98 GT40 rpms drop when engine warms up
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=40597
Printed Date: October-07-2024 at 2:32am
Topic: 98 GT40 rpms drop when engine warms up
Posted By: Norman
Subject: 98 GT40 rpms drop when engine warms up
Date Posted: February-22-2017 at 7:32pm
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98 GT40, Super Sport, The boat sat for 4 summers with no water under the lift. After the lake came back up and after replacing the fuel and fuel filter the boat ran OK although it was missing at higher RPMs. It ran this way for one summer and then I started working to repair the miss. I have replaced the plugs, wires, rotor, cap, fuel pump relay, EEC relay, fuel filter and the fuel line in the fuel control cell, throttle position switch (the only code that showed up), and one exhaust riser gasket. This has taken place over 25-30 hours of engine operation and every new part seemed to solve one the various problems that appeared that second summer. When it's running it's running better than it has in years, however after about 20 minutes of cruising at 3000 RPMs the RPMs will drop slowly (just like pulling the throttle back) to about 1000. It is not missing, surging, or backfiring. After 2 to 3 minutes at idle it will accelerate strongly back to 30-35 MPH and run well for a short time and then the RPMs drop again. I have repeated this process several times so I think something must be getting hot, what should I be looking at next? Thanks ------------- Norman |
Replies:
Posted By: DVskier
Date Posted: February-22-2017 at 7:44pm
| I had a 97 SN with a GT-40 engine that did the exact same thing. Try replcing the small 1.5 inch tube in the FCC. This can be obtained from your dealer or White Lake marine. |
Posted By: Gary S
Date Posted: February-22-2017 at 7:46pm
I would get a fuel pressure reading while all this is going on.You have not mentioned how many hours are on the boat or if any fuel pumps have ever been replaced. 19 years on the original pumps might be a record  ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Posted By: Gary S
Date Posted: February-22-2017 at 7:49pm
I would suspect the low pressure pump,but I'd check pressure first ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Norman wrote:Posted By: Norman
Date Posted: February-22-2017 at 8:02pm
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663 hours, original fuel pumps. I have not tested fuel pressure as I don't have a gauge, I guess it's time to find one. ------------- Norman |
Posted By: Gary S
Date Posted: February-22-2017 at 8:13pm
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Harbor Freight I understand has a set that has worked for others. I'm thinking the boat runs fine the low pressure pump starts failing to supply enough volume to the high pressure pump,boat slows. Many here have replaced that pump let's see if they concur. I bought my 95 with less than 300 hours and at 18 years old,LP pump had recently been replaced ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Posted By: quinner
Date Posted: February-23-2017 at 10:23am
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Might be worth the effort to check and clean the anti siphon valve also ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1143" rel="nofollow - Mi Bowt |
Posted By: Norman
Date Posted: February-23-2017 at 1:29pm
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Did not mention it in the original post but I did remove and clean the anti-siphon valve ------------- Norman |
Posted By: Norman
Date Posted: February-23-2017 at 1:35pm
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Since I sitting at the computer I want to thank all those that have posted various repairs on this forum, I have been a regular reader for years and the repairs so far have been based on my limited shade tree mechanic days and the information posted here. Thanks to those that take the time to share their knowledge. ------------- Norman |
Posted By: Norman
Date Posted: March-16-2017 at 12:32pm
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Problem finally solved based on 40 minutes run time without a problem. Fuel pressure was fluctuating while it was running poorly so I replaced both pumps (figured 19 years was enough service out of them) however that did not solve the problem. I went back to the fuel pickup and finally got it loose, using an inspection mirror in the fuel sender opening I could see something stuck on the end of the fuel pickup, when I finally got the fuel pickup out of the tank it turns out that the screen on the end of the tube was crushed. I finished removing the screen (almost ripped it off getting the tube out of the tank) reassembled everything, ran it for about 5 minutes and immediately plugged the anti siphon valve. Until I can find a new pickup tube I've installed an inline fuel filter between the anti siphon valve and the tank and it is running well with fuel pressure steady, The inline filter is catching what I consider to be a lot of junk from the tank, it appears that there may have been a mud dobber nest someplace in the system, I'm going to check the vent since it's the only thing that is normally open. ------------- Norman |
Posted By: Bri892001
Date Posted: March-16-2017 at 5:36pm
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Glad you found it! I added your fix to the Poor Man's GT-40 Diagnosis thread here: http://www.correctcraftfan.com/forum/forum_posts.asp?TID=25584&PID=524681򀆉" rel="nofollow - http://www.correctcraftfan.com/forum/forum_posts.asp?TID=25584&PID=524681 |
Posted By: Norman
Date Posted: March-16-2017 at 7:29pm
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Part number s538 Pickup Tube for fuel tank, Plastic at White Lake Marine (nautiqueparts.com). I may have crushed the screen while trying to remove the pickup tube as there is some flex in the tank where it's mounted. The real problem was plugging the screen which is about 1/2 inch tall and 3/8 inch in diameter, the height of the crushed screen was about 1/8 inch. The problem was originally occurring after about 20 to 30 minutes which was about how long it took for the temperature in the motor box to get hot, therefore I was thinking temperature failure of something electronic. After replacing the low pressure pump which is probably flowing more fuel due to being new the problem would occur after about 5 to 10 minutes at idle. The stuff in the tank that I could get out and the stuff being captured by the temporary inline filter looks just like the stuff that was caught in the internal screen in the old low pressure pump. ------------- Norman |