Chevy Suburban

When accelerating the car backfires a little from under the hood. This backfire is not from the tail pipe. It does this when speed exceeds 65 mph and this is not a constant problem. The problem occurs every once in a while during the course of a day. Then sometimes it does it after 25 mph it seems that is won’t get over that speed. I have taken it to a mechanic and both times was told it was something different. The first time I was told that is was the fuel injection pump, this was changed but the problem persisted. The modulator was then changed but the problem persists. The mechanic also changed the part that cleans the air and fuel fixture with no solving the problem. Can anyone help, please? Welcome any suggestions that might be helpful

Years ago, I had a similar problem with my 1954 Buick. The problem turned out to be in the ignition system. A service station that had a scope found that the firing voltage was too low for the spark plugs to fire reliably. Scopes were not a common item back in garages back in those days. At any rate, an ignition analyzer might either pinpoint your problem, or eliminate the ignition system as the cause. Your symptoms sound similar to what I had with the Buick.

I think that what you ascribe as “backfire” through the intake tract, is actually “misfire”. Spark plugs and spark plug wires might take care of that problem (whether it’s “backfire”, or “misfire”). If not, it’s, most likely, still spark production system.
This brings up an interesting question: Is it even possible for an engine to backfire at those higher rpms? I don’t think it is.

What year vehicle are we talking about, here, and what kind of engine?

Could this sound that you’re interpreting as a backfire actually be a rattle, like as in pinging? If so you may have a malfunctioning EGR system. The EGR system allows a wee bit of inert exhaust gas to mix in with the intake air and displace a bit of oxygen, preventing the cylinders from getting too hot and pinging under load.

There was a TSB (Technical Service Bulletin) on some make of car of the pcv valve hanging open, leaning the fuel/air mixture, and causing misfire. A pcv valve is very cheap. Change it and see if there is a positive result.