The Caterpillar 3126 Engine

This is a summary post of a bunch of research I’ve done….  Hopefully it is a reasonably accurate distillation of many many hours of research.  Everything in this post is based on hearsay (i.e. internet forums posts) so take it with a grain of salt.

TLDR

Here’s the thing, even though the 3126 is bad mouthed all over the internet there are thousands, and likely 10s if not hundreds of thousands of these engines around and still in use.  If they really were THAT BAD there would be a lot more first hand accounts online.  My conclusion is that I believe the 3126 is a very good engine.  If you try to use it for a heavy duty, continuous duty cycle application you’ll be disappointed.  But if you are just using it in your RV, and you don’t have a lead foot, I think you’ll be just fine.

Overview

The Caterpillar 3126 engine is a CAT engine that was used for light and medium duty uses.  It was used in everything from boats, cranes, over the road trucks and ambulances to RVs and buses.  It is based on an earlier design (3116), and when it was retired, the CAT C-7 was based on it and was it’s replacement.

I found a lot of diesel enthusiasts and truckers who really slammed the engine.  Said they didn’t last, would have cracked blocks, dropped valves, etc…  The marine community said similar things, but I could only find one instance of an RV user who said they had to have the engine replaced at 69k miles.

Now, CAT builds good stuff so what’s the truth here?  I think it is the following…

Why does the CAT 3126 have a bad rap?

The 3126 is based on an earlier engine (3116), it appears that earlier engine had some bad problems and this started it out on a bad foot.  I read some posts saying that the early 3126 blocks were weak, and cracked frequently (Something about a french foundry making bad castings) but I think that was actually the 3116 and CAT pro-actively recalled those blocks.  Also, there are some stories about dropped valves (when a valve in the head of the engine comes loose or breaks and gets repeatedly smashed in between the piston and the cylinder head (not good)).  CAT being CAT they took care of their customers and replaced a fair number of engines (I don’t know how many, but my rough estimate would be somewhere between 2 and 5 percent of the early marine engines (but, that is totally a SWAG.  I suspect it is actually less, but I have no idea).  How come they seemed to fail more in Marine use?  The engine is rated all over the map, from 170HP to 300+HP.  Same block, but with a combination of parts and / or ECU programming CAT can make it perform differently.  A company called Sea-Ray put pairs of the higher power Cat 3116 and 3126? in some of their boats, from what I read to get the boat up on a plane you had to run the engines at near 100% power and past what seems to be the normal redline.  I read somewhere that a user had to run 2800+ RPM, but 2200 RPM seems to be the typical redline for these engines (https://cptdb.ca/wiki/index.php/Caterpillar_3126) .  A LOT of these engines failed and were replaced under warranty, I even read an account of an account where CAT replaced both engines in a boat twice.  CAT seems to have gotten this issue solved, but not before the damage to the engine’s reputation had been done.

Also, it seems like the CAT 3216 engine was used in a few heavy duty applications where it just didn’t quite have the power to do what was asked of it and a larger engine should have been chosen.  People whose experience with this engine is limited to Heavy Duty use (Truckers) also think they are under-powered and trash.  (Hint, if someone thinks something is under-powered it probably also means they were running it very very hard in an attempt to get enough power for the job.)

What’s really bad about the CAT 3126?

It seems to me that CAT got the weak blocks and dropped valve problems squared away quickly, but there are still 3 “Defeciencies” in this engine.  These aren’t really deal killers, and I don’t think they are bad, but people should be aware of them.

The engine is a “parent block”

This means that unlike most heavy duty diesels (remember, this is a light to medium engine anyways, not a heavy duty engine) the cylinders aren’t pressed in sleeves (aka wet sleeves).  Instead they are machined in the block.  In a wet sleeve engine, with the engine still in a vehicle you can press the cylinders (sleeves) out of the block and press new ones in to replace a worn out cylinder.  This means that when the engine is worn out, if you want to rebuild it, you pretty much HAVE to remove it from the vehicle.  CAT does make a sleeve kit for the engine, so you can still “replace” the cylinders but afaik it can’t be done in the vehicle, it has to be removed, cylinders bored out and new sleeves pressed in.

This doesn’t really impact most users at all.  It just means that when you get to 200k or 300k miles expect a large bill to rebuild the engine.  (Again, not good for a trucker).  Probably fine for a Crane, Fire Truck, RV, etc…  See – https://www.steelsoldiers.com/showthread.php?134730-Caterpillar-3116-is-a-bad-Engine&s=0b1b00bed88b0e5d2ad3a25272fab800&p=1662012&viewfull=1#post1662012

New HEUI Fuel System caused erratic behavior if oil isn’t maintained

This is a very very minor issue.  CAT used a new fuel injection system called HEUI to allow computer control of the injectors.  A HEUI injection system (also found on Ford Power Strokes, as well as other engines) uses a second oil pump to boost oil pressures to 1500+ PSI (I don’t know the maximum) and essentially use the high pressure oil as hydraulic fluid.  And the computer would control the high pressure oil which then controlled the injectors.  This allowed CAT to do some really cool stuff, like make the engine run much quiter, better cold starts, more power, better fuel economy (and that is really true, not just a marketing thing), better emissions and possibly more.  However people would have problems if the oil wasn’t properly maintained.  (Atleast, that’s what I heard).  There was also a small possibility that a failed injector o-ring could get fuel in the oil system but I only heard of a single occurrence in passing about this and I don’t know if it was common, or even pertained to this engine or not.

I’ve also heard that the HEUI fuel pump can go out, if you have an older engine (2002ish and older) the pump is cheap ($1k) if you have a newer engine like a C-7 (not sure if they ever put the new pump on the 3126 or not, but I know the 99’s I looked at had the old pump) it costs more around $3k.

Again, not a huge deal, like any engine take care of it and it will take care of you.  CAT brand filters are some of the best of the best and they are cheap for the quality.  I think CAT wants people to use them so they have fewer warranty claims, they could charge $50  apiece, but instead, they are around $11.

ECM Problems

Because the engine is controlled by a computer, sometimes the computer dies.  Personally, I think they should be able to build them so they never die but they don’t.  I’ve heard that sometimes if the engine isn’t starting you can pour ice water on the ECM to get it to work.  And on a unit I was thinking of buying a previous repair invoice said the tech’s tried to put it in the refrigerator for a while to communicate with it but couldn’t.  (If they can communicate with it, they can “clone” it to a new one, if they can’t, they have to look up options and make sure the new one is programmed right… so they really really want to be able to talk to it).

If your ECM goes bad, it’s a $3500 repair at CAT.  BUT, it’s fairly easy to replace yourself, and a rebuilt ECM only costs $1k.  If you are going to put a lot of miles on one of these engines or you run a fleet that has them I would definitly buy a used / rebuilt one BEFORE you need it and have your current ECM cloned by CAT so you have a spare on hand and don’t rack up a large towing bill just to get your rig taken to a CAT dealer and for them to install a $3,5k ECM.

This is not an issue that is just for this engine, or to CAT in particular.  ECM’s go bad, I just think $3.5k for an ECM is a bit much to pay.

So what’s good about the CAT 3126 Engine

Here’s the thing, even though the 3126 is bad mouthed all over the internet there are thousands, and likely 10s if not hundreds of thousands of these engines around and still in use.  If they really were THAT BAD there would be a lot more first hand accounts online.  My conclusion is that I believe the 3126 is a very good engine.  If you try to use it for a heavy duty, continuous duty cycle application you’ll be disappointed.  But if you are just using it in your RV, and you don’t have a lead foot, I think you’ll be just fine.