Mercruiser 260 5.7L in the shop for carb overhaul - got bad news about compression

[Blueghost924]

My boat is in the shop getting a carb overhaul and the stern drive reworked (they replaced the Alpha 1 shift spool). The shop technician called and said that the engine idled rough, and he said over the phone that 3 of the 8 cylinders are exhibiting low compression. He said the other 5 have almost no compression. He seemed in a rush, and didn't give me the exact compression numbers, which I'll call back tomorrow and press for. He mentioned real quick that as a minimum, he will have to remove the valve covers and watch how it operates from there to determine if there's something obvious. Makes sense to me.

My concern is that I may be looking at some extensive engine surgery to determine why 5 of the 8 cylinders have almost no compression. I'm going to authorise the tech to remove the valve covers and see if that reveals anything. Beyond that, any recommendations? Out of curiosity, how easy is it to swap and replace the head assemblies in these 260 5.7L V8 engines (if it is that issue)?

 

[Bt Doctur]

4 - 5 hours

 

[Scott Danforth]

I would do your own compression test. My experience is motors do not run with almost no compression on 5 cylinders

 

[Blueghost924]

Yes thanks Scott - it's a little odd for a shop to tell you verbally over the phone that 5 cylinders have almost no compression and the other 3 have low without giving you actual numbers...unless they're trying to just give a quick update. Couple of other things I was told - the tech said that the intake manifold was an aluminum manifold, that it's not stock and may cause valve corrosion issues over time. The other thing I was told is that he had an almost impossible time getting the motor to idle below 1200. That's with a new rebuilt carb installed and an overhauled sterndrive with proper alignment (he checked and adjusted alignment as well).

The most frustrating part is the lack of communication with me (the customer). Problems come up...I understand. But to go 3-4 days with no phone call to the customer...even to give a quick 5 minute update...is shabby. If a shop is overwhelmed with work - I understand that too, but just give me a quick call and tell me! I think I need to drive down there and perform a compression test myself with my gauge...or watch him perform it. It is my boat after all.

 

[hemi rt]

A V8 with zero compression on 5 cylinders and very low on the other 3 would not even start, something is wrong here. Also when you have low compression on any cylinders a good mechanic will do a wet compression test which will tell you whether it is the top end or the bottom end. How was it running before you took it in?

 

[Fishermark]

... Couple of other things I was told - the tech said that the intake manifold was an aluminum manifold, that it's not stock and may cause valve corrosion issues over time.....

Cannot see how an aluminum manifold would cause valve corrosion issues over time. The manifold itself can corrode and cause some problems... but not necessarily the valves.

 

[NHGuy]

Yup, it's high season for him. He's doing fast profitable stuff and keeping the most people happy. Boat shops don't do engine stuff quick at the end of July. They do faster stuff.
Get the boat back. Maybe tell him you want to think things over. And do your own compression test, dry then wet.

 

[Blueghost924]

Aye, thanks for the responses, and I think it will be a good thing to pick up the boat this Saturday. Yes, he mentioned quickly on the phone that the 3 cylinders were about 80psi, and the other 5 were zero. 0. NH Guy - what you said hits the nail on the head...he has other high level projects that they want to turn around - I saw lots of outboard boats there too (they are PRICY).

Before I brought it in, it would start and actually run. Not very well, but it would run. I don't want to be the untrusting type, but I'm not into letting a shop turn my boat into a cash cow.

 

[hemi rt]

All that taking the valve covers off will tell you is whether the valves are moving, won't really tell you if they are closing, that's what the dry/wet compression test will tell. Take it home, use a screw in compression tester and do a compression test. The engine should be up to operating temp to do it properly but if you can't get it started the compression tests will still tell you what is wrong. I'd like to know how he got it started with zero compression an 5 cylinders though.

 

[NHGuy]

duplicate

 

[NHGuy]

Don't forget to do your compression tests with the throttle open, spark plugs out and ignition off.

 

[Pete104]

So, they did the shift spool & carb overhaul and now you have a compression issue!? Pay with a VISA card.

 

[Blueghost924]

Well, I did get the boat back yesterday evening and last night did a quick check of compression. The tests are going to be somewhat "flawed"....or not the most accurate because I forgot that I have the neutral safety interlock, and couldn't figure out a way to bypass around it. So, the quick compression test I did last night was with the throttle at neutral (with all plugs out), but here's the numbers:

2- 150 psi 1- 0
4 - 135 psi 3- 138 psi
6- 0 5- 0
8- 0 7- 0

So...looks like the shop was correct, but didn't quite give me the entire story or details. With those 3 cylinders having decent compression, I guess it's possible to have the engine run - but run cruddy. I was tempted to put the boat in the local river and just see what it would do right next to the dock, but I'm thinking that may not be a good idea. Should I even try to start it here on the trailer at my house?

Time for diagnosis - and to keep with the spirit of the forum, start a new thread soon.

 

[boatman37]

have the valves been adjusted? maybe they are too tight and not closing. shouldn't have changed by theirselves but if somebody tried adjusting them and messed them up that could be a simple fix. worth checking into

 

[Oshkosh1]

There's more to this story...with those numbers either someone screwed with it, it overheated, or something catastrophic happened like water intrusion. Did you just buy the boat?

 

[Oshkosh1]

With a solid lifter/cam follower tappet the valve clearance is more crucial. With hydraulic lifters you'd REALLY have to screw the pooch to set them so tight as to cause that sort of catastrophic loss of compression. They've got a lot of "room" for variance. As a matter of practice...most of the time you simply set them by feel and ear rather than a feeler. It's not that unless someone just monkeyed them tight. But at that point...the thing probably wouldn't turn over and certainly wouldn't run.

 

[Blueghost924]

Yes - I just bought the boat 2 months ago, so I'm learning all the little quirks about it. Water intrusion may have been an issue, but I won't know until I tear it down. I know that the previous owner said that him and his friend tried to overhaul the carb and screwed that up big time, so I really want to remove the port valve cover and see if I can tell anything simple from there.

 

[Oshkosh1]

Yes - I just bought the boat 2 months ago, so I'm learning all the little quirks about it. Water intrusion may have been an issue, but I won't know until I tear it down. I know that the previous owner said that him and his friend tried to overhaul the carb and screwed that up big time, so I really want to remove the port valve cover and see if I can tell anything simple from there.

I don't think he told you the whole story...0 compression on an entire bank says at least a head gasket and/or a cracked head. You need to do a cylinder leak down test. Don't bother with a tester just find a fitting to screw into the plug well and put a little pressure into it. You should see right away if it's not holding. If it overheated, it could have blown the port side gasket(explaining the entire loss on that side) and it could have burnt the exhaust valve on #1(explaining why the rest of that side has acceptable numbers). However...everything(including this) is just a pretty wild guess without more info and testing. Just maybe a direction to start looking toward. Probably not lower end though...unless he ran it out of oil or something REAL bad. Ensure the pistons are actually moving up and down in their cylinders as well...everything at this point is on the table.

 

[HT32BSX115]

Yes - I just bought the boat 2 months ago, so I'm learning all the little quirks about it.

Howdy,

I might have missed it but, Did this engine EVER run right since you have had it? And if it doesn't or didn't, you might consider just cutting your losses and find a long block so you can at least get it on the water for AUG.

A good choice would be to go to a wrecking yard and finding a similar year low miles pickup truck engine and swap all the marine "stuff" over. With your current engine, you just might be going down a "rabbit hole" that ends up costing a LOT more time and money.

Regards,


Rick

 

[Rick Stephens]

I like Rick's thoughts here. Myself, it would be about an hour and I'd be tearing the motor down on an engine stand to see what was wrong and what I ought to replace. I'm waaay too cheap to just jump on a replacement motor without I tear this one down and inspect everything. A leakdown test before teardown is very smart as it will tell you what to expect when you open it up. That also can be done a lot easier on an engine stand.