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Looking for somewhere to start. 98 AN with GT 40 no PP. Occasionally idles around 1500 rpm. Occasionally gets a loopy idle surging from 500-1100ish. Both symptoms can occur at start up or after coming off the throttle, shutting down and firing back up usually smooths it out. At first it was really random, days apart with it running just fine in between but now it's getting very frequent to the point we weren't sure we would get off the lake Sunday morning. Any help with a direction to start is greatly appreciated.
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First get the pro boss service manual it's under manuals here.
I would check for a vacuum leak only one hose going to map sensor and fuel pressure regulayor.
Then check fuel pressure.
Then idle air control valve make sure it's clean and moves smooth.
If none helps check all sensors there is only a few.
How does it run besides idle under load high rpm.
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Thanks for the reply, I'm going to get a new manual I can't find mine. Good to have a starting point. Up until Sunday under load was good, seemed to hold pretty steady rpm. Then Sunday morning it felt off while I was driving and we were surfing, felt like it was trying to speed up or slow down and then ran really rough on the way in with no one behind the boat but still around surf speed.Comment
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What's the last time you did a tune up (spark plugs, wires, fuel filter, distributer cap+)Originally posted by Puddle View Post
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Plugs are new, distributor cap and rotor look good but I have a new set so will change them out, wires were back ordered but they will get changed out when I get the boat back. We took it to the marina to let the mechanic look at it while we are back home for the week. Fuel pressure comes in at 32psi on the rail so low, waiting for a high pressure pump to come in and hopefully that solves it, fuel filter will get done same time. I did check for vacuum leaks and all was good.Comment
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How well versed is your mechanic on gt40’s ? Depending on how it was tested 32 lbs is within spec. 31 lbs + or - 3 psi at idle is what is called for. 39 + or -3 cranking as well at WOTComment
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I know he was talking with a mechanic from the Nautique dealership when they determined it was low, he said 40 - 44, not sure if it was tested cranking or idle though, I'd hope the dealer mechanic would have asked those questions when determining to order a $375 part.Comment
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Just spoke with the mechanic, fuel pump brought the pressure up and smoothed it out some but it still starts to surge after it gets warm, I'll pick it up on Friday when we go back to the lake and start going through sensors ect. see what I can find. I wish I could talk to the computer, that would make life so much easier. To recap high pressure fuel pump and filter have been changed, vacuum lines checked, air filter cleaned, idle air control checked, plugs are new, distributor and rotor look good but I'll change them and the plug wires this weekend although it seems unlikely that's the culprit. Other than sensors anyone have more suggestions for me to check? ThanksComment
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I'm thinking your new pump is putting out more pressure than your old one but that the increase in pressure is just masking the original problem that has not been corrected. I think I'd be looking at the fuel pressure regulator. 5.0 Mustang forums are good places to check for these kind of problems,those guy's have come across all these troubles beforeComment
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The fuel pump is not responsible for the pressure the pressure regulator on the end of the fuel rail is.Originally posted by gary s View Post
You can talk to the computer you need a odb1 tester like a innova 3110 or 3120.
If you get one make sure it has the yellow ford obd1 adapter.
However there are only a handful of sensors they can be tested quickly with a multimeter. MAP sensor puts out frequency so you need a advanced tester.
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Found Peter's sensor test video in another thread, thanks for that super helpful. Here's my results.
Limp mode measured 42ohms and 12v key on if I'm not mistaken that points to a bad oil pressure switch?
ACT under 1ohm I forgot to write down the actual reading 4.5v when unplugged and 1.8v key on
ECT under 1 ohm again, 4.5v and 1.8v same as ACT
TPS .9v rising with the throttle to 4.9v wot
My meter doesn't have hz so I didn't do the MAP sensor I'll borrow one from a buddy. When I changed out the plug wires and cap and rotor the cap and rotor the mechanic said was fine looked horrible so I'll pull the IAC he also said was fine when I'm at the lake this weekend.Comment
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I agree that it sounds like a vacuum issue; they can be hard to find and change with temp.
How did you test for it?Ken
2001 DD Air GT40Comment
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Realized I never posted our solution and hate threads with no finish good bad or otherwise. So things changed last summer the hunting and high idle went away and a new problem formed in their place. We started having a cold start issue, anytime the boat sat for more than a couple hours it would fire up run for a couple seconds and die, if we pushed the throttle in neutral to about half it would fire up and run fine while warming up, once warm it would run for hours without much issue, just the occasional stall coming off throttle. Chased a couple things through a 5.0L forum (IAC was one) with no luck. Finally got a OBD 1 on it and it spit a TPS low voltage code at me. "Local" Nautique dealer said it's no longer available so the NAPA version got put on and it's been running great ever since.Comment
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Sometimes a faulty ignition coil will cause these issues. Worth replacing since you have checked the rest of the ignition items and the price is right at your favorite local parts house.Comment
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