1985 6.9 IDI major oil leak, probably rear main seal?

J_C_S

Registered User
Joined
Jul 5, 2025
Posts
4
Reaction score
0
Location
California
Hi everybody, was recommended this forum by a friend for diesel repair help. Wondering if anyone has done the rear main seal on a 6.9/7.3 idi, pretty sure that's the source of my leak but seeing a ton of stuff online saying I need to pull the motor to fix it which seems insane, surely there is a better way to do that right? Would appreciate some advice if anyone has done that before on that gen. Thanks in advance

Truck has been sitting for about 15 years and I bought it last spring, while I'm in there anything else I should do?
 

IDIBOBS

Registered User
Joined
Jan 13, 2025
Posts
444
Reaction score
287
Location
Southern California
Either motor or trans has to come out for sure. The motor could come out in an hour or so. If your doing all that I’d buy a reseal kit and do heads gaskets, studs, and all the other seals.
 

IDIBRONCO

IDIBRONCO
Joined
Feb 5, 2010
Posts
13,880
Reaction score
12,975
Location
edmond, ks
Before you go hog wild and start pulling major components, make sure that your leak is actually oil and that it's not coming from the top side of the engine. You can have a fuel leak (oil leak too) on top of you engine and it will run down the back side of the block and onto the starter. That's by design. If your truck was sitting for 15 years, I'd be thinking fuel leak first. Welcome.
 

asmith

Full Access Member
Joined
Jan 28, 2012
Posts
763
Reaction score
312
Location
Atascadero, Ca
I agree with @IDIBRONCO. My valve covers were leaking a ton of oil, combined with a small fuel leak and it runs all down the back of the engine. I would clean real well back there and figure out for sure where it is coming from. If it is a rear main seal, either the motor needs to come out, or the transmission needs to.
 

DirtyWood

Registered User
Joined
Apr 20, 2025
Posts
80
Reaction score
50
Location
Earth
I've had two IDIs with leaky rear main seals but they were such small leaks it wasn't worth fixing. Maybe you have a nice clean concrete driveway and you want it to stay that way?
 

Jesus Freak

Full Access Member
Joined
Apr 3, 2022
Posts
3,708
Reaction score
4,376
Location
Crestview, FL
I've had two IDIs with leaky rear main seals but they were such small leaks it wasn't worth fixing. Maybe you have a nice clean concrete driveway and you want it to stay that way?
Do people that drive ancient diesel trucks have clean driveways? That might make a good thread "show us yer oil slick......" Lol
 

Old Goat

Full Access Member
Joined
Jan 14, 2021
Posts
2,268
Reaction score
2,409
Location
Northern Nevada
Welcome to the Forum.
Lots of good correct info here.
Lots of misinformation floating around out there.

I have a copy of this manual , 88 - 93 Engine Service Manual.
Has a lot of good detailed info. It is for the 7.3 Engine, but both
engines are pretty close to the same.
This is a paper manual,

there is also the same manual but for the 83 - 87 6.9 IDI.
I have both and the 6.9 is lacking in a lot of info, but good to
have both.

Some of the manuals are on CD, or you have to down load their PDF File.
I prefer the paper one.

Power wash the engine to get the crud off it to see what you have.
I have a Hotsy hot water pressure washer, really blasts the crud off.

Forget the using the Car Wash DIY spray wands. They are not
powerful enough. But better than nothing.

Goat
 
Last edited:

DirtyWood

Registered User
Joined
Apr 20, 2025
Posts
80
Reaction score
50
Location
Earth
Do people that drive ancient diesel trucks have clean driveways? That might make a good thread "show us yer oil slick......" Lol
I grew up doing concrete work with my dad so I got an education about the evils of leaky vehicles and people who think it's cool to write their initials in someone else's concrete.
Thank goodness I have a gravel driveway--it always looks clean!
 

Jesus Freak

Full Access Member
Joined
Apr 3, 2022
Posts
3,708
Reaction score
4,376
Location
Crestview, FL
I grew up doing concrete work with my dad so I got an education about the evils of leaky vehicles and people who think it's cool to write their initials in someone else's concrete.
Thank goodness I have a gravel driveway--it always looks clean!
Yeah, people do get fussy about an oil leak......I like it myself. It's a simple way to start diagnostics......"hey, what's that puddle?....." "Is it green? Brown?.......no, it's yellow.....oh, my dog just walked away from here, we're good, let's get going....."
 

J_C_S

Registered User
Joined
Jul 5, 2025
Posts
4
Reaction score
0
Location
California
I've had two IDIs with leaky rear main seals but they were such small leaks it wasn't worth fixing. Maybe you have a nice clean concrete driveway and you want it to stay that way?
Main issue is that the leak just isn't that small- it's seriously just ******* all over seems like, way worse than any vehicle I've owned before
 

J_C_S

Registered User
Joined
Jul 5, 2025
Posts
4
Reaction score
0
Location
California
Welcome to the Forum.
Lots of good correct info here.
Lots of misinformation floating around out there.

I have a copy of this manual , 88 - 93 Engine Service Manual.
Has a lot of good detailed info. It is for the 7.3 Engine, but both
engines are pretty close to the same.
This is a paper manual,

there is also the same manual but for the 83 - 87 6.9 IDI.
I have both and the 6.9 is lacking in a lot of info, but good to
have both.

Some of the manuals are on CD, or you have to down load their PDF File.
I prefer the paper one.

Power wash the engine to get the crud off it to see what you have.
I have a Hotsy hot water pressure washer, really blasts the crud off.

Forget the using the Car Wash DIY spray wands. They are not
powerful enough. But better than nothing.

Goat
Thank you for this, great info! Will check out the listings
 
Last edited:

J_C_S

Registered User
Joined
Jul 5, 2025
Posts
4
Reaction score
0
Location
California
I agree with @IDIBRONCO. My valve covers were leaking a ton of oil, combined with a small fuel leak and it runs all down the back of the engine. I would clean real well back there and figure out for sure where it is coming from. If it is a rear main seal, either the motor needs to come out, or the transmission needs to.
I figured I could do it just taking out the trans, just haven't ever actually seen it recommended for that gen. Information seems so few and far between for them which is just crazy to me, the OBS trucks theres millions of forum posts and videos showing you how to do every little thing. I guess these are just slightly too old

Thanks for the info, much appreciated
 

IDIBRONCO

IDIBRONCO
Joined
Feb 5, 2010
Posts
13,880
Reaction score
12,975
Location
edmond, ks
I figured I could do it just taking out the trans, just haven't ever actually seen it recommended for that gen.
There's no reason why it isn't recommended. It's basically the same process for any generation of our trucks. If your truck is 4X2, then it's probably easier to drop the transmission. If it's 4X4, then that's your call because there's more to do to drop a 4X4 transmission. People have replaced clutches by dropping the transmission. If you go that far, you should have the flywheel resurfaced. Once that's removed, you're at the rear main seal. Either way you go, just be aware that oil will leak out once you remove the flywheel/flexplate. The bolt holes go through the crankshaft. Just plan on an oil change at the same time.
 

DirtyWood

Registered User
Joined
Apr 20, 2025
Posts
80
Reaction score
50
Location
Earth
And when you do the rear main seal it seems a lot of people need the seal kit that comes with a repair sleeve since the rotating seal wears the metal surface a tiny bit and a new seal often leaks if installed without the sleeve.
 

Old Goat

Full Access Member
Joined
Jan 14, 2021
Posts
2,268
Reaction score
2,409
Location
Northern Nevada
I did the Rear main on my 86 probably year N half back.
I have the T-19 and that thing weighs 150 LBS.
If you drop the trans, just make sure you have a trans jack to safely remove it.
I have this one, picked it up off CL for a killer price. Guy used it once, then put it on CL. Think I paid $85.
Just make sure it is safely secured on the plate. The chain that is supplied is to secure the trans is a joke. Lube up the 2 screw adjusters good.
Haven`t worked on a auto tans, probably close to the same.

You say you have a fuel leak down the back of the engine.
Could be the IP leaking, or one of the hard lines screwed onto the pump leaking. If the Vibration dampners are not on the hard lines, vibration can cause them to crack, any fluid drips onto the valley pan then to a drain at the back of the pan.

Note: scroll the bottom of the page up, and there are 3 related rear main Seal Threads.


Goat
 
Top