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7.4 mpi surge at 1100rpm

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Old 07-23-2012, 08:00 PM
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Default 7.4 mpi surge at 1100rpm

I'm new to the forums and need some direction. I have a 1999 7.4l mercruiser saltwater series multi port and surges at 1100 rpms. Idle is fine and above is fine. I changed the MAP sensor and checked the IAC valve for carbon, where should I go from here?
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Old 07-23-2012, 11:38 PM
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Any codes? Check there first... if you dont have a scanner hit me up and I will explain how to get the codes with a jumper wire and led light.
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Old 07-24-2012, 07:25 AM
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I don't have a scan tool.How do you go about getting them with a light?
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Old 07-24-2012, 03:44 PM
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You need to locate your computer port... Should be sitting right under starboard side engine cover... Take the cap off... Get a small wire and jump the two holes on lower right (I believe its the D and E ports) they sit horizontally next to each other.... Then take a small led light (got mine from radio shack... A tiny little led with two wires) and put those wires in the two holes on left side (I believe A and B, they sit vertically next to each other). If u have all right, when u turn your key one click u will hear the fuel pump cycle and the light will glow steady for a couole of seconds. Then the light will start to blink... First is code 12... One quick blink, then a pause then two quick blinks... Then it will pause. If it repeats the 1 and the 2 blinks there are no stored codes (the system will always read code 12... It just means the system is working and does not detect a signal from the distributor... This is normal). If u have stored codes... It will blink say 4 quick blinks then 5 quick, this is code 45... I don't have my book with me but if this all works for u and u get a code other than 12 let me know and I will tell u what the code is.

Your problem sounds to be like an IAC that needs to be re calculated. That's a whole other procedure. Let's see if u have any stored codes.
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Old 07-24-2012, 06:58 PM
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Thanks. I wont be able to check till friday or saturday.
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Old 07-24-2012, 10:36 PM
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check the FP regulators vacuum reference line for failure or being real soft and prone to collapse.
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Old 07-25-2012, 06:24 AM
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I found some slightly swelled by the front of the intake last week. I cut them back a few inches to the good part of the hose.
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Old 08-01-2012, 07:06 AM
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Finally got to check, no codes. should I replace the IAC valve even though it never stalls and only surges at 1100 rpm's?
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Old 08-05-2012, 02:09 AM
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Hmmmm... Still could be a vacuum leak somewhere, also get a hold a fuel pressure tester and make sure when you turn your key and the pump cycles that it gets up to near 45 psi... then check while running, should stay over 40psi near 45. Also, when you checked the codes with the led flashes, did you let the code 12 go at least 3 times. I forgot to mention that each code will show 3 times then change to next. If all is good the code 12 will just keep repeating.
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Old 08-05-2012, 07:29 AM
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look at tps, check with meter to see if readings are steady rise/fall.
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