i know i know, another gt40 idle problem

i have recently made the switch from my beloved 88' SN to a 97' super sport. but i have a feeling there is someone on here who can point me in the right direction with the problem im having. motor is a gt40. recent tune up included plugs, wires, cap and rotor which made the boat run significantly better and faster. however. i am now experiencing an intermittent problem. boat starts almost instantly with flick of the key everytime regardless of hot or cold and NEVER stalls. but, after the boat has been running for a few minutes, idle rpm reads about 850 (which i believe is high) and then every few minutes rpm will surge up to 3k-4k for a few seconds before dropping back down to 850ish. when it bounces up like this, the engine definitely kicks up a bit but not as much as i think it really would at 3-4k. does this in neutral and in gear. when in gear and idling around it still does it. when i get on it it picks up quick until about 3/4 throttle. between 3/4 and WOT it seems to hesitate on and off and tach reads between 5k and 6k which does not seem right to me. today when i went to the boat, started it up and stayed at a nice 600 rpm idle, went out and opened it up and it was smooth and fast with no hesitation. took it back down to idle and it started bouncing again. i have ordered new tps, new idle air controller, new ecm relay and new fuel filter (plus 3 inch hose inside canister) as the boat is new to me and no idea when it was last changed. im leaning now towards thinking it is the tach that is causing the issue. i checked the back and all connections were tight and secured. i also spun through the cylinder settings on the back a few times with no success in the problem going away. it was set on 8c 4p, should it be on just 8c? also should the manual controlled dial on the back of tach click for each setting? it just spun freely through all settings... any and all input is appreciated

thanks for the suggestion, will check later today. does anyone have a napa part # for a complete distributor that works

got it! do you or anyone happen to have cross refrence number for ecm relay?

Since you were asking about NAPA,If you want an Echlin relay, the number is AR-174 like mentioned above Just as good as any of the others mentioned.

https://www.napaonline.com/en/p/NRD482891?impressionRank=1&keywordInput=distributor

You really need your fuel pressures to see what is happening in that regard. Key on/ Engine Off pressure, Engine Running pressure.

also, when the rpms start jumping and engine kicks up a little, fuel pressure doesnt change and stays spot on. someone mentioned the wire mesh ball filter inside idle air valve... can it be run without this to see if it makes a difference?

there is no in line filter from my tank to the pumps. have i read somewhere that there is some sort of screen in the low pressure pump?

Did you replace the 2.5" hose inside the FCC?

i also read another post on a different site where someone had the same issue as me. wouldnt happen till 10 minutes after the boat had been started, would idle fine then start sporadically surging up on and off. and he said his fix was the injector connections had gotten corroded and were giving off 30 ohms instead of 12 which makes sense to me because it seems like when im at a certain speed the boat kicks itself up a notch here and there

Since you're worried about proper fuel pressure, fuel pumps etc, have you looked at the anti siphon valve and the in tank suction line for any restriction/plugging.

replaced coil back today and the problem is gone. idles smooth, no hesitation or random kick up while under load anymore. before replacing coil i was just barely at 12 volts on the gauge, at a steady 13.5 now

ugh, i wish. but unfortunately i did NOT solve the problem. when i put the new coil on, i had the battery unplugged for a half hour which i believe allows the computer to reset. connected everything back up and she ran great like she was supposed to, no idle flair, no surging, no missing at high speeds. took it back to the dock and parked it. came back the next day and problem was backed. next i changed the TFI. also unhooked battery for a while while doing this. connected everything back and bam. running great again. perfect in all aspects. for about 35 minutes... problem back. with everything i have done and changed, i have narrowed it down to its got to be the ECM. bit the bullet and ordered a new one from gt40marine.com it will be here thursday. my reasoning is this. everytime i change something and unhook the battery allowing the computer to restart, it runs great like it's supposed to for 30-40 minutes before the problem returns which tells me the computer resets and is good but then goes bad again shortly after. fingers crossed the new ecm does the trick. not sure what else it could be. i was thinking distributor. but it just seems too convenient that it runs great for the first 10-15 mins until something gets hot, and the fact that everytime i disconnect the battery for 20+ minutes it goes back to being good again for a brief while...

KENO wrote: Sounds to me like "you done good" BB

I've always wondered for $780 what refurbishing and testing steps are done to the used ECM's by this company.   Don't be too anxious to send your original back for the core