Engine has rough idle and missing on acceleration
 

Et. al,
496HO MPI no modifications. Normal engine maintenance schedule followed.

This is what I am experiencing. This has never happened before and just started last week.
At cold start-up it has a rough/choppy idle and seems to smell a little rich. When I start to accelerate (while not at operating temps) to plane it has the same feel as if there are dead cylinders with a definite loss of power. Once on plane and with the remote at between full and 3/4 throttle it has intermittent missing then will clear itself up and run normal with no missing, with normal idle.
Gauges: Oil pressure ~ 40-50psi, volts 12(+) Temp <160.....
Once at normal operating temps 160-170 it wont duplicate the problem and runs like normal.

Anyone experience this problem that seems to be related to first start-up and disappears when at operating temps?

No service bulletins related to this problem that we could find.

Here is what I have done so far:
- Checked plugs (all are fine)
- Ohm the plug wires (all are good)
- All connections to injectors and coils are fine
- Siphoned all the non-ethanol gas, replaced with new non-ethanol (just in case. Put the siphoned fuel in my vehicle and it ran fine)
- Fuel filter has ~ 25 hours on it. (there is no fuel/water separator)
- Flame arrestor is fine.
- Scanned for any codes, no codes found or produced (this was done by my local Merc dealer) we were thinking temp sensor causing timing changes.......
- Anti-freeze was changed last year.

Any ideas??????????

I am about to swap out my ignition coil which is causing the exact symptoms you are giving. I have not taken it out yet with the new coil but after much research I am pretty sure this is causing my issues. The issues are exactly to a tee what you are saying. I can barely get on plane until the engine warms up.

Bad injector...take a long screwdriver and put it on each injector and have a listen..you should hear a clicking noise,,which ever one is not or has a faint clicking noise i would pull that injector and replace it..you can also ohme out the injectors to see if you have a bad one..

I will look into the coil(s) but wouldn't it keep missing regardless the operating temps?

I will definitely try that.

Do you know the ohms range for the injectors?

Could be an injector. Are you getting any misfiring?

fixx
 

have no clue,,when you check then i usually go by lets say 3 of then ohme out at 12.9 then ill use that number as a referance,if i find one that is more then 1 ohme of from the others then i will suspect that one as a problem.. i will find the bad cylinder and ill hook my scope to it and look at the pattern..

Just rough idle that continues when you try to get on plane.

Once at temp 160-170 it idles smooth with no missing when going on plane.

What sucks about trying to trouble shoot this problem is I have to let the engine get cold again.

All the injectors ohm-out around 12. Some were 11.8

Are you getting black on the transome. If you are it may be too much fuel press.

No soot.

Cylinder compression is 158-162. Can't check fuel pressure, because I don't have the equipment, so I will have to try and barrow or have someone check it.

The IAC ohmed at 10.0 (normal per merc is 10.1) ECT sensor is difficult to get to to ohm it.

I'm think maybe the ECT is bad and just didn't display a fault code, but I want to check some other things before I start "guessing" even though I can pick-up an IAC from NAPA and a ECT locally for about $90.00 total.....

UPDATE:

I practically ohmed every possible part and sensor I could get to and they all were within the Merc manuals limits.
Plugs, wires, injectors, ECT (which is a pain to get to) IAC, MAP.
Also, I pressure tested the cylinders all were fine (158-162) fuel press was fine.

So I took after all the testing with and without a scanner, I took a shot-in-the-dark and did:
- Put some Mercury Quick clean in the tank with 10gal; pulled the cool fuel filter assembly out and poured 1/2oz in the reserviour.
- Replaced the plugs with AC 41-983.
- Replaced the IAC with one from NAPA.
Total cost = $125.00

Ran it cold yesterday and did not duplicate the problems I was having...
It idled smoothly, accelerated without any missing or lack of power. Once I got to operating temps, there was no changes, ran great.

So, I don't know what made the difference, Fuel/injector cleaner? new plugs? new IAC? or maybe a combination of all three? Who knows.

I am just like the majority of the DIY people that don't "throw" money at parts and HOPE it solves the problem, but after so many dead ends I guess I got lucky.

I will run it again Monday cold and see what happens. This is what was so frustrating, the that I had to wait for the engine to be in a start-from-cold state.

IMO: I think it was a bad IAC even though it ohmed 10.0 (manual calls for 10.1)

Nice work! Hope it stays running well for you.

I bet it was the plugs. Lots of people on here have had that problem on the 8.1 engs. Artie