QuestionQUESTION: I have a 1978 F-100 351M(engine) C6(Trans). No matter what speed, hauling a load or under it's own weight, the truck seems to stall. When it does this it sounds like the truck is back firing through the carb. I checked the timming, no problem. While trying to simulate the problem in my driveway "power braking" until it happened again, I noticed smoke comming up through holes in the floor (something mounted in the floorboard by previous owner, since removed). I "power braked" again until it happeded again, I threw it into park and jumped under to see what was happening only to notice that the transmission was smoking from the vent. Re-cap, "power braking, engine backfire, trans smoking." I have already changed the fluid and the filter in the trans and the fluid in the torque converter. Fluid was not burned prior to being changed, maby changed by previous owner. Un-verified history - told by the used car lot that the previous owner tried to pull a 32ft. trailer on a 2in. ball. I am assuming that the trans is shot, I am hoping just the torque converter is shot. Any Advise? or refferals?
ANSWER: Hi Sean:
The backfiring sounds like you are running too rich (air/fuel mixture). That could also account for the smoke underneath. If you are running too rich, that means excess fuel is being dumped into the exhaust system and burning there, which would create a backfire due to lack of oxygen.
However, the tranny problem sound strangely like the transmission is over-filled. As a standard practice, you should always under-fill the tranny by one quart when changing fluid, then drive the vehicle for 15 to 20 minutes and then check the transmission fluid level again while she is still hot. Then top her off if she is still low. Over-filling the transmission is a common error, because it is usually cold when it is changed and checked.
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QUESTION: Hi Vern,
The back firing is coming out of the carburetor. I checked the air fuel mix for the carb, I found that someone else set it to full lean (there is a fool proof twist adjust on the side of the carb) so I backed it off a little bit (made it richer). It seemed fine for a day, then started backfiring again the next day.
I have adjusted the timing before, and back since, but that did not help. Could I have a sticking valve causing the backfiring?
On another note, a friend of mine said something about a vacuum line coming off of the transmission that could possibly cause the carb issues?!?!? Could that explain the smoking transmission (it is not overfull, and the smoke is coming out of the vent on the top of the transmission)?
I am in dire need of something to help.
Thanks again, Sean.
AnswerHi Sean:
This really looks like a timing problem. If you have set the timing correctly, then it may be a loose timing belt that keeps slipping. If this is the case, you will have to adjust or replace the timing belt/chain. The question is, have the timing belt ever been replaced? On a truck of that age (do know the mileage), it may be time. Keep in mind, once you drop the front, may as well replace the water pump.
But, before you consider doing this, check to make sure that the intake valve is not stuck open and you are getting the right air/fuel mixture. Also check that your spark plug wires are all on tight, in good condition and that they are not crossed (e.g. the correct firing order).
If all of this checks out, you most like have a slipping timing belt. Especially if you notice it more on heavy loads.
