454 compression test question
 

The question is for a 38 foot Bertram motor yacht I put a deposit on.... its no performance boat but would like opinion from the engine gurus.

The boat was repowered in 1997 and the 454 330hp mercruisers have roughly 900 hours on them.

The boat is in heated indoor storage so the compression test was done on cold motor.

All cylinders are 125psi except cylinders #5 and #6 which are 95psi.....both port and starboard motors have identical readings.... all 125 except the #5 and #6 cylinders. The motors do sit on a slight angle due to them being inboard and straight shaft drives. Are the 5 and 6 cylinders the richest running ones resulting in more rings wash from more gas? The boat is pending a sea trial and the owners says boat runs great. The owner is a friend the family so he is credible.

Would like opinions.

just my guess, but i would think they would compress closer to 145-150.
so all 125 is simply because they are not new obviously, the other 2 lower, either less new, or a bit tired, but might also be just ''carbonized''. maybe a magic cleaner in bottle could help, might be too dirty on a valve to close properly... Should do a leak down test to see where the leak goes, or add some oil in cylinder to see if the ring would seal more. if so, ring is the problem, if not, ... head gasket? valve? or...

might work like that for years cruising around a slow speed !

At 900 hrs, their in need of work even if they run fine, as been said, all the compression is down but two are worse than the others.

I would try another compression gauge first to make sure it isn't a gauge issue. I had a gauge that would read a good 20 psi less than others. Damn Matco junk.

Wish I had that kind of luck. There is something to be said though about those 454/330's. regardless time for a rebuild. probably running on borrowed time. Does seem rather odd both engines are exactly the same. Your not having problems getting the compression tester in all the way on those those cyl are you?

As mentioned above leak down would help but I seriously think it's time to pull em.

150 pds is normal I believe.

before you get all worked up and pulling stuff apart ,Why don't you service motors and run the boat..JMO
Went back and saw it was subject to sea trail ..Although 900 hours is a pretty good bit as has been said ,if you just cruise they could go for years

I was just in ft Meyers on a pairasail boat. The motor had a 454 bravo 3. It lasted 3000 hours.

did you have the throttle wide open? if not you will get a bit lower numbers

Budget for a repower, but you have two engines, so run it until one breaks. You might get ten years of free money, lol. At least you know they run. No guarantee with new motors.

I was going to post same thing.

the 454s in my sea ray cruiser had 1200+hours on them before rebuild but, one of them would loose oil pressure after about 10 minutes run time.ended up being a cracked crank that wiped out the center 3 main bearings.the valve guides wear and the valve leaks at the seat when they get a ton of hours and i,d bet that is what is causing the low cylinders.

The batteries were fully charged and all the plugs were out but the throttle blades were closed. Going back tomorrow with a leakdown tester and will try with a throttles open... seems very strange all cylinders are same except 5 and 6 on both motors... did it three times and numbers are same.

Those cylinders are on the back side of center riser exhaust and may see some reversion. Probably need a valve job. If taken care of and not ran hard the bottom ends may be fine.

I wouldn't bother opening the throttles. Did you notice any rpm difference last few times out last summer/fall?

I would open the throttle up maybe the number will come up a bit. Think he is looking to buy this boat.

Never thought of that but good point.

Yes I'm looking to buy.... have deposit just being thorough on inspection before sea trial.....I will do the leakdown and redo then low cylinders tomorrow with blades open. A warm motor may seal up another 10 psi. As someone mentioned, its just a cruiser and will only see 3000-3200 rpm...

True. May come up a little but with that said I'd make sure the choke isn't closed either. I've seen that lower some numbers but doubt 5 and 6 will be within 10 % of the others.

Has it always been a fresh water boat?

How much difference can you expect from the compression test being done on a cold engine vs a warm engine?

ok, whatddya mean by repower? new engines? or new rings and rod bearings? whose engines? meaning whose exhausts are on the engines? how many hours on the exhausts? my hatteras had new Indmars put in it in 92 or 93. bought it around 03 to 05 and the exhausts with 400 hours were rotted away even though manifolds had antifreeze in them. yeah mine was salt it's whole life, but lake Michigan has no corrosion inhibitors in it. I would budget for all new manifolds and risers. oh yeah; fuel tanks. still orig fiberglass tanks? if so, they gotta go. the ethanol eats the resin out of the glass and it acts like sugar in the gas tank and makes the valves stick in the guides.

Opening throttle won't do anything for the number other make it come up slightly quicker. It pumps what it pumps. If it has been sitting I would run it first then redo comp check. IMO it can not be accurately done on a motor that has been sitting.

It can vary a little from engine to engine depending on internal parts however 5-10%. Test should be done with engine warmed up to near operating temps however when done cold sometimes any known problems may exaggerate and show lessor comp. So some may argue its best to do cold however most mechanicss will perform when warm.

Well...just did the leakdown....all the cylinders at 125psi were at 20%...all 4 cylinders at 95psi were at 70% and the air was escaping through the carb...meaning intake valves.

Hopefully just pulling heads and a valve job would fix things. Again the tests were done cold. I redid the compression tests with two different gauges both 90-95psi on the same 4 cylinders. Maybe the one who posted about the center riser manifolds is correct about slight reversion.

Supposedly the boat still runs good...so we will see. Not a deal breaker if the price is adjusted to compensate for work.

I wouldn't be surprised that with fresh heads and fixing the manifold situation that you'd be good for another 900 hours.

yeap, needs valve job now, as said before, do that and good for a long time as long as was maintained well, about those hours heads need work

unless he has recent receipts, replace the exhaust manifolds...

the 330 merc was what like 7.9:1 static compression? 125psi is probably within range

Thanks for all the input so far.... no receipts for manifolds but saw notes in a log book for rust on the manifolds and a receipt for riser gaskets. Sucks buying boat knowing it needs works..... if the sea trail goes ok... may use it for a short Michigan summer and do the heads next winter.... if it comes pulling motor not fun in the style Bertram (flush deck motor yacht)... motors are mid shop under the salon floor, headliner has be removed and 6x6 panel removed from cabin top and motors hoisted out.

Per mercruiser recommended ranges and test procedures regardless what many might not know or think. Just putting it out for info.

http://www.boatfix.com/merc/bullet/97/97_25.pdf

Well at least it's better knowing going into rather than hitting you unexpectedly and hopefully the sale price will warrant such to be done. Just keep in mind IF it's a salt water boat and given its 5 and 6 on each engine with the lower compression just be prepared for the worse and hope for the best. Reversion seems unlikely in a boat like that with 330's however I just saw a set of heads that came off salt water boat with reversion issues and didn't look pretty. Just a thought but maybe post an ad in wanted section for 330 peanut port heads. there use to be a a lot of take offs and they sit. I had to sets I just gave away with 5 hours on them. Not only that in a weekend you could have old ones off and new ones on. I just have to see you take out a piston or cylinder wall from a potential broken valve. Its your decision of course but at least think about it anyway.

Almost forgot....I think they are 454 340hp mercs....I'll have search difference between the 330hp I thought it was and the 340hp they are.

Good info. Makes me wonder how I've managed all these years. Just playin.. No matter how or what way I've done them in the past usually when a cylinder or two gets red flagged it turns out I'm I'm wrenching regardless.

They should be the same, the bobtail inboard motors were 340 horse and the stern drives 330 horse

Mercruiser in the service manual for 1993 thru 1997 big block 454 and 502 listed the compression pressure at 150 psi for all black production side engines. Then in 1999 Merc listed the SB as I already posted as they wanted a min of 100 psi and no more than a 30 % difference between the lowest and highest psi cylinders.

With that said many marine techs go by a 15 % to 20 %. from the highest to the lowest cylinders.

That's surprising. Not doubting it at all just surprising especially with an expected engine running for long periods time at higher rpm. So in this case here at 24% they'd without a doubt further diagnose.

I was just going off general engine knowledge. I know the manual says 150psi, but it also says 8.0:1 static in one section, and 8.5:1 in another section, for the 330.

The 330 engine was the L19 GM shorblock, with peanut port heads. The only real difference was the cam, was larger than the truck engine's cam, longer duration. That engine was 7.9:1 static. I don't see how they would get 150psi cranking with that engine combo. I had a couple of those engines in trucks over the years, and 130psi on the starter was the norm, and that was with a really short towing camshaft they had, shorter than the 330 merc camshaft.

The higher compression "mag " engines, with the dome pistons, would crank closer to 150ish.

If a motor cranks with 100psi across the board, she's tired haha. I do agree, the 330s I've compression tested were about 130 in good running condition.

everything depends on what it was repowered with. in 97, it could have been a round port 8 to 1 motor or L29-ish oval port 9 to 1 motor. and were they bought complete new as a package from merc or crusader et al, or rebuilt by bob's engines with whatever bob may have had laying around...

I agree on everyone about the numbers and some versions being closer to 150 and others closer 130...heck my 67 pontiac has 13to1 comp motor and cranks at 240psi with big roller cam. I'm just confused and trying figure out why the 5 and 6 cylinders and both engines are 95 when all rest are acceptable.

Dereknkathy...you mentioned the fiberglass tanks. Yes there still in there and that was the one thing I planned replace and how I got the sale price to 30k for a very clean and updated bertram. The prior owner had it 8 years and has never run ethenol in the tanks. The majority of the gas docks in my area are non ethenol.

I spoke second owner who owned boat from 1977 until 2000. The repower was takeouts from a new Chris craft charter boat his friend had. The gas engines were used slightly for one season and taken out to put diesels in. The second owner bought the entire gas setup and installed it in the bertram.

If the heads need come off I can deal with that but even at 30k for nice solid bertram if the engines need come out may have reconsider or pass if the seller doesn't want drop price due to low compression.

the leakdown test already showed you it was valves. and there are SOOOOO many sets of peanut heads laying around. I am not sure if I have 1 set or 2 right now, plus a couple sets of l29 heads. and that is just 1 guy on here. and no I am not trying to sell them to you. OK, so they are merc engines, exhausts. start shopping for 4 of these. they are no longer available new. http://www.ebay.com/itm/MERCRUISER-5...dW1jFm&vxp=mtr I assume closed cooling system? are the exhaust manifolds antifreeze or raw water cooled? you are still not sure which engines you have. gen5 round port or gen 6 oval port. how many bolts on the timing covers? 6 or 10?