Mercury 25l 200hp EFI miss fires at low RPM and runs rough

"Hi there, can someone help. M

"Hi there, can someone help. My local merc dealer in Cape Town can not find a fault using the diagnostics tool. Here is the problem.

About 3 months ago the engine sounded like a steam train. I changed the plugs. No change. Checked all the ignition coils. Cylinder one had a faulty ohm reading. Replaced it. Seemed better yet not 100%. Over the next month she seems tired. Struggles a little to get to WOT. At 4000 rpm she seems to have a tinny noise (more engine noise than usual). If you push the throttle.. she goes. Pulls skiers out with not much problem. Yet, when you idle and chug alone at 2000rpm with a wakerskater, after time (15 min) she miss fires. Stutters then gets going.

So essentially, she runs rough till about 3000 rpm and then noisy to WOT (avoiding WOT for now), and miss fires at low RPM after much idling.


She's just not 100%. Battery seems good and gold connectors (car audio modification).

I have checked all the ohm readings in the manual and all are correct except the ignition coil point a to d reads 10kohm on all 6 coils. The dealer checked and said my multimetre was faulty...
Fuel pump pressure was also tested... normal.
I only use 95 octane lead free with premium plus quicksilver oil. Fuel filter replaced and thermostats and impeller last year.

What could it be? The diagnostics say nothing is wrong. Failing coil?

HELP!

Regards
Kenny"

First thing I would do is chan

First thing I would do is change all the sparkplugs - even if they are "new" right now.

Your symptoms could indicate a bad plug.

If they are not the problem then what is happening is indicative of a Throttle Position Sensor (TPS) that is malfunctioning...

Dear Graham

Thanks for the

Dear Graham

Thanks for the speedy reply. I'll get 6 new plugs and ask the dealer if they can check the TPS voltage while running.

Kenny

"Hi Kenny,

Sparkplugs coul

"Hi Kenny,

Sparkplugs could fail under compression cycle. Had it before with our offroad bikes, 2 stroke & 4 stroke.

Good idea to keep some spare on the rig. Oil them lightly to avoid corrosion. Not nice when you need them and it's all rusty...


Have a good one!

From a sunny Jozi, Cheers!"