Weird gt40 miss/stumbling - please help! |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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So I have an intermittent issue I am experiencing. My 1995 gt40 will miss and stall when warm but when it happens the tach will bounce all over the place despite the engine running at 1k rpms or less. Of note is that I burned an alternator belt recently and replaced it. It has had a smoking belt issue since I’ve owned the boat. Seemingly my tired mando alternator was putting out a steady 13.5 volts after replacing the belt but only around 12 before. In Thinking this through, my stalling problem began after the new belt was put on so on a whim I took it off and the boat ran flawlessly immediately thereafter off the battery. Didn’t skip a beat.
I guess I am going to order the 100amp alternator upgrade kit tomorrow with the hope that it was entirely the alternators fault sending a strange voltage or spike to the system causing the ecm to act oddly. Anyone have a similar issue or thoughts on what else I should be checking? I changed the relays just to be safe but Initially I was also thinking possible distributor issues, pip sensor, iac valve, tps, ignition module or ecm, but if removing the belt solved the issue, I think it’s the alternator. Given the wacky tach, I am certain it is electrical based. My hydrophase also registers odd tach readings when the issue presents itself, so it’s not the 6-8 cyl selector on the tach. Also both the Tach and digital rpm read outs function flawlessly when the issue isn’t happening. I’ll try to link to a video I took. I appreciate any thoughts or insight! Thank you, Kyle Kyle |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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8122pbrainard ![]() Grand Poobah ![]() ![]() Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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Hey Pete it definitely does, but I don’t think I captured the miss great. Sometimes it will stumble at the bottom and almost die. If I try to add throttle when this is happening it will shutter.
Kyle |
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8122pbrainard ![]() Grand Poobah ![]() ![]() Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Kyle,
I wish I knew more about the GT's. All I can do is suggest going to the GT diagnostic thread in the FAQ's. Also, I know once more members come on, you will get some help. |
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Gary S ![]() Grand Poobah ![]() ![]() Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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I'm just guessing and only by the tach jumping could it be the pip sensor? But yet the tach signal comes from the TFI module...... Makes no sense why it clears when the alternator is disconnected. Lewey,Ken ???
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gun-driver ![]() Grand Poobah ![]() Joined: July-18-2008 Location: Pittsburgh, Pa Status: Offline Points: 4117 |
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I would think it’s a low voltage issue these motors don’t like low voltage.
Watching the video your volt meter is only showing just over 12 volts it should be at 14.5 if the alternator was working correctly. As for the 100 amp alt. I wouldn’t waste the money unless your running a bunch of amps on your stereo, the stock one is more than sufficient. So I would start with the alternator and make you’re getting 14.5 to the battery and also check all the grounds to make sure they are clean and tight and see how she runs after that. |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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Thank you. I was thinking about this a little further, and doesn’t the tach signal come from the e coil? If so, maybe that’s what is causing my problems which may be voltage related or heat soak related?
Going to get my alternator tested this am. I’ll see if I can also pick up a coil. Thanks, Kyle |
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Gary S ![]() Grand Poobah ![]() ![]() Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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It’s not with me but I printed up the gt40 manual. It’s much easier to look things up in it than thumbing thru pages here. Tach signal comes from the TFI module not the coil. Go thru the manual before you go changing parts, but because of the engine running fine with the alternator disconnected is suspect so that might be worth checking first
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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Cleaned the IAC valve, put in a new e-coil, and had the alternator tested (allegedly good). Hooked everything back up and same symptoms immediately before engine was at operational temp. Pulled the alternator belt and the boat runs smooth again off the battery. Thinking maybe the voltage regulator in the alternator is shot and shorting out causing a potential voltage draw issue, thus starving the ecm of the needed voltage to run normally. Is this possible? Batteries both show 12.5 v at rest.
My battery setup is as follows: 1 interstate mt 34 starting battery (1) and 1 interstate srm 29 deep cycle (2) hooked to an isolator switch. Stereo and ballast always run off of batt 2. I have a yandina c100 combiner relay which auto combines the banks when voltage of system is +13.3v. This is why I was thinking maybe I need a 100amp alt. Upgrade But the 55 has been doing okay despite the issues I am now experiencing. Thanks! |
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8122pbrainard ![]() Grand Poobah ![]() ![]() Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Kyle, Did you go to Autozone for the test? If so, I wouldn't trust anyone except Roger (with the English accent) |
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KENO ![]() Grand Poobah ![]() Joined: June-06-2004 Location: United States Status: Offline Points: 10811 |
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I think you have a bad alternator that's causing too much AC ripple in the alternator output and it's affecting your electronics for the fuel injection/ignition system.
If you took it to Pete's favorite place and they slapped it on a machine and said "yup it puts out good voltage" you didn't get a good test. To test the shape of the output wave you need an oscilloscope It'll tell you if one or more of the output diodes is bad. along with a lot of other things. When you're just on the battery there is no ripple, just steady DC voltage (which will drop over time supplying your fuel and ignition systems) but for the short time you were just on the battery it would be pretty much the same voltage. Here's a link to a lot of reading if you happen to be bored. It explains ripple and what a normal waveform looks like and causes and effects of different looking waveforms. link |
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tryathlete ![]() Platinum Member ![]() ![]() Joined: April-19-2013 Location: Lake Villa, IL Status: Offline Points: 1797 |
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Keno makes a lot of sense. Get that alternator tested on an oscilloscope and report back. Running fine on the battery but wonky on the alternator leads me to agree about the bad diode. I did see you’re trying to get ahold of a used alternator.
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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Hey all. Thanks for the input. It was tested at oreilley and I don’t trust the test very much. The marine alt doesn’t cross with anything in their alt. Test computer so we had to find a similar model 3 wire 55 amp auto alternator of a different make to enter in to the computer to hijack it to run the test.
Kyle |
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gt40KS ![]() Gold Member ![]() ![]() Joined: August-05-2017 Location: Wichita Kansas Status: Offline Points: 943 |
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+1 - My first thought. Haven't heard of "surging" that would cause such wild rpm changes, and so quickly - usually only 200-400 rpm rises/ falls and definitely wouldn't surge to 4000+ let alone peg the tach at 6000 as I saw in the vid - certainly didn't sound like more than a couple hundred rpm's either. You may have more than one issue here too. The alternator spikes would explain the the gauge and very likely explain the uneven idle. But with a new alt, if the surging/ idle rolling continues I'd consider Gary's thought of a loose/ corroded PIP sensor. Also, cleaning the IAC doesn't always do the job and it may need to be replaced. The rolling idle and surging issue that has sometimes plagued the GT40's was also common with Mustangs, Lightnings and some Thunderbirds of the same era and unfortunately after exhaustive research I've found that there are several possible causes - sometimes more than one at a time, compounding the issue. Sure makes it tough to diagnose ... and fix. |
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JCCI
1995 Ski Nautique GT40 |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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Thanks Joe. You are right in that the tach isn’t actually showing what the boat is doing in terms of rpm. It is running really slow - almost stalling at idle. So much so that limping home the damper plate was clattering because the rpm was so low as to barely engage the transmission. If i gad to guess it was at 500 rpm or less and real rough.
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8122pbrainard ![]() Grand Poobah ![]() ![]() Joined: September-14-2006 Location: Three Lakes Wi. Status: Offline Points: 41045 |
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Kyle,
I'll throw this to you knowing I'll get the normal criticism from Ken but an alternate to a new alternator is a diode and brush replacement. Depending on the alternator, diodes are available individually or preassembled in their heat sinks. |
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KENO ![]() Grand Poobah ![]() Joined: June-06-2004 Location: United States Status: Offline Points: 10811 |
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What you sayin' there Pete ![]() Delco parts and kits are available everywhere and they're relatively easy to work on. But as far as a Mando, first you have to find the parts or a kit to do the job |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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So the new alternator failed to solve the issue. At this point I don’t know where to go and am really frustrated. Just to confirm the issue I started the boat with no alternator before doing any work. Started and ran fine. Installed the new alternator, adding a new and beefier ground to the engine block and running the larger + lead to the battery side of the starting solenoid under the ecm, and splicing the green wire in to the old field lead. Go to fire it up and it starts but runs like garbage with the same dancing tach and barely idling. I turn it off. Thinking maybe the culprit is the battery combiner relay, I disconnect it, then crank and no start. Frustrated, I remove the alternator belt and try again. Same result - crank and it won’t fire. Not sure where to turn next other than to start diagnosing a crank and no start issue. Fuel pumps are priming and the ecm/fuel pump relays are brand new. The e-coil is also brand new. Anyone have any good ideas or suggestions? There’s nothing t much summer left...
Thanks, Kyle |
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KENO ![]() Grand Poobah ![]() Joined: June-06-2004 Location: United States Status: Offline Points: 10811 |
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Might not help much but since it won't start, do you have spark now?
This may be pointing at your distributor like mentioned earlier Maybe whatever component was affected failed with the voltage from the new alternator.; You've replaced a lot of parts, if you decide that the distributor needs work internally you're probably better off on that one just replacing it like Joe (gt40KS) did. Maybe he'll give you a name and part number but other people have used a Dorman FD-14. It sounds like you're going at it methodically . Hopefully not an ECM issue. |
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KENO ![]() Grand Poobah ![]() Joined: June-06-2004 Location: United States Status: Offline Points: 10811 |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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ThanksKeno! So my plan today is to get a fuel pressure gauge I can attach to the rail just to double check I do in fact have fuel pressure when pumps prime and while cranking. Also am going to check for spark during cranking. If I don’t have spark but have 12v on the + side of the coil, I expect this points to a distributor issue?
Thank you! Kyle |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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Good news - I have spark so it’s not the distributor. What I found is that I don’t have any fuel pressure at the rail. Down side is that I have voltage at the carter hp fuel pump plug and independently the fuel pump runs when you put 12v to it on the bench, but isn’t working in the boat. Cleaned contacts and no dice. Low pressure pump does what it is supposed to. I guess I am going to order a new Carter hp pump and lose the 2nd input on the pump coming from the rail tank return and see where that gets me.
Anyone have any other ideas? Kyle |
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Gary S ![]() Grand Poobah ![]() ![]() Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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On thing you could try is having a helper turn the key while you take a hammer and hit the fuel pump,it could have a bad spot on the commutator and happened to stop on it. Back in my workings days when one of our trucks would get towed in I'd help the mechanic get them started that way. They were always dropped off somewhere in the lot and he had to somehow get them into a bay by himself. I'd start it while he'd beat on the gas tank with a rubber hammer. It seemed they always had a full tank too which he'd have to drain so he could remove it to get to the pump. If it then works order a pump!
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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So I guess I was mistaken about the pressure. Seemingly I have pressure. It just isn’t registering on the gauge that I borrowed. If I turn the key and push the pin in the Schrader valve, fuel squirts out. Next thing I checked is injector plugs for voltage when cranking. Nothin. The needle bounces a little but it’s less than a volt...
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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So I took out the schrader valve and got the gauge to work. I can build rail pressure to 40psi by cycling the key switch on-off like 5 times. If I crank after that, rail pressure will hold at 40 but if I let off, it immediately drops out and eventually doesn’t have any pressure til I re cycle the key. I’m thinking this points to the high pressure pump, the fuel pressure regulator,or the anti siphon valve. The engine will just hiccup if I keep cranking but won’t really fire. Do the injectors need sustained pressure to be able to fire?
Thank you, Kyle |
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MrMcD ![]() Grand Poobah ![]() ![]() Joined: January-28-2014 Location: Folsom, CA Status: Online Points: 3636 |
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When I need to check Fuel Pressure I grab my AC gauges, they do dual purpose work in my garage. They connect and work well for checking the Fuel Pressure.
Use the Low side gauge as it measures the lower numbers better. A fuel pump issue would show up less with more voltage but yours shows up as your voltage increases. The battery will run the engine but it is limited to 12.6V. The alternator will bump voltage to 13.9V approximately. This is odd to me, I would expect the fuel pumps to work better at 13.9V. Based on your jumping gauges I suspect you have an electrical issue that is affecting more than the gauges. I caution you about bench testing a fuel pump. They are fuel lubricated and I am told running one dry for even 10 seconds will damage the internal bushings the motor runs on. This is the reason you never want to run out of gas with an electric fuel pump. |
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Gary S ![]() Grand Poobah ![]() ![]() Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096 |
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I would suggest you download the GT40 pdf manual . I downloaded it and went to FedEx/Kinko's and had it printed,bound and made into a manual,was about 55.00
In it are the test procedures, 3H-3 which has you test for fuel pressure leakdown and 3H-15 in which they have you vacuum test and visually inspect the regulator,gaskets and orings. |
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kylem428 ![]() Senior Member ![]() Joined: May-31-2005 Location: United States Status: Offline Points: 263 |
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Thanks for the link to the manual. I used a few tests from 3h to confirm some things - I borrowed a noid tester and was able to confirm the injectors are firing. On to the fuel pressure test-I when I jumped the fuel pumps I could get to 35# of pressure but it wouldn’t hold. It immediately goes away when I disconnect the jumper wire from ground. As such, per the manual I replaced the fuel pressure regulator , re-test, and same thing. I checked the anti siphon valve on the supply side thinking maybe it is being drawn that way but the valve seems” good, however the pressure in the fuel rail has to be going somewhere. Any thoughts on what to check next? Vacuum system? Does the 35psi from the pumps suggest my hP pump is toast? If you recall, it had a nice squeal when I got the boat but went away once I changed the fuel filter.
Kyle |
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gun-driver ![]() Grand Poobah ![]() Joined: July-18-2008 Location: Pittsburgh, Pa Status: Offline Points: 4117 |
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Try plugging the Tee fitting where the stainless line from the HP connects to the return line and cap the stainless line. Curious to see if your losing pressure through that line.
Also when mine was going bad I was showing pressure but it was mostly air in the line, I had to bleed off the air out of the rail to get the motor to run. I used one of these to bleed the rail. https://shop.advanceautoparts.com/p/actron-fuel-pressure-tester-kit-cp7818/9030328-p?c3ch=PLA&c3nid=9030328-P&adtype=pla&gclid=Cj0KCQjw5NnbBRDaARIsAJP-YR9qn7qbUoC-RAmzZ0GXxOXv7j_GhjvGzM44bQca_q6ZZT5gkyqiRd4aAk5aEALw_wcB&gclsrc=aw.ds&dclid=CI7LoZ6X9dwCFdQONwodMjAJnw |
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