I have a stock 502 Mag - Gen 6 and it is running fat. Its not a horrible problem, but I end up with a lot of soot on the transom and swim deck.

It appears that the motor is running on the cold side and the ecm is pumping more fuel that whats needed.

Anyone have any ideas? I am going to try a compression test in the next couple days but I thought I would see if anyone had any ideas.

It not the gauge or thermostat either. I had it on a garden hose and tried to cut the water back a little trying to get it to warm up and no change from WOT either. Any ideas?

I am not sure I understand. Its kind of weird???

heard of a lot of them doing that...set well rich from the factory

yea, but it has never been like that before. Just started the last 6 months. At first I thought I had a thermostat stuck open but after a couple thermostats I found that wasnt it.

Have you checked the temp sender for the ecu? I don't have the book with me but you can unplug it and get a resistasnce value for a given temp. Best option is if you have a Rinda or similar to see what looks funky. could be temp or map sensor. It also could the fuel regulator. You can check for a bad one by using a piece of clear tubing in place of the black rubber vacuum hose to see if the regulator ruptured internally. Just a few ideas

Replaced the temp sensor twice with no change and took readings on it. Didnt try the map sensor but also dont have any codes in the ecm either. We had it hooked up to the laptop and everything looks fine except for the temp is low.

I wounder if maybe I have a bad or leaky injector? All of the plugs look the same though? I guess it wouldnt hurt to send the injectors in and have them gone through anyway.

What do you mean the temp is low? What temp does the ECM see? Have you checked fuel pressure? I had a regulator fail and was running 62 psi, talk about rich

I did look at the fuel pressure but I dont remember what is was. I will look again.

I want to say we were seeing temps in the 120 to 130's and couldnt get anything higher on the lake at WOT or with a garden hose. motor still has a circulation pump and not a cross-over. Its just weird???

If you know that the thermostat is good, you changed the temp senders(one for ecm and one for guage), (I would get a laser temp gun on the hot side to be 100% sure myself) the only explanation left is that water is getting past the thermostat somehow. Did you drill holes in the t-stat? did you put it in correctly with the stat on the correct side of the plastic "cup" in the housing? what temp stat are you running?

Carb or EFI/MPI 502?

The carb model uses a 142 degree thermostat where as the EFI/MPI
engine uses a 160 degree. You did not mix these up did you? The
EFI/MPI engine will run rich all day long with the colder thermostat.

Sorry guys, it is an EFI motor with a 160 degree thermostat in it. It is a stock thermostat no holes drilled into the thermostat.

Yes, I have put in a couple thermostats a couple from napa and even merc thermostats with the gasket and oring. I have also used new plastic cup things in it. I did shoot it with a laser and it was only at about 120F.

I cant seem to put my finger on it???