Tuesday, April 24, 2012

Cat 3126 Hard Starting & Running Rough

The CAT 3126 Diesel Engine uses electronic fuel injection and they use HEUI (Hydraulically actuated, Electronically controlled Unit Injector) which require high pressure engine oil to actuate injection. The high pressure oil pump is mounted on the driver side and is engine driven. The hard part of this problem regarding our fleet is we do not have any CAT 3126 software to troubleshoot this problem.

So as with DT466 electronic engines the most common failures is the o-ring seals on the injectors. If high pressure oil is bypassing, the injection cycle is incomplete and the hard start poor running condition develops.

Side Note:  By varying the oil pressure, injection can be controlled independently of the position or speed of the engine crankshaft or camshaft. The injectors are controlled by a Powertrain Control Module or PCM. Injection Pressures can reach as high as 18,000 to 24,000 PSI.

Since this particular engine has accumulated lots of  miles without having a major tune-up besides a valve set we pulled all of the injectors and found the problem. The o-rings failed on a couple of the injectors enough to cause a constant problem every day cranking over the engine excessively to get it started and having a bad miss when warm.

Cat Engines specify that you need a Cat tool specifically designed to install these injectors, well since we only have a few of these engines in our fleet (and want to save money on tools) we found another way to push the injectors into the bore.

Use an appropriate sized combination wrench to gain leverage using the open end, coat the o-rings with clean engine oil and seat the injector using the injector hold down or body casting. (whatever you use take care and push the injector into the bore without force making sure it goes into the bore evenly). using the hold down bolt to install the injector is not advised but you can lightly snug it up as you progress.

The main point here is to not pry on the very top of the injector where the solenoid is located.We found this technique in our International installation manual. The HEUI injectors are used in their engines as well.

I hope this information helps you out the next time you have a CAT 3126 Diesel Engine with a hard start or rough run condition.


Robert (Rob) said...

Our ProLink helps us identify HEUI leaks I have observed Cat pump % on the top output, no leaks, engine hot, fresh rebuilt pumps, is in the 55% range, max. is 100%. IH around 50%, max. is 85%. So if your percentage is running high especially if its worse when hot you likely have a leak.

Ben said...

Hello,first and foremost thanks for this site it has been a huge help with our diagnosing our engine problems, so thank you. We have the engine starting and running great now but it has more smoke than we would like coming out of the engine crankcase vent and a little out of the exhaust. Both of which seems to vary sometimes it is not too bad other times it is very annoying. Could this be caused by injector orings being bad? or is it more likely just a worn engine. The engine was a swap done prior to our buying the truck so I don't know the miles on it but was told they were low.
The engine is a 3116 heui 1997, it starts right up and runs like a top thanks in huge part to this site.
thank you.

Oficina Mecanica Sorocaba said...

Wow very knowledgeable blog.. Thanks for sharing...
I really love to read about bus mechanic. Is this is similar to car mechanic or difference from that..?