8.1 power issue

Bad IAC would have nothing to do with a low WOT RPM number.

Verify the mechanical health of your engine with a compression test
 
Ok, I now have new plugs and wires and still have a problem.

Both engines run completely smoothly and strong up to 3400 RPMs, even under hard acceleration from standstill, but as soon as I hit 3500 RPMs the port motor has a slight occasional popping sound, like a very small backfire. As soon as I drop it back a hundred RPMs it runs completely smoothly again with throttles in same position for both motors and same RPMs
 
I would set up a fuel pressure gauge in a way that you or some else could read the fuel pressure as you increase the rpm.

This needs to be done under load, out on the water. Not in neutral. Look to see if the pressure remains within spec as you approach and pass 3500 rpm. Dropping pressure could cause a lean condition and loss of power. If the pressure stays within spec, I'd say that the fuel pump(s) and filters are ok, But doesn't necessarily mean that the injectors are operating properly, they would need to be flow tested off the engine.
 
the port motor has a slight occasional popping sound, like a very small backfire. As soon as I drop it back a hundred RPMs it runs completely smoothly again with throttles in

Your running out of gas at hi power settings. The boost pump is not supplying the gas to the high pressure pump
You need 38-41 psi on the rail ,under full load
 
Here's an update after running it with a fuel pressure gauge.

Firstly, I misspoke about it not popping under hard acceleration. I must not have been accelerating that hard the first time. It pops constantly under hard acceleration and reaches 39psi under full load.

It ran at a steady fuel pressure all the way up to 39 psi under hard acceleration. Ran at consistent fuel pressure if 33-35 psi between 3000 and 3600 RPMs with no drop-off. There was zero popping up to 3400 RPMs and constant popping at 3500 RPMs with a consistent fuel pressure.
 
Popping back thru the intake system could be a broken valve spring inner or outer. Intake valve not seating from a stick hyd lifter or a lifter pumping up.
Cross-firing if you have the plug wires tied together in a bundel
Carbon tracks inside the dist cap.
Failing coil or impulse to the coil
 
Ran injector cleaner through the fuel system with no change.

I was considering changing all the fuel injectors unless there's a way to identify if any one particular bad injector?
 
copb8tx said:
Ran injector cleaner through the fuel system with no change.

I was considering changing all the fuel injectors unless there's a way to identify if any one particular bad injector?
Click to expand...
Could it be paint from the cool fuel module clogging the injectors? Mine is laid up right now for, among other things, replacement of the all injectors for that reason.
 
Funny you mention that because between the time I posted the above and now I found a string of postings about the paint issue in the cool fuel module.

I think I'm going to pull all the injectors and inspect them for paint.
 
sarbog said:
I'm having an issue on my port engine ('05 8.1 Horizons). Last week I noticed significant throttle "lag" on the port engine. I cruise around 3400 rpm and to get the port engine up to 3400 to match the starboard requires significantly more throttle. Temps, pressures, etc., all OK and runs smooth with no alarms. It just won't turn up. Had the fuel filters changed and it was fine. Next day the same problem. Could it be the Cool Fuel module issue? Before I spend a lot of bucks and loose a lot of water time, if not the rest of the summer, having a mechanic tear into it I thought I'd seek opinions.
Click to expand...
Had the same problem two weeks ago. Two bad injectors
 
If you pull the injectors you can try to have them professionally cleaned and flow/balanced tested. Maybe they can steer you to some aftermarket injectors. The merc replacement are ridiculously expensive.

Here are some DIY videos, but the pros use ultrasound which may be the key.


 
saopm said:
If you pull the injectors you can try to have them professionally cleaned and flow/balanced tested. Maybe they can steer you to some aftermarket injectors. The merc replacement are ridiculously expensive.

Here are some DIY videos, but the pros use ultrasound which may be the key.
Click to expand...

Looks to me like that cleaning method is similar to what we did with a professional injector cleaning kit. It hooked through the fuel rail port and ran high-pressure cleaner through the system using a compressor. However it doesn't reverse the flow to back out any debris.
 
T
paulswagelock said:
$200 to clean, or $349 new, I might go in for the new ones as well.
Click to expand...

That's what I was thinking. I ordered them.

If I'm going to rip apart the top of the motor and remove the fuel rail I just want to plug all new ones in and and not jack with sending off the old ones and waiting to find out if they can be cleaned. This way I can immediately button it up and I'll know immediately if that's the issue.
 
I ordered a set as well. I will sell the old ones on ebay or keep for a spare.
 
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