Bottome Line
Intermittent irritations
There is nothing more frustrating than an intermittent problem that will never arise when you're at your service provider.

By Paul Abelson, senior technical editor

Q. About a year and a half ago my 2005 Peterbilt with a Caterpillar C15 ACERT motor started having this odd power issue. At first it would happen once a week or once a month. Now it does it several times a day, and still my local Cat dealer and my local Peterbilt dealer are completely unable to figure out the problem.

When I'm on the throttle, either light or heavy, the engine will just jolt, like a hiccup. It happens so fast I don't have enough time to see what's happening on the gauges because it only lasts for a second or less.

Also, because the event happens so fast, I was told by the service writer at Cat the ECM does not throw a code. Events have to last for several seconds to register, so no one knows where to look. Everyone is convinced this is an electrical issue of some kind, but what and where?

One of the biggest concerns now, aside from damaging the engine, is that the "jolt" may also be damaging the transmission, driveshaft and rear ends. I have gone through several u-joints and two hanger bearings in the past year and I have no doubt this is making it worse.

Any suggestions you may have at this point would be very helpful.

A. I discussed this with my TMC "brain trust," Carl Tapp and Tom Tahaney. They're the retired experts I've mentioned in previous Maintenance Q&A columns.

The intermittent nature and failure to record an error are strong clues. There is nothing more frustrating than an intermittent problem that will never arise when you're at your service provider.

We think the strongest likelihood is that it is an electronic problem caused by an electrical source, with your engine controller getting a momentary short or an interruption of signal.

It could be in the wires leading to the module from the sensors, or from the ECM to the injectors. It could be a chafed wire that is momentarily grounding to something, or a bad connection or connector pin. Problems other than the wiring would typically last longer and should throw a code.

We recommend old-fashioned troubleshooting, starting with a very careful visual and physical inspection of the wiring and harnesses.

It is also possible that the ECM has had its computer code corrupted. If it's happening several times a day now, soon it should become very obvious.

Since you didn't describe what work had been done, our guess is they just checked for codes. With modern, computer-controlled engines, the first thing technicians do is to plug in to the J1939 data port. That usually tells them where to look for problems. If nothing shows up in the reader, many of them are clueless.

First-generation (EPA 2004) emissions-controlled engines created a great deal of excess underhood heat because of the exhaust gas recirculation (EGR) that every maker used to some degree. That heat was often enough to soften insulation on wires. Wiring that could easily withstand chafing when cool was now subject to damage through the slightest abrasion.

"Tug tests" with the engine running can help determine whether the problem is in the wiring or at the connections due to physical damage, like bent or broken connector pins. Heat also acts on connectors where they are spliced to wires. Thoroughly going over your wiring to/from all sensors, the ECM and the injectors should solve your intermittent problem.

You were kind enough to write back to say that the problem was found in wiring from the sensors. Several were worn through, but only rarely made contact with anything – thus the intermittent hesitation. You had all your underhood harnesses replaced with upgraded wires and the problem was resolved. LL