So you want to buy an IDI

Discussion in '6.9L IH & 7.3L IDI Diesels' started by nelstomlinson, Jun 12, 2018.

  1. nelstomlinson

    nelstomlinson Full Access Member

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    A friend is planning to travel to Arizona this summer and purchase his first IDI. He wants an older diesel with no computer, so that's a reasonable thing to do. He asked me to tell him what he should know, and this is what I came up with. Does anyone have additions or corrections?

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    Engines:

    The Ford mechanical, indirect injection (IDI) diesels (the engines were really made by International Harvester/Navistar for Ford) were made from 1983 to 1994. The engines were 6.9L from '83 through '87, and 7.3L from '87 through '94. There is nothing at all wrong with the older 6.9L engines, but the 7.3L engines have some incremental improvements, since they came later.

    In '93 and '94 Ford made turbochargers an optional extra on the IDI. The turbo'ed 7.3L engines were built stronger and made a little more power. They are pretty unusual today, and usually go for a premium price.

    There are quite a few trucks with aftermarket turbos. If that's done right, the builder will have beefed up the engine quite a bit to handle the extra power, and you can make more power without letting your exhaust gas temperatures get too high. If it wasn't done right, there is a danger of blown head gaskets and so on. Probably, if the conversion was done back when the truck was newish, the problems have already been corrected.

    I believe that it was '92 when they went to serpentine belts on the engine. Serpentine belts are nicer, but v-belts are OK.

    The earlier trucks use Ford second generation alternator (2G). I believe that they changed to the third generation alternator (3G) in '93, so the '93 and '94 IDIs have the better alternator. The 2G uses two connectors and several wires, and the connectors are weak points. It's worth looking at the connectors on the back of the alternator for signs of melting. That's not a show stopper, but it might be something to fix before you start to drive from Arizona to Alaska. The 3G alternator has a single wire, starts charging at a lower RPM, and is available in higher powered versions. It's a bit more robust and the single connection is much less likely to give trouble.

    I plan to slowly upgrade my trucks to use the 3G alternator. I'll begin that with the plow truck, which needs a big alternator to run the electric plow motor.

    Things to look for:

    The IDI engines have some blow-by, and the oil will be dirty, even right after you change it. Take the dip stick out while the engine is running. If there isn't much air flowing out, that's a good sign. If it spits oil out the dipstick hole, that's a bad sign. My truck that does that will burn about one quart of lube oil in 50 miles.

    Look for exhaust smoke. A little black smoke when you are accelerating hard is OK. Loads of black smoke when you aren't accelerating hard is likely to be bad injectors or the injector pump turned up way too high. The injectors and pump should be replaced together, and are likely to need replacement after 100,000 miles or so. Blue or gray smoke is not OK. They indicate dribbling injectors or burning lube oil, or both.

    I've mentioned checking the alternator and headlight switch connections for signs of melting elsewhere.

    Trucks:

    The trucks from '80 through '86 had separate headlights. From '87 through '91, the front clip changed and the trucks had headlights with replaceable bulbs. From '92 through '94 the front clip changed again. The doors may not interchange between the three periods.

    For '83 through '86, the manual transmission was a four speed with no overdrive. In '87 the five speed overdrive manual transmission became available. The four speed was eliminated in '89. The five speed manual requires ATF, NOT gear lube!

    For '83 through '88, the default automatic transmission was the C6, which is all mechanical and has no overdrive. From '89 on, the default automatic was the E4OD, which is an electronic four speed overdrive. It's not a terrible transmission, and it gives decent mileage, unlike the C6, but I like the standard transmission much better than the automatics.

    It's probably a good idea to put relays on the headlight circuit so you don't have the full headlight current passing through the headlight switch in the dash. That switch and its connector are another weak point on the Fords, and it's worth looking under the dash at that switch and connector for signs of melting. Again, not a show stopper. Bronco Graveyard and LMC both sell a plug and play kit to put relays in the circuit.
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    Last edited: Jun 12, 2018
  2. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    The earliest 6.9's (called the "A" block) did have an issue with cracking externally by the block heater, but it's not like it's just going to decide to crack on a whim so if it's not cracked, it should be fine if you're careful on how you use the block heater. The block on the factory turbo has very minor changes to it compared to a N/A block. It's the crank, rods, pistons, balancer, and flywheel/flexplate that's different. I believe that the earlier alternators were 1G instead of 2G. A lot of black exhaust smoke could mean retarded timing. Maybe someone just installed an injector pump and ran with it wherever it was at. Don't forget to mention that there's only 3 places to buy quality pumps and injectors. I'm fairly sure that the doors will physically interchange at least between the brick nose and the OBS. I'm not sure about the bull nose. The inside door parts may or may not be similar. Overall, your list was pretty good.
     
  3. JAKRANCH

    JAKRANCH Registered User

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    Bullnose and bricknose interchange too. I think all 3 interchange
     
  4. MtnHaul

    MtnHaul Supporting Member Supporting Member

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    As far as "beefing up" the engine I don't think there is much besides head studs--to keep those head gaskets in place-- unless someone did a full performance oriented rebuild. That being said if I was in the market for another IDI and found one with head studs I would definitely take a look at it. One thing I would look for is aftermarket gauges. To me, a person who was aware enough and took the time to install aftermarket gauges is a good indicator that they MAY have taken good care of the truck. And of course if the seller is aware of the need for SCAs in the coolant that is also a good sign. Just my .02

    And as far as high mileage goes, I bought my truck with 303k and it runs great!! I fully expect to get quite a few more miles out of the ol' iron steed.
     
  5. catbird7

    catbird7 Full Access Member

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    Whatever he buys, I'd suggest spraying some type of protective coating to the underside of the body as well as the frame. Very likely will find a nice rust free truck in Arizona however if they use salt or brine on the roads in Alaska, she's gonna rust quick if it's unprotected. Also, may need to "beef-up" the batteries as more cold cranking amps are required up north.
     
  6. saburai

    saburai Full Access Member

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    Good advice all around.

    If he's serious, the best thing for him to do is join us here. That's what I did, lurking and studying for quite some time before buying my truck...
     
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  7. towcat

    towcat Administrator Staff Member

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    the other "nice" thing about joining and participating in a forum, is the relationships that are built from members all over. let's say your friend joins and gets familiar with which member is in the area they are looking at a truck, they can come to a deal where the member can put an extra pair of eyes on the truck before purchasing. I've assisted in quite a few sales and at the same, discouraged many sales due to "flippers" selling a vehicle they know nothing about but are asking top dollar. Even after purchase, you never know if someone is willing to let you use their driveway for prepping the truck for the long trip home.
     
  8. nelstomlinson

    nelstomlinson Full Access Member

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    Thanks for the thoughts and kind words, folks. I think you may be right about the early IDIs having a 1G alternator. I know they were still using them in '78, and probably into the early '80s.

    I'll definitely mention the SCA and coolant test strips.

    The door issue probably doesn't matter to my friend, I may just drop that. He doesn't have a stable of rigs to swap parts between. The undercoating is a good idea; things rust fast in Juneau.

    Are there any other things that he should look out for when evaluating an IDI rig? Any other interesting trivia that might help him choose between this one or that one? I'm far from an expert. I have bought six IDI trucks in the last three years, and I'm just starting to figure out what kind of things I should look for, in large part thanks to you all.
     
  9. chris142

    chris142 Full Access Member

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    Tell him that they roll fine on flat ground. Any grade and forget it.
     
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  10. 79jasper

    79jasper Chickenhawk

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    Also a standard 3G is NOT a single wire.

    Sent from my SM-G892A using Tapatalk
     
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  11. nelstomlinson

    nelstomlinson Full Access Member

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    Oops! That's good to know. Can you recommend a resource for me to read up on the 3G alternators, and how to make them single wire? Am I right that they solve the lousy connector problem that the 2G alternators have?
     
  12. 79jasper

    79jasper Chickenhawk

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    Google search "3g alt single wire conversion" will show plenty.
    Still won't be a true "single wire."
    And I don't know what connection problems you speak of.
    Not really important information for him though. Should be a FAQ somewhere covering most all of this, if he's really interested in learning.
    Sent from my SM-G892A using Tapatalk
     
  13. nelstomlinson

    nelstomlinson Full Access Member

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    OK, thanks. That search turned into a fun rabbit trail. 3G still looks like a better thing, and worth upgrading to, for the higher capacity. It looks as if the voltage sensing wire for the onboard regulator can be hooked up at the alternator end of the charge wire, if your charge wire is big enough that voltage drop is negligible. I've got some 1 gauge reserved for that.

    The connectors on the 2G alternators melt often enough that some of the alternators come with a new connector in the box. I've had one melt on me, and of course the only alternator I could find didn't come with a new connector that time.
     

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