alter ego

My engine is getting some moderate detonation around 4200rpm. I had a nasty tank of gas that I ran out and I changed the water seperator when I refueled. I put some fuel treatment in and octane boost but every time I get up over that 4000-4200 rpm range I get a ping and have to back off. The engine is a 502mpi with some modifications but no blower. If anyone has any suggestions I would appreciate them. I don't have to run wot all the time but it would be nice to know I could. Thanks in advance.

paradigm shift

There are some really experienced folks here on the board but some times it take a little while. I am no expert on fuel injection but we need more info to help.

Do you know what your fuel pressure is running.
Do you know what your total ignition timing is running.
You mentioned modifications, exactly what was done.
What compression ratio - cam - exhaust are you running?
Water temperature and pressure would be nice to know also.

Electronic Fuel injection is great but for engine modifications it can be tricky but possible. Like I said need more input before much can be recommended.

Vinny P

If the motors didn't detonate before this bad tank of gas, chances are things will return to normal after this tank of fuel is used up. If you are confident that the fuel is bad, just use it up at a nominal speed and rpm, run it down low then fill up with fresh good fuel. Change the filters and give it a run.. Hopefully things will return to normal.

alter ego

Thanks for your input, I am starting a new tank with an additive cleaner and I will try it this weekend. I have been running it down and adding fresh fuel so the bad gas should be almost diluted to a minimum. The timing and fuel pressure are my next check but it seemed to run fine until this tank. I didn't do the modifications myself, I bought the boat last fall with them done and I am researching exactly what is done. I never received a grind sheet for the cam but from the idle it definitely isn't stock. I have Gil offshore manifolds going to thru hulls right on the waterline. I also just switched to closed cooling so it runs warmer than it did before. About 175 degrees compared to a little over 160 degrees. This made me think it may be a temp issue but 175 is pretty normal from what I know. I just don't want to hurt anything with this fuel if it is the problem. The fuel pressure is adjustable and was running just under 40 last time I checked it. I am afraid my injectors are getting varnished and that would also effect the spray pattern so the fuel doesn't atomize correctly and causes more of what I'm trying to get rid of. So far it seems like a problem that if a couple of fresh tanks don't remedy I could be chasing it for a while. I'll hopefully have it running back to normal soon. any other ideas would be appreciated. Thanks!!

Vinny P

I would not worry about the temps. I run closed cooling as well. The temperature is always about 175*, never a problem..

paradigm shift

YEP get that gas out! I thought you had already ran it out and changed filters so miss read. If it ran OK prior it is probably just the gas. 40 degrees total if it is correct on timing is up there. You might try pulling some timing out of it till you get things sorted out. It might help with the pinging. What octane gas?

alter ego

Only gas available around here is 89 at the docks. I put octane boost in but I havn't gotten a good run in yet.

mcollinstn

Unless the guy raised your compression, 89 should be just fine.

Stock compression is fine with 87. Stock compression and a hotter cam will be even "safer" on 87.

Just put gas-dri in it and some fuel system cleaner. You'll need to run that tank thru it pretty soon so the alcohol in the gas-dri doesn't stay in the hoses too long (makes them brittle).

bobl

The knock sensor should be picking up the Knock and retarding timing. Put a scan tool on it and make sure everything is working correctly. You should not be "hearing" it ping, even with bad gas.

paradigm shift

DOH! I hate it when I forget to check the obvious first. Yea make sure knock sensor is there and working.

bobl do all 502mpi motors have knock sensors in stock form? Nice catch and actually I do not know if I would hear mine ping with the engine hatch shut. But my exhaust is not very quite.