502 low compression
 

I have a 00 32 Sunsation with twin bone stock 502mpi 190hrs. on engines. When I purchased the boat 9 months ago it had 120hrs. I had it in the shop to have the engines checked over and a compression check came back with rather bleak findings!

Starboard Eng.
cyl. 1 100psi cyl. 2 105psi
cyl. 3 100psi cyl. 4 100psi
cyl. 5 85psi cyl. 6 95psi
cyl. 7 110psi cyl. 8 110psi

Port Eng.
cyl. 1 120psi cyl. 2 50psi
cyl. 3 115psi cyl. 4 90psi
cyl. 5 100psi cyl. 6 65psi
cyl. 7 115psi cyl. 8 60psi

I had reservations about the findings so when I brought the boat home I had to check for myself. I performed a dry and wet compression test and had the same results. I was hoping their gauge was inacurate, but no such luck! The wet test did not change the results....
The bizzarre thing is the engines appear to run good, no miss, good idle, pulls 5k rpm @ WOT, Both engines use 1 qt. after about 10-12 hrs of use, oil pressure 30psi hot idle. It seems to me that with compression like this it should hardly run.....Any thoughts on most probable cause? Thanks in advance.

Re: 502 low compression
 

More than likely you have a bad head gasket on the port even# cylinders.

Overall, I'd say all the compression #'s are weak.

Have a leak down test done to determine where you're loosing compression. Most likely, the engines both need valve jobs. All those #'s are low, especially with that low of hours. Have you ever had the ecm's scanned to verify the hours. It will tell you how many hours and at what rpms they have run, if they've been overheated, or banged the rev limiters a lot.

Re: 502 low compression
 

pull the heads ....good valve job ...maybe add roller rockers ....new head gaskets ..... zoom zoom .....

however .... Griff is correct ... leakdown first ....

Re: 502 low compression
 

I appreciate the advise...I will do a leak down before a tear down...And hopefully the problem remains on the top end! Typically when a head gasket blows will it have heating - cooling problems or erratic temp gauge? Temp gauge seems steady around 170.......Thanks again!

Re: 502 low compression
 

eratic temps.... can be steam pockets ....maybe due to bad sea pump ..hense not enough cooling ....hense excessive heat .... and a blown head gasket ....might have started the hole process ....m

Re: 502 low compression
 

be sure to do your comp check on a warm motor (preferably warm after being up to operating temp on the cooling down side of the curve). Make sure you have the throttle plates wide open when you do the compression test.

I'd love to see a re-test. That's low hours for any sort of issue like this.
Unless it spent some time underwater..

Re: 502 low compression
 

Chances are the head gaskets are bad between the cylinders--the thinnest point and where they get the hottest. They are more than likely leaking from one cylinder into the other.

If the head gasket was bad where there is water circulation, you would know it real fast. You would have lots of steam and water in the oil.

Re: 502 low compression
 

The cylinder leakage test did determine a valve problem....I pulled the heads on the port motor and the cylinders appear to be in good shape. I pulled a couple of exhaust valves and it appeared that the sealing surface was toward the leading edge of the valve, not centered in the face of the valve. It also appeared that on 2/3rds of the valve was seating properly. I reassembled heads and took them to a reputable performace engine builder to go thru them....I recieved a call from the builder about a week later and was told that the heads had 454 exhaust valves in it!!! On top of that he said one side had Manley 454 exhaust valves with other side having stock 454 exhaust valves. He said the intake was correct and they where 502 heads... Has anyone ever heard of this happening before? The original owner of the boat bought the boat with 10hrs as a demo and requested 502mpi's installed before he would purchase the boat...So I am assuming that some sloppy reman motors was installed or at least heads anyway!!! The builder installed the correct valves and performed and performed a 3 way cut on the valve and seats.. He states that this process seals better than stock. So if I am living right all might be ok! Can anyone give me the proper torque on the cylinder heads and should the head bolts be replaced. Thanks everyone for your help...

Re: 502 low compression
 

first rotation 30lbs ...Second rotation 59lbs ...third rotation 89lbs on short bolts and 92lbs on the long ones ...

I am not sure about replacing em .. ARP's to be sure if you do ...but I'm sure someone will have imput ......m

Re: 502 low compression
 

ARP bolts with ARP lube. 65 lbs. do it in 3 steps
25
45
65

or ARP bolts with 30w oil. 85 lbs 3 steps

35
60
85

Re: 502 low compression
 

Well, I don't know what the guy doing your heads is talking about. You can't say valves are for 454 or 502. If they are Merc rectangular port heads from Merc black engines it makes no difference if they are from a 454 or 502. The heads and valves are the same. The intakes should be 2.19 and exhaust are 1.88. some Blue engines use 2.25 intakes.

Head bolt torque depends on the gaskets, bolts and lubricant used on the bolts. The head bolts also need to be torqued in a SPECIFIC ORDER!!!!!!! You probably also need to use a sealer on the head bolts as many go into water jackets. I would get new ARP bolts and there sealer and follow their torque recommendations. I don't remember the specifics on the torque#'s but ARP's are lower because of the lubricant sealer used.

Re: 502 low compression
 

Looks like duke posted as I was typing.

Re: 502 low compression
 

Here is the bolt torque sequence to get proper sealing.

Re: 502 low compression
 

I'm with you. That raised my eyes/ears.

I'm hoping that was a case of what was 'thought what he (mechanic) said" vs this "is exactly what he said."

BTW: 502's have blind bolt holes, meaning they do not run into water. Bolt sealer not needed nor recommended. Use proper lube or oil (depending on bolts used) while also making sure that the bolts and bolt holes are as clean as possible.

BTW #2 - you may want to check the cyl head surface and block surface for straightness while you can. No reason for putting thi stuff back together if they won't seal well.

Re: 502 low compression
 

I'd make sure they check the valve guides carefully, valve seating problems are sometimes caused by badly worn guides.