6.9L, 7.3L IDI Diesel Review

Summarizing our Experiences with The International IDI Diesel

We acquired a 1991 Ford F-250 several years ago and it has been used as the foundation for various projects here at Diesel Hub. This F-250 is unique in that the previous owner had replaced the original 7.3L IDI with an earlier 6.9L IDI. For what the truck is and what it has been used for, we have not been disappointed in the engine's performance and reliability. For an engine with well over 300,000 miles on the odometer, it continues to take whatever we throw at it. This is truly an engine you can beat on with little concern. It has the close ratio ZF 5 speed manual transmission and a 4.10:1 rear differential. With factory sized tires, this translates to roughly 2,000 rpm at 55 mph.

Towing with the IDI Diesel

90% of the time this IDI is used to tow. In fact, it's been hitched up for most of its life here at Diesel Hub. It frequently pulls a 25' flatbed equipment hauler and 12' dump trailer. The equipment hauler weighs in at 2,500 lbs and the dump trailer weighs a whopping 4,200 lbs unladen. With the flatbed, the max load would be our 6,200 lb tractor. With the dump trailer, however, we've grossed 19,000 lbs with truck, trailer, and load. While this was a lot for the old IDI, she pulled at 55 mph with little effort and 800° F on the pyrometer. We have other trucks to tow with, including a 2004 Dodge Ram 2500 and 2003 Ford F-250, but we can take the IDI anywhere (landfill, back roads, steel yard, scrap yard, etc) without worrying about scratching or denting the body. That's a huge bonus and thus the reason we use the truck so frequently.

With a factory rated 165 horsepower and 318 lb-ft of torque, the 6.9L IDI is severely underpowered for heavy loads; however, it gets the job done. The naturally aspirated 6.9L and 7.3L IDI are definitely flatlanders - it's the hills that really drags them down. This truck will not pull a trailer of significant weight up a hill in 5th gear (OD) without losing speed and pegging the pyro. However, it seems to pull moderate grades in 4th gear at 45 mph just fine. The biggest problem with long grades is that we actually have to pull back on the accelerator to keep our EGTs from spiking. Gas temps rise slow in this truck, but 1,200° F will sneak up on you if you're not paying attention.

This truck is a lugger. It has no problem lugging down to 1,000 rpm in a tall gear and recovering, loaded or not. While peak torque is nothing to write home about, it's incredible how much of that torque is available at low engine speeds. Lugging an engine is not the best thing for it, but having the ability to do so helps in certain situations.

IDI Performance Upgrades

There is minimal support for performance upgrades for IDI diesels. Ours is naturally aspirated, so a turbocharger is an option but it comes at a steep price. The last thing we wanted to do was pony up for a turbocharger kit only to pop the heads off our 30 year old engine. We did, however, make the following modifications:

Dynomax 4" exhaust system - This exhaust upgrade replaces the entire system from the manifold outlets to the tailpipe. Coming off the exhaust manifolds, the tubing diameter is larger than stock and the bends are not so invasive. Shortly after the pipes meet, the system opens up to 4 inches. We finished it off with an attractive 5 inch tip and left the muffler off. It's obnoxiously loud inside and outside the cab, but there was a noticeable improvement in power at higher RPMs. The provided muffler is a straight through design and thus there is no performance difference between the muffled and open configurations.

Open element air filter - Your options with regard to reducing intake restriction are to trim the soup bowl from the factory filter housing, install an aftermarket air cleaner, or fabricate your own cold air intake system. We opted for an open element filter. Be warned that it creates a lot of cabin noise. There's a small, but noticeable improvement in power until the engine and air cleaner become heat soaked. Once the air cleaner assembly gets hot, so does the incoming air charge.

Turn up the injection pump - This is where you get the most noticeable performance increases, but there's an obvious point of diminishing return. We hooked up the truck to a trailer and turned the pump up until visible smoke become excessive and the EGTs would spike, then we started backing it down. What we ended up with is an increase that is noticeable, but remains practical with minimal smoke. Turning the pump to its maximum fuel limit results in nothing more than plumes of black smoke. These engines are limited by airflow, not fuel.

Highlights and Verdict on the IDI Diesels

While most of the experiences discussed herein are based on the 6.9L, we've had plenty of time behind the wheel of many 7.3L IDIs as well. This is an honest engine that has been left behind in the wake of modern technology. Anyone's opinion on these engines would be entirely different if they test drove one long before we had direct injection, common rail engines. It's slow. It's underpowered. It's noisy. But it still has its place and makes a good engine for the younger generation to learn the fundamentals of diesel operation, service, and repair.



Noise. Unique exhaust note.

Noise. Sometimes you just want to be able to have a conversation. Between the open element air filter and lack of a muffler, it can be difficult to communicate.

Simple mechanical system that is relatively easy to diagnose and work on.

Slow and sluggish. Trucks equipped with an automatic transmission are worse. A stick shift is the only way to go. If you have no patience, this is not the engine for you.

Inexpensive to find a good used Ford with an IDI diesel, low cost of ownership. Maintenance and most repairs won't break the bank.

Underpowered while towing. Sure, it does fine on flat roads, but even small grades reveal its shortcomings in the power department.

Engines run extremely cool. The cooling systems on the IDIs are well designed and overheating is not common. Ours runs at 190 to 195° F whether it's 100° F or 32° F outside, loaded or overloaded.

Parts availability is becoming a problem. Service parts are readily available, but certain items for the 1980 to 1991 F-Series are becoming obsolete and difficult to find (steering columns, pedal assemblies, accelerator pedals, throttle cables, etc).

Gets the job done. If you're not in a hurry, these engines will pull just fine. They don't "struggle", they're just slow.

Reverse in the close ratio ZF 5 speed is extremely tall (high) and thus you need to babysit the clutch in order to back up a trailer.

Better fuel economy than a comparable gas engine, especially the big block 460s of the same era.

TTB front axle. Buy a F-350 with a Dana 60 solid axle if you desire a 4x4.

Doesn't leak or consume a drop of oil, despite its age and mileage.

Not a lot of performance upgrades available.

All-in-all, not a single regret in purchasing this old, beat up Ford. In fact, we've even acquired a spare engine for the truck...just in case.