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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..
-Christopher
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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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Quote:
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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Quote:
Originally Posted by cadillackid View Post
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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