380sl Engine Reconditioning
 

I just recently bought a 1982 380sl, so far I love this car, but I'm having small (which might be major) issues with the engine. It runs very good above idle, but at idle, was running very rough. The car was in storage for about 7 years and does have over 150000 miles on it. At first I thought it was just the old fuel, so I changed out all the fluids, and added Seafoam to the gas and ran the tank down to 1/4 and added fresh fuel. It seemed to improve, but still ran poorly at idle.
The P.O. stated he did a tuneup on it, but I think he ment 7 years ago, so I changed the plugs and found they were very old looking and one plug looked like it wasn't firing at all. So I did a compression check. It read 60 at idle and did increase with exceleration up to about 110/120. I figured either the valves are carboned up or the rings are gummed up from sitting so long (at least that's what I'm hoping).
I added some more Seafoam and Lucas engine cleaner and have been trying to drive the car on the expressway as much as I can to try and clean or free up the valves and rings. It does seem to be improving at idle and runs great at speed.
Does anyone have any other suggestions on how to clean the rings and valves and also freeup the rings. I know an engine overhaul would solve all this, but I don't have that kind of money at this time and to be honest, the engine runs great, but just idles poorly due to the one cylinder. I'm hoping that freeing up the rings and cleaning the valves will solve the problem. Also, it doesn't burn or leak a drop of oil, so I do believe the engine is in fairly good shape.
Any one have any other suggestions on what type of engine conditioner to use. I've read about ENGINE RESTORE as a possibility.
thanks

How does one do a compression check at idle and then during acceleration. All the compression checks I do are engine not running with plugs out. My 560SL compression is about 160 - 165 PSI.

John Roncallo

* 82gray380, you need to first determine whether you have a problem related to cylinder sealing (rings, valves) or whether you have an "external" problem (fuel management, vacuum, ignition parts or adjustments). Loosely speaking, whether the problem is "inside" the engine or "outside" the engine.
* The most useful indicator of cylinder sealing is the cylinder leakage test. This requires a piece of test equipment (leak-down tester) which is hooked up between a compressed-air source and the cylinder. Each cylinder is tested at TDC on the firing stroke (of course, the engine is not running). If you do this test with a cold engine, it will give you the "worst-case scenario" readings. Compression tests are also useful, but can give false negatives (i.e., readings will indicate a good engine when there is in fact a problem).
* No cylinder should leak more than 25%. Part of each reading will be leakage past the rings, and part of it will be leakage past the valve seats. If you get a reading higher than 25%, the next job will be to determine whether it's from a ring problem or a valve problem.
* If all cylinders test below 25% you can forget about an "internal" engine problem. Your problem won't have anything to do with cylinder sealing. It will be related to something "outside" the motor.
* You can find out more about clyinder leakage testers by google searching or asking a passing Snap-On or Mac tool salesman.

I know my compression test wasn't the correct way, but it gave me a rough indication of the cylinder. I have a long hose extension that attaches to my pressure gauge and I just removed the one plug and faced the gauge into my windshield so I can see the pressure readings. Not accurate, but lets me know good cylinders compared to bad. All the other cylinders read between 120-130 at idle and went up to 160 or 170 when I gave it gas. I wasn't driving the car. Anyways, it isn't perfect, but it let me know that the one cylinder is having issues. I just hope it's just carboned/gummed up, not a burnt valve. (Just hoping for the best) cause I will have to drive it the way it is for now. A engine rebuild is way out of the question at this time.

* 82gray380, I want to applaud you for wanting to learn more about your new car. Surprisingly, many people who say they care about their car couldn't even tell you where the spark plugs are. Then, when there's a malfunction, they're completely dependent on the nearest helping hand (and that doesn't always turn out well).
* The compression test is a good test. The compression test is done with the engine NOT running, because compression specs are given for an engine not running. You just crank the engine over till the gauge reaches the highest reading (ground the coil HT wire so that the engine won't start while you are cranking). The pressures associated with combustion in a running engine would make the compression test readings unuseable (what specs would you compare them to?). The compression test specs in the book are given for pressure developed while CRANKING the engine, not running the engine. You're trying to measure the pressure that is created WITHOUT combustion.
* What I was trying to point out was that there are a few ways that a compression test won't tell you the truth about ring and valve sealing capacity. Such as, if the piston dome/combustion chamber has a generous layer of carbon, the higher pressure readings can mask a cylinder sealing problem. Generally, though, the compression test is a useful test. The cylinder leakage rate is just a more precise metric and doesn't have those loopholes. The Mercedes technical literature has always paid more attention to cylinder leakage, as a diagnostic tool, than compression (very wide acceptable pressure ranges in their specs).
* But data is good. And more data is better. So do a compression test AND a cylinder leakage test. And did you know that each of these tests can be done with either a cold engine or a hot engine. That's four sets of data. In addition to discovering a problem (if there is one), your data will provide a great "baseline" to compare any future test data to.
* It would be interesting to see the two sets of data side-by-side, compression (cold engine) while cranking vs. compression (warm engine) while running.

Just a note....testing the compression with the engine running does not really change the compression much. There is no combustion pressure because the compression tester is screwed in place of a plug and will not ignite the mixture.

I have done it before with the engine running and not and the difference is not noticable. Trying to test it with the engine running is not recommended though because of the danger of getting something caught in the fan belt.

At that mileage, the bottom end should be still in great condition if the car has had decent care. Your idea of running it to free stuck rings might work, but you should determine whether it is leaking into the exhaust manifold; an exhaust valve leak, the intake; an intake, or the crankcase; a ring leak. This can be done with a leakdown test, I believe.

Also be sure to get the correct, non resistor plugs for the car (I believe that model calls for them).

Good luck.

Tom W

I'd like to thank everyone for the replies. I own 7 different model classic/sports cars and am a forum member of each model and this forum is very helpful and informative. I do work/restore my own cars and sometimes I take shortcuts, so I can afford these cars. Thats way I'm trying/hoping that I can resolve this problem without an engine rebuild (cause at this time it can't be done). I appreciate all the good advice and will let you know the final outcome, but it seems that the engine is starting to smooth out the more I drive it and I LOVE DRIVING IT. Thanks again for all the good/informative advise.
If you interested in seeing my other cars, go to www.cardomain.com/ride/816145
thanks again

Great discussion on compression testing.
You might be looking at nothing more than a vacuum leak though. Look real close at all the rubber parts, especially the hoses/connections, the injector seals (spraying quick-start or similar to find vacuum leaks works for me), and the large air chamber piece at the front. Mine had several broken hoses and a crack in this piece when I got it home. Silicone worked to seal the air chamber inlet (until I can replace), and some fresh hoses, it idled 60% better. I tested and cleaned the idle air valve, and put in a used but correct part control box to cure the other 38%.
Back to compression, how do you 'ground' the high tension lead from the distributor? Would just disconnecting it hurt the electronics?

Not for the faint of heart
 

There is (or was) some stuff called X-66 aka "GM Top Cleaner" that was discussed a while ago. Here's one of the posts:

http://www.peachparts.com/shopforum/tech-help/19836-delco-x-66-de-carbonizing-agent.html?highlight=x66

* walgamuth, good point! Without the spark there shouldn't be much of a contribution from combustion pressures. But then I wonder why he got such a large difference in compression between his idle reading and his off-idle reading? Some other contributor? I've never actually tried to do a compression test with the engine running. What useful information do you think could be gained by doing the test with the engine running? Despite the safety hazard, there might be something worthwhile in doing the test this way.
* donbryce, "grounding" the cable means connecting it to ground. That is, don't just pull the center lead out of the distributor and let it hang in mid-air or hang close to a grounding point (where it can arc with a much wider gap than the spark plug). That's what can damage the control unit. Just use a little cable with alligator clips at either end, one end to the HT lead end-terminal and the other end to a ground. In normal operation, the secondary (high-tension) current goes to ground anyway, but only after it has jumped across the small rotor-to-cap gap (engines with caps) and the (.032") plug gap. So, in essence, you're closing the gaps by connecting the cable directly to ground. Problems arise opening the gaps, not from closing the gaps.
* Speaking of safety, you don't want to hold that coil wire when the engine is being cranked. Granted, secondary current is a fairly low amperage, but it's a very high voltage. (You can use the water analogy; if amperage is volume of water and voltage is pressure of water, then HT current would be like a pencil-thin stream under enough pressure to go right through you. Using the same visual aid, the current required to run your TV, maybe 7amps at 120volts, is like a lot of water under fairly low pressure, i.e. like a wave moving very slowly). If you provide a ground with your body by holding the lead end-terminal, HT current will clear the wax right out of your ears (ha-ha) and maybe stop your heart in the process.

I don't think there is any useful info from testing with the engine running. IIRC, I found it did not make much difference. I did do it though and wanted to report my findings as well as I remember.

Great news that it is smoothing out.

A leak should affect all cylinders, not just one and it could not affect the compression test, I don't think.

Good luck.

Tom W

* 82gray380, in the interest of learning something from this thread, why don't you run some careful tests on your new car and then post the results. Be sure to include test conditions. If you have your numbers from the running compression test, put those in too.

Robert Squires, Good suggestion about doing some research. I don't have the time for detailed research, but in the interest of helping other owners and learning about my engines situation, I did get some numbers. I removed all the plugs and did a quick compression test on all cylinder. All were around 140-150, except cylinder#2, which was around 90psi. I then added oil to cyl. 2 and did a compression test and it increased to 120psi.
I then replaced all the plugs except plug #2 and did a running compression test. At idle it was at around 90 and as I increased the RPMs it increased to around 120.
As for the compression test, I feel the running test was as accurate as removing the all the plugs. So if you suspect just one cylinder, a running test might be the answer. I did not do a leak down test.
By adding the oil and getting an increase, leads me to believe that the rings may be the problem, as adding the oil to the cyl. would improve the ring seals for a short time.
My hope is that the rings on that cyl. are gummed up from being in storage for years and that cleaning additives might free the rings up.
I did go to a local repair/parts shop and discussed this theory with a mechanic and he agreed, it is a possiblity. So to hopefully free up the rings, he recommended using Roslone engine cleaner for the rings and Seafoam to the fuel for cleaning the valves and drive the hell out of it for several hundred miles (high speeds as much as possible).
I added both cleaners and got on the local expressway and drove around 70-80 miles today. I can't say that anything is that different yet, but I'm figuring it might take 200-500 miles before I should really notice a difference. Crossing my fingers.
As you know, this is all just theory and I'm not recommending these products to anyone. But I'm more then willing to try these products out in hopes of solving my idle/compression problem. I'll let you know if it works.

A good friend who built a lot of drag engines says that a leaking valve also can be raised with oil in the cylinder.

Tom W

t walgumuth, I have heard that also, but the oil procedure usually leads towards the rings. Just in case it is the valve, as long as the valve is just carboned and not bent or burnt, the Seafoam will hopefully clean it up. Believe me, this is all just trial and error. If it works, I save myself a rebuild, if it doesn't, I drive it till the engine completely stops running and then start saving for a new engine.