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Low end ping

Hi guys,
my truck is a deluxe model pickup circa 1993 3vz v6 with 150k in it, runs very very well and gets close to 24 mpg. I drive like an old man so that sorta explains the mileage. Problem is a ping below 1500 rpm, engine hot,light load going mostly up hill or light acceleration. Seems to be getting worse. It used to ping badly and I upgraded to top grade gas and the ping went away and the mileage increased 9%.

I have checked timing which was a tad advanced (2 deg), increased spark gap .010, tried a new knock sensor in a remote location with a new pigtail which the ecm did not see at all & read the 52 code. No code on the old sensor, no change in the ping with the new one, but slight change in performance perhaps. The wiring to the ecm showed no resistance.
denso platinum plugs gapped @ .042, mass air flow meter checks out, egr checks out, tps checks out, temp gauge reads normal, plenty power throughout the rpm range.
Only thing I see not right is when I check the timing with te1 and e1 shorted, when I remove the sst the timing does not advance the 5 to 7 degrees the book says it should. it advances plenty when I rev it though.
I have swapped put the ecm, and igniter from my daughter’s 94 which is the same truck. None of this shows up on her truck even with my parts installed.

egr valve passes tests and is spanking clean inside. Passages are clear.

I have run seafoam as a top end cleaner both through the tank and directly into the intake.

What puzzles me is that the truck runs so well at higher rpm & load, but pings under 1500 rpm under light load. Even when I push just a little bit more on the gas, not enough to speed the truck up, the pinging stops.

My friends tell me to just drive around with my foot in it and stop driving like grandma.

“engine hot” I think is where you have to start. Pressure check the coolant system, or squeeze the top radiator hose when cool, then again after it is warmed up. If you do not feel a firm hose when warm you have a coolant system leak. Are you using coolant? pull the radiator cap when cool and see if coolant level is to the top of the radiator. Next most likely suspects, is coolant fan electrically operated and kicking in? if so then move on to the thermostat and water pump. Running hot is the red flag for me!

I think “engine hot” meant “engine is fully warmed up.”

low end ping can often be caused by either overly advanced timing, or carbon deposits on the pistons, which themselves are caused by the incomplete combustion of fuel. The carbon deposits get hot, and then pre-ignite the gasoline before the spark plug does it. You haven’t, by any chance, been feeding it premium all these years thinking that was good for the car, have you? That can cause carbon buildup on engines not designed for premium.

Try filling it with premium for a tank and see if the pinging goes away. (ironically, the cure for carbon deposits if seafoam doesn’t work and you don’t want to pay to have the engine torn down and cleaned for real is to put premium in it, since it has a higher resistance to ignition and therefore won’t be ignited by the hot carbon)

Sounds like you are “lugging” the engine…Give the engine (downshift) 2000 RPM so it has something to work with…

You may be on to something Caddyman. That truck might need to pass 3,000 before shifting and always be above 2,000 except when cruising at a constant speed. My old 4.9L Ford likes 1,200 rpms but those rice burners seem to like life on the high side.

“when I remove the sst the timing does not advance the 5 to 7 degrees the book says it should”

Does this truck have a distributor?
If so I think there’s something fishy with the vacuum advance.
Try swapping the distributor with your daughters.

I’ve driven many “rice burners” and none had a problem moseying along and shifting at 2000 rpm.

If he upgraded to top-grade gas (which I missed in my initial read-through) and it fixed the problem for awhile, then he’s pinging, not lugging. He has a carbon problem.

And btw, the gap for the 93 T100 with the 3vze engine is .032, not .042.

The OP stated that the timing was advanced by 2 degrees and that could be enough to cause a ping. They did not state if the base timing was reset to where it should be.

Since most pinging is due to a timing or EGR issue it seems to me the problem would lie in one or both of those areas.

Other vague, possible causes could be oil consumption (lowers octane so to speak) or a partially clogged converter.

ok, I’m back. Thanks for the input.

“Engine hot” indeed meant fully warmed up-after 7-10 miles. It does not run hot.

I have been running premium for years. It pings like mad on anything less. The service guys at Toyota say that some of these engines need to run on premium and some don’t. My daughter’s truck runs fine on the cheap stuff.

The engine will lug below 1000 rpm but is smooth at anything above that, with plenty of guts. Pinging is between 1000 & 1500 where it goes away, and only under a light load, like up a 15% grade (6-7 degree slope)in second.

I have done the seafoam thing, no difference, so I wonder about the carbon on the pistons. Certainly still could be, and a couple of my friends have encouraged me to stick my foot in it. Most of my driving is freeway, very little stop and go. Steady 2500 rpm for 10± miles each trip.
The distributor has no vacuum advance. The base timing is now correct, no difference. Engine does not burn oil. I checked the resistance in the distributor pick-up sensor cold and it was fine. I forgot to check when the engine was hot, and will do that today, providing I remember. I changed the cap and rotor just for good measure. No change.

I changed the gap to .042 on advice of an excellent mechanic friend of mine. He thought by lowering the spark temp it might help because the engine has so many miles. It didn’t.

I there a way to check for a clogged cat? That is a possibility but that would show up at a higher rpm and load more than at 1000. This thing runs sweet up higher.

A Seafoam treatment through a vacuum line might help it. So might an 'Italian tuneup"–basically floor it and drive it at high speed for a while. It might help get the carbon out.

We used to dribble water down into the carb to get rid of carbon deposits, I have no idea what introducing water into the intake plenum would do to a modern engine with cataylitic converters would do.