TPS...adjust or replace??
 

99 454 mag mpi, idles great, runs great when cold. After it gets warm, & above 1000, surges real bad, & loads up to the point of stalling, unless I play with the throttle. Above 2000 seems to run ok. I checked all plugs, wires & cap, all ok. I removed the throttle body, cleaned the butterflys, & replaced the IAC ( it didn't look real good). No change, I'm thinking it may be the TPS, but don't have access to a Scan Tool, is there a way to check with a digital multi-meter, & if so, how? Should I try to adjust or simply replace it? I don't think it's the MAP sensor, due to it idling ok, but any insight on this would be a great help.

If it idles OK the TPS is not so likely to be faulty. Does it surge under load or only when not in gear? Does it behave OK at higher rpms under load?

your TPS has 3 wires, should be grey,black and blue, the blue wire if i remember right is the signal wire from TPS to mefi computer, back probe this wire with a digital meter red lead and take the black lead and ground it , you should have .50volts at idle, move the throttle and watch the meter, should sweep up to 5 volts at WOT

It surges under load, & without a load, it does seem to clear up above 2000rpms. The troubling thing is that is loads up so bad that it will stall if I don't play with the throttle. Off idle, if I move it just a click on the throttle, at times there is no change in rpms, & other it revs up & will surge bad, that's what was making me lean towards the TPS. I do appreciate any input, being that good Mercuriser mechanics in the Gulfport area are tough to find right now.

Thanks, I will check that tomorrow.

I would look at the fuel system, if you have access to a fuel pressure guage that is where I would start. I believe that engine is suppost to run 49 PSI at idle with the regulator unhooked, the one down on the side of the engine not the one on the rail, it's just s dummy and is capped from the factory. I'm not positive about the presure spec on thast though mybe someone with the manual can verify that for you. If your engine has a VST pump and filter check the filter screen in there, they get plugged up and take out the pump.

43 psi according to my manual (Seloc) with the vacuum hose unhooked. I think you will need a scan tool. It is possible to trouble shoot with a DVOM but it may take quite some time. It could be basically any sensor, a short somewhere, an open somewhere, etc.

That is a direction I had thought of, but didn't go into any pressure checks. I'll have to do that, my fuel pump is easy to get to, so that is a quick check. I'll do that today, thanks.

Will a scan tool for automotive work? I do have access to one of those, but I've been told that it wouldn't work for Mercuriser.

No it wont work. You need either the Quicksilver or Rhinda scanner. There is a way to jump the connector and read flash codes, but I dont remember how . I am sure someone here knows or just do a search , it has been discussed here before.