Problems with NOS Cummins 6bta

Long story, but I can use help.

I have a Cummins 6bta, 250 hp. 1990 cpl 1241. The motor was never in service, and I bought it last fall to replace my ailing 6bt. Kept the same transmission and drive train from my 210 hp bt with the intent to deliberately have the boat underpropped.

In preparation for service I did a few basic things. I don't have a shop or good work space, so it was pretty cursory, but all looked good. Motor barred over smoothly, everything nice and clean.
- changed fluids
- replaced some hoses and front seal
- checked valve lash
- new impeller

I pulled the old motor myself, sold it, and put new seals in the old VD 72 transmission which I kept. Also put in a flexible torque adaptor plate. Bolted it all together and installed in the boat.

After launch the motor wasn't getting fuel to the injectors and wouldn't start. Not completely unexpected. So pulled the rotary pump and all injectors and lines and visited the local injection shop. They cleaned the pump and lines. Said there was rust in the pump but seemed confident they had remediated it. Injectors tested fine but the tips had pitting corrosion so replaced the tips. This was a bit unexpected and got me concerned about the internals... Motor had been run on the bench for an hour or two years earlier, and I had visions of nasty deposits left for a long time causing corrosion. No I put some ATF in the cylinders while I had the injectors out thinking it might help free up rings if they had corrosion as well. I also replaced the lift pump.

After getting the motor running - lots of smoke at first from the ATF - I was planning to break it in as a new motor. But a few other issues came up with fuel delivery and guages. I ran the motor for a few hours under light load, 1200-1500 rpm, and had what seemed like fuel starvation issues at higher power. I should also say that the motor was very difficult to start cold. I attributed this also to possible fuel supply issues. Had a mechanical oil pressure guage in the ER that showed good pressure from the first start.

Finally got the fuel system in good order and turned my focus back to the motor. Went out for a few hours. Couldn't get above about 2000 rpm, light white smoke, nothing too alarming until oil pressure started dropping gradually and oil and smoke started coming out of my CCV system. Checked oil and found it up by a lot, limped back in at low speed.

Back to the shop with the fuel pump. Assured that no leak in the shaft seal and fuel delivery specs are correct. Checked new lift pump for weeping fuel, none observed.

At this point my amateur diagnosis was that I had unseated rings, low compression, and incomplete ignition, and that unburnt fuel was washing into the crankcase.

Changed the oil, went out again to try to run the motor hard. I was pretty nervous about fuel dilution of oil and didn't stay out more than two hours total. Even then I was coming back with oil level up by a couple of quarts. I was trying to vary throttle between bursts of WOT and 1500-1700 rpm, still couldn't get above about 2000. I did this twice.

I did see some changes. Started getting grey smoke, then black at WOT. Along with the black smoke came what looked like oil out of the exhaust. White smoke seemed less at lower throttle. Had fuel in the exhaust all along.

After the second day of this I noticed that the paint on the exhaust manifold was heat discolored on all but the front cylinder. Pulled that injector and noticed that the nut came loose very easily, and the injector tip was wet and looked like new, no sign of ignition. I sprayed some Seafoam Creep into the cylinder, left it for a couple of days, and checked the torque on all the other injectors. They were fine. Not sure what happened with this one, but I think the injector wasn't seated properly when I reinstalled the injectors.

Changed the oil again and was out yesterday. Behavior is similar, but no more oil in exhaust, and the rate of fuel being dumped into the crankcase seems to be much less.

So, current state:
- motor still doesn't want to start cold without a good shot of starter fluid
- lots of blowby. I disconnected my CCV and essentially smoke comes out of the breather vent at a high rate all the time.
- not able to get above about 2000 rpm. Heavy black smoke at WOT, white smoke the rest of the time, some blue at startup/idle.
- as noted fuel dilution is occurring, but at a lesser rate than earlier.
- seems to be firing on all cylinders most of the time.

I now have about 20 hours run time. Temperatures have been unremarkable. I have boost and EGT guages new this year, and see 10-15 pounds of boost at WOT (1900 - 2000 rpm) so don't think there is a turbo issue. Max EGT I have seen is about 940.

My theory is still that I have a problem with seating the piston rings, but throw this out for debate. Are there other likely possibilities? Have I missed something in my DIY approach and diagnoses? Assuming that this is the case is there something else I can do short of a major disassembly? Right now I'm reluctant to keep doing what I've been doing. I now have a couple of barrels of dirty oil Am considering pulling all the injectors and dumping some top end cleaner in the cylinders to soak for a few days in hopes of freeing up what I suspect are rings that are frozen in the piston.

I should also add that I am most definitely not overpropped, and there are no issues with throttle linkage or anything like that. Motor gets to full rpm in neutral, and I even got up to about 2200 in reverse yesterday.

Thanks in advance for any and all advice/commentary.

Jeff

Does sound like stuck rings. With engine idling and oil fill cap off, see if blowby is puffing in sync with one cylinder firing. If no paint cook on #1 exh port, that may be the problem hole. Can also crack injector fuel line nut and see if there is any change in rpm or exhaust smoke.

Can you post a vid with audio at idle with oil fill cap off?

High blowby is almost always rings. Might have gotten some corrosion in bore and on startup that stuck the rings.

Hope the video come through. Have cracked injectors and all cylinders seem to be firing. There is now some heat discolouration on #1 after my fix as well. Thanks for your help on this.

Take 2 on video

If the rings are stuck, would it show on a compression test? I would think it would, and be a means of confirmation about what's happening.

Marvel Mystery Oil is what I would pour in for stuck rings. If some of them are broken, however, that is an entirely different kettle of carp.

Years ago I bought a 1960 MGA with the same symptoms. When I pulled the pistons the rings fell apart on the floor.

That is a BUNCH of blowby. Not a happy engine.

Talked to seller, he had a motor shop do a thorough inspection last summer, said they had heads off and all was perfect. Motor was well sealed up when I got it.

I'm going to check the front gears tomorrow to make sure injection timing is correct. Seems like that would account for poor starting, fuel washdown, white smoke and poor power, and may have prevented rings from seating properly.

Thanks for your comments.

Cummins are famous for blowby, I wouldn't give up yet. What I would be wary of is a runaway; have a way to block the air intake near at hand until you can get the fuel issue sorted. If its not your fuel timing, check that the tank return line is free and clear, check the air filter is too. Starting could be the air supply and perhaps even the heat coils aren't working. The 6CTA doesn't need them but the B might?

Break a leg...

Xsbank said:

Starting could be the air supply and perhaps even the heat coils aren't working. The 6CTA doesn't need them but the B might?

Break a leg...

Click to expand...

A Cummins B should easily start at any temp above freezing without the heaters (even lower). I had a couple of them.

Good luck!.

High blowby is symptomatic of internal engine failure of the rings Vs the cylinder walls.

Hard starting, inability to make full power, fule diluting oil, smoke, are other symptoms of a cylinder or cylinders that have failed.

Unfortunately the 5.9L cummins is not an engine that can be rebuilt internally in place. This is most likely going to require removal and tear down.

Jeff F said:

Talked to seller, he had a motor shop do a thorough inspection last summer, said they had heads off and all was perfect. Motor was well sealed up when I got it

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Did they run the engine on a stand or dyno?

I'm starting to doubt that is a NOS engine. Seems to run more like a clapped out high hour takeout from a sportfish. How sure are you of the history of the engine?

I have been around lots of engines that sat for years and unless moisture got in a hole and rusted things up, pistons, rings and liners usually are fine.

Often on B's with high blowby, the cylinder bores are not worn much and a quick hone to re-establish cross-hatch and new rings can get it back into good shape.

Now have a root cause analysis. I was assuming that problems were compression related, but it turns out the camshaft is out of time. So most of my run time has been with the cam and IP synched with each other but out of time with the crankshaft.

I omitted one piece of the story earlier, mostly out of embarrassment but also because I didn't see how it fit in the overall narrative. After I reinstalled the IP the first time and got the motor running there were some pretty awful noises from the front gears after a few hours of running. While I was taking the gear cover off I caught a glimpse of what looked like a bright shiny nut, but it disappeared when I got the cover completely off. Must have gone down the oil drain. There was damage to the cover, the case, and one very small chip in the edge of the camshaft gear. Could see where the object had bounced around the IP gear then made its way down to the camshaft gear and rattled around there. Did an inventory, and no bolts or pins missing from inside the case, so I came to the conclusion that I must have somehow dropped something in when I had the IP out the first time. Put a new cover and gasket on and moved on. It appears that this trauma is what threw the cam timing. There are accounts of this happening in Dodge trucks when the Killer Dowel Pin comes loose.

I should say that I'm at anchor out in the middle of nowhere, still hoping I can resolve this myself without finding a port and hauling the motor. Big question is whether there is a reasonable chance that I can come to a full resolution without doing this. I'm very patient and perseverent and don't mind figuring things out as I go, and am secure and self-sufficient here. I'm hopeful that with proper ignition and full power I can get the rings to set and solve the blowby.

To resolve, I think I need to
a) remove the cam gear, either with a really good puller or by removing the shaft through a hole cut in the bulkhead and having it pressed out. My preference is for the former, but need to find a puller and confirm clearances. Reinstall and retiming seems simple enough. I have an oven to heat the gear.
b) make sure no damage to valves or pushrods. Guess I'd need to pull the head to be sure.
c) change the oil again
d) get underway and load things up.

I should also add that I got a more detailed account of the motor history. It was never installed. When the owner was getting it ready to sell he checked it carefully, ran it, and discovered there had been a leak in the aftercooler core. He replaced the core, and fearing damage to the internals he had it sent to a shop. I have an invoice from 2016 showing complete disassembly, cleaning, and checking against tolerances, including new rings, gaskets and head bolts. Seller wanted to sell the motor in perfect shape and is a straight shooter, and I'm confident that the issues I've had apart from the dirty IP are all of my own doing.

Will post progress. Comments/advice will be read with interest!

I'll try to include some photos where I can. Here's the original gear cover that got mangled by the unidentified foreign object



It was actually punctured slightly in the area where the object got seriously caught in the camshaft gear.

When I took this off I was anchored out in Long Point on Lake Erie. No car. Not much in the way of local services, a decent grocery store is a 45 minute drive away. Local boat people don't do diesel. But as it turns out there's a one-man Dodge Cummins used parts operation about 15 miles up the road. Has an eBay store and ships all over. I called him in the morning, and he delivered a freshly painted cover to me that afternoon. He drove down to meet me at a bridge over a creek that I dinghied into I had a spare gasket and crank seal in my onboard bin. Less than a day of forced downtime.

If there's one really compelling case for the old BT this is it.

I can't see how you could possibly change the camshaft timing without something breaking, even with a nut rattling in there. How can it possibly jump a tooth? Are you sure its out?

Xsbank said:

I can't see how you could possibly change the camshaft timing without something breaking, even with a nut rattling in there. How can it possibly jump a tooth? Are you sure its out?

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It didn't jump a tooth. I had checked all that when I replaced the cover. The camshaft gear is driven by the driveshaft and then drives the IP. And the common TDC reference is based on the camshaft gear position. But the gear goes on the camshaft with a press fit. Not normally ever adjusted or changed outside of a major overhaul. It's that press fit connection that allowed the cam timing to change. There was some rotation between the shaft and the gear caused by the nut getting briefly caught up between the gear and housing. Hard to imagine - rotational inertia on the shaft? - but well documented in the pickup world.

I determined it was out by looking at the valve positions when locked at TDC. Knew exactly what to expect because I had checked this just before I installed the motor. Also have full shop manuals, and the Dodge pickup boys have broken everything than can possibly be broken on these motors. The valve positions now are not at TDC, and this is the only possibility.

sunchaser said:

Did they run the engine on a stand or dyno?

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They ran but not under load.

Jeff F said:

It didn't jump a tooth. I had checked all that when I replaced the cover. The camshaft gear is driven by the driveshaft and then drives the IP. And the common TDC reference is based on the camshaft gear position. But the gear goes on the camshaft with a press fit. Not normally ever adjusted or changed outside of a major overhaul. It's that press fit connection that allowed the cam timing to change. There was some rotation between the shaft and the gear caused by the nut getting briefly caught up between the gear and housing. Hard to imagine - rotational inertia on the shaft? - but well documented in the pickup world.

I determined it was out by looking at the valve positions when locked at TDC. Knew exactly what to expect because I had checked this just before I installed the motor. Also have full shop manuals, and the Dodge pickup boys have broken everything than can possibly be broken on these motors. The valve positions now are not at TDC, and this is the only possibility.

Click to expand...

I'm making some of this on the fly here and much of my research has put me in the non-marine world which can be dangerous. Just reread the service manual and they say it's a keyed connection. So maybe I'm all wrong here. But the valves definitely don't line up against pinned TDC, and I'm sure they did before. What could have happened? Time to sleep on it.

I thought it was keyed as well, the only engines with gears I've seen were keyed, hence my question.

My Cummins book is on the boat, is it possible that the valve train is assembled incorrectly? Have you set the timing pin to check correct valve settings?

Yeah, thanks for keeping me honest. I'm really learning as I go here, but that's the way I roll. Would usually rather puzzle it out myself than outsource to a pro. I was focused on cylinder #1 yesterday and didn't find any lash in the intake valve at pinned TDC and then made a leap too far . Lash had been within spec when I did my initial inspection. After barring the motor over I found that the rocker doesn't unload at any point on that valve. Do valves seat during break-in? I had sort of assumed that lash would maybe need adjustment after running for a few hours. Not going to sweat that right now unless someone tells me I should. Doesn't seem like the core narrative.

I keep coming back to timing, and am going to narrow my focus to the fuel pump removal, service and reinstalls.

I admire your determination and effort Jeff! Good luck getting this resolved.

Do I now understand that one or more valves never close all the way as demonstrated by there never being any clearance? That could explain quite a bit.

No, the clearance should not change on running the engine, though most manufacturers call for resetting clearances after 1000 to as much as 5000 hrs. But the gap shouldn't close up.

When you set the valves, are you sure you were at the top of the compression stroke and not the top of the exhaust stroke? It would be an easy mistake to make, either now when rechecking, or before when you set them initially.

And is it just me? I would really want to know where that nut came from, and where it is now. Unauthorized metal objects inside an engine is generally a bad thing.

twistedtree said:

Do I now understand that one or more valves never close all the way as demonstrated by there never being any clearance? That could explain quite a bit.

No, the clearance should not change on running the engine, though most manufacturers call for resetting clearances after 1000 to as much as 5000 hrs. But the gap shouldn't close up.

When you set the valves, are you sure you were at the top of the compression stroke and not the top of the exhaust stroke? It would be an easy mistake to make, either now when rechecking, or before when you set them initially.

And is it just me? I would really want to know where that nut came from, and where it is now. Unauthorized metal objects inside an engine is generally a bad thing.

Click to expand...

Thanks. Agree on the nut. Can't even say for sure that's what it was as I only caught a glimpse of it. I should have been more deliberate in removing the cover, but assumed it would drop into the bilge. Can only assume it went into the oil pan since I didn't find anything under the motor. I may disconnect my oil removal hose and try fishing around the pan when I drain the oil. I did poke around the oil drain at the bottom of the front gear case with my pickup stick but didn't get anything. Assume it's pretty much a straight shot from there to the pan. Its path appears to have started out on the motor side of the IP gear, and my workspace hygiene is generally not great, so I'm willing to accept as a working hypothesis that it was my doing.

Re valves, I haven't actually made any adjustments. At this point I'm looking more, doing less . Will continue, and your comments help.

Do I understand that no.1 has no lash? If the valve is held open there would be no compression and that would explain the making oil.

Jeff F said:

Thanks. Agree on the nut.

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What might that nut have been attached to? Or what was it attaching to the engine, if anything? Is that widget still attached? Properly affixed? Sans nut?

-Chris

Is your oil pan steel or aluminum? If AL, you may be able to move nut with a strong magnet to somewhere you can grab it.
X2 on are you sure you are on the right stroke for TDC. Compression is right, exhaust is wrong.

You can tell if you are on the right stroke by barring engine over and watching rockers. Rotate til intake valve opens then closes, then turn about another 120deg. No valves should be moving and that is comp stroke.

Also another hint is that 1 and 6 pistons move together, 2 and 5, and 3 and 4 same. The piston of each pair is always in the same place, but in opposite strokes in the cycle. So roll over til 6 exh valve closes and intake just starts to move. Now you are at #1 tdc firing or very close to it.

engine won't run 180 off and blowby would not be affected by cam timing.

the video audio sounds like a very unhealthy bottom end to me.