Compression problems - please help remedy

[thysonsacclaim]

I checked the compression. This time I got shoddy results:

[Firewall]
140  130
90    130
150  120
210  145
[Radiator]

Units are PSI. The right bank is OK. All are above 108 and the variation does not exceed the allowable amount. The left bank is wacky.

We decided to take off the valve cover to see if it would help us diagnose the problem. To our surprise, the 10mmx155mm head bolt that is located next to the #3 cylinder was missing the bolt head!

Luckily, it was not stuck. We manage to put a vice grip on the end of the bolt to unscrew it.

Our theory is that the bolt, not being entirely screwed in, allowed flexing to occur during cam rotation. It is conceivable that the bolt not being fastened in that area could allow it to move upwards, also possibly forcing the areas left and right of it downwards.

The cam was in good condition, although there was a slightly rough spot on one of the lobes (not the cylinder in question).

However, the other anomaly in the left bank is that the #1 cylinder is showing 210 PSI. We triple checked all of these readings, by the way, with the air cover off, the throttle wide open. We lubed the #3 cylinder, but it did not change the reading of 90 PSI.

Also, the #3 spark plug was noticeable cleaner than all the others, which all looked the same. However, it did show signs that it is indeed combusting.

We think the head bolt, which I believe goes straight down to the block and near the head gasket, allowed the head gasket to leak.

I don't have a coolant detector, but we are draining the coolant tomorrow to check it out. I think a small amount of water leaked into this cylinder, which combined with low compression, caused a crappy ignition. The untightened, broken head bolt caused the low compression.

So I am buying:

1) Timing chain
2) Timing chain guides
3) 1 x 10mmx155mm head bolt
4) Head Gasket Kit
5) Water pump gasket

We are taking the top end apart tomorrow, starting with the front dressing.


Any suggestions? What else should I look at?

[oscar]

I think you're pretty well on the money about #3.  Whether there's distortion or just that compression escapes as it builds up I dunno, or a combo of both.  Thing is, I'd imagine the gasket's kaput.  But it's going to be interesting to see what you find when you take that head off.  You said something before about a lean condition perhaps in that cylinder.  There's a slight chance of damage to the piston or valves for that cylinder.  Of course, finger's crossed there's none.

I can't really think what's doing with #1.  It's like a constant wet test reading, yet if there was more oil in that cylinder you'd be smoking all the time out the tailpipe.  Would it be coolant?  Anyway, heads off will reveal all.  Good luck man.

[thysonsacclaim]

Is it okay or recommended to replace only the valves in 1 cylinder? Will it not affect the others?

Also, are you suggesting a coolant leak into #1 would cause higher compression?

Its a good thing I caught this early. It doesnt smoke at all, white or otherwise, even at full throttle. However, I had noticed I was losing a little water.

[oscar]

Also, are you suggesting a coolant leak into #1 would cause higher compression?

Sort of, but also asking as I'm not sure if that's what happens.  I was wondering if it's a possibility.  You loosing coolant kind of adds to that suspicion but I'd hasten to add that my first thought was your coolant is most likely sucking into #3.  But maybe there's a leak in #1 as well.

Is it okay or recommended to replace only the valves in 1 cylinder? Will it not affect the others?

Valve operation is independant and replacing one or two or more has no affect on the other valves or cylinders.  New valves only affecting firing of the cylinider they open into.  Important things are to have gap adjusted correctly and the valves to be seated correctly.  I'm assuming that means grinding valves and lapping seats.  No experience here, it's just I've watched a heap of youtube howtos
FWIW, I think the valves in #1 should be ok.  Assuming some fluid, oil or coolant, is blocking the piston rings making them air tight, you're getting this large 210psi constantly.  If your valves in #1 weren't seating properly or they were damaged, you wouldn't get that high for a big leak or you'd hear air escaping via valves.   Your gauge only records a peak pressure and not a slow drop, or rather mine doesn't, but a leakdown test would be beneficial here to show in #1 and #3 the condition of the valves. 

Extreme case for #3 is that the damage I referred to earlier could be burn or pitting holes in the top of the piston or valve/s.  Hence why the low 90psi reading remains unchanged during wet and dry compression test.  However, my feeling is that it's not that bad.  I really think the compression is just escaping via the damaged head gasket and #3 piston and valves will be ok.

[thysonsacclaim]

I meant to say they didn't change much when wet.

Dry, the 90 was about 80 PSI.

What else would cause a high or low reading? Is it possible the rings are bad on the low compression cylinder?

[oscar]

True, not much of a change. 

Is it possible the rings are bad on the low compression cylinder?

Yeah, they might.  But I was under the impression that the oil creates a temporary air tight seal despite what condtion the rings are in, therefore you'd expect a huge jump in PSI if it was rings only.  Way back when I did the red 280 I found #6 to jump from 60 to around 200psi I think.  I really think the compression is escaping back into the coolant channel.

What else would cause a high or low reading?

For a high reading; I'm hoping someone else can confirm the coolant question plus add other thoughts.

For low reading;
Again, worst case scenario, apart from burnt or pitted holes in piston or valves, the only other doomsday thing I can think of is a pitted or cracked cylinder.  With the 6.9 engine in my shed it was confirmed that coolant ingress caused a crack and pitting like a delineation of the cylinder surface.

[thysonsacclaim]

Tomorrow, I am going to run cleaner down through the air intake. I am hoping the high compression cylinder is partly due to carbonization of the walls. We are still going to do some top end work, regardless of the results.

I will then check the compression again to see if there is improvement. I am almost certain the #3 is due to the head bolt and/or the gasket. I do have to be careful as to not dislodge a massive chunk of carbon, which could cause issues. The cleaner is pretty good, and said to cause lots of tiny chunks to come out the exhaust for a while.

The car doesn't seem to run that bad, so I am thinking (hoping) it isn't too serious, such as bottom end stuff. There really is no smoke at all, even if I floor it. Water loss is very minimal

Yeah, they might.  But I was under the impression that the oil creates a temporary air tight seal despite what condtion the rings are in, therefore you'd expect a huge jump in PSI if it was rings only.

You're right. It didn't change much. We used a light oil, check, and then added tiny bit of penetrating lubricant and checked again. So I am thinking the rings aren't the problem.

For a high reading; I'm hoping someone else can confirm the coolant question plus add other thoughts.

I am also thinking the higher compression could be excess fuel in that cylinder, but I'm not sure. the spark plug looked like all the others (except cylinder 3, which was pretty clean). If it were coolant, I'd think it'd be clean also. Oil I would think would be more sooty/oily. Either way, that head gasket is going, just to be safe.

It is possible that the coolant or oil leaking in is somehow not affecting the combustion, but I have no explanation for that.

[thysonsacclaim]

There is definitely coolant leaking in the low compression cylinder. The bolt that went there was the one that had a broken off bolt head. So, I took another head bolt and screwed it in and torqued it down.

Then, I cranked the engine over (all plugs removed and coil lead off). Unscrewed the bolt and voila! White colored oil on the end of the bolt. Then I put everything back together and dumped carbon build up remover down the intake.

My car doesn't normally ever smoke. I have never seen it. Ever. However, upon dumping this in I got quite a bit of soot, black smoke and a big black mark left on the driveway. I am going to check the compression again to see if this clears up the issue with the cylinder that is running too high of compression.

[thysonsacclaim]

I fixed the issue on cylinder #1 which was reading 210 PSI.

I took some heavy duty carbon build up remover and put that down the intake as it was revved to ~3000 RPM. Large bits of soot and dark smoke ensued. Then, we allowed a small amount to sit in the cylinders for about 35 minutes. Started it up, and did a combination of city driving and then took it on the highway to clear out the exhaust more.

I took the compression tests again. Cylinder #1 is now at 155 PSI. Cylinder #3 reads at 95 PSI now. Going to replace the head gasket and see if there are any other issues.

Thank goodness the issue with cylinder #1 was just build up.



Thanks for your help Oscar!

[etmerritt33]

I'm curious as to why you did not do leakdown tests on the questionable cylinders? I had a similar problem on my 6.9 only the variances were not so great. Using the leakdown tester in conjunction with a smoke tester allowed us to figure out with some reasonable degree of confidence where the compression was being lost. I had three cylinders with 120, 130, and 130 with the rest being over 150-155 psi. Differences exceeded MB max of 22 psi. Still have not figured out what caused this to begin with. Doing an engine transplant project right now. Based on my analysis that was the best option but there is some degree of risk with it. Should have the euro donor engine back in the car next week and running.

In my case, we had #7 leaking out the exhaust valve, #8 past the rings, and we didn't leakdown #2 as since it was on the other bank and both heads would need to come off we didn't bother. We tried all kinds of soaks and GM Top Engine Cleaner. Nothing made any difference. Unless you are doing the work yourself this gets expensive fast. At my shop the cost is over $ 3500 just to R&R the heads. Add $ 2K for a basic valve job at the machine shop. We then thought about trying to pull the oil pan with the engine in the car and re-ringing all 8 cylinders. That takes you in the $ 6-7K range with lots of risk. Could get it all back together and really not get the compression on all 8 cylinders back to a satisfactory level. No idea as to how the new rings would mate with the old cylinder walls.

[TJ 450]

Just another point... don't assume all that needs to be replaced when you remove the head is the gasket, as on a recent model. Remembering that these cars are getting on a bit, you may be in for a rude shock.

Think along the lines of the head(s) needing replacement, or at least a full recondition. Every engine I have opened has had rotten heads.

Of course, you may be lucky, in which case you can celebrate.

Tim

[thysonsacclaim]

I'm curious as to why you did not do leakdown tests on the questionable cylinders?

I'm not finished with my tests. The main thing I wanted to find out was if there was a head gasket leak or not. A leak down test is a good idea, however, and I probably will pick up a tester soon. The thing is, the bolt being loose on that cylinder and there being coolant present there pretty much confirms water is leaking from the jacket into chamber.

The high compression cylinder seems to have been an anomaly. But, I will be testing everything more the next few days just to be sure. I won't really know until stuff is apart.

I didn't initially think I should expect any other major issues, either. However, I am questioning more and more. I don't know a machine shop in the state I live in, only my home state (Texas), where I could get it done cheaply or free if I gave them a hand. But... I don't have the car there so if there is anything major that takes me out of my money zone, I will have to part the car out.

Just another point... don't assume all that needs to be replaced when you remove the head is the gasket, as on a recent model. Remembering that these cars are getting on a bit, you may be in for a rude shock.

I'm just being optimistic. I'm worried there is more wrong... I just really hope there isn't. Because if it is too major, I would have to let the car go.

What else should I be on the look out for? I'm thinking I will forgo buying the parts until I actually have it apart. Then I can prioritize the needs.

I wish I had $5k, then I'd just drop a new engine in. If I could buy a reasonably well running engine and drop that in, I'd consider it if it didn't cost too much. Problem is, I have yet to find a W116 where I live at the scrappers.

[thysonsacclaim]

At my shop the cost is over $ 3500 just to R&R the heads. Add $ 2K for a basic valve job at the machine shop.

Hmm... I made some calls today about milling and truing the heads and my quotes ranged from $35 to $100. I'm sure if there was warping or something, it would cost more. Perhaps I didn't ask about the correct thing?

[TJ 450]

The heads shouldn't be warped, unless the engine was overheated.

The main issue is corrosion, and most will have some level of it. It becomes a nightmare when there are huge holes, and welding becomes prohibitively expensive.

There is also no point in removing the head unless you are prepared to do a valve job as well. The guides will almost certainly be worn and the stem seals will need replacing.

This is where the dollars come into play.

Corrosion will be more likely in cars that have done low miles, as they have inevitable been sitting around for some time.

Genuine Mercedes coolant, or equivalent use is arguably critical in preserving these alloy heads.


Of course, this is just my opinion on the matter.
Tim

[thysonsacclaim]

As far as installation and/or removal, I plan to do all myself with the help of my brother, step-father and perhaps our neighbor.

All three are mechanics and far more knowledgeable than myself.

Machining, however, will be done by whatever shop I can find which is reasonable and and good.

Im pulling heads off tomorrow. May post pics.