1 dead hole and 1 with low compression

[NorthernMaverick]

So we have a 6.9 in a International 1654 small straight truck.
Trouble was a few weeks ago we started it up, it was cold out, and immediately we hear a miss. Motor smoked under a load, and idle, and on hills. We were low on power, but there was no horrible knock sounds grinding etc. So we checked fuel filter, injectors, and then a comp test this morning.
Results are:
#1 420
#2 400
#3 240
#4 380
#5 420
#6 20
#7 410
#8 400
We pulled the drivers side valve cover, and everything was working, no broken springs, etc but we haven't pulled the passenger side yet. We don't have time or shop space till later this week to look into it more, but I'm trying to figure out if it would be rings or valves. With good comp on 6/8 cylinders, could rings cause problems on just 2 cylinders?
Is this a more common symptom of valves or rings/ pistons?
The truck sat for quite a few years before we got it, but its been running good for about 2-3 months. We did push comp air in at the glow plug holes and found that the dead cylinder #6 had air coming out the intake. We did the same at # 3 but it did nothing except move the piston slightly/ turn the motor. I'm no diesel mechanic, and my dad has mostly experience with gas.
Trying to decide if we should pull the whole motor, or just pull heads in the truck to start with. Time and money is very limited so we're no trying to rebuild everything, just fix enough to get back on the road, but not have it back in the shop in a week.
Its a mid size truck with a forward flip hood, so you have walk up access to the sides ( no inner fender in the way).
Advice, help, and experienced input is appreciated.
I'll update as the project goes along also.

 

[IDIBRONCO]

Air coming out the intake indicates a bad valve or valve issues of some sort (of course you know this). That's where I'd start. Pulling off the heads and checking them out.

 

[NorthernMaverick]

That's what we figured. The one that threw me off is #6 @ 240 psi. Kinda hard to figure that rings would go bad on one cylinder and all the others be good. The truck only has around 250k miles. If a valve was to burn up what happens to the burnt pieces? Do they usually disintegrate enough or do they score up the cylinders bad?
Also is the 6.9 a sleeved motor?

 

[IDIBRONCO]

These aren't sleeved engines. They're more like a 302 gas engine in that respect. I think that a burned valve is actually just an overheated one. It could be warped and not sealing. This would explain your #6 with 240PSI (to me). A bad (warped) valve could be mostly sealing, but not completely. The one with 20 PSI could have a bad valve guide and is tipping to the side, sealing even less than the theoretically warped one. Bad valve guides aren't as common with 6.9s, but can still happen. 250,000 miles on the odometer could be along with 25,000 hours of idling. There may be more wear on your engine than on a pickup engine with 250,000 miles.

 

[Scrench]

Try a wet or dry test

 

[Black dawg]

Before you pull the head, look at the intake spring on that low psi cylinder.

 

[NorthernMaverick]

So here is the first update.
We pulled the heads and found the following:
The piston on cylinder #6 with no compression has a very nice crack the width of the piston.

Cylinder #3 that had low compression has a pitted hole in the piston and also #8 has some mild "chips" on the top edge of its piston (although it has good compression).

Valves looked ok (not burned up anyway) to me, but they'll go to a shop to get checked tomorrow. The fuel precups do have some cracks in them.

What would your suggestions be?
Pull the motor, change 3 pistons, send the heads in to a shop, and then putting it back together? Change all the rings on the other pistons while its out? Could any of this create problems somewhere else?
Complete rebuild? Something else?
Thanks for all your help.

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[IDIBRONCO]

The simplest thing to do would be to buy another, good running engine. Swap your International parts over to it and install it. If you can't find one for a reasonable price, I guess the next best (cheapest) route would be to replace the bad pistons and put it back in. If you go that route, I would recommend putting new rings on the other pistons too. Have all of the cylinders honed. This is assuming that there isn't a lot of wear in the cylinders. If there is, then you're looking at a complete rebuild. That's my opinion.

 

[IDIBRONCO]

Of course, you should still have the heads redone at this point. It's kind of foolish to not do it since you have the engine apart.

 

[NorthernMaverick]

The simplest thing to do would be to buy another, good running engine. Swap your International parts over to it and install it. If you can't find one for a reasonable price, I guess the next best (cheapest) route would be to replace the bad pistons and put it back in. If you go that route, I would recommend putting new rings on the other pistons too. Have all of the cylinders honed. This is assuming that there isn't a lot of wear in the cylinders. If there is, then you're looking at a complete rebuild. That's my opinion.

What should I expect to pay for a used good running motor?

 

[IDIBRONCO]

I can't say. It varies by location. Some people are just trying to make a little bit of money and help others at the same time. Others are trying to get rich every time that they sell something.

 

[NorthernMaverick]

I can't say. It varies by location. Some people are just trying to make a little bit of money and help others at the same time. Others are trying to get rich every time that they sell something.

I found a '88 7.3 idi n/a near me. Thinking of swapping it out next week, if all goes well. Going to check it for compression. From what I'm reading on here, the swap looks relatively easy/simple. Anything in particular I should be aware of when changing it?

 

[IDIBRONCO]

Not that I can think of. When you have the two engines side by side, it will be apparent which parts to swap.