Logo
FULL SITE MENU

How screwed am I? Low compression in 1 cylinder after overheat

B

Bowenlhzn

Member
I’ve got a 98 Johnson 150 fast strike. Brand new power head last year only took it out a handful of times since I rebuilt it with the new power head. I took it out this week and had to go through a channel connecting 2 lakes. In that channel somehow the water intake got clogged by some weeds not letting any water in and when I took off across the other lake the motor overheated to the point that it shut off when I went down to an idle. It was hot enough that the rain hitting it when I took the cowling off was sizzling and it melted the temp sensor. After letting the motor sit for around 2 hours or so I attempted to start it and i after cranking a few times it started right up ran it the rest is the day no problems, even trolled with it a while with a large drift sock in the water slowing the boat down and putting a bigger load on the motor. I couldn’t then and still can’t tell if it’s idling a bit rough or if it’s my imagination and it’s just a normal 2 stroke idle. I did a compression test today (couldn’t find my hose attachment so I couldn’t let it warm up and the test was done cold, very cold it was about 40 degrees this morning) and 5 of the 6 cylinders came in right at 90psi and one came in at 73ish (I believe normal is 110-120 but assuming lower numbers due to how cold it was). So I guess I’ve got a couple questions. If I ran it as is since it seems to be running ok still maybe just a little rough idle will I cause much extra wear/damage to the motor, will that cylinder continue to drop compression or will it stay about where it is at, would honing the cylinder help at all? I haven’t taken the head off yet but I have a camera I put in the spark plug hole and I could see 2 light scoring marks on that cylinder.
 
During the rebuild did you find / correct the cause of the failure ?-----Why did overheat warning not sound?----Why did S.L.O.W. not kick in?------I would say it needs to be taken apart again.
 
racerone said:
During the rebuild did you find / correct the cause of the failure ?-----Why did overheat warning not sound?----Why did S.L.O.W. not kick in?------I would say it needs to be taken apart again.
Click to expand...
Yeah I corrected the issue, the vro pump went out originally on the previous owner. He said he thought carbs just needed cleaned I bought it to fix and get a cheap boat I’ve done quite a few rebuilds on cars and trucks before and did some time as a diesel tech so I figured I could easily handle some carbs, but quickly diagnosed the oil pump wasn’t working and 2 cylinders has around 35 -40 psi when warm. since I rebuilt it I’ve just been mixing gas and bypassed the pump. The previous owner installed a different throttle control box looks like it’s the newer model, I looked at it this morning and it doesn’t look like it has the warning horn built in like the original ones had. Correct me if I’m wrong but the original ones for this year had the horn built in and the newer ones have an external horn and I don’t see that anywhere. so that’s one thing I’ll need to fix. The only thing I could think of for S.L.O.W was when I took the cowling off the plastic nuts that holds in both temp sensors had melted and the temp sensors popped out of the head and were dangling in the back. I’m wondering if it did kick in while I was in the channel but they popped out and the ambient air temp inside the cowling was low enough that it no longer set it off
 
Last edited:
Took it out on the lake today so I could do a compression test on a warm motor. 5 of the 6 cylinders tested about 105 and the low one tested at about 85. Took it around the lake a few times and it seemed to be just fine besides maybe a slightly rougher idle. 85 is definitely low and outside the 10% that I typically look for, but it seems to be running fine. If I kept running it like this do you guys think compression will continue to drop on that cylinder significantly quicker than the others or has the damage been done and as long as it’s getting lubricated should it stay right around there? Around me there is pretty much nowhere that can bore this block, when I originally attempted a rebuild I brought it to 3 different shops over the course of 2 years for all of them to tell me they couldn’t do it which is why I bought a new power head but I can’t find any for sale anymore. Do you think that a hone could bring it up at all even just a few psi to get it on the low end of that 10% range
 
Bowenlhzn said:
5 of the 6 cylinders tested about 105 and the low one tested at about 85. Took it around the lake a few times and it seemed to be just fine besides maybe a slightly rougher idle. 85 is definitely low and outside the 10% that I typically look for, but it seems to be running fine. If I kept running it like this do you guys think compression will continue to drop on that cylinder significantly quicker than the others or has the damage been done and as long as it’s getting lubricated should it stay right around there?
Click to expand...
With 5 of 6 cylinders okay my guess is you have a bad O-ring from the overheat or the head slightly warped.

It's easy to pull the cylinder head on these engines. Remove the head on the bank with the low cylinder and visually inspect the cylinder wall. If the wall looks fine the next thing is to check the head for flatness. If the head is warped I would have it milled or sand it on a piece of glass to get it flat. Once the head is confirmed good, then reinstall it with new O-rings ($15/each) and retest your compression.

Also, as Racer said your warning horn, HOT ENGINE light, and SLOW should have kicked in when before things got catastrophic. You need to track down some wiring issues as well to make sure this doesn't happen again.
 
kevinj said:
With 5 of 6 cylinders okay my guess is you have a bad O-ring from the overheat or the head slightly warped.

It's easy to pull the cylinder head on these engines. Remove the head on the bank with the low cylinder and visually inspect the cylinder wall. If the wall looks fine the next thing is to check the head for flatness. If the head is warped I would have it milled or sand it on a piece of glass to get it flat. Once the head is confirmed good, then reinstall it with new O-rings ($15/each) and retest your compression.

Also, as Racer said your warning horn, HOT ENGINE light, and SLOW should have kicked in when before things got catastrophic. You need to track down some wiring issues as well to make sure this doesn't happen again.
Click to expand...

Yeah I was hoping that’s what it was. I’m not usually that lucky though, but your comment inspired me to check it. Popped the head off and about 1/4 of that ring was completely missing. Not sure how I got any compression readings with how much was missing. A marine shop down the road has a bunch for $6 a piece so I’ll pick one up tomorrow. Did a visual inspection of each cylinder and they look good so hopefully I got off ok. Ordered a warning horn I took the throttle box apart and found the wires that are supposed to go to it that were wrapped up inside going to nothing so that should fix that issue as well.
 
Bowenlhzn said:
Yeah I was hoping that’s what it was. I’m not usually that lucky though, but your comment inspired me to check it. Popped the head off and about 1/4 of that ring was completely missing. ....Did a visual inspection of each cylinder and they look good so hopefully I got off ok. Ordered a warning horn I took the throttle box apart and found the wires that are supposed to go to it that were wrapped up inside going to nothing so that should fix that issue as well.
Click to expand...
It sounds like the O-ring is your issue. Make sure you check the head for flatness as well before reassembling. Definitely get that warning horn connected so you avoid future issues.

Does your boat have a System Check Tach or 2" System Check gauge? If so, the WATER TEMP light should have come on when the engine got hot and SLOW should have engaged limiting the engine to 2500RPM.
 
kevinj said:
It sounds like the O-ring is your issue. Make sure you check the head for flatness as well before reassembling. Definitely get that warning horn connected so you avoid future issues.

Does your boat have a System Check Tach or 2" System Check gauge? If so, the WATER TEMP light should have come on when the engine got hot and SLOW should have engaged limiting the engine to 2500RPM.
Click to expand...
Yeah it was definitely the O ring, picked up new ones for all 6 cylinders just to be safe and sanded the head on a piece of glass and the head seemed good still, I put a light coat of sharpie and it all got sanded off within a few passes on the sand paper. After replacing the o rings got compression numbers of 105,105,103,100,100,100. Seems pretty good to me.

The boat doesn’t have any warning lights on it, only gauges it has is fuel, a tach without any lights and a speedometer also without any lights. May look into picking some up though to prevent any future issues. I think I got off pretty lucky this time but don’t want to chance it in the future. Thanks for your help kevinj and racerone!
 
Okay, even readings on the compression is good and around 100PSI is right on these engines. You got very lucky and I'm glad it came out well.

At minimum get the warning horn working. If you have the wiring harness for a System Check gauge, you can find aftermarket 2" gauges for under $30 and BRP branded for about $60 here --> System Check Gauges. I try to use this MarineEngine for this stuff, but they show NLA for that part.
 
Back
Top