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Mercury 90 four stroke quits when warm

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Bruce2012

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I got one of those dreaded 2000 Mercury 90hp four stroke outboards on my nitro. Starts and runs great at first. Will perform normally until gets warms (around 15 mins of run time). Spits and sputters, won't idle, dies, can restart with full throttle as it floods out once warm. I let it cool for 20 mins and it starts and runs like nothing was wrong with it ( that's is, until it gets warm again).

Fuel will keep keep coming out of the throttle air intake when it starts acting up. Was told that it may be one of the coils. I just ordered both. Just incase it doesn't fix the problem: do any of u guys have any suggestions? CDI maybe? Cold start/Fuel enrichment staying on? I don't have any tools to test any of this stuff. Any help is greatly appreciated. Thanx
 
lol not everyone at once. Either way, I'll try the new coils when they show up and post the results. It might help others that run into this same problem
 
You have to do some detective work when this happens, to separate an ignition problem from a fuel problem. Leave the hood off, take an extra spark plug along, and when it dies immediately pull a plug wire off quick as you can. Stick it on the spare plug, ground it to the block, and crank it over. Is there spark?

Jeff
 
Fuel will keep keep coming out of the throttle air intake when it starts acting up
Click to expand...

A ignition or spark problem will not cause this!! More likely a problem in VST tank as Merc had problems with hoses splitting on regulator and tips of needles swelling/falling off.
 
What do u mean? What is the VST? I thought it may have been coils as I have had dirt bikes with similar issues. I figured if a coil was dying out, the fuel that wasn't being burnt would be loading up and passing. The coils just showed up in the mail today. I will try it out Wednesday night and get back to u. All help is greatly appreciated. Thanx
 
Also, once the motor is warm and starts acting up, could I simply just unplug the fuel enrichment wires to try to rule it out? Or would I have to jump the wires together to bypass?
 
Bruce2012

I have a 2000 60 HP EFI with the same problem. Have taken it to several Mercury boat motor repair shops - had fuel lines replaced, a new fuel pump and several guesses. (One offensive one suggested that i failed to flip the kill switch - really ???!!!) So... with acute interest i read your post. Has the issue been resolved?? The VST (?) post sounded informed. If fixed - what was repaired and how much did it cost? Thanks in advance -- GK
 
Grandpakeys

i put the new coils on but haven't had a chance to try it out yet. I too am wondering about the "VST", but faztbullet hasn't replied back to help tell me about that theory. I did do a bit of research based on that: I think only the efi models have that? Mine is carburetored. I will 100% post the results of the new coils once I get a chance to try it out. What kinds of stuff did your mechanic already rule out? Between your motor and my motor, we should be able to eventually figure this out. Thanx
 
Tried it out with the new coils. Seems better. Fuel isn't dumping out anymore and it doesn't quit when warm now. Still acts up at idle, but, apparently these motors are notorious for weird idle issues that never seem to get solved based off the info I can find online. I'll keep u guys posted.
 
No - haven't tried new coils ... I looked up the VST. That looks promising but expensive. I had my fuel pump replaced - I don't think the VST was replaced. I'm limping along now during fishing season - reluctant to take it in while the fish are in. I'll drop it off at the shop in October -- until then ... hope your problem is solved. Thanks for the update. I'll keep the board informed with my results... I'm surprised that someone doesn't have the definitive answer as there are so many Merc owners who have similar complaints with early 2000 engines. MERCURY Company!!! WHat's up!!! GK
 
Update:

Tonight is the first time I got to try the boat out since my last post. Worked amazing! Better than ever. I didn't know that It could work this good. Idles perfect, no hickups, pulls hard, and starts great.

So what did I do? I put some mercury Quickleen in the fuel tank. Also, for some weird reason, the oil level was way too high. Seriously, almost twice as much oil in it as their should have been. Which is really weird cause when I bought the boat the oil level was spot on. I checked it while the motor was trimmed level. Either way, I drained the oil to the right amount. Runs great. Well, for now...

. I hope this can help anyone in the same situation. Oh, and yes, the coils did fix the prior issue with it quiting once warm. Thanx
 
Thanks for the updated info. I have been out about 6 times this month - each time same no start results after 15 - 20 minutes run time . I'll try the QuickSilver Fuel additive suggestion but think it's another unknown issue. I'll take it in next month. Thanks for the update.
 
GrandpaKeys,

does it quit on on its own once warm? Or just hard to start once u shut it off and try it again? If it's quiting on its own once warm, I wouldn't rule out the coils. That fixed that portion of the problem for me.
 
Starts immediately first attempt everytime. Idles perfectly. Runs perfectly at lower RPMs. After running wide open for 15-20 minutes, I turn off the motor, switch to 9 hp trolling motor and run for several hours. 60 hp fails to start. Sounds like it's fuel starved - cranks perfectly - no sputtering - no start. Discovered yesterday top speed has dropped from about 25 to 15 at full throttle when running out to trolling allocation. My plan is to take it in, of course, just frustrated because it's been at a very reputable shop several times and they they can't pinpoint the problem. Thx for you thoughts.
 
I have a 2004 Mercury 4-stroke that is doing the same thing-runs perfect at low RPMs, but after 15-20 minutes, if I increase the RPMs she starts acting up. I have replaced the fuel filter system, removed all of fuel, changed fuel lines, connected to a temporary fuel tank, and rules fuel out as a problem. Next I replaced once coil, which didn’t help, so I ditched the new coil and replaced the other oil,still I improvement. I removed the thermostat and still nothing. I feel like it should be the coil. CouldI have purchased a faulty coil? Or do I need to replaced both coils to see a difference. This is so frustrating.
 
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