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Old 07-30-2018, 03:15 AM   #1
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89 Crown DD 6-71T missing on one cylinder

DD experts, I need some help!

SO, I bought a Crown that is missing on number 3. I bought the bus at auction from Fresno USD for a great price knowing that it had developed the miss after a run away that was stopped by shutting it down with the key. This is first hand knowledge from the head mechanic at the school yard. The bus runs but smokes and is missing. He thought it was likely that the injector was damaged during the run away causing the miss. I am aware that it could also be a valve. The oil is clean so it was originally assumed to be either the injector or a bent valve. I picked the bus up in February and its been parked ever since.

Also, the bus is in Fresno and I live in Hawaii so I am not there often. I try to go as much as I can to work on it and getting as much info as I can really helps me prepare for the small amounts of time I have to work on it.

Fast forward to last month. The engine starts right up every time. I pulled the valve cover and did the injector press test to figure out that cylinder 3 is misfiring. I brought an injector with me just for this purpose. The head mechanic guy was kind enough to give me his DD service manual and a list of every part ever replaced on the bus, including part numbers. It was awesome! So, I swapped in a fresh 7G75 injector but just like a noob (which I am for the DD) I forgot to bench bleed the injector and did not prime the fuel system. It was hard to start, which was about the time I realized I hadn't bleed the injector, DOH! Anyway, I got it to start but its still missing and remains really hard to start.

The questions I have are:
Can the injector be air locked and not work? Even though its still missing after the swap could it still be the problem?

Is there a relatively easy way to get a small camera inside the cylinder to look at the valves? Will I be able to get a camera in through the inspection ports? Will they be on the underside of the engine or will I need to pull the top cover? (Crowns are mounted on their side mid-engine) So, top or bottom for inspection plates?

What else am I forgetting to do?

And finally, thanks for your help!

P.S. If you want to watch me work on the bus and make fun of me, the link for my Youtube is in my sig.
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Old 07-30-2018, 03:43 AM   #2
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Can you do a compression check?



good luck J
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Old 07-30-2018, 04:14 AM   #3
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I suppose that would be the next logical step. I haven't done one yet because the tester costs more than the injector did. Ha

I thought maybe a leakdown would be a good idea as well. But again, prices for the tools for single use are ouch. Off to ebay I go.
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Old 07-30-2018, 07:10 AM   #4
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a "quick and dirty" "compression" check.. is that you turn off the fuel supply to the engine.. and you crank the starter.. 5 secinds or so.. let it rest.. then crank it again... trick here is you want no fuel in the system to reach an injector..



now crank the starter again and listen.. every engine has a rythm as it cranks.. your 6-71 does too.. re-ne-ne-ne-ne-ne etc.. you know the sound..



every 6 of those is a complete engine revolution (its a 2 cycle).. listen for a miss in the rhythm.. re-ne-reaaah-ne-ne-ne re-ne-reaaah-ne-ne-ne .. a "miss" in that crank thythm says that the starter never has to work hard to compress one of the cylinders.. so in effect the motor just spins right by that cylinder with little effort causing a different sound.. thus a compression issue in that cylinder..



if you just have a bum injector, the cranking will be even and in rhythm as all the cylinders are compressing normally.. just no fuel to that one..



remember you need there to be no fuel in the system (ie fuel valve closed, or if it has a supply solenoid, that is disconnected)..


its quick and dirty and not as good as a real compresion test however it needs no tools..
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Old 07-30-2018, 03:54 PM   #5
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Great info Christopher! I'll give that a shot on my next trip there before I drop the money for the tester.


Thanks
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Old 07-30-2018, 04:04 PM   #6
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How did the old injector look? Was it intact? If the tip was gone there will be serious valve, head, and piston damage. Is there more leakage around the third cylinder air box over? This could indicate excessive fuel dumping in that cylinder leading to excessive piston ring & liner wear and reduced compression.

Does the oil smell or feel like it has been diluted with diesel?

I'm a little rusty on my Detroit trouble shooting, I'll try to clear the cobwebs and see what else I can think of.

Good Luck
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Old 07-30-2018, 05:52 PM   #7
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Originally Posted by Lamacose View Post
How did the old injector look? Was it intact? If the tip was gone there will be serious valve, head, and piston damage. Is there more leakage around the third cylinder air box over? This could indicate excessive fuel dumping in that cylinder leading to excessive piston ring & liner wear and reduced compression.

Does the oil smell or feel like it has been diluted with diesel?

I'm a little rusty on my Detroit trouble shooting, I'll try to clear the cobwebs and see what else I can think of.

Good Luck

The old injector looked okay. The tip was still on it but I thought it looked a bit burnt although I don't know what normal should look like. The oil is clean as well. Is it possible for the tip of the injector to be jammed and not opening?



Small world, I grew up in St. Bernard.
Thanks
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Old 07-30-2018, 06:01 PM   #8
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swap it with a known good injector and see which cylinder doesnt fire
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Old 07-31-2018, 01:43 PM   #9
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swap it with a known good injector and see which cylinder doesnt fire

Another great idea, I'll try the cranking with no fuel thing first. Then I'll try this next.
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Old 07-31-2018, 02:14 PM   #10
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40 years of diesel experience leads me to think it has a bent valve if the runaway condition jacked up an injector it would be visibly damaged. Also in a runaway condition the engine could possibly turn 5to 6000 rpm causing all kinds stretching (valve interference or cylinder sealing) problems.So even if you determine it is a compression problem you still really won't know the cause. I would plan on a head removal project. Due to your travel constraints I would have a recon cylinder head nearby. Gene
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Old 07-31-2018, 02:37 PM   #11
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I'm with Gene on this one - the runaway is the cause of your problems. Why exactly did it runaway? Leaking blower seal, stuck injector control rods, what? If what caused the original runaway isn't diagnosed and fixed, it could happen again, the next time maybe with disastrous consequences. If the engine were sucking in oil past leaking seals, how could it be shut down by the ignition key? Does your engine have the emergency shutdown flapper near the blower?

John
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Old 07-31-2018, 05:57 PM   #12
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Amazon.com sells a cheap endoscope that links to a smart phone. $40. pull the air box covers, at least on number 3 and turn the engine( by hand if you can) till number 3 is all the down. Now you can stick the scope up in the cylinder and see the valves.

You have a bunch of great suggestions from others, but thought this might be helpful too.
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Old 08-01-2018, 12:00 AM   #13
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Originally Posted by Gdog 5651 View Post
40 years of diesel experience leads me to think it has a bent valve if the runaway condition jacked up an injector it would be visibly damaged. Also in a runaway condition the engine could possibly turn 5to 6000 rpm causing all kinds stretching (valve interference or cylinder sealing) problems.So even if you determine it is a compression problem you still really won't know the cause. I would plan on a head removal project. Due to your travel constraints I would have a recon cylinder head nearby. Gene
Thanks Gdog, my original plan was to try and swap the injector first and if it didn't correct the issue, I would assume it was a valve and plan to pull the head.

I have found reman heads online in the 1700 range with no core needed so I am prepared to go that route if necessary.

Would it be possible to pull the head and just swap in new valves in number 3, since its the only cylinder with a current problem. Or would it be a wise investment to just swap the head at this point?
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Old 08-01-2018, 12:35 AM   #14
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I'm with Gene on this one - the runaway is the cause of your problems. Why exactly did it runaway? Leaking blower seal, stuck injector control rods, what? If what caused the original runaway isn't diagnosed and fixed, it could happen again, the next time maybe with disastrous consequences. If the engine were sucking in oil past leaking seals, how could it be shut down by the ignition key? Does your engine have the emergency shutdown flapper near the blower?

John
Sorry for the novel here. But thanks in advance.

Before I get into this long post, would you do as Gene suggests and just swap the head? Or would it be feasible to order 4 valves and just swap all of them in number 3? I will probably order a head and just swap it.

The head mechanic said the runaway was due to excessive idling. The bus was waiting for a few months to be auctioned and during the lead up he said there were many people that came and started it and simply let it idle for rather long periods. He said none of them actually drove it.

His thoughts were that due to it idling so much it slowly built up, either oil or fuel in the intake, I can't remember which one he said since it was back in February. But he said it had built up to the point of causing a runaway that was able to be shut down by simply turning off the key. He said it would not have happened had it been driven normally. It seemed plausible to me but I am new to detroits. He was super nice and really didn't have a stake in the sale at all since I bought it directly from the school district. He has over 30 years of experience with detroits and in particular this bus along with its sister bus which sold at the same auction for over 5000.

When it went up for auction after the runaway, it was said to be missing on one cylinder and smoking very bad. So I took a flyer on it and ended up getting it for 1750. So short of an in-frame, I think I'm ahead of the game. Especially seeing these buses on Ebay starting at 10K. I think that's nuts but people are apparently buying them for that and more.

So when I picked it up he thought I should be able to drive it the short distance to the place where I am storing it. It was a little less than a mile away. He said if it started to run away again to just kill it with the key. It started right up and did smoke a lot. Twice on the drive to the storage place it started to rev a bit more than I felt like it should so I just quickly let off the gas and it calmed down. I caught a few red lights on the way and by the time I made it there it had nearly stopped smoking other than the small amount you would expect from one cylinder not firing. I would say about 10 percent of when I first started it. The guy told me if I drove it a bit it would probably burn off whatever was in the intake and it appears it did that. After about half way it no longer felt like it wanted to rev on its own and I was able to drive it normally.

On our last trip I was able to drive it around the storage lot a couple laps just to get it running and driving and it did not feel like it wanted to rev at all other than normal.

I think he was correct on the cause of the runaway.

I don't think it has the emergency shutdown and I also have the manual from Crown that came with the bus and it doesn't mention it in there. That was why I asked him what should I do if it tried to run away again to find out if it had one, but he just said to shut it down with the key.

I did bring a bath mat with me to throw over the air intake/filter box as I was told that would keep it from getting enough air to detonate and possibly stop a runaway all together.

Emergency bath mat.
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Old 08-01-2018, 12:37 AM   #15
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Amazon.com sells a cheap endoscope that links to a smart phone. $40. pull the air box covers, at least on number 3 and turn the engine( by hand if you can) till number 3 is all the down. Now you can stick the scope up in the cylinder and see the valves.

You have a bunch of great suggestions from others, but thought this might be helpful too.
I have a small engine inspection camera but I didn't know if the covers allowed access to the cylinder above the piston, though I suppose I could look in through the intake ports as well.

Thanks Ronnie!
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Old 08-01-2018, 09:04 AM   #16
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I'd use an endoscope and have a look inside.

If the valve is the culprit, swap the whole head. With a bent valve, you have no idea if the seat or guides are jacked up as well. Repairing everything involved will take time. Getting a recon head means that you're eliminating any valve related issues altogether.
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Old 08-01-2018, 11:27 AM   #17
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Quote:
Originally Posted by Ronnie View Post
Amazon.com sells a cheap endoscope that links to a smart phone. $40. pull the air box covers, at least on number 3 and turn the engine( by hand if you can) till number 3 is all the down. Now you can stick the scope up in the cylinder and see the valves.

You have a bunch of great suggestions from others, but thought this might be helpful too.
I have 4 of those endoscopes for sale for $15ea/ps. (around $3.50 shipping)
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Old 08-02-2018, 01:45 AM   #18
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I'd use an endoscope and have a look inside.

If the valve is the culprit, swap the whole head. With a bent valve, you have no idea if the seat or guides are jacked up as well. Repairing everything involved will take time. Getting a recon head means that you're eliminating any valve related issues altogether.
I think I will just use the endoscope on the next trip. If there is a bent valve I will plan on a head swap.


Does anyone know if using a fuel pipe as a jumper to bypass the injector on the dead cylinder would reduce the smoking? If no fuel is getting into that cylinder I would think it would eliminate the smoke.

Also, would driving the bus short distances like it is risk any further damage to anything else? I have read about these engines in boats running for years on 4 and 5 cylinders, or is that just tall tales?
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Old 08-02-2018, 06:42 AM   #19
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You can jump over the injector with a fuel pipe, and it may help with smoke. However then the injector is not getting lubed, so really would not be inclined to do that.

As far a driving on the bad cylinder you will likely get away with it for a bit. My concern would be in time snapping off the head of a bent valve, assuming this is the problem. This would likely take some time to happen.
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Old 08-04-2018, 05:49 PM   #20
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Thanks Ronnie, I will just leave the injector as is.


If I drive it, it won't be far and not very often. I just wanted to make sure it's safe enough to move when needed. I found a place I can rent to work on the head if i comes to that. But its less than a mile from the storage place so I would think it would be okay.


Thanks again everyone! I noticed when editing my video for tomorrow that the smoking appears to have decreased after swapping in a new injector. I did a side by side view to get some opinions on. I did not set the injector timing when I put in the new one.


How much smoke could I expect if the injector timing is off? I thought it sounded better after I swapped it. I'll post the video when I finish it and get it uploaded tomorrow.
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