I bought a used 1998 Merc 200 EFI. Compression is 125-135/140. I took it to the lake for the first time today. It started great. It idled great. It runs real smooth up to around 2K RPM. It runs great at 4K and above. However, if I back off to just under 4K it drops real fast to just below 3K. If I start at idle and slowly increase the RPM's it starts surging and backing off at just above 2K. IF I try to hold it in the 3.5K range, it surges then backs off..surges then backs off. Any ideas? Like a said, it runs great at low RPMs and great at high RPM's.

Thanks,
Mark

Sounds like a weak fuel pump not the high pressure pump but the mechnical fuel pump it sits right behind the vapor separator right next to the thottle arm sqaure in shape Part# 21 857005A1 probable distorded rubber check valves or stiff diaphams....OPTI

Hey, Opti. If the mechanical pump is week to the point of causing surging problems in the midrange, wouldn't it fall on it's face at high speed? I'm no Mercury Tech, but I do understand 2 stroke engines very well. My first thought would be that the midrange controls in the ECU were not right. I have a 200 EFI, and I have had a few problems with Merc ECU's. I have been surprised at how things can just start causing problems. My engine wouldn't run on top end, and I had to install the BUZ plugs and low RF plug wires to fix it. It was never a problem before, and then it was. And it was the same with 2 different ECU's. Like I said, I'm no Mercury Tech. But does the engine have the right plugs? wires? Merc EFI's are great when they are right, but if they are not, they can be terrible to diagnose. If you are a Merc Tech and have insider info, be kind and share!

Mark, I hope you get this figured out. The 2.5 liter 200 Mercs have to be one of the best V6's ever made.

Artie

Opti, I wish that I knew you were at Grapevine Bass Pro. I would've had you troubleshoot it.

Thanks for the input.

this could be a TPI problem,have a tech hook it up to a meter and see how smooth the values work.