Hunting for Idle 02 GT-40

[jjgag60]

I just got my rebuilt 2002 GT-40 with EFI installed and it won't idle. Mark at PCM said if you go more then 20 over in boring, your ECU will have a fit with the extra vacuum it is produceing. We had to go 40 over. Has anyone found a solution to this problem. The boat runs just fine above 900 rpm. When the RPM are set at 600 or 700, it hunts back and forth and then dies. Then it is flooded and I have to go to open throttle to get it to start.

Thanks

[DanielC]

Stock displacement of a GT-40: 351.858. A GT-40, .040 over, 366.073. If my math is correct, that is about a 4% increase. Not really that much. Changing altitude 1000 feet, is about a 3% increase in air pressure, and the GT-40 seems to handle that just fine.
Mark, at PCM has given me good advise before, and it is certainly possible the displacement change could be confusing the ECU, but I am still a little skeptical this is the root cause of the bad idle.
Does the fuel pressure drop on the running engine at an idle? It should.
Have you measured the vacuum on the engine?
I would be more suspicious of a vacuum leak, or something else that was not assembled correctly, or some part that might have gotten some hidden damage during the disassembly and reassembly process, before I would think the ECU cannot handle the minor displacement increase.
Download the GT-40 service manual from this site. It does have a good troubleshooting section.
A good friend of mine is a mechanic at a Ford dealer in Gresham, OR. I also have friends at the Auto mechanic program at a local community college. I will try to remember to ask them about this.

[nms1991]

I also would check for proper vacuum at idle and also check to see if the map sensor hose is connected properly. Actually a .030 over bore would take it to 356 and a .060 over bore would be 360. But the ecu should compensate for a slightly larger motor. Also check to see if your bypass air system filter to see if it is plugged or restricted. To check the filter you have to remove the bypass air motor and the adapter plate to check the screen in the adapter plate.

[jjgag60]

Gt-40 Idle

What is the best way to check for a vacuum leak. I know you can spray carb cleaner, but I do not want to make a mess on my new motor.

[AirTool]

Re: Gt-40 Idle

What is the best way to check for a vacuum leak. I know you can spray carb cleaner, but I do not want to make a mess on my new motor.

Propane....carefully.

Put your cigar out first.


Another snag might be lifter adjustment. I'm not sure how that is done on old Fords.

If the lifters (even hydraulic) are adjustable, they can have an affect on performance at idle.

[DanielC]

More thoughts on this.
Do a compression test on the engine.
Check the fuel pressure, with the "key on, engine not running". Then hook up a vacuum gauge with a "T" into the hose that goes between the fuel pressure regulator, and the manifold. When the engine is started, the fuel pressure should drop, unless the engine is at wide open throttle.
The GT-40 has to have the correct fuel pressure, because there is no feedback to the engine's computer, the computer just assumes the fuel pressure is correct.
I made a slight error on my calculation for displacement. .040 over makes a 358.930
I would think that because the engine's computer thinks it is feeding a 351.858 engine, the slightly bigger engine would be running slightly leaner, than before the engine was bored out.
The fact that the engine floods at idle tells me something is wrong. A vacuum leak at the manifold would cause the engine to die at idle, from a too lean condition, not flood out. As the engine speeds up, or gets more load, the effect of the leak would go away because so much more air is coming through the throttle plate, and that masks the vacuum leak. A vacuum gauge at engine idle would show the leak, by an abnormally low vacuum.
Are all the cylinders flooded, or is only one flooded? Pull spark plugs to check.
You can also put the end of a long screwdriver on each injector, and hold the handle end up to your ear, and actually hear the injectors "ticking" as they switch open and closed.
If one cylinder is rich, and/or the injector sounds different, maybe a bad injector.
If all the cylinders are flooded, I would then suspect the computer is either bad, or getting some wrong information from a sensor, and the computer is going to full rich because of some bad information.
Again, download the GT-40 manual, and use it to help.

[DanielC]

I talked to my friend who is a mechanic at a semi local Ford dealer. Boring out the engine would cause the engine to run very slightly leaner. His thoughts, if the engine is flooding out to point of stalling the motor, the problem is something else, not the fact that the engine was bored out.
If the fuel pressure regulator is stopped up, the fuel pressure would rise to whatever the high pressure pump could supply, possibly 90 PSI? If the diaphragm in the fuel pressure regulator ruptures, that would allow a path for gas to travel from the regulator, through the hose to the intake manifold, and that would dump raw gas into the manifold, causing flooding.
If the MAP (Manifold Absolute Pressure) sensor is disconnected from manifold vacuum, that would cause the engine's computer to run full rich. May also cause flooding at an idle.

[jjgag60]

hunting for Idle

After printing the shop manual i see there are some good checks for the MAP sensor. Ford has discontinued the MAP sensor so I will have to go with a NAPA aftermarket.

Mark said to disconnet the vacuum to the fuel reguator and see what happens. the engine went to 1200 RPM and ran smooth. Mark said that would make it a vacuum issue. I dont know where to go from here, so I will pick up a new MAP sensor, idle motor and fuel pump relays today. I dont think that will solve my problem but it is a start.

[jjgag60]

Idle

Well I gave up and went to NAPA and order all three sensors. I could not trouble check them on my limited electical testers. $230 later I had a new MAP, Idle motor and throttle position sensor. I know that is expensive, but going to the dealer and having him look for the problem and replacing the part would have cost me the same or more. Plus I would have to wait for three weeks to get in to the shop.
A huge mistake I made was I should have disconnected the battery after replacing each sensor and then start the boat. I didn't and after replacing the MAP and idle motor the ECU i think was till using its memory to start. Once i disconnected the battery and let it sit for 12 hours, and replaced the TPS it started and idled. It was very exciting to see it run on its own in neutral on a cold start without having to start the motor with the throttle handle open 2 inches. After it warms up it idles at 1,000 which is a little high, but as the ECU learns, I hope it comes down. Thanks to Daniel C and others for their advice and thier post on the problems they have had. I now had some extra parts (old MAP, Idle motor and TPS) i can use as test equipment in the future. "life is better on the water".