454 mpi hard cold start
 

I have a 26 ft mirage with a 98 gen 6 454 mag mpi. Ive had the boat out about 5 times this year and it ran great. But the last time I had it out, it had a hard time starting (cold start) I had to give it some throttle to get it going and had to keep it at about 800 rpm to keep it going. after about 45 seconds I could back it down and it ran great. Yesterday I started it on the muffs and it did the same thing. Im not getting any codes.

Also, when I shut it down and started it up about 15 minutes later, it starts up like normal.

Any ideas?

Thanks

Dave


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Great advice from 3pointstar I would also look at your idle control valve, if you have to give it throttle to start the motor is start to go bad.

+1 iac

I put a new cap, rotor, spark plugs and spark plug wires on it. everything looked good but it was time for all of that to be replaced anyway. I am going out right now to change the fuel water filter. I don't think that could be the problem but its due for a new one. I have been reading a lot about the IAC valve (I am new to fuel injection). I think I would like to start with the IAC. is there any way for me to test the IAC I have before I buy a new one?

from looking at exploded views. it appears that the IAC is in the throttle body. yes the plug is difficult to get to. if im unable to get to the plug, could I just remove the throttle body and unplug and replug the IAC and listen for the motor to move?

Only way I know to read the IAC is with a scanner. The IAC on your motor is the same used on many motors like the Ford and Chevy. Don't know which you have without a serial number

the serial number is 0l014516. the number for the IAC is 805224A 1

Search that one and Walker 215-1026, one on ebay for 72 and also at amazon and newegg

My 502 was the same way. I bought new IAC. I think I could have cleaned the carbon off it and would have worked, but was cheap enough to buy a new one.

Now if you want to save some money and not pay Mercury's outrageous price's. You want BWD PN: 21755 for the IAC ($38.99). And the gasket you want Felpro PN: 17852 (also comes with top cap gaskets) ($11.99).

Thanks 92 I will put this info in my file

Thanks for the alternative part numbers. I did the "check" for the IAC that is called out in the merc manual and I determined that the IAC is bad.

My neighbor is a very good auto mechanic and he brought up that if it was a truck, he would think it would be the TPS. Dose anybody know what threw symptoms of a bad TPS sensor are? Are they similar to a bad IAC?

Thanks

I replaced the IAC valve with the part number that 92 suggested and everything is working great.

I will save the info for testing the TPS incase I need out in the future. Thanks

I may have another issue though, when I bought the boat there was some idle surging issues. I had no idea on how an mpi engine controlled idle so when I was having this issue I started adjusting the "idle screws" on the butterfly valves in the throttle body. I know now that was the wrong thing to do, dose anybody know what the factory settings are?

I can not find any info on, I assume because you are not supposed to adjust them.

No idea on how to re set the screws. I just know to never touch them If you need I have a spare TB Ill sell. PM me if needed

Merc uses timing of all things to control the idle speed. If you ever have a scanner hooked up to it look at the timing when idling. It is all over the map. I asked the same question years ago. Here is what the computer is doing to hit target idle speed/RPM's. Well besides IAC. But you get the point.

My memory is not what it use to be, but I bought my MPI setup from a guy on ebay many years ago. The intake was dropped off the UPS truck and broke the plastic linkage between plates and also bent the piece which the set screw adjust. Had new pieces made at a machine shop, including new allen head screws and jam nuts. At the time I did not have my Rhnida scanner, MEFI Burn, Scanner Pro, in other words no special tools. Short story long, but got mine working just fine.

I adjusted the plates so they just touched closed, maybe a tad more open (been a long time). Fired it up and got the engine warmed up. Then adjusted the screw to idle at 600 RPM. Drove it around a while at which time I might have tweaked it. Years later it still going, and even been repowered and still going strong.

I just mesured my set screws for the idle. The out side / second butterfly one was out .248" and the one in the center/first opening butterfly was .250". This is mesuring from the throttle linkage side

So I'd say set them both at .250" out and should be close.