Hello all

See if I can get some input on what to check and how to move forward. I have a 1999 Mercruiser 502 Mag MPI. Serial Number OL352200. Boat has twin 502. This is the port motor issue. Here is what is going on and would love some help on things to look for. I am thinking Timing chain but want to see what else to look for:


Wednesday, put the boat in. Some issues on that port motor but found it was a electric solenoid that the wire came off. Put the wire on and it ran. Went for a small ride and then put it on the mooring and motor seemed fine.

Saturday got to the boat and the motor didn't start. All day tried getting it started and nothing. Here is what I tried:

- Fuel is getting to the fuel rail. I didn't get the pressure but it is getting up there.

- Sprayed starter fluid in the intake and nothing at all.

- Spark was weak. Changed cap and router and spark got better.

- Pulled #2 plug out and was wet (getting fuel) and is sparking.

- Put my thumb on #2 and pressure was there, wasn't much

- Noticed that wasn't much suction on that when I put my finger over the hole.

- Noticed not much suction at the intake.

- Put the motor to TDC and noticed the distributor wasn't at TDC. Put the starboard motor in same position and that distributor was correct (and different than port motor).

This leads to me to leave the timing chain jumped or teeth are gone.


So I am getting fuel and spark, have compression (didn't put a gauge on it yet). I am thinking timing chain jumped but wanted to see before I pulled and check anything else anyone recommends. Please let me know your thoughts on this before I dig deep into this motor.



Also- I believe I can get to the timing chain without pulling the motor. There is plenty of space in the engine compartment but wasn't sure about the oil pan. Heard I might have to drop it a inch or so, but I didn't think I had to. Thoughts?

There are 2 TDC’s. One for compression stroke and one for exhaust stroke. You want to be checking rotor position on compression stroke tdc

i haver never seen a timing chain slip/skip a gear on a steel gear.

So it is either TDC for compression or exhaust. Even still the rotor position would be 180 of the compression. And right now it is like 90 degree off. The rotor position is so far off it is telling me chain is off. Unless someone has another idea?
also the fact there isn’t much suction and compression makes me think it….I think.

Your serial number comes up as 454 MPI

No for the most part. The piston does come up where the notch on the vibration damper shows TDC, timing is done on TDC of the compression stroke. You have a 4 stroke motor, so there is TDC on exhaust stroke and a TDC on compression

Right now it sounds like your going off the deep end and the troubleshooting process is haphazard.

Fuel pressure is low, so find out why (should be 43 psi)?
Turn key ON, does the fuel pump run for a couple seconds?

If you are using mark on balancer it could have spun on the rubber hub. Seen this lots of times over the years. Take spark plug out. When it’s at TDC you can feel top of piston

Ya I don't want to go too deep without getting other ideas.
I will check the fuel pressure. The pump is going on and running like it should. I sprayed Starter fluid in the intake and stillnothing. Plugs are wet as well with fuel. Seems to be getting fuel, but Ill get the PSI.
I will check the compression but from initial feeling it seems like there isn't much. This making me wonder if the timing chain is off. The motor ran fine. Put her to bed and came back 2 days later to 0 starting. Nothing at all. Cranks over fine too.

Ill double check the TDC but if I spun the hub, it wouldn't effect the timing. It would effect me timing the motor, but prior to this the motor was dialed in fine and running fine. I think spinning the rubber hub would throw off my marking, which is fine, but it wouldn't throw off the motor timing. I never touched the timing.

Don't see it being the timing chain. If the cam was not turning, the piston would make contact with the valve

The tach should show at least 300 rpm while cranking. If it doesn't then the distributor is not firing. The ECM must see there is spark before fuel injectors are turned on

tach is not moving at all during cranking. The running motor, the tach doesn’t move till started.
I have spark when cranking at the spark plug. I am getting spark on all cylinders.

spark plugs are wet so fuel is getting there.

Possibly worn dist gear and/or cam gear is much more likely than a timing chain issue.