1 cylinder, high compression, the others are equal

hey guys...
i was checking the compression in my '83 258 the other day, and all cylinders have a pressure of 172psi, with the exception of #5, which is way up at 195psi. what gives? i am attributing the high compression in the consistant cylinders to carbon build-up, and will be trying TeamRush's steam cleaning, but what could be the causes for that 195 psi, and should i be concerned about it?

thanks!

[Pog]

Thats a big difference but it could just have a bit more carbon then the rest. I cannot believe how high you compression is.. Is that a rebuild? The last I checked mine it was at 115 for all except number one it was 105. This is due to the cold coolant hitting it first = more wear. I have since retired it. As for cleaning out the carbon there is a engine cleaner that GM makes works really good just runs a little hot and burns any stuff up that is unwanted..

i know, i thought those numbers were high myself. i was prepared for low numbers, because the motor has 60K on the rebuild, and i am starting to get a little oily goop in my coolant [img]images/icons/frown.gif[/img] which i think is the head gasket, but then again, there is no coolant in the oil, and are no bubbles in the radiator while running.

[RRich]

Steam Cleaning?

Does that mean pouring water down the intake to make the valves look like pretzels?
Or is it just to dislodge a chuck of carbon so it lodges somewhere else more damaging?
Or maybe that technique is used to get the oil off the cylinder walls so the rings wear in faster? (How do you clean the outside of your engine -- steam?)

Use the GM addative in the tank, or Chevron's Techroline - they dissolve the carbon gently and safely.

There's a reason the carbon's building up - too rich, timings slow, plugs' too cold etc. Cure that and the carbon will not only stop depositing, but the excess will go away eventually, and SAFELY.

Use stock original plugs - none of the so-called "trick" stuff that doesn't work properly. If they are yellow, pink, or blue, made in Germany or Japan, use "exotic" metals like platinum, or have fancy shmancy things done to the electrodes like knurling, splitting, etc - they are gimmicks made to sell, not to use. Stock, boring plugs work like the engine's design engineers built engine around.

[LEVE]

<a target="_blank" href=http://tellico.off-road.com/ubbthreads/showthreaded.php?Cat=&amp;Board=offroadjeepshort&a mp;Number=290255&amp;Search=true&amp;Forum=offroad jeepshort&amp;Words=Carbon%20clean%20washer&amp;Ma tch=And&amp;Searchpage=0&amp;Limit=300&amp;Old=all posts&amp;Main=287664>Here's</a> TR's explination...

well, now i dont know where to turn....
i have taken teamrush's and leve's advice many a time before, all to great results, and now i am getting an answer that contradicts these two guys. TR and leve say to steam clean, and RRich says no way. as with doctor's advice, i'll take as many second (or should i say fourth) opinions as you guys can dish out.

RRich: you never gave me your thoughts on why my readings were so high.... do you think that it is because of carbon build up? and what about that 5th inconsistant cylinder?

im sorry but i personally dont agree with the water injection cleaning method,if carbon buildup is that much of a problem then chances are good that the engine needs to be dissassembled and freshened up .another major cause for preignitionthat i didnt see mentioned in TRs post is ignition timing,thats an easy one to screw up folks so watch how much timing you dial into your rigs.

[RRich]

I hadn't read TR's cure, he says to do it at higher RPM under load and cautions against doing it too much. That's much safer that what most people do.
Most folks run it at high idle and pour it in a little at a time from a cup.
I had one of my mechanics do that as a "favor" for a customer. A chunk of carbon dislodged and then lodged on the top of the piston - creating a terrible knock. I made him rent a car for the lady while he pulled hers down to fix it. Fortunatly it didn't do any damage, slightly larger it could have really made a mess. It was just banging against the cyl roof.
Most of the time I agree with TR's diagnosises and cures, obviously he's had lots of experience. I have too, but from a slightly different aspect. This is one time I differ a little.

I just don't like adding water.
The idea behind it is twofold - one it does the same thing as steam cleaning, cleaning all the gum and dirt out. The rings actually don't touch the walls, they ride on a slight film of oil. Take away that oil - well you can imagine!
Also the cold water hitting the glowing red hot carbon chunks shatters the carbon, breaking it off the chamber. It gets it out, but if there's big pieces one may get stuck under a valve, when the piston comes up ----.
Or, like what-his-name's did, lodged on the top of the piston and hit the roof.

It's best to add something that will dissolve and remove it slowly. But that's only a temporary fix if you don't cure the original reason it's carboning up in the first place.

Reasons: Anything that causes incomplete combustion will create excessive carbon. Carbon is just the result of incomplete burning, usually a lack of oxygen for the amount of fuel. If there was enough oxygen, it would turn the carbon into CO and CO2 gasses.

Rich mixture, cold plugs, timing way off etc, possibly even a partially plugged exhaust.
Plugs I've already addressed, timing either fast or slow can cause improper burning, or improper advance.
Usually it traces back to too much fuel for the air = Rich mixture. Either at idle or higher speeds or both.

Now I'll throw a curve - can also be a vacuum leak. (Vac leak? That's lean! HUH? IS THIS GUY NUTS?)

Let's take a simple straight line 6 cylinder with a log type intake manifold. Let's introduce a vacuum leak at the rear - a loose line or even a gasket. Air gets sucked in through that leak, affecting the rear cylinder - it's lean, not enough fuel.
The engine runs rough.
Abdul says "Me Fix" and adjusts the carb richer, till the engine runs smooth again. "Fixed, $30 please!"
But what really happened? Now the rear cylinder is getting enough fuel so it no longer misfires - good. But, the others are now too rich. After a while those build up lots of carbon - all except the one that's running right. (Hmmmm, sound familiar?)

Computer controlled cars are famous for that. Depending on which side the leak is affecting, and which side the O2 sensor is on, it could run either rich or lean. If the leak affects the same side the O2 sensor's monitoring, then the sensor sees the excess oxygen and richens all cylinders - you never know it till later or by checking your mileage records. If it's on the other side, it just runs rough.

The best method to find a vacuum leak is use propane, a controllable fuel that burns like gasoline.
Take a propane torch's orfice and head off. Slip a vacuum hose over the end about 4 feet long.
Holding it upright you get vapor propane, upside down you get liquid for more volume.
Let a little vapor go down the carb or intake - with or without air cleaner on. Note the engine's reaction.
It normally should "like" the propane slightly, RPM should increase just a tad, but no more than about 100 RPM. If it "likes" it too much, it's too lean. Same applies for high speeds, and even under load. I use it all the time with a chassis dyno and an engine dyno. It's a real easy way to see how a richer condition helps or hinders.
If it slows down it was too rich - you are making it even richer.

Vacuum leaks - simply spray the propane around places you suspect a leak - if the RPM changes you've hit it. Remember the propane blows around, so don't be fooled by it blowing in the intake when you spray a heater hose. Keep the fumes away from the distributor - I think you can guess why.

If you have trouble locating a leak, you can lean the carb down till it starts to run rough, then when you hit that small leak it will be really obvious.

Computer controlled cars are a little harder, when the propane hits the leak there will be a slight RPM change, then it will drop back to normal as you hold the propane going. The computer sees the rich mix and leans the system out as well as readjust the idle speed. Then when you stop applying propane, the RPM drops for an instant as it has to compensate again. You just have to listen more carefully.

I like propane rather than a liquid, like a burnable carb cleaner, ether or whatever. Liquids puddle, then if they ever catch fire you have a problem, propane just flashes, scares hell out of you, then goes out.
Also a liquid can wash dirt into the hole temporarily plugging it. "It was there a moment ago!"
Base gaskets, hoses, power brake boosters, trans vac modulators etc can all be letting excess air in.

I know this is much more detailed than you asked, but hopefully this helps a few other folks as well.

Almost always the carbon will gently and safely burn off as you drive it after the cause of the problem is cured. The addatives simply help it a little faster.
Let us know what you find.

RRich....
thanks alot for your insight.... i always like a long detailed explaination on things.... makes it more thorough. i gotta print this out and go grab a beer and digest this. thanks again.

[Jim_Lou]

A terrific post! One thing to add: The "Abduhl" cure for carbon buildup is "Take it out on the highway and blow it out." Sudden high temperature in the combustion chamber can turn relatively soft sooty deposits into a tough glassy conductive deposit that will foul the plugs and, because of it's toughness, never go away. You've trashed the brand new plugs you just put in trying to cure the miss. And you'll not think of them as a problem because they're brand new.