454 Merc Idle Problems

I have a 1998 Cobalt 293 with a mercruiser 454 magnum mpi (efi) with a dual prop bravo 3 out drive. Recently we started having problems when idling, rough idle and once the engine comes up to temp it will stall at idle. Runs great under power, though. Thought it was the IAC valve, as you can hear it working hard to regulate the air being fed into the engine, so we replaced with a new OEM mercruiser IAC valve and the problem still persists. Also installed a new MAT sensor (manifold air temp sensor) as the old one was shot. New plugs, cap, rotor and vacuum line also put on. We tested the throttle position sensor based off instructions online and seems to be putting out the correct volts. It'll idle all day long at the dock (still sounds rough) but as soon as the engine is warmed up and you put it in gear and idle under load it sounds like it starves itself for fuel and stalls. Put it in neutral and it will fire right back up every time no problem, but will stall as soon as you put it back in gear under idle speed. Any ideas on what we should try next? We were thinking the MAP sensor might be the next step but we are open to any suggestions at this point!

 

HI Sarge, thanks for the help! If the fuel pump was failing, would that mean that it would have engine performance issues more than just at idle? It seems to run like a top once it gets under power and up on plane. We were thinking if it was the fuel pump or lines then it would starve out for fuel with more throttle?

 

HI Walt, thanks for weighing in, we appreciate the help! The plugs, cap, and rotor was our first thought along with the fuel water separator, so we went ahead and changed them all. The plugs we took out looked pretty good but the cap did have a bit of corrosion.

 

Had a friend stop up that works at the local marina and hooked it up to their computer, faulty MAP sensor code popped right up. Cleared the code and going to order one tonight. Hopefully a new sensor and we'll be back in business (fingers crossed)!

 

Turns out the map sensor was not the problem, it was unplugged when my husband was chasing wires and that's why it threw the code. Had another run at putting the computer on it, this time while it was on the lake and running. The mechanic said it is only running on 6 cylinders (he thinks 1 and 3 are not firing) and may be a head gasket. Just did a compression test and all cylinders were 140-145, with the exception of cylinder 1. The first time we tested it, it was at 180 and tested it again and it read 160. So we're thinking not a head gasket. Not sure what would cause cylinder 1 would be higher than the rest? Also did a fuel pressure test off the rail and it read 32 and then would drop back down to 18 once the pump kicked off (they did it without the boat running and only turned the key on and off). Any ideas?

 

Thank you for your help, that is definitely something that we will look into. This problem will have to be put on the back burner for a bit as we discovered a ton of metal shavings in the out drive oil this weekend. Tore the thing apart and it appears to be coming from the upper portion somewhere (maybe a shim that got chewed up?). The oil also smelled burnt. Seems like we bought quite the project boat with this one and are now in the market for a new out drive. When it rains, it pours...

 

Well, it's been awhile but I wanted to circle back and give everyone an update. The source of the metal shavings was a thrust washer and some small shims that were chewed up in the outdrive. Luckily all of the gears looked ok and only one or two of the teeth had minor damage that we were able to clean up. We tore the outdrive apart and sent everything we could through the parts washer to get the metal shavings out, ordered up the new thrust washer and shims, and put everything back together in time to make it out for Labor Day weekend. Turns out the stalling issue we were having was a result of the metal in the outdrive binding up and causing the motor to stall at idle. Still have a very minor vibration from the outdrive but we are going to keep an eye on things and make sure that it does not get worse or we don't start seeing more metal in the outdrive oil. We appreciate everyone's help and suggestions!