Tell me about 6.9l IDI diesels

I've never owned a diesel, never worked on one and have only driven one a few times but....

An 85 f350 showed up with a for sale sign on it on the highway I drive home on. It was actually parked next to another truck that I originally stopped to look at so I didn't give it too much of a look over but it's a dually flatbed manual tranny butt simple truck. I don't even think it had any options. Looks like a longer wheelbase than the normal 8ft bed too. 2wd means that it won't get a lot of people looking at it up here either. Price was a bit higher than I was expecting but still very reasonable.

I don't really need another truck but a 1 ton flatbed for cheap could come in great handy for my life at times. Anything I should know about the 6.9? Afaik they are pretty reliable but slow. It's a 4 spd manual so t-18/np435 is pretty much unbreakable. I'm assuming Dana 60 or 70 so that's beefy too.

If I decide to look into it more indepth, anything I should watch for?

Nothing to know other than glowplugs every now and then. They just get slower and smokier until they die one day.

I have nothing to offer, but I do own a 1985 truck, although it's a Chevrolet.  Back then, trucks were simple beasts that would run forever with just basic maintenance.  If it isn't overly rusty, I'd say go for it.  How many miles on it?

Ford IDI's are really common in the alternative fuels world if that interests you at all.  Waste motor oil and waste veggie oil are both pretty common.  Turbo kits are available and apparently are really effective at waking them up even though you can't run the boost pressures you would run on a direct injected engine.

Best truck I ever owned was a $200 '85 F250 with a 6.9 diesel.  Put over 100k mi on it as my daily

Couple little nuances that you learn real quick but overall slow, quite efficient for a fullsize, and bulletproof.  They can do brutal work but they just do it slow.  They only have 5 digit odometers but as far as Im concerned mileage is irrelevant on them at this point in time.

Manual trans will help, mine was a C6 auto.  Check the rear gear, if it s a 4.10 ratio you will likely be near the governor at highway speeds.

It has been a while, but the biggest thing that sidelines them is air intrusion into the fuel system.  It is pretty easy to re-do all the return lines solve that issue.  Biggest ticket wear items are injection pumps, they usually last 100-150k and are around $500 or so to reman.  Put a set of motorcraft glow plugs in it and put it on a pushbutton.  The cheap glow plugs will swell and get stuck or drop tips into the cylinders.  Stock glow plug timer probably doesnt work at this point in its life, and chances are it is already on a pushbutton.  2 GOOD batteries and good cables to get it to start quickly and reliably.  People complain of headgaskets and water useage but I never had an issue in the number of IDI's i've owned.

Agreed on battery cables, I think that's the biggest thing other than glow plugs. These are farm equipment- extremely reliable and not fast but bulletproof. I think there can still be a place for that. 

What mpg's do they get and how well do they tow? 

yupididit said:

What mpg's do they get and how well does do they tow? 

my 84 f250 with the 6.9 and 4 speed would get 11ish mpg towing and 16-18 not towing. It towed well everywhere except for onramps and steep hills (going up). Because the diesel has limited rpm range, you couldnt use third until you were going as slow as 45-50 or so. So if you couldn't pull the hill in 4th, you either had to hope you had the momentum or be the really slow guy on the interstate. 

That engine seemed invincible though. 

Ah so not great for this area possibly. I'm sure a turbo kit would help with that issue.

As mentioned, by everyone slow, but tough. A turbo definitely helps, but cans add a ton of boost, so it's a bit less slow, but still shockingly slow! Farm equipment decries them well. 
They do shine in the alt. fuel dept., and can do much better in the mpg dept. if you find someone (or learn) that can optimize the injection pump. I am not one of those guys! But 90% of the guys that think they are, at not. A friend of mine bought the factory tech. alignment stuff, and found out none of the techs knew how to use it correctly. Most brand new trucks were not set up correct, and never got the pump right, resulting in a engine that ran lees smoothly, less mpg, and less life... and still lasted longer than the rest of the truck

Even set up wrong, a very tough engine, if ya ain't in a hurry!

yupididit said:

What mpg's do they get and how well do they tow? 

My C6/6.9 would get 16-17 empty.  

Define "well".  As a reference point, if you hooked the same car trailer to an IDI and an early 2000's 5.3 silverado, the silverado would walk away from it.  The IDI however would do it for decades without complaint loafing along at 2800rpm where the silverado would be doing 2nd gear downshifts to 4500rpm all the time.

The key to towing with an IDI is to keep it wound up.  Turning the fuel up a bit and nudging the timing helps them out a fair amount, but you absolutely 100% have to have a pyrometer to monitor.

The C6 hurt a lot because of the limited RPM range.  You don't have the option to bang 2nd until you are down around 40 mph.  

Turbo helps quite a bit.  My '88 IDI with a banks sidewinder turbo and ZF 5 speed was much better.

My F250 has a 7.3 IDI and C6

It'll tow damn near anything just not at 70mph.

Slow down on the hills, feed it all the fuel it will take without smoking, put on your 4-way flashers and some music and just follow the big rigs up the hills.

You won't be the first guy there but you'll always get there.

I'm very experienced towing with the 7.3 that was in my old Excursion but I really love the body of the 80s-mid 90s trucks. 

In reply to yupididit :

The 7.3 IDI I was talking about isn't the same as the Powerstroke in your Excursion. It's just a bigger version of the 6.9

We had a 7.3 Powerstroke in our Superduty and it was a far more powerful engine than my old IDI is.

In reply to ShawnG :

Got it. I just thought it was a N/A smaller version of the 7.3 

In reply to yupididit :

IDI stood for indirect injection - a type of combustion chamber. DI is the direct injection that ford ih and cat developed with the high pressure oil pump, to shove the fuel out of the injectors at super high psi (16,000 psi iirc)

Thats when ford coined the powerstroke term. Made it in some '94s, but considered to start in '95.  
'83 6.9 started with 160 hp, and 307 tq.  Last year of 7.3 IDI turbo'd was up to 190/400. 
Last year of the similar 7.3 powerstroke ('03) was up to what? Double a '83 6.9?

I have a 1991 F250 4x4 with a 7.3 n/a.  with an  AOD trans.  It slow as dirt but will get you there.  Last time I checked the fuel mileage I was getting about 18mpg.

In reply to mjlogan :

Very solid response! I should add that our 86 manual trans 4WD averages nearly 20 mpg. And that my sons would rather drive it than just about anything else we own. Never discussed in a vehicle review is: How does this vehicle make you feel? Answer: old truck on the right day is sublime. 

My kid #1 has a 7.3 IDI with 5 speed and 3.55's. It keeps up with traffic just fine, empty. The 3.55's are the better choice unless you're working it hard all the time and long for the return of the national 55 mph speed limit. I confirm the following comments made upthread, based on my kid's and my experience with the truck:

- New, fat battery cables and 2 big batteries are very good ideas. A fresh starter helps, too.

- Make sure the fuel return lines are in good shape

- Turning up the fuel flow and advancing the fuel timing helps power.  Be careful and conservative with this.

- It will chugga chugga forever.

Also, they're really noisy. He took the muffler off the truck and it really didn't make much difference in overall noise.

The 6.9, with its smaller bore, is better than the 7.3 if you're running LOTS of boost. If not, no difference except a little less power.

They are slow up hills, but won't stop.  But much slower than the stroke. 

N/A Diesels feel like you've left the parking brake on, all the time.

I bought one recently after looking a dozen to find one that was even worth paying for.  They seem to usually have rust holes around the rear window and the steering column be very broken.  I have only driven mine once but it has a banks sidewinder kit on it and it's slow but not much worse than an idi Mercedes diesel.  Also one of the engine mounts appears to be NLA or at least indefinitely backordered.

ShawnG said:

N/A Diesels feel like you've left the parking brake on, all the time.

and when you do leave it on, you won't know. 

At altitude (Rockies) you might get a ticket for rolling coal.  They are seriously smokey at elevation.  You might consider adding a turbo if you can still find the Banks kit.

In reply to Robbie (Forum Supporter) :

Lol because I did that today moving a horse trailer.