A4 not working under load

[Phil_E]

About 3 weeks ago I went to start my engine and found that 2 of the valves were stuck open. I removed the side plate and got the 2 values working again. Since then the engine has started twice and run pretty good under load, smoothly but not lots of power which is the norm for my boat since I got it (everyone waves to me as they go by!).

Last weekend I started the engine and it was idling perfectly. I rev’ed it a few times in neutral and it sounded great, put in reverse and found no problems. When I switched to forward, the engine runs rough like it’s missing and there is a huge lack of power, but it never dies. I shifted back into neutral and the engine runs perfect, rev’ed in neutral and it goes up and down with the rpms perfectly, back in drive it’s no power again and running rough. I repeated this many times always with the same result. I went out for the day using the engine working poorly for about 20 minutes in the morning and ran it again at the end of the day on my way back to the slip with the exact same problems.

Normally I would think it’s a problem with the transmission since it only happens in forward, but since this is right after the problem with valves I wonder if it’s a problem with the head gasket failing under load and letting a little mosture in to make the valves sticky.

Also, I have had problems in the past with jets in the carb which I have fixed with a brand new carb and filters. This is not that problem. Pulling the choke out does nothing to help this.

-Phil

[JOHN COOKSON]

The first thing to do if you suspect a trany problem is to let the shift cable loose and shift by hand then go from there. Hopefully all you will need to do is adjust the shift cable.

TRUE GRIT

Edit: An exhaust system problem is a possibility. Are you running on all 4 cylinders? Maybe a valve is sticky as you suspect.

[Laker]

It could be that one valve is stuck open. The engine will run fairly well on three cylinders , but bog under load. Reverse is 2 to 1 reduction , I believe , so that might mask the problem somewhat.

[Phil_E]

I replaced the cable about 2 months ago. I can feel it go into the "detent" position and there is no way it can go any further.

[Phil_E]

Originally Posted by Laker

It could be that one valve is stuck open. The engine will run fairly well on three cylinders , but bog under load. Reverse is 2 to 1 reduction , I believe , so that might mask the problem somewhat.

I have heard this from others as well. If this is the case how do I fix the problem? I'm already running MMO in with my oil.

[Mo]

I'd check fire, plug wires and plugs. Remove plugs do a visual inspection.

http://www.xs11.com/xs11-info/tech-t...look-like.html

There is one sx they didn't mention in the above link...water. If you have water entering the combustion chamber the plug usually looks very clean and might have a water bubble / bead on it...gas doesn't bead.

Clean them if necessary, ensure gap is correct and that they have all have spark at the end. If you don't find a problem there I'd look to the valves and ensure they are all working.

Another thing I might check is the prop if the boat hasn't been used.

[Laker]

Phil ,

To check for stuck valve , just do a basic compression test - disconnect coil wire , remove spark plugs , don the safety glasses , place thumb firmly onto #1 spark plug hole , and briefly crank the engine. The compression will wiff your thumb off of the hole , unless a valve (probably exhaust) is stuck open. Repeat for each cylinder.

If a valve is stuck , you may be able to fabricate some sort of a bent tool with which to reach through the plug hole to the valve head and push it down , but I have not personally had luck with that technique. What I did was remove the head , lube the offending valve stem , rotated the engine 'till the same cylinder intake valve is standing open , then tapped the stuck exhaust valve down using a section of softwood dowel as a buffer between the valve head and a small hammer. Crank the engine and see if the valve is free ; if not repeat the process. I had one particularly stubborn valve which required me to place a longer dowel right on top of the valve , then push down hard as I cranked the engine through many revolutions , pushing the valve down with each revolution. THAT finally did the trick. (after 4 head removal removals)

It CAN be done. Good luck.
P.S. - I love the Triton.

[Loki9]

While it's running, try pulling the plug wires off one at a time. This will tell you right away if any cylinders aren't working.

[edwardc]

Originally Posted by Laker

... Reverse is 2 to 1 reduction , I believe , so that might mask the problem somewhat.

Actually, I believe it's 1.3:1, but you're right, this will help mask a loading problem.

[tenders]

This sounds a lot like it could be a blocked exhaust. Troubleshoot by disconnecting the exhaust at the manifold, plugging your ears, and running the engine in gear.

Also: have you completely ruled out a fouled propeller? By "fouled" I mean "fouled with growth." A few barnacles make a big difference, and a lot of barnacles make the engine behave very badly.

[sastanley]

Originally Posted by tenders

This sounds a lot like it could be a blocked exhaust. Troubleshoot by disconnecting the exhaust at the manifold, plugging your ears, and running the engine in gear.

Also: have you completely ruled out a fouled propeller? By "fouled" I mean "fouled with growth." A few barnacles make a big difference, and a lot of barnacles make the engine behave very badly.

+1
I agree with tenders on a blocked exhaust, or a dirty prop.

[Phil_E]

I am going to check all these things out this afternoon. I'll post what I find out.
Thanks!

[Kurt]

In my experience, an exhaust restriction (even a small one) will result in bogging under load AND in neutral when revving the engine. As you say the engine runs normally when revved in neutral (eg. doesn't bog), I would venture to say you don't have an exhaust restriction resulting in elevated exhaust backpressure.

[Mo]

Hi Guys,
I understand that exhaust back pressure can cause loss of power, however, it is very important to rule out the easy fixes. We are looking at an hour to do the obvious checks. I think he needs to check for proper ignition, fire getting to the combustion chamber, check compression as suggested by someone here. Water in fuel can cause it to run decent at idle and then bog under load...I know because it happened. I'd even take the carb apart again and blow out the jets before I started hauling the exhaust apart.

Once you go at that exhaust something is going to break...a nut strip, stud twist off, create a hole etc....then it becomes time consuming and it might not be the problem. Go for the exhaust once you have ruled out everything else...because you will likely be rebuilding it. ....and the first check is the prop.

[Laker]

Ah , dear Watson - this episode began with stuck valves.

Spark plugs are easy to pull. Moreover , the above mentioned one by one spark plug wire removal test is easiest of all. Start there.

[positron]

I had a similar set of symptoms with rough running when I put it into forward. It ran OK at lower RPMs, but above about 2800? RPMs (3.8 knots) it got really rough. Reverse and neutral worked fine. I adjusted the transmission to get a tighter clamp on the clutch plates and the problem went away. Now the engine runs nice and smooth (except for some valve clacking), but I can't get over about 4.2 knots. If you haven't adjusted your transmission in a while, try this easy fix first. It took about 30 minutes including refering to the Moyer manual a couple of times.

[sastanley]

laker, good point.

Run the motor under load (tied to the dock if you don't have a driver) and see what happens when you pull & re-insert one plug wire at a time to see if there is any change in engine RPM...that will diagnose sticky valves. I had a sticky valve, and squirted MMO into the cylinders after each run for many months until the pesky #4 valves finally freed up.

However, I would confirm the prop is clean first. Also from personal experience I can tell you a dirty prop will yield 1,100-1,200 RPM max and simulate a very poor running engine.

I agree that the exhaust restriction pull apart & check can be time consuming and tedious (I've done that one too.) I'd do the above two first.

[Phil_E]

Only bad news I'm afraid. Exhaust valve is stuck open, but this time it's stuck pretty hard. I filled the cylinder with carb cleaner but I doubt that will help. The plug has little water dots on top of very heavy carbon. I just changed the plugs about 1 engine hour ago.

Now for the really bad news.....oil has water in it!

Next the head comes off to see what's going on and to tap the valve loose.

I have a new respect for the A4....with all that wrong, it still made it back to my slip from out in San Francisco bay and never quit once.

[Laker]

Phil -

The good news is : you are getting somewhere. Now you know. You will get that valve free , once you get the head off , but - stating the obvious here - you need to find out how water got into that combustion chamber. (Which cylinder was it?) Hopefully it is a simple failed head gasket.

My head came off with all studs intact , (two threaded out of the block) , but it took a massive effort to separate the head from the block. You now need advice from those who know about what measures to take regarding preserving the studs and reducing the degree of difficulty of separating the head from the block. ...?

[Marian Claire]

Phil: How is the access to the head/block joint? I was able to remove my head with all the studs in the block. Tap Tap Pry Pry and work your way around the entire outer edge. I used putty knife, old wood chisel, hive tool, flat bar, wood shims etc. Just go very slowly. Access and patience are key. Dan S/V Marian Claire

[sastanley]

Phil, sorry to hear the bad news.

Just to make sure, you didn't have any 'hard to start' issues did you? If you crank the starter for too long with the raw water intake open, it will back water up the exhaust and dump it into the cylinders...starting with #4 & a sticky valve in that cylinder is an indicator of that possibility.

If my engine won't start within 5 to 10 seconds of cranking, I close the thru-hull, get the engine running, then open the thru-hull again.

3 or 4 oil changes will be needed to get the water out of the crankcase once you get it running again. (I'd use cheap oil for this procedure.)