Piston Slap: The Undiagnosed, Fatal Discharge?

Sajeev Mehta
by Sajeev Mehta

TTAC Commentator mr_mike59 writes:


I’ve been following you and Sanjeev for many years on TTAC. Until now, I have mostly been interested in your advice and input related to other folks, and their sometimes obscure issues. At this time, however, I feel the need to reach out to look for some much needed advice.

My father-in-law is a Suburban man. Has been for over 35 years now. Unfortunately, the one he has at this point is suffering from some hard to track down electrical gremlins. He currently has a 2008 Suburban with the 5.3-liter and close to 150k on the odometer. For the past 3 winters, on their way to their snow birding location (an on the way home), they have been beset with charging issues that several shops have been unable to properly track down. The car will run fine for a good portion of the trip, with the only indication of a problem being a charging gauge that doesn’t sit still. However, when they are a good 6 or 8 hours into day one of their travels (pulling a 3,500 lb trailer), the voltmeter starts to sag, and all of the electrical systems start to shut down. This has caused them to either spend a fair amount of time on the side of the road, or cost into dealers up and down the east coast.

Many items have been replaced so far, and the car does not exhibit any of these issues, either in Florida during the winter, nor in the North during the summer season. (The needle does move around on the dash, but never far enough that the car shuts itself down).

Items replaced so far:

Alternator (3 or 4 times)


Positive Battery Cable

Negative Battery Cable

While these might be items that were ready to be replaced, none of them has shown to be the magic charm. None of the dealers have been able to recommend further places to look, beyond the many replaced parts, and we’re running out of ideas and patience. Prices have gotten insane on the newer ones, so as a retired minister, he isn’t replacing it any time soon, so we’re hoping you and the B&B can help make their next trip south just a little more relaxing!!!

Sajeev asks:

Has anyone tested the replaced alternators to see if they are running below 13.7 volts? Sounds like it might just be that re-manufactured alternators are sometimes poor quality and he needs to pack a spare with his luggage.

That’s what I do with two of my vehicles: a total of four alternators, and two of them have lifetime warranties from parts stores. (The other two are rebuilt locally and seem to last 4-8 years before dying) They certainly come in handy when you keep cars for decades, not years!

mr_mike59 responds:

I don’t believe that they have been checked. However, a little more to add… In all cases, once parts have been changed, or it has sat long enough to cool down, it will start right up. So the battery is not drained by the low voltage condition, nor is it contributing to keeping things going when the alternator isn’t.

While his recent luck doesn’t preclude substandard replacement parts in several states, it just doesn’t seem to be the answer.

Sajeev concludes:

It doesn’t sound like the answer to me, either.

And newer vehicles (in general) are harder on charging systems too, especially to the ol’ 12-volt battery.

More to the point, your father-in-law needs to keep a plug-in voltmeter in the Suburban’s console, plugging it into the cigarette lighter whenever he goes on a long trip. Actually just plug it in every time the Suburban leaves the lot, we need to see long term data on when voltage drops, when the battery charge drops, etc.

If no valid data shows up, perhaps it’s time to check every large ground wire (engine to body, body grounds) and install a marine (deep cycle) battery with the same amount of cold cranking amps instead. Or not…Best and Brightest?

[Image: General Motors]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

Sajeev Mehta
Sajeev Mehta

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  • Namstrap Namstrap on Sep 28, 2019

    I assume the alternator is also charging the trailer battery while travelling. It's either linked by a solenoid or an isolator. If it's a solenoid, I'd dump that puppy and get an isolator.

  • Cbrworm Cbrworm on Oct 01, 2019

    There is a current sensor somewhere in the power path between the alternator and the battery. If the trailer wiring is connected to the wrong side of the current sensor, the battery voltage can be allowed to drop, or cause a charging error due to the ECM not being able to see the battery reach a proper charge. I believe you are not supposed to hook anything to the battery post itself, only to the terminal on the other side of the current sensor (shunt). I would also wonder if they are towing in a lower gear than they normally would drive on the highway - causing higher sustained RPM and higher heat. Also, while towing, engine compartment heat will be higher than normal driving, even if they somehow stay in top gear. There could be some other sensor going out of range causing the ECM to start depowering systems and limiting power. It is possible the coolant/oil temp is actually too hot, but not being reported by the dash idiot gauge.

  • Steve Biro Frankly, while I can do without Eyesight and automatic start-stop, there is generally less B-S with Subarus in terms of design, utility and off-road chops than with many other brands. I just hope that when they adopt Toyota’s hybrid system, they’ll also use Toyota’s eCVT.
  • The Oracle These are all over the roads in droves here in WNC. Rarely see one on the side of the road, they are wildly popular, capable, and reliable. There is a market for utilitarian vehicles.
  • Stephen My "mid-level" limited edition Tonino Lambo Ferraccio Junior watch has performed flawlessly with attractive understated style for nearly 20 years. Their cars are not so much to my taste-- my Acura NSX is just fine. Not sure why you have such condescension towards these excellent timepieces. They are attractive without unnecessary flamboyance, keep perfect time and are extremely reliable. They are also very reasonably priced.
  • Dana You don’t need park, you set auto hold (button on the console). Every BMW answers to ‘Hey, BMW’, but you can set your own personal wake word in iDrive. It takes less than 5 minutes to figure that that out, btw. The audio stays on which is handy for Teams meetings. Once your phone is out of range, the audio is stopped on the car. You can always press down on the audio volume wheel which will mute it, if it bothers you. I found all the controls very intuitive.
  • ToolGuy Not sure if I've ever said this, or if you were listening:• Learn to drive, people.Also, learn which vehicles to take home with you and which ones to walk away from. You are an adult now, think for yourself. (Those ads are lying to you. Your friendly neighborhood automotive dealer, also lying to you. Politicians? Lying to you. Oh yeah, learn how to vote lol.)Addendum for the weak-minded who think I am advocating some 'driver training' program: Learning is not something you do in school once for all time. Learning how to drive is not something that someone does for you. It is a continuous process driven by YOU. Learn how to learn how to drive, and learn to drive. Keep on learning how to drive. (You -- over there -- especially you, you kind of suck at driving. LOL.)Example: Do you know where your tires are? When you are 4 hours into a 6 hour interstate journey and change lanes, do you run over the raised center line retroreflective bumpers, or do you steer between them?