We are working on a 93 KW that has 415K original miles on it. 3406B PEEC Caterpillar 4CK20959. The story is the truck was bought by an owner operator, then he went under, so a farmer bought it and let it sit in his barn for years. Then the farmer passed away and the current owner bought it. While sitting, it rusted through the liners and had to have an inframe overhaul. After that ran great.

Now, the current owner took it to the local truck wash last weekend, and afterwards the engine had no power and ran poorly. So he dropped the trailer at the truckstop and bobtailed to our shop. At some point before he arrived, he changed the fuel filters, which seemed to help some, but not much. When we checked things over, we found we could not communicate to the ECM. Check engine light flashes a 55, signifying all is well, so we didn't worry about communicating that way. Found a few loose connections at circuit breakers, cleaned and tightened, all look good now. Continued testing, found that when the engine runs, it has what sounds like a random, single miss, and other times sounds like a single knock or ping, as would a mechanical if you opened the rack suddenly. But it's just one miss, or one knock, at random times, with no apparent stimulus. May be 30 seconds after starting, then three seconds later a miss, then 20 seconds later a miss, then a minute later a knock... there is no pattern to it that I have found. This is just an example of the randomness, not necessarily what it has done time-wise.

We checked the rack motor, found it to be sticky, so it was replaced with a new one. This made no difference in our issue. We verified that the timing motor operates, observed it and tested the motor, all is well. From time to time, the rack motor will flick, but there is no change heard in the engine running, as you would expect from a timing change. It flicks towards the rear of the engine, and flicks back to original position very quickly, seemingly at random, the whole time the engine is running. Does not seem to correspond with the engine miss or knock.

I checked the rack sensor on my graphing scanner, found that SOMETIMES when the knock occurs, it shows a slight voltage spike and back to normal very quickly, and sometimes when the miss occurs, it shows a slight drop then returns to normal voltage. I would guess this to be .5V change in either direction when this happens. Sometimes when we hear the knock or the miss, the graph reflects, other times we see the graph spike or drop but hear no change in engine idle. Other times the knock or miss will be heard but the rack sensor shows a steady signal.

This miss or knock occurs randomly at any RPM range, and doesn't necessarily happen more often at higher RPMs. Sometimes the square wave going to the rack motor would look funny, or not quite the same as it had awhile before, but had no correlation with the miss or knock. Rack motor seems to run between 1.6v to about 3v wide open, and increases off time of pulse width to increase RPMs.

We did find the J1587 datalink wires have 1.1V drop on them with a 4 amp load. That's why our scanner won't communicate, but should have nothing to do with the engine running fine on a PEEC ECM, right? Should only be for scanner communication I believe.

I cannot find anything definitive here that will scream "I'm the problem!" and I'm not completely sure that it's in the electrical system, but with a base mechanical fuel system, could the plungers in the fuel pump have taken on rust with the truck sitting so long? Seems to run very well if that were the case, but not right. We think it sounds like a timing issue, but the timing sensor reading stays put when the miss or knock occurs, and the timing motor does not get told to change the timing from what my graphing scanner shows. By the way, this engine does not smoke when the knock or miss occurs, just makes the noise. And the driver said when he pulls a hill, it'll sometimes start knocking until he lets out of the accelerator, and getting back into it then will resume normal engine behavior. Makes us think timing, but all seems to test ok!

Any input would be appreciated, we are going nuts here, running out of things to test! Sorry for the long post, just want to make sure I include all the details I can think of!

Tyler Coressel
Technician
Rosencrans Truck Repair
Columbia City, Indiana, USA