Hi All, I am new to iboats and hoping that you all might be able to help me out. I have a 1998 Cobalt 293 with a Mercruiser 454 Magnum MPI and a dual prop Bravo 3 out drive. We started out suddenly having rough idle issues where it would surge quite a bit. Had a friend come down and hook it up to the computer (he wanted to do it on the trailer hooked up to the hose). There were not any codes and he said everything looked within range on the computer so he recommended changing plugs, cap, and rotor to see if that fixed the problem. The old plugs looked good but the cap did have a bit of corrosion on it. Also replaced the fuel water separator and added K-100 to the fuel.
Took it out the next weekend and it was still having the same issue but now started stalling at an idle after the engine has come up to temp. The surging has calmed down a bit but it idles rough and will stall out if we idle after the engine is warmed up. Once it stalls, it will fire right back up no problem. It also seems to idle (rough) but will not stall itself out if it is in neutral. As soon as you kick it into gear and put it under a load it starts acting like it is starving itself for fuel and will stall. It also runs great under power once you get the RPM's up out of an idle.
We thought it was a bad IAC valve, as you can hear the IAC working hard to regulate the air being fed into the engine, so we replaced with a new OEM replacement and the problem still persists. I think the hissing noise we are hearing is the IAC working to overcompensate for another problem. We also installed a new MAT (manifold air temp sensor) as the old one was shot and barely screwed in. It looks like the previous owners had recently installed a new PVC valve and seems to be a bit of residual oil in the throttle body. We tried to clean the throttle body as best as we could. As previously mentioned, new plugs, cap, and rotor were installed and also put on a new vacuum line. We tested the throttle position sensor based off of instructions online and seems to be putting out the correct volts.
We are hoping to get it put back on the computer on the lake and have him look at it while it is stalling. Also considering the MAP sensor but we are throwing a lot of money at sensors that may or may not be bad and, so far, have not fixed the problem.
Any ideas would be greatly appreciated!
Took it out the next weekend and it was still having the same issue but now started stalling at an idle after the engine has come up to temp. The surging has calmed down a bit but it idles rough and will stall out if we idle after the engine is warmed up. Once it stalls, it will fire right back up no problem. It also seems to idle (rough) but will not stall itself out if it is in neutral. As soon as you kick it into gear and put it under a load it starts acting like it is starving itself for fuel and will stall. It also runs great under power once you get the RPM's up out of an idle.
We thought it was a bad IAC valve, as you can hear the IAC working hard to regulate the air being fed into the engine, so we replaced with a new OEM replacement and the problem still persists. I think the hissing noise we are hearing is the IAC working to overcompensate for another problem. We also installed a new MAT (manifold air temp sensor) as the old one was shot and barely screwed in. It looks like the previous owners had recently installed a new PVC valve and seems to be a bit of residual oil in the throttle body. We tried to clean the throttle body as best as we could. As previously mentioned, new plugs, cap, and rotor were installed and also put on a new vacuum line. We tested the throttle position sensor based off of instructions online and seems to be putting out the correct volts.
We are hoping to get it put back on the computer on the lake and have him look at it while it is stalling. Also considering the MAP sensor but we are throwing a lot of money at sensors that may or may not be bad and, so far, have not fixed the problem.
Any ideas would be greatly appreciated!
