87 300D low compression on one cylinder, what to do?
 

The car is missing at idle and at stop light otherwise it drives smooth with a lot of power on the freeway. I check the injectors and they are all good. I check the compression and there is bad news. All cylinders read 340-360psi when hot except cylinder 3.

Cyl 3 = 150psi.

The car does not smoke, fuel burns clean ( no black smoke at idle or at speed ) and drives well. The only issue is missing at idle on occasions. It does not seem to miss at high speed. What shall I do? Keeping driving it? What is the downside of doing nothing? Or shall I overhaul the engine?

Investigate. You could check the bores with a scope, check valves for excessive carbon buildup. You could Hook a compressor up to the cylinder and listen out for the largest amount of escaping air be it on the intake side exhaust or dipstick.

Good luck

I had the same problem on a 85 300sd. I used the compressed air method and heard a hissing from #1 exhaust valve. All other valves were quiet. Checked the valve lash and it was ok.
I removed the head and found out the valve guides were toast. Replaced the guides and got a valve job. Put it back together and all was good.
My car had 284k miles and it didn't appear to have ever had the head removed.
You can drive the car as is until you discover the compression problem.
Roddy

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Put air in the cylinder and identify the leak. If it is only valves do a valve job, if it is rings try a solvent soak, oil change and a hard hot run to see if it is a stuck ring that might free up.

Running it from full throttle to full long high vacuum coast after a solvent soak helps too loosen stuck rings. This is how rings used to be "seated-in" back in the day. 20-60 MPH @ full throttle, then coast from 60-20. Repeat 10 times.

Good luck!!!

I agree with #3 and #4 first paragraph.

Do you have any valve lifter noise/tick tick?

Do a cylinder leak down. On an engine with miles, you will get blow by past the rings so look for consistency across cylinders to determine if rings are worn too far in one cylinder.

I'd do a mini " Junk yard rebuild " after a leak down was performed and if the car isn't worth a full rebuild.

Valve grind / replace guides that are excessively worn, slightly loose guides are OK.

( Added ) Mill the cylinder head.

Pull the pistons and check for cracked ring lands other damage. Inspect the rod pin bushings.

Lightly hone the block.

Install new piston rings , rod and main bearings / front and rear seals.

Grind the oil pump cover flat, check gear to end cover clearance and mill down the gear housing if needed. This end clearance is where most loss of oil pressure occurs.

Roll in a timing chain , replace guides / tensioner if they typically fail at / near this mileage.

Congratulations, you now that the mythical " good used engine " .

I'm with the routine overhaul, it's hardly "junkyard" work if you do it carefully and completely and is far cheaper to boot .

Is this one of those moments where some diesel purge in the fuel filter and an Italian tune up might hel

In addition to my " junk yard rebuild" , a " routine overhaul " would consist of :

New pistons.

A rebored block or new liners as applicable.

Resurface the block deck , maybe line hone the main bearing bores.

Reconditioned connecting rods, ( rebushed piston end / resized crank end )

Reground crank.

Valve guides / valves / valve seats replaced to get clearances to factory new.

New hydraulic lash adjusters.

Cam / crank sprockets if they show any wear.

New oil pump.

New water pump

All of this adds $.

I've rebuilt / repaired countless engines over the years to varying levels of quality. Future use of the engine dictates how far one goes in a repair / rebuild. A car that only has 50 K more miles / 5 years of life left does not need a factory fresh rebuild. The only exception in this case is if someone has a second car that will receive the rebuilt engine once the first car rusts away.

How would this fix a mechanical problem?

By "slightly loose" SL320 is referring to clearance between the valve stems and the guides, NOT to "slightly loose" as in the fitment of the guides into the cylinder head.

Or if you can locate a really good engine cheap is another alternative. We had it made locally until a few years ago. Low millage rusted out cars where all over the place. Lower millage relative to the year of course.


Another way taking a chance. If it is a valve. Just do the one valve. Depends a lot on the general condition and overall miles already on the car. Plus if it is a number 14 head on a 603.


To me then it is not worth the cost of a total valve job. In fact if the car is still good it might be a good time to put a better head on it.

Thanks you all the info. The car is in very good condition, no rust and everything works. It has 365k and we put in 35k miles a year so it is well used. My plan is to take the top and bottom off, push the pistons up and examine the rings and piston landings. I suspect it is a broken ring(s) as I added oil into the cylinder and compression did not go up. The tester held the pressure for a long time so the valves probably are seating tight. It is still a lot of work to push the pistons out. I will drive it locally for a while and do it later or in the fall. It doesn't worth much so a engine swap or big overhaul is out of the question.

The exhaust valves on these engines take a beating especially if the engine has been run on WVO.
https://i.imgur.com/SLzwFOg.jpg
Every single one was leaking prior to lapping.

Adding oil to the cylinder while doing a compression test with a no increase result indicates good rings and bad valves. Putting air in the cylinder will show the location of the leak.

Good luck!!!