problem.

It may just be easier to go ahead and try replacing the seals... it will either work or not and it's the cheapest/easiest option. If not fixed, then you're probably going to have to remove the head anyway. Oil control rings are low tension and their job is to scrape most but not all of the oil off the cylinder walls, and unless damaged and combined with worn out compression rings they don't really wear that much (per conversation with reps at Total Seal). I had a broken ring in #4 and it didn't show up on a compression or leak down test though the plug did burn differently but wasn't wet.

All of this makes me think it's more a head issue vs a ring sealing issue. Bad seal, bad valve guide, blown head gasket, or a crack...

[david premo]

SMHertz,
The problem may not be what you think it is. Sometimes when you have incomplete combustion that can be attributed to excessive oil consumption or overly rich fuel mixtures is, excessive carbon buildup on the piston crown or combustion chamber and even both. Back in the day, it was not uncommon to bring the engine up to operating temperature and then begin to introduce water via the carburetor at a very high idle. The water quenches the carbon breaking it free from the piston and combustion chamber and then it goes out the exhaust. I once had a 472 with excessive compression and pinging. FYI a 472 is a Cadillac engine of the 60’s and early 70’s. I poured a lot of water slowly down the Rodchester carb at about 1800 RPM and it was making all kinds of noise and I kept going and at one point it sounded like a rod bearing went bad. After several minutes of the treatment it was time for a test drive, a lot of the carbon had broken free and one good full throttle peer run on the Simi Valley freeway and it was good as new. Some people like to call it an Italian tuneup, but it can work. What you might want to do is scope the cylinder, number four for excess carbon first then proceed with the procedure. The other way is to I’ll the head and take a look.
Dave

[redroadster]

Whats the 1st pump readings , cold . it's likely a warped head that got milled instead of heated and straightened , at least that makes sense usually the valve seals go /harden at the same time # 3 cyl is the hottest cyl , unless it's been run low on coolant , and it probably has in its life , really needs to have it pulled and check the head
# 4 exhaust valve cam lope worn ?
Likely the oil ring isn't functioning piston passages clogged oil ring are pretty precision items to work @ 3000rpm or 50 strokes per second it would have to scrape a quart or more off the cyl wall in that 1 sec.
How bad is the oil smoke out exhaust?

[redroadster]

Dang button

[SMHertz]

So Ive replaced the valve seal three times now thinking that I have an issue at the seal, I now don't believe its the issue. I'm incredibly gentle getting the valve seal on now. The head was rebuilt in the 90s and then no miles were put onto the car. The valves guides are basically new and the valves have zero wobble.

My timing seems good as the car runs really well on the other three cylinders. The plugs look perfect there.

Also smoke only happens when you can hear it ping down low off idle. No matter how gentle I am coming off the line it pings around 1800 to 2000 RPM and does a full 007 smoke screen.

Ill try the water trick and see if if its just full of carbon and oil. Now I just need to figure out where the oil is coming from.

Thanks

[Gregs672000]

So it sounds like you eliminated seals etc. It's detonating and smoking at 1800rpm under relatively minor load. I assume it's an SU engine that's de-smogged and running an initial timing of 17. I'll assume a stock advance curve.

My motor is 215 215 210 225 cold compression, estimated compression ratio is 11 to 1 (long history with this engine). I have a crank fired ignition so I can set what ever timing map I need and can data log it all... I run an idle timing of 17. As the engine launches from a stop under a typical load, at 1800 rpm I have 23 degrees of advance, at 2000 rpm 25, and at 2200rpm 26... no detonation at a trypical load. Under high load it will pull down as low as 20, 21 and 22 respectively at those rpms, but that would be full throttle at low rpm (not what the engine was designed for and exactly where it would knock!). Point is, I don't think my initial curve is a lot different from stock under typical (lighter) load, so I don't know why yours would knock and SMOKE so bad due to compression alone assuming timing is correct. I would suggest a check in timing just to be sure. Other than that, I fear that something may be broken or compromised, as in an oil ring. Got nothing to lose trying the Dave method or other ring decarbonization stuff like seafoam or chem tool...
Good luck and let us know what you find.

[SMHertz]

So last night I took David's advice and got the motor hot and poured water down the carb. Made a ton of smoke and steam and seemed generally angry for about 5 minutes. Took it for a test drive and still had the smoke down low issue but it seemed better.... pulled the plug and it was still oily. The car used to detonate on shutdown like crazy, this is the first time it has actually shut off without trying to run in reverse.

I then did an oil change to see if I milkshaked the oil, but it all seemed fine, it looked like normal oil. I did notice that during the water down the carb stunt I started to get some bad blowby out of the PCV hose, makes me think that either the valves are still leaking into the valve cover through the seats or through the stems. I think ill try the water trick again tonight and toss in a new plug. I also ordered some Yamalube ring free, I figure ill shock treat the gas and see if that frees anything up.

Thanks for all the help.

[Habitat.pat]

I didn’t see that anyone recommended a leak down test, but it couldn’t hurt & might reveal something.

Peace, Pat

[david premo]

Alright this is a long shot, the 4 studs that mount the top of the intake manifold are wet studs. I for years have seen tons of U-20’s with oil pooled between the #1&2 cylinders and 3&4 because they did not know to put thread sealer on the studs going into those holes. If you look inside the head with the valve cover off you will see the threaded holes that I’m talking about. Try putting thread sealant on those 4 studs. If no that I’m going with valve seals.
Dave

[david premo]

I forgot to ask, did you do a post water induction compression test? It should have come down to within a few pounds of the other cylinders if you got all the carbon blown out.
Dave

[JT68]

I'd say you are likely just postponing the inevitable. Just take the head off and find the problem. External measures are not going to correct this. Most likely you have water or oil leak to the cylinder or something else very wrong. Water and oil are not compressable, so any significant amount can increase the compression significantly. Carbon build up would not account for 220psi.

[redroadster]

I'd be cautious about thinking the blocks of carbon are going to loosen up then go quietly out the exhaust. Like good little boys ,if it gets jammed in the valve opening. The rocker will play baseball batter with the valve , knocking it into the piston .
Pull the 4 plug and idle it if it has a flow of oil that keeps coming you have oil ring probs ,which likely are on all cyls

[spyder]

Check the cam and make sure cyl #4 is good. If the duration is smaller it will increase the pressure.