Why do EFI vehicles have engine knock?

I’ve got a 2000 GMC Sierra 5.3L engine with about 65,000 miles on it. Oil has been changed faithfully every 3,000 miles with synthetic. It doesn’t burn any oil. The power train has never had any issues.

The only maintenance item due on it is a coolant flush and fill with dex-cool (engine temperature is fine).

About 2,000 miles ago, the engine started pinging on acceleration. I’ve used 87 octane gas from day one and it’s only recently that the pinging started.

If I run 91 octane the pinging is all but eliminated.

I think I understand the technical reason for engine knock. Pre-ignition.

How does this happen on a vehicle with electronic fuel injection and a knock sensor? On a vehicle with adjustable timing, I can understand that it may be out of adjustment. I don’t believe that’s possible with EFI. Am I wrong?

A couple of mechanics heard me say “engine knock” and responded “higher octane gas”. But that seems like a bandaid to me.

Are there perhaps other components that may have failed?

Is this the beginning of the end for this all too young engine?

Generally as engines get older, they tend to knock less because as the engine wears, compression is reduced. Lower compression = less chance for knock.
Now on to your issue. While there is a chance that it is a mechanical issue, I would guess that it is more related to the air fuel ratio, with one or two cylinders being too lean.
The very first thing I would do is a full inspection of the intake system. Pull the intake hoses off and check for any leaks, a split or hole can cause issues. After that I would look for a vacuum leak in the intake track somewhere. Next I would pull the plugs, and inspect. All of them should look the same. Google for pictures of various conditions. If there is any doubt screw in a new set.
Next would be a check of the injectors for spray pattern. A dirty injector can cause the fuel for one cylinder to not atomize properly and a ping.
If all of that checks out and you still have a ping, GM Top Engine Cleaner has a great reputation. Or maybe you want to start with the top engine cleaner.

Sounds like some Click and Clank advice. Although true, it may help, but you’re right, that is a bandaid. If a car designed for 87 ran good before, and now needs 91 to get by, something is up.

EFI still controls timing adjustments. You could have a component failure, or some other rogue problem. I like to start with the basics, do you have a CEL (check engine light)?

No check engine light. No perceived loss of power.

Rick’s advice sounds like a good starting point. Thank you.

The truck’s just getting old enough that hoses may be an issue. Reminds me that I haven’t changed the PCV valve too.

Spark plugs are only at about 15k miles - looking at them should give me a good indication about what’s going on in there.

I haven’t pulled injectors before. I’ll have a look at my service manual to see if it’s at my level of DIY ability. If the spray pattern is compromised are they salvageable or is it time for new injectors?

I haven’t heard of that method of engine cleaning before. Given the fact that the truck is an urban warrior that spends lots of time in stop and go traffic and idling in the winter, the top engine cleaner sounds like a reasonable routine maintenance event.

I’ll get to digging into it in the morning.

Are you certain you even have a knock sensor? EFI != has a knock sensor, as my Mustang is very happy to remind me.

About pulling injectors, to find out how they spray you are going to have to trigger them somehow, which is going to put finely atomized gas all over. You need to make sure that there is no ignition source available for that gas. If you have a bad spark plug wire, you could wind up with a new vehicle as your old one could burn to the ground.
Or in the (only) words of MooNiE the Magnif’Cent* Don’t screw up!

Seriously, don’t take any chances. If you are not 100% sure on how to do this safely, leave it to a pro.

Una is correct. EFI != knock sensor. I often forget that not all EFI systems are as sophisticated as the ones I am used to working on.
You might want to move the top engine clean up to before the injectors and put it right after spark plugs.

*MooNIE is a ren fair performer who does his entire act with only whistles, and gestures. At the end he has an audience member assist him in his final bit. The bit involved lit torches and after instructing the AM with just gestures and whistles, he says the above linked line.

It is possible for carbon to build up in the combustion chambers effectively increasing the compression ratio, higher compression ratios typically require higher octane gas. If this is the problem there are many inexpensive additives that could help, A product called ‘seafoam’ is popular on many car forums. You can add ‘seafoam’ to your gas and/or directly to the intake through a vacuum line.

It might not be pinging. The GM 5.3 engine of 2000-2004 vintage is known for having piston slap which manifests itself around 40K miles. Many people experience the knocking and there is a class action lawsuit (naturally). I have one, I have the slap.

I am on several GM boards and it is quite common on this engine.

If I had a nickel for every time I heard him say that, well, I’d have a fair handful of nickels. I’m just a bit startled that anyone here knows who he is.

For the OP - you’re not going to have an audience member on stage holding a small bucket of water to toss on you in case things go “foooosh…AAHHH!” If you want to try this at home, it would not be a bad idea to have a friend standing by with a fire extinguisher.

And hope that they’re not like the guy at his 10:30 show today that didn’t even flinch when a still-burning torch was tossed onto the stage. :smiley:

So I pulled the plugs today. They look good. Nothing to indicate that any of the cylinders are running lean.

There’s no obvious air leaks anywhere.

I’m going to put my hopes on a carbon build up and do the top engine clean. If no luck with that, I’ll defer to safety and pay someone to inspect the injectors. No sense putting on a show for the neighbours.

MrFloppy, unfortunately, I’m familiar with piston slap too. The engine does do that when the temperatures hit about -10 (15F). It lasts a few seconds from a cold start until the metals start to expand from the heat of combustion. This knocking’s not the same as the slapping though. Poor engine.

I think I’ll get through this winter with the truck and then consider selling it in the spring.

Thanks everyone for the information and advice.

Have you run a code scan on the engine? A defective EGR valve can cause knocking.

I haven’t done that, but I assumed that the check engine light would have alerted me to the need to do so. It hasn’t come on. Is it prudent to check the codes if the computer isn’t throwing any warnings out?

I don’t know how out-of-spec the emissions control system needs to be before it sets the CEL. But, IMHO, it’s always worth doing a code scan.

On review, I don’t see anything that indicates that the truck does have it. I assumed that it was a normal component of anything with an electronic ignition.