QuestionI have a 1988 1 ton dually with a 7.4 TBI that started backfiring during light and hard exceleration. I changed the fuel filter today and it improved but is still present. The engine is low miles and starts and runs well besides the backfiring through the intake. Does not have HEI, has the newer ignition.
I'm not sure what I can check next, any input would be appreciated.
AnswerHello Darryl,
It has throttle body injection, and I think there is a good chance you need a throttle position sensor.
It is mounted on the side of the throttle body, and connected to the throttle plate.
It is testable, and I sure would suggest testing it before just replacing it.
Another possibility is fuel pressure. Especially since a new filter improved performance.
Problem is, the pressure is a pain to test on 88 models, cause they didn't include a test port.
One way is to get a test adapter that goes in place of the filter.
But however you tap into the system, it should have 13 PSI whenever the pump is running, which is all the time that the engine is running, plus three seconds before the engine starts. You can make the pump run by finding the fuse on the right side of the firewall, under the hood, and with a jumper wire, jump battery voltage to the fuse.
With a tester in place, and the pump running, if there isn't 13 PSI, squeeze the fuel return hose with some vise grips and a rag,(so you don't damage the hose). If squeezing the hose makes the pressure come up to 13-14 PSI, you could need a fuel pressure regulator, which is on the throttle body also.
If the pump runs, but won't put out 13, then you might have a hose split inside the tank, where the pump connects to the line, or just a bad pump. But I have had that hose split, and spray everything above 6PSI out inside the tank. That truck bucked, stumbled, and backfired till I replaced the hose, and is running great still. That was about 6 years ago.
Van
