Low WOT 4.3GL (AQ)

[MikeD105]

After overheating my engine trying to get off a sandbar (stupid) my maximum WOT dropped to 4000 RPM's. Immediately afterwards I changed out my exhaust manifolds and risers only because of their age. Other than the loss of RPM's at WOT the engine does start and run good/smooth. Prior to writing this message I did check the butterfly's in the carburetor to insure that they are fully opened at max throttle. They were. I then attached a vacuum gauge to the carb to check the vacuum at various RPM's. 800-900rpm - 16in HG, 2000rpm - 20in HG and 3000rpm - 14in HG. At all rpm's gauge fluctuation was + or - 1/2in HG. Short of checking the engine timing I have run out of troubleshooting options and could sure use a little help. Forgot to mention the compression in all cylinders is normal with #1 being on the low side @ 125psi. Any ideas?

 

[jimmbo]

How badly was it overheated, by how much, for how long? Did the horn sound?(detects overheated exhaust elbows) If so, how long was it run before being shut off? Which cylinder is the low compression? How do the spark plugs look? Lots of things can happen when an engine is overheated. What year is the engine? Not all VPs have them but some engine had backflow shutters in the down pipes. If yours has them they may not be fully open. There might be some scoring in the low cylinder, it won't affect low speed operation yet. Cylinder head gaskets often leak after an overheat, depending of course on how long and how hot.
Have you inspected the raw water pump? The impellers suffer when starved of water and/or fed a diet of sand

 

[MikeD105]

Thanks for your reply Jim. It overheated to the point where the rubber couplings attached to the riser began to melt sending black smoke into the air. (no backflow shutters installed) That's when I shut the engine down. I did not hear the horn (engine temp) possibly due to the speed I was traveling. Replaced the raw water impeller prior to and checked again afterwards. That was OK. My engine was manufactured in 1999 and has 300 hours.

Wouldn't low vacuum readings reflect a bad head gasket? Would you take a peek at the vacuum readings I mentioned in my first post and see if they reflect healthy valves and gaskets?

#1 cylinder had the lowest compression at 125lbs the rest were near 150 or slightly above. Thanks so much for your help

 

[jimmbo]

Sounds like it got pretty hot. Is the motor making any knocking, or new clicking noises? Since the oil was the only heat removing agent during the overheat period, I would recommend you change it as it too may have been overheated and possibly compromised.
The horn is located on the engine so it is not always heard.
While 16 inches at idle would have some people worried, it might just be normal depending on cam specs(overlap, and lobe separation angles. The +-1/2" fluctuation could be an indicator of a weaker cylinder, or a rich carb mix. Is the fluctuation rapid, or slow? Do you have the records of any earlier compression tests to compare this one to? Overheating the engine can cause manifolds and/or heads to warp, resulting gasket problems. It can also burn the valves. Put a little bit of oil into cylinder number one, then take another compression test. If the numbers increase, the rings and cylinder are sealing better, which could mean scoring/scuffing. If the numbers don't increase, then there might be a leaky gasket or burnt valve. You say you hit a sandbar, any chance the prop got 'modified'? That could be your rpm variance. BTW what was the peak RPM you used to have?

 

[MikeD105]

Thanks for your reply Jim; although I do wish your answer would have resulted in an easy fix. No such luck.
Regarding your concerns: a) yes I did change the oil afterwards. b) The vacuum fluctuations were rapid but minimal, c) I did put a new prop on the boat and no change in RPM's, d) prior to my stupid mistake my engine ran at 45-4600rpm's at WOT. Today 3900-3950rpm's with a reduction in speed of almost 10mph and e) no engine knock although I can hear a slight tapping of one lifter.

When it stops raining in SW Florida (a rare occurrence this time of year) I'll go out and recheck #1 cylinder before and after shooting oil down the bore. I did this before with #1 but never wrote the readings down. Thanks again for your reply.

 

[MikeD105]

Took the boat out on the lake this afternoon and ran it at WOT, 3900rpm for about 10 minutes. When I slowed down I heard a pop sound, poping on every engine cycle. It sounded like a blown head gasket. Upon examination I couldn't believe what I saw! The spark plug in #3 cylinder blew a hole at the very top of the plug where the electrode meet the bakelite. It then appeared that the insert that the spark plug screws into came out of the block about 1 inch. I always though that the engine block was cast in one piece with the exception of freeze plugs, etc. Can someone explain what happened and if a new engine is in my future?

 

[jimmbo]

It sounds like someone stripped the spark plug hole threads and use a insert type repair kit. The overheating may have have compromised the repair. My dads outboard had an insert repair on the spark plug, and it too let go.
In you case, it might be possible to get it reinstalled, or replaced with a new one, or a different insert type tried. Worst case is a replacement head for this. Removal of the head let's you get a look at the walls of #1

 

[mklearl]

Helicon let go. Most a put in and secured with locktight. Depending on which lock tight product you use the only way to remove a helicon is with extreme heat. .. which you had. I'd start there.

 

[MikeD105]

No Title

I agree. It must have been an insert that was compromised by the overheating. Please confirm per attached picture. I already considered installing a re manufactured engine but since this insert let go I may as well take the head off and examine the cylinder walls. Then for sure I will know if the block needs to be replaced.

 

[Baylinerchuck]

That is strange looking indeed. Attached is an image of a helicoil. I used a similar system numerous times to replace stripped threads in aluminum housings at work. I have also used the carbon steel inserts a couple times in heads with great success. I always considered them a permanent repair.

 

[Baylinerchuck]

No Title

Forgot attachment.

 

[Fishhead-1]

Were those extenders used on all cylinders?

 

[MikeD105]

Good question. I guess I won't know until I take the heads off but I'm guessing all 6 cylinders will be the same. What would be the purpose of using "extenders" in the first place? This certainly has me baffled
Now the hilicon pic: That is exactly how I pictured a threaded insert to look, not like what I removed. Sunday will be my tear-down day. Any tips?

 

[mklearl]

New head might be in order. Helicoil (sorry for the auto correct of helicon) are about one size bigger than the stripped threads. They come with a tap and they basically screw into the re tapped head. Really easy to do. Unfortunately it looks like someone tried to manufacture something to fit in there and they drilled a hole way bigger to fit their "fix" in it. I know they make different size helicoils but not sure if you'll find one to fit that.

 

[jimmbo]

I don't know what the insert that Baylinerchuck showed is, but Every Heli-coil I've ever seen looked like this

 

[Fishhead-1]

I call them extenders, maybe non foulers I think there called. Used to keep the spark plugs from fouling from whatever bandaid the installer was trying to prevent happening. I used to see them in forklift engine that were usually tired.

 

[Fishhead-1]

I have seen some thread inserts the machine shops use that are solid, threaded inside and out and have 4 tapered pins on the outside that are driven in to hold it in place. Problem with a helicoil is knocking that tang off after threading in falls into cylinder.

 

[jimmbo]

I've just grabbed it with needle nose pliers and it snapped off

 

[MikeD105]

Showed the extension to a Russian friend of mine who mentioned that similar extension inserts were used in the old Soviet Union to compensate for octane variations in the gas. Now that made sense except for the fact that it would have been much easier to retard or advance the timing.
Anyway, I'm pulling the heads today and hopefully the cylinders will be clean and smooth upon close examination. If I have to buy new heads so be it. Better than buying the entire engine. $$$saved.

 

[jimmbo]

I've seen shorter extensions used in Mosquito Control(burning oil) engines. The theory was the extra length kept the plug from cooling down and fouling. In reality air/ fuel didn't always find its way to the plug and it would misfire anyway and then foul. It also effectively retarded the timing on that on cylinder.