7.4 MPI serging @ idle and cold
 

I just got my boat out from under the snow and took it out on lake Tahoe (6000MSL, water Temp39F). I don't recall the serging just after cold start and stumble off idle occurring last year in warmer conditions. I have read my service manual and see that I should have a scan tool or other reader in order to properly diagnose what might be going on. the boat seemed to run fine once off idol but I am a curious performance nut and I like to understand what is happening even if it is a characteristic of the system. Should I get a reader/scan tool? Any ideas about the serge?

baronbob,
I am hoping for some feedback to your questions too.
I just got my engine running yesterday after rebuild and bigger cam. I have a re-calibrated ECU from Arizona Speed & Marine.
Same problems, Engine is surging and then dies out completely below 1000 rpm. Seems to run OK on the garden hose above 1000rpm. Hope to go for a test ride this weekend...
I am not sure if I have an ECU calibration issue or a problem with the idle air control valve.
I have had similar problems with my Ford Ranger and my Mustang. Changing the idle air control valve fixed it...
I read through my manual and see only info on how to change the idle air valve. No info on how to test the valve.
I have the cheap code tool that is also in the manual and there are no simple codes related to idle air either.
Not sure if a scan tool will give feedback or not.
Lets hope a fuel injection expert will come back with something for us...:(

Patrick This web site is the best for getting info from experts who have been there. I am @ work right now and my service manual is @ home but I think I remember a whole section on testing with out a scan tool. I believe there was a piece on testing the air control valve with a meter. I will look when I get home and post a reply regardless of my findings.

Patrick I have searched my service manual (#23) and have found reference to "check the proper operation of the IAC valve" on page 5d-23 of the injection section. It then refers to Table A-8 in the (gen. sys. diag. section). I can fax you this Table if you do not have it.

Thanks BaronBob,
I am going to try swapping the IAC with one that I have on a spare MPI system. If I have the same problem I may ask for that page.
I did speak to AZ Speed and Marine today, they told me to get the boat out on the water to check the top end performance. Then I am going to send my ECU back for tweaking...
From past discussions on this Forum, I may have to ditch the MPI for a carb to realize a performance improvement from my cam and exhaust change anyway.
The season is young, Don't want to do anything rash yet. We are at strike one though...:rolleyes:

Nothing to Say???
No Ideas???:confused:

I think you guy's have a little different problem.
baronbob, it sounds like yours maybe the TPS on the edge of where it needs to be. It is located on the throttle body and is held on with 2 allen head screws. You have to loosen it and turn it until the correct setting is acheived. You can use a scanner, but you can do it with a volt meter. The voltage should be in the range of 0.55 - 0.65 volts. Use very small wire, like 30 gauge and put in the connector to hookup your digital volt meter to.

Pat , yours is probably due to the modifications to the motor. The motor needs more air than the AIC can deliver. The setting on a scanner should show 20-50 for this to work properly. On the bottom side of the throttle body, there are 2 set screws that the throttle linkage rest upon when the throttle is closed. You adjust the set screws to raise or lower the idle speed. Adjust the secondary butterfly ( one closest to the front ) clockwise to allow more air in so it will idle. I would keep track of how far you turn it so you can always go back if it doesn't fix the problem, but I think between these 2 settings you guys should get your problem fixed.
Let us know if this works!;)

:D ... Always do a search for your problem.....see:

http://www.offshoreonly.com/forums/s...tps+adjustment

"How to adjust IAC & TPS" On Merc EFI


A full primer on the fix!! I had the same problem.

blown formula, nice job on the procedure. I must have missed it last summer.

Thank You for the help!
I going to give it a try tomorrow...:)