I Smell a Rat
Gonzo Jan 2010
A 2002 Lexus LS 430 came in the shop, stumbling, missing, and sounding pretty rough. It had a slight misfire but overall the owner said he couldn’t feel it vibrate, just that the check engine light was on and it didn’t seem to have the power it used to have. Mileage wasn’t that excessive, about 150,000. Miles or so. Nothing I would call extraordinary. The owner had some impressive service records and there was no doubt he kept care of his pride and joy. (Note: Keeping records of your cars history is priceless whether it’s for your own benefit or for the eventual sale of the vehicle.) There were 2 codes, P1310 and P0330 – Misfire on #3 cylinder and a knock sensor code. I figured I better start with the misfire code and see what that leads to. It could be that the knock code was a result of the miss. After removing the engine cover a glance at the #3 coil showed a problem. 3 of the 4 wires to the coil were broken off at the connector. Seems I found my problem, in the mean time, I might as well pull a plug and check them while I’m at it. It could use a new set, not all that bad, but we’re here, might as well price it out. I’ll call the customer and gave him the news. “Will that take care of the service light?” he asked curiously. “That I can’t say for sure until I repair what I see wrong with it at the moment,” I told him. I’ve learned over the years even though you see an “absolute-for-sure” problem in front of you… never, ever assume it’s fixed completely till you have test driven, and checked again. Just when ya think you’re done, you’re not. There’s always that “Murphy rule” that can spoil your day. I got the job approved and changed the plugs, cleared the codes and started it up… Ah, smooth, in fact very smooth. Sounded like a new car again. The miss was gone, the service light is off and the engine was running perfectly. It’s time to take the Lexus for a little test drive. As soon as it shifted into second and I gave it a little more pedal… the service light came back on and the engine power dropped off. Geez, now what? Here I was thinking I was done, ready to get to the next job… so confident that I wouldn’t have to do anything else to this Lexus, but here I am again. I’m wondering if that Murphy guy is around again, that rat, always making my job harder.
OK, back to the shop and get it on the scanner. The p0330 code was back again, but the misfire code was gone. The engine wasn’t missing; it still was as smooth as ever. What I didn’t quite understand was why the power drop happened exactly when the service came on. I went back to the computer to the theory and operation section.
Seems that as the vehicle passes 30 mph and the rpm’s are above 2500 the PCM reads the knock sensors mounted on both sides of the engine. The computer wants to know whether or not the engine is in good condition and that there are no “knocks” that would harm the engine. If it doesn’t receive a signal then it assumes there’s a problem (mechanically) with the engine. The PCM’s answer to this lack of knock sensor information is to lock the spark advance to a nominal value and retard the timing enough to allow you drive it, but not full throttle.
Rather than going for the wrenches and start taking the intake off I thought I might take a look at the knock sensor first. Since they are buried under the intake I grabbed my video scope and stuck it down into the engine valley below the intake… hmm, I don’t think there is supposed to be dog food, fiberglass insulation, a chewed on drinking straw, two bottle caps, and mouse droppings down in there. I think I smell a RAT! – It’s MURPHY! Found him! I took pictures of the whole thing, called the customer, and got the job approved. I took the intake off and clean out the pack rat house. And, as expected he had chewed thru the right hand knock sensor wiring and made a pretty good meal of the main battery cable leading to the starter.
After the repair was completed another test drive was in order. This time I wasn’t worried about old Murphy the rat, if he’s still in the car, he better have his little seat belt on… because I’m giving this V8 a little stomp on the throttle test drive this time.
One thing is for sure, there’s no holding back on that timing advance now… “Hang on Murphy; we’re going for a ride!”
The owner came by the next day to pick up his car. I saved all the junk I took out of the intake for him to see. He said he was going home to move the dog food and set some traps so if that little wire chewer shows back up he’ll have to wear a new type of seat belt, a permanent one.
On this repair job, Murphy really was a rat. I guess the thing to think about is not to give up on the diagnostics just because the two codes “sound” like they are related, but aren’t. A test drive to confirm the repair should also be a part of every repair. I’d like to make one more suggestion to the manufacturers and engineers, how about a code for “Rat in car”, or “Mouse in the engine bay”, maybe even a diagnostic tree with the final results as; “Remove Rodent”. I could have used that information on this job.