PCM GT40 Idle surging Please help!?!?

[Marinshamrock]

Just rebuilt my GT40 it’s a 2001 and I’ve done all the sensors throttle positioning MAP sensor and intake air control valve fuel regulator new fuel control cell new fuel filters checked for leaks and the fuel pressure is running @ 45PSI it runs great until it get warm and starts surging at idle it has the 300M ECM that came stock on it in 2001 please help the problem is making my head explode.

[Peter6000]

45 psi is quite high check the manual I think it should be 32 at idle.
Make sure the vaccum hose is connected to the fuel pressure regulator.
Was it drilled to oversize?

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[Marinshamrock]

Yes the hose is connected the the regulator and it was not drilled oversized when I disconnect the hose and plug it the surging seems to subside it did the same thing before I replaced the regulator I believe it could be a failing ECM

[shag]

If I remember right, a failing ECU will cause surging in the GT40. Otherwise it sounds like a vacuum issue maybe..

[Marinshamrock]

So if I pull the line off the top of the regulator and plug it with my finger the surging stops

[Peter6000]

I have repaired several of these ECU's and never had one that creates surging.
90% of them are completely dead or do not turn on the fuel pump.
Measure your vaccum, unplug regulator and block the hose remember vaccum level.
Reconnect regulator vaccum level should be the same. If not your regulator has a leak.
Monitor fuel pressure at the same time psi levels should go up when you remove line.
If I remember right 38 and 46 but check the ProBoss manual to be save.

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[Peter6000]

Just looked it up it's on page 38
31 psi at idle 39 at full throttle
Also if you have not done already, check all sensors there are only a few, all can be checked with a regular multi meter.
Only the map puts out frequency.

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[gary s]

Surging at idle could be the idle control valve, it's "screen" could be dirty or the idle control valve is sticking. First thing I'd do is clean the screen with carb cleaner. Here is where it is-

[ibelieve]

I have a 2000 SN 196 that is doing the idle surge thing. I just bought the boat and giving it a good once over. Mine looks slightly different that the posted pic (see attached) and I am wondering if the white circle is the filter on mine?

[MN Ryan]

ibelieve I'm no expert, but to me, it looks like if you pull the marked bolts and remove the circled piece, yours will match what gary s posted.

[ibelieve]

Agree. The discussion has been about the breather screen being clogged or dirty and I was wondering how that might apply to this component. The white circle to the left above your X almost seems like a breather hole and I am wondering if I should try to clean it out. Also, I have no idea what this component is or what it does?

[JayG80]

Here are some data points on fuel pressure from my 2002 GT 40 from my notebook in 2015.

Key On & Engine Off fuel pressure is 39 psi, turned off the key and at 8 minutes later the pressure had dropped to 35 psi, and then dropped to 18 psi after sitting for a total of 15 minutes.

Started Engine in the water, 2,000 rpm while in neutral and the pressure is 35 psi. Turned off the engine and pressure leaked down to 32 psi, at 2 minutes it was 25 psi, at 3 minutes it was 20 psi, at 4 minutes it was 15 psi.

Started boat again and and at 2,000 rpm in neutral I pulled the vacuum line and the fuel pressure went to 39 psi.

Ran boat in the lake at around 3,500 rpm and fuel pressure was 40 psi.

[gary s]

I have no idea what this component is or what it does?

It's the air by-pass valve. Rather than opening the throttle plates like on a carbureted engine the computer opens or closes the air valve to adjust engine speed. Remove those 2 screws remove the valve and that "filter" screen is under there. Clean it and check to make sure the piston in that valve does not stick.

[2002sansac]

I had the same "hunting idle" condition on my 2002 PCM GT40. Idle would fluxuate between 600-950 rpm. I removed the air bypass valve and cleaned it with carb cleaner, followed by isopropyl alcohol, the seals may be effected by harsh solvents. Cleaned the mesh filter with solvent and reinstalled the valve. Issued was solved, I'm no expert, just my results.

[Emil]

I have this same issue, but with one big difference I think, I have surging AND am running quite rich at idle. I’ve replaced all the sensors, relays, etc. Has anyone else had this issue plus the rich fuel mixture? My theory is currently leaky injectors, because my plugs were noticeably blacker on cylinders 1,4, and 8. I have 40psi on the rail with the key on engine off, regulator is new as well.