Low compression in #5 cylinder

In the process of trying to make everything right i.e. eliminating leaks, cleaning, polishing, etc., I did a compression check and discovered that # 5 cylinder only produced 60psi, as opposed to the others that were between 100-105 psi. After squirting oil in #5, the result was 105psi. I was surprised to discover this because the engine runs so smoothly at idle. I am only a backyard mechanic. Does that mean that I have a broken ring? If so, is repair within the capability of a socket set and basic wrenches, or will my 1963 220 SEb overhead cam M127 engine require special Mercedes tools and expertise? I have had the car for ten years and it has what I believe to be an actual 90,000 miles on the odometer. Does anyone out there know what a qualified mechanic would charge to do this?

In answer to your question "has the car been sitting for awhile without being run?' No, Before I got around to taking the compression check, I had been driving around town the previous two days. I didn't make the check until the following day, this morning, as I had to get my wife to assist me in turning the engine over because I couldn't find a place to attach my remote starter button with the starter hidden in the deep right side of the engine.

[Tomguy]

Gregg: If it went up that high in PSI after oil, well... how much oil did you put in it? Would you think that, when running, the oil in the cylinder would raise the compression to 105? If so, you're fine. Remember, a car sitting overnight even has the oil drain out of it. It just may have been too dry!

Yes, I am sure it did sit around for awhile as I bought it from a specialty car showroom that had it on consignment in San Diego. I have only put a little over 10,000 miles on it since purchase 10 year ago since I garage it during our Salt Lake City winters. They use a lot of salt on the roads and it is happily rust free. I only drive it for outings or nice days.

I'll go along with the "stuck ring" possibility and follow your Rislone suggestion as the first step. It sure is going to be cheaper and easier than the alternate. Does Rislone have any advantage over the Mobile 1 synthetic that I have been using for the past 6000 miles? I just finished warming up the car an took another compression check after shut down. #5 still reads 60psi, so the previous low reading wasn't due to having a dry cylinder from sitting.

[ctaylor738]

Do a search on "XP33." This is some nasty stuff that seems to fix problems like this.

Did a seasrch on XP33 on several search engines without results. What is XP33?

[ctaylor738]

I'm sorry - it's X-66 and X-66A. Dunno what I was thinking of.

Here's a link to a good post:

Delco X-66 de-carbonizing agent

MPNYE,
Today I completed your suggested method of a 24 hour run with full Rislone, but did it in two 12 hour periods. Unfortunately the compression result in #5 remained the same at 60psi, so it looks like I have a major decision in store. I am going to drive it around for a few more hundred miles and take another reading, just in case it is a stuck ring, that might free itself, before deciding whether it is cost wise to have a Mercedes specialty garage complete the labor of disassembly and rebore #5 in the car. This would result with one cylinder larger than the rest on an engine with almost 90,000 registered miles. As removing an engine longblock and installing another is within my simple mechanical ability, since it doesn't require the special MBZ tools and techinical skills, I'll have to check on the price of a long block from someone advertised in STAR magazine like Metric Motors or Noel's as an alternative and compare. I wonder if my M127.984 135 HP engine components and accessories would work on the higher 185 hp 6 cylinder M130.983 used in the 280 and if it would be a matching swap? In any case, I appreciated your suggestion.

[ctaylor738]

But if the car runs well and is not smoking or using large amounts of oil or fouling plugs every week, then doing nothing is always an alternative.

Realize that I am not very good at following my own advice!

I checked the #5 cylinder plug and it looks fine. Tan colored without oil or soot. The engine doesn't use oil and doesn't leave a trail of smoke. You would think that with a stuck ring it would lead to a fouled plug, use oil and smoke! I'll check again after a little time and mileage before making the big cost decision.

What a relief! After pulling the head, it was discovered that the source of the low compression in #5 was a burnt/defective valve and not a broken or stuck ring. The valve had a tiny hole in it just inside of the lip. I have seen damage on a valve face before but never a hole in the tulip area. I assume that the valves are sodium filled and this may have been a defective forging. On my initial email thread I made a wrong assumption after pouring oil into the cylinder, that the rise in compression was the oil sealing a bad compression ring, when in fact, the oil was temporarliy sealing the tiny hole in the valve. Now instead of being faced with possible cylinder scoring/damage, resleeving & boring or possible engine replacement (big bucks), I find consolation in just a relatively less expensive valve job. It is just as well, as even though the engine only has 89,000 miles on it, upon inspection the valve seals have hardened and the timing chain has developed slack over the last forty-years. Other good news is that the cylinder walls don't even have the slightest hint of a lip/ridge worn in at the top. Maybe using Mobil 1 all this time helped. I am curious though about the design of the valves. The face area of these original valves leave very little material for a refacing/regrind. Has anyone ever reground this model motor valves or is it standard to just replace them during a valve job? I hope this info helps and relieves others that find themselves with the same initial diagnosis. I'll take a picture of the valve and post it later. Does a quote of $200 for a head gasket sound reasonable for this motor? I know it is a Mercedes but I almost gagged when told. Anyone know of a source for still good quality but cheaper price?

[1967250s]

If your engine is a 127.981, same as 230sl. The price range is $87 (elsewhere) to $110 (fastlane) for a head gasket set.

Dan

Thanks, but no it is an M127.984. I'll check with Fastlane though. Maybe they might be the gaskets are the same and I'll make out on the price compared to my previous call.

[1967250s]

Gregg if you need a pic for your sig, you can take this one....

Dan