Toyota Truck Pinging Problem

I have a perplexing problem with a 1990 Toyota Pickup with 125K miles on it. This truck is equipped with a carburetor. It has been very well maintained over the years. When the weather starts to become cool and dry as it has been here recently (I live in a rocky mountain state at about 4,200 feet in elevation), this truck will ping incessantly after a cold start as it approaches normal operating temperature. It only happens this time of year (Oct-Nov). It?s been happening for at least the last five years and I have not been able to solve this problem. I do not hear any pinging in the winter, spring or summer months, and tuning the engine has had no effect.

I hear the pinging most notably when I start from a stoplight or when I shift into higher gears (third or fourth)?and only under light load. If I step hard on the accelerator, I can?t hear the pinging. However, if I?m driving at moderate RPM, it sounds literally like marbles in a tin can. The odd thing is, after driving it for several miles at normal operating temperature, if I turn it off for a few minutes and then restart it, the pinging is totally gone.

I?m thinking of three possible causes. First, the combustion temperatures could be excessively high before the thermostat opens. It has a fresh factory 195 degree thermostat in it and I have always used the Toyota red coolant. It has the original water pump, but a new radiator and fairly recent fan clutch. Second, I may have carbon build up in the cylinders causing hot spots. I was going to try a Seafoam or BG 44K treatment to see if that makes a difference. Third, I may have a lazy valve somewhere in the emission system or possibly and EGR valve problem. I was going to change out the vacuum lines (they are original), and I was thinking of pulling the EGR to see if there is any carbon buildup in the tube.

Am I on the right track here? Are there other suggestions?

Little confused here…Pinging will be MORE prominent under load. It should be NON existent while idling or slow acceleration. I’m wondering if what you’re hearing is pinging? I wonder if it’s valve clatter. Have you ha d a professional look at it??

I’ve driven this truck since 1992. There’s a lot of valve noise in these engines, but this is definitely pinging. Happens under light acceleration or steady driving immediately after shifting into higher gears. Hard throttle causes it to diminish signficantly, or go away altogether. Right now, I’m running mid grade gas. High octane helps, but does not completely eliminate the pinging.

Pinging is often caused by an EGR valve that doesn’t open. You do have a lot of carbon in the area of the EGR valve too, if it is anything like an 87 Mazda. By now, the EGR valve is probably worn out too.

Kind of leaning that way too. It does seem like something is sticking closed, like possibly the EGR. Only happens with cold starts in the morning or after work late afternoon when it’s had time to completely cool off. Ran it today at lunch and no pinging.