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...:)

Be sure to tell us how it works out!!:D

Rockie You know your stuff. I set up the DVOM like you suggested and found an initial reading of .55V. I adjusted the TPS to its stop and achieved .638V. My service manual said I should have .7V @ idle stop. I decided to set the idle stop up slightly to achieve the .66V you had suggested. It ran great in the driveway, I still have a little hesitation just off idle but I think I will try it on the lake before I draw any conclusions. It runs good Thanks for the suggestion.

COOL, I'm glad that fixed your problem. I only know this because I have had to do this on a few boats. Now you know and can help your friends.
If you looked at blown formula's procedure, you will notice what he said, that there is kind of a relationship between the 2 settings and you may need to set the other IAC also to get the hesitation out.
At least you can do it without buying a $500 scanner or being an aero space engineer.
Pat McPherson, did you have any luck on yours over the weekend?

Hi Guys,
I did adjust the throttle plate stop some, did help a little. The 7.4 MPI only has one throttle plate that I could see. I could not change out my IAC though, one of the screws was stripped. I did not want to pull the throttle body off because I didn't have another gasket. I stuck my finger were the IAC should let more air in, no change. I don't think the IAC is opening at all so I do need to change it.
Got out on the water, and the boat ran quite well. No stumble and awesome throttle response out of the hole.
The engine seems to stop pulling hard at about 4200 and max rpm was about 4700. I was warned that this would happen with the GM MPI... I was hoping for 4800+. Still we are up about 150-200rpm from last year.
I pulled a couple of spark plugs to read. If anything it looks to be running rich. I'm going to take some picture and post them for comment.
Thank Again!

All of the EFI plenum gaskets are reusable. You do not need new gaskets for the throttle body (unless someone used sealer on them).