502 Question

[Jim]

I had some engine trouble this weekend and after doing some
checking I have zero compression in Cyl. one. I'm going to pull the valve cover off before I pull the engine to make sure I didn't have something happen with a rocker.

My real qestion is this, if I do have a hole in the piston or a bent valve should I just replace it with stock parts. I haven't seen too much on upgrading 502 with fuel injection. I already planned on doing the exhaust this winter so besides that is it possible to go with a little more aggressive cam and set of heads? I have seen some threads were people actually go back to carbs when they have built their engines. Can you reprogram and go with bigger injectors?

I dont' want to jump the gun here since I might also have a blown headgasket but I was running about 3800 when I lost the cyl. and it ended up heating up pretty good once I started to skip (now I need to see why the alarm didn't sound either when the temp started to climb). I have a feeling I will have to at least have the heads checked and then it will probably go from there.

Thanks Jim
By the way its a 2000 26ZX with a 502 with about 120 hours

[RickR]

I would suspect a bent valve stem or a 1/4 moon chip on valve.

To diagnose your cylinder problem.
If your compression tester has a hose with a air chuck fitting, remove the air valve at the spark plug end.
MAKE SURE THE CYLINDER YOU ARE CHECKING IS AT BOTTOM DEAD CENTER
Insert in spark plug hole and attach the other end to your compressor air (about 100#)
Air out exhaust-exhaust valve
Air out intake- intake valve
Blow by out valve covers-piston :(

Use Manely Iconel for exhaust or Severe duty for intake.


OE Pistons are made by JE/SRP #123475 (New pistons have shallow oil ring lands) Call and see if that is a dircet replacemant for yours. If ring lands are different from your 502 (and you are only going to replace one) I would balance rotating assembly. http://www.jepistons.com/

Pistons and valves available @
Competition Products http://www.competitionproducts.com/

[jeffnstef]

What if the cooling system is bubbling ?

[RickR]

jeffnstef
Cooling system bubbling???
I would suspect
Head gasket or
:( block or head rusted through

[Jim]

Thanks for the info, eitherway the head will have to come off.
My concern is how hot it got once I lost the cyl, not sure if it was already like that but my black flex house that connects my exhaust tips to the exhaust logs is soft now from being over heated. My alarm didn't go off either which I have to check that out as well, I did get a bunch of smoke out the exhaust right before I shut it down which could mean the exhaust valve is stuck open I guess.

I probably should just take the motor out and take both heads off to have them sent out and checked, hate to just work on the one head and then find out I have a warped head on the other side. I did get good compression on the other side though 130 -140, is this where is should of been?

Thanks Jim
PS. Sea Tow will be getting me to the boat ramp this weekend which should make it a little easier to work on once I have it home.

[BUIZILLA]

For sure remove both heads for COMPLETE inspection. I believe the 502 has a head gasket misery history, maybe due to delamination issues... it seems like lack of usage is more of a culprit on these 502's, than mega usage.
depending on where the temp sender is, it is very possible to get steam from that side, and not have the alarm go off... unless you have senders in BOTH heads, that tie into a common alarm trigger.
as long as the oil didn't milkshake, consider it a blessing..

JH

[Jim]

JH,
The oil looked good but the rubber house you could see actually started to ouze(sp) a little. I know the previous owner had some heat issues before I got it so I'm not sure when it really happened. I did have a smell once I opened the engine hatch that I think was the rubber melting a little.

It wouldn't run after I shut it down but the next day it would start but run real rough with only having 7 cyl firing. The good side of the engine actually gets much warmer then the bad side which must be from carrying that extra cyl. but thats why I think I'm just going to rip the top side of the engine down.

Thanks for the input.

[BUIZILLA]

Once you get it apart, send me the 8 injectors. I have a complete flow bench for them,
and we'll make sure that hole didn't go lean...

JH

[gold-n-rod]

Since the previous owner had a heating/cooling "issue" and you have smelled the signs of trouble, I'd say you blew a head gasket first. One cylinder filled with water, hydrolocked and bent/broke a valve.

Just my 2 cents.

another Randy

[Cuda]

In my experience, no compression is always a valve issue. I've gotten "some" compression with blown head gaskets, torn up pistons and such, but if I got 0 compression, it has always been a valve problem.

[Cuda]

Pull the valve covers and run the engine. Look at the rockers to see if they are all moving up and down the same distance.

[BUIZILLA]

JIm, I seriously doubt you've bent a valve, or hydrolocked the cylinder. The 502 is notorious for bad hemmorage gasket arenas, resulting in zero, or near zero compression. Particular areas to observe would be the ends and the top 12 o'clock of any one hole. It's obviously adjacent to a LARGE water port however, resulting in the steam. If your theatre of failure activity was salt, time is of the essence in preserving the integrity of that specific cylinder.

JH

[MOP]

Once you get it apart, send me the 8 injectors. I have a complete flow bench for them, and we'll make sure that hole didn't go lean...

Jim not knowing much about the newer systems I thought the new computer controlled stuff automaticly went into a limp mode if an injector went down.

[BUIZILLA]

Jim not knowing much about the newer systems I thought the new computer controlled stuff automaticly went into a limp mode if an injector went down.Not to say it can't, but I have yet to see a Merc ECM recognize (code to limp) a specific performance fault in that direction for just a weak injector.

[Jim]

Thanks for all the input and JH I might take you up on your offer. I have a friend that's a marine mechanic and he suggested to check the rocker and mentioned that he has seen the stud that the rocker is bolted to actually pull out
or that the rocker slipped of the valve or push rod. I don't hear and type of noise when I run the engine (just skipping) so I'm thinking its a whole in the piston, bad headgasket or a bent valve stuck partially open (not enough to hit the head). Luckly I was in fresh water when it happened but the steam/smoke makes me nervous. When I run it now It seems like I'm almost getting a little blue smoke out the good side of the motor, not sure what the computer is doing with the injectors since I have one cyl. out.

Before I had taken delivery of the boat in the spring I had the previous owner replace the exhuast risers and gaskets, not saying the exhaust log gasket didn't go but I would hope I would just get a year out of the new gasket. I had the work done due to some external water leakage and I had the mechanic then check the heads to make sure I haden't ingested any water especially since it was being used in salt water. This was one reason I wanted the new stainless exhaust since that gasket is so dam thin where the water chambers are on the exhaust manifold and where the Exhaust Log bolts on. Hopefully I said that right :)

Once I get it home I will pull the valve cover off and see what is going on but if I only had to do the head gasket or a valve Job would you try to leave the engine in the boat?. My point is I'm not sure If I'm just wasting time here or should just pull the motor and get the top end ripped down. I guess it can't hurt to at least identify the problem and if its a bent valve with the rocker still on it should be pretty easy to see. I just don't know if I can get the valve cover off without taking the exhaust manifold off.

Thanks Jim
PS. Shame on me for not doing a compression test on the boat when I got it but I'm wondering if I had an issue with this cylinder and that's why I seemed to not be able to get the rpm's that some of the other 26ZX owners were getting.