Yanmar 4JH3-HTE down on power - revs slowly drop off

[Sam McD]

Hi guys,

I have a Yanmar 4JH3-HTE in a Bene 523. We have had a few most likely related issues since we brought her.

Firstly, some details.

• Boat is 2005 and motor original
• 2400 hours
• Cont rating - 92hp @ 3700RPM
• Max 100hp @ 3800RPM
• Max RPM 4300RPM
• Max prop - manufacturer recommends 18 degrees of blade angle for our boat/motor combo

When we got the boat, the prop was set at 22 degrees. The motor would make around 2500 RPM and do ~8-8.5kts.

The skipper said they intentionally over pitched the prop for long motor sailing (boats been to the islands a number of times)

The motor slowly over time started loosing its ability to make over 2000 RPM, and developed an issue where it would stall at idle - not all the time, but regularly, especially after long runs over 2 hours

We spent days checking the fuel system from tank to just after the last filter (we haven't touched the pump itself yet). We relocated the Raycor filted what was the highest part of the fuel system, to become the lowest bar the tank itself, and that seems to have fixed the idle issue - it hasn't stalled since.

We figured the low max acheivable RPM was down to the over pitched prop, so we quick slipped her yesterday and set the prop at 18 degrees as recommended.

As soon as the boat was lowered in the water, we went straight out the marina and put the hammer down. She made a nice looking 3200RPM and 8.5kts, then after a minute or two (assuming when she heated up a little), the RPM slowly started to fall away, down to about 2500 where she held.

We stopped the boat and let her idle a bit, then tried again. This time she only made just over 2200RPM and wouldn't go any higher. Shes defintiely down on power big time.

We have since checked the following:

• Out of gear, she will free rev to max RPM - 4000+
• Checked the throttle control and its moving full motion and not springing back or anything
• We removed the air filter (its just a mesh cage, so no restriction there), and checked the turbo - no broken blades, no shaft play, spins find while running. Checked the exhaust side too and no major carbon build up
• We did a turbo wash as per the manual just for good measure - no change
• While we slipped her, there is no unexpected resistance in the prop shaft - she turns smooth and easy
• Both engine and transmission were just serviced so clean oil in both (not really a fault finding step, just worth noting)
• Fuel filters were replaced recently, and tank inspected for bug - all looks clean
• The motor does have a bit of blow by - there is vapor coming out the breather, but no notable level of oil accumulation in the catch tank
• Just an observation, but it doesn't really feel like she's sucking a great deal of air into the turbo inlet. I had to get the turbo detergent pretty close to the neck of the turbo for it to even vaporize and suck in at ~2500RPM

When we purchased her, an oil sample was sent away and came back clear. No compression test was done

Below are the comments from the inspection:

"A sea trial was carried out and data collected throughout the engine RPM range (copies of which will be sent to you). The engine operation did not meet full rated RPM (3800), only reaching a maximum RPM of 3400 before dropping to 2500. The boat captain then replaced the fuel filters and a second sea trial was carried out, reaching and maintaining a maximum RPM of 3027. This suggests that there is a possible restriction in the fuel system that will need to be rectified. I also believe that the reason for the engine not able to reach full rated RPM is due to the propellor being over pitched."

So I'm looking for the most logical diagnosis process from here. What do you think of this and have I missed anything?

1. Fit boost gauge - check boost/vacuum
2. Check valve clearances
3. Compression test

If these come up clear, I'm guessing fuel pump/injectors are next?

Are there any other simple tests I should look at before pulling parts off?

[Sam McD]

Sorry I should also mention the engine starts first time with the flick of the key. No cranking required. She just starts right up as fast as can be.

[Dockhead]

Sounds like fuel, although - that's a trivial observation since 80%, maybe 90% of all diesel engine problems (in my experience at least) are fuel.

Classic diagnostic move in this case is to fuel the engine from a clean jerry can of clean fuel, bypassing the primary filters, fuel pickups, tank. Use a piece of new fuel hose. Use a new secondary filter. If the engine now runs right, work your way back.

Note that these engines are exquisitely sensitive to even tiny air leaks in the fuel system. I had similar problems with my own 4JH3 HTE and after a long and maddening search, finally found a tiny air leak, which was the cause.

[osprey877]

I'm not a mechanic and may be wrong, but I think I read somewhere that the usual symptom of an air leak in the fuel system is hard starting because of loss of prime in the injection pump. The OP reports no problem with starting, which also suggests lack of compression is not the problem.
You mention you checked the fuel line and filters. Did you also check the return line? Disconnect it at the injector pump end and you should be able to blow the fuel sitting in it back into the tank and it should then be clear. If not, check the tank end.
If that doesn't work, I would follow the advice given above and pump clean fuel directly from a jerry can. If it's still not working, fit the vacuum gauge on the fuel line and it should tell you if your injector pump is working and/or you have an air leak as mentioned above. Good luck.

[Dockhead]

Originally Posted by osprey877

I'm not a mechanic and may be wrong, but I think I read somewhere that the usual symptom of an air leak in the fuel system is hard starting because of loss of prime in the injection pump. The OP reports no problem with starting, which also suggests lack of compression is not the problem.
You mention you checked the fuel line and filters. Did you also check the return line? Disconnect it at the injector pump end and you should be able to blow the fuel sitting in it back into the tank and it should then be clear. If not, check the tank end.
If that doesn't work, I would follow the advice given above and pump clean fuel directly from a jerry can. If it's still not working, fit the vacuum gauge on the fuel line and it should tell you if your injector pump is working and/or you have an air leak as mentioned above. Good luck.

One data point on this -- I had an air leak problem with my 4JH3 HTE and had no problem whatsoever starting it. My Yanmar has always started with half a revolution; just look sideways at the key and it starts. I've never used the air preheater even in below freezing temperatures. I'm not sure it even works! So I wouldn't take easy starting as a sign that there is no air leak.

[Seasic]

I've had the same problem, in my scenario I had just had the fuel tank polished, all filters changed and a couple weeks later did the same thing, my case it was the exhaust mixing elbow was clogged.. in neutral engine reved and performed well but in gear would not go above 2000rpm, it was a one hour job for me to remove and clean the mixing elbow, problem solved..
Best of luck!!

[Mike Banks]

Blocked exhaust elbow would be my first port of a call.

[Sam McD]

Hi guys,

Thanks for the replies and advice.

Re the mixing elbow - I have had that off in the last 2 weeks to weld up a pin hole leak and that was free and clean. Its spotless now after a wash before welding. So we can rule that out. I haven't checked anything further downstream in the exhaust though, so that could still have a block somewhere. Ill add that to the list to check.


When I was trying to track down the idle issue, I did run the motor from a bucket of clean fuel, but we never took her out of the dock for a run like that, so maybe thats the next thing to try. Will rule out the tank, pickup, primary filter and return all in one hit. I think I will chuck a new secondary filter on and give that a go.

Compression testing her before Xmas might be a bit of a mission - I'm trying to track down an injector blank for the clamp in style injectors (or an old injector I can modify for the tester) with little to no luck here in New Zealand. I can order one from oversea's, but the way shipping is at the moment, it wont be here for months. Plan B is to find someone with a metal lathe and make one.

Keep any other ideas coming if you got them. The more I can rule out next time Im at the boat the better!!

[Mike Banks]

Maybe there is a blockage in the fuel tank breather pipe. If you are on the hard wasps can plug them up.

[Sam McD]

Originally Posted by Mike Banks

Maybe there is a blockage in the fuel tank breather pipe. If you are on the hard wasps can plug them up.

Thanks Mike - will rule that one out as well. Easy check will be to remove the fuel fill cap and see if that makes a difference.

[kenbo]

After you rule out fuel supply by remote fueling my thinking is dirty fuel injectors. Prop pitched to 2500 is awfully low for those high revving Yanmars. That engine has been lugged a lot. Diesel power is produced by fill timing and fuel amount. Assuming you haven't changed the timing it's all about fuel amount.

How did you confirm the pickup tube is clear, did you open the tank and physically inspect it? Some of your symptoms lean toward pickup clogging. So with remote fueling, if the engine produces power throughout the rpm range I'd open the tank and check the pickup. If power is not produced then injectors would be next. If their not too carboned up they pop out fairly easily

Blowby is what I would expect to see from a lugged engine. Not an encouraging sign.

[Sam McD]

So I swung by the boat tonight and did a couple of things in the limited time I had. Will be back again later this week to try the fuel in a bucket test.

I replaced the secondary fuel filter with a new one - just in case.
I installed a boost gauge on the inlet manifold and remembered we installed a clear hose in the return line outlet on the fuel pump so took a look at the returned fuel while running.

Took the boat for a run and no change. Maxed at 2500 RPM and was only making 2 PSI of boost.
The fuel in the return line was clean of any bubbles and flowing what I assume is well. It was moving a fair amount of fuel - i.e. maybe a cup full every 10 seconds at a guess. Definitely not dribbling.

I did notice a bit of white/light colored smoke at startup/warm up at idle. There was no wind at all so nothing was blown away before I could spot it. Nothing major, but noticeable. Drifted half the length of the boat before disappearing.

I also took the fuel cap off while we were out and that made no change, so I think we can rule out a blocked tank breather.

We checked the feed and return lines when looking for the idle issue a couple of ways. First by blowing through them. They seemed clear. We then took the feed line off the Racor primary filter and put that in a bucket. We also swapped the whole fuel system with the generator intake/racor and return.
On top of that, we opened up the fuel tank inspection port and had a look, then stuck a endoscope camera in the tank and had a real good look at the pickup tubes (they don't have strainers on them in this tank. Just a straight pipe in a baffle that's about an inch or two off the bottom). I would think it unlikely that both the engine and generator pickups were blocked (genny runs fine on both fuel systems), but anythings possible.

So lack of boost - What do you think I check next? Downstream exhaust blocked? Turbo bearings shot? Blocked intercooler? Still a possible fuel issue? Any chance valve clearances could cause that much of a boost shortage?

I will still do the fuel in a bucket test later in the week and report back, but the complete lack of any boost is interesting. I might see if there is a way I can fit the boost gauge between the turbo compressor housing the the intercooler to see if there is much more pressure there.

[blondjohn]

We have a Yanmar 4JH2-TE, and I'm not familiar with your engine so I don't know if this is applicable, but a couple years ago our engine gradually lost power over a few months time until it would not run in gear, yet it ran to full rpm out of gear. I checked everything you are checking until I figured out it was a fuel supply problem caused by the governor. The old 4JH2-TE has a mechanical governor on the end of the injection fuel pump that is controlled by rpm and turbo inlet vacuum. If your engine has a cold start mechanism you might try running the engine with the cold start fully engaged. On our engine this disconnects the fuel limiting function of the governor and it immediately solved our problem. I was told by a trusted mechanic that running our engine with the cold start on would not hurt it and we ran it that way for months (RMI to Fiji to NZ) before I rebuilt the governor. Good luck!

[LittleWing77]

Originally Posted by Dockhead

Note that these engines are exquisitely sensitive to even tiny air leaks in the fuel system. I had similar problems with my own 4JH3 HTE and after a long and maddening search, finally found a tiny air leak, which was the cause.

I dunno, guys, this problem has been coming on gradually, hasn't it, so my gut is with Dockhead.

LW77

[Sailmonkey]

The two psi of boost seems awfully low to me. Do you have an after/intercooler on this engine? It may be clogged. Another option may be a loose or missing hose clamp on one of the after turbo air hoses.