gt40p heads have low compression
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=41963
Printed Date: October-06-2024 at 6:44pm
Topic: gt40p heads have low compression
Posted By: joshjanae
Subject: gt40p heads have low compression
Date Posted: September-21-2017 at 8:55pm
|
Earlier this year I blew a head gasket on one side. I replaced the heads with gt40p head that I pulled from a junkyard explorer. The boat starts and runs but has noticeably less power. Before pulling the heads with the blown head gasket, all cylinders were showing compression numbers around 135psi, except for the two where the gasket had gone. I checked the compression today and with the gt40p heads I am at 90psi on every cylinder. Is this where I am losing my power? Boat is a 1983 sn2001. |
Replies:
Posted By: gt40KS
Date Posted: September-21-2017 at 9:11pm
|
Very likely is the heads if this is the only thing you did at the time of your swap. I would assume you machined/ rebuilt the heads before installing them? does the engine consume oil or smoke? In what condition was your base engine before the swap - i.e. how many hours/ how much use? ------------- JCCI 1995 Ski Nautique GT40 |
Posted By: KENO
Date Posted: September-22-2017 at 12:49am
|
Back in June you said it was running great with the head replacement So is this something that just happened recently or has your definition of great changed in the last few months?  Sounds like you probably didn't check the compression back in June when you put the heads on? |
Posted By: KENO
Date Posted: September-22-2017 at 1:30am
|
If this just happened you may want to consider timing gear/chain issues. Ford (and most everybody else) used an aluminum camshaft gear with nylon coated teeth for a number of years and they wear out eventually and your camshaft timing may be off causing the low compression in all cylinders. I don't know if by 83 they quit with the nylon coated gear and went to all steel or if they were still using it Edit They were still in use in 83 |
Posted By: SNobsessed
Date Posted: September-22-2017 at 9:15am
|
Push rods not correct? Sounds like the next step would be to do a leakdown test to find out what is not sealing - rings vs. valves vs gasket ------------- “Beer is proof that God loves us and wants us to be happy.” Ben Franklin |
Posted By: gt40KS
Date Posted: September-22-2017 at 1:54pm
|
+1 on what Chris said. This was my first thought just hearing a brief description. If the heads were used, and not rebuilt before you installed, I would strongly suspect valve guide and/ or valve seat issues first. If you did have them rebuilt, how long after installation did this issue present itself? I've seen bad rebuilds turn south within a very short time. Then there is the question which may not have been address yet of what caused the head gasket to blow to begin with. Generally there is a root cause that unless addressed first will cause other problems later. If I'm not mistaken, the GT40 heads are a slightly higher compression head. If swapping these to an standard engine without a rebuild first, an existing weaker ring condition could have worsened the problem. Just throwing out some ideas to explore. ------------- JCCI 1995 Ski Nautique GT40 |
Posted By: MrMcD
Date Posted: September-22-2017 at 4:32pm
|
Do all parts work on the 351W stock heads when changing to GT40P heads? Same Pushrods? Same Rocker Arms? Might be a compatibility issue not opening the valves correctly. When I swapped my 78 351W stock heads for a set of ported and polished 289 Hi Po heads 25 years ago, I did have to get custom push rods to keep the proper geometry. It is not likely that all 8 cylinders have bad wear on the valves causing all 8 to have similar bad compression. I know many on here have swapped to GT40P heads and someone will chip in this answer. ------------- |
Posted By: Gary S
Date Posted: September-22-2017 at 6:39pm
|
For all practical purposes. You need to adjust lifter preload,if the shim's cant do it you need different pushrods but thats all. ------------- 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: joshjanae
Date Posted: September-25-2017 at 1:15pm
|
I did not have the heads rebuilt, as a poor excuse, I was in a rush because I needed the boat as support for a Boy Scout canoe trip. The boat starts and seems to run fine. I did not do a compression test when I put the heads on earlier and only really noticed the "lack" of power when I loaded it up with tons of camping gear. To be honest, this is the only inboard I have ever driven and I don't even know what I should be expecting. I have to amend my above post as well, I have 90 psi in most of the cylinders, the two nearest the flywheel are reading 120psi. I used the push rods from the original heads and I have not done a leak down test. I believe I used the rockers from the gt40p heads. I kind of feel like a goon, I was so excited the boat was running after the head swap but it doesn't feel right. I know I say that without anymore experience than the 10 times I used the boat prior to the head swap.. I appreciate all the help from the forum and I love the boat. |
Posted By: joshjanae
Date Posted: September-25-2017 at 1:19pm
| There is 500 hrs on the boat. It does seem to use a little bit of oil but not a lot. I will try to do a leakdown test when I get the chance. Luckily, I live in AZ and we still have some months of boating before it gets coldish. |
Posted By: GottaSki
Date Posted: September-25-2017 at 2:11pm
|
Consider one very quick and dirty test, comp test one jug thats low, loosten the rockers one turn. Retest. If it goes up alot, pushrods are bit too long. You testing with the throttle open, yes? Did you drill out the bolt holes yourself? ------------- "There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing." River Rat to Mole |
Posted By: MrMcD
Date Posted: September-25-2017 at 5:17pm
|
I drilled out the bolt holes when I swapped heads on my 1978 25 years ago. Did it in my garage with a hand held drill. I was younger and dumber but it worked fine for many years after. As I recall the old bolt hole grabbed my bit and guided it right through. I did buy a new bit to do this job and it did all the holes in less than 5 minutes total time. I think if I had messed up I would have entered a water jacket but would not have lost compression due to bad drilling. If the valve cover is off the engine just rotating to top dead center on each cylinder would allow you to spin the push rod by hand and feel if it is loaded our unloaded. They should all be unloaded at TDC for each cylinder if you follow the firing order. ------------- |
Posted By: 64sleeper
Date Posted: April-16-2018 at 1:40am
|
I read an old post you had about swapping gt40 heads. I seem to have the exact same problem after swapping g40 heads onto a 1995 bronco....low compression about 90psi on most cylinders except the back ones have 120psi. Just wondering what you figured out on yours before I start tearing things apart. Thanks, Erik ------------- EW |