Originally Posted by dalesy
A buddy of mine has been having trouble with his late 90's Ford van. It has a 302 engine. He's been having rich running problems with it (rough idle, smoking, excessive gas use) that the dealer can't seem to fix.
He's wanting to take off the injection and put on an older 302 intake and carb, using regular linkage, etc. and go back to simpler....
Can he do this? Does the computer control more than just the engine? I'm trying to help him get this thing fixed without it becoming a money pit. The engine runs decent, but has pretty many miles on it. It's just a work truck.
Thanks for the help.
How late 90s, the technology goes thru big changes at times on a yearly basis. 95 and older it's probably an OBD I system which is simpler to work with. 96 and up it's an OBD- II system which is more comprehensive but pretty much shuts the backyard mechanic out unless they're very computer system savvy. But depending upon model, you can find OBD II systems as full or partial beginning to infect designs as far back as 94. So year and model is important data.
Both systems have a lot of on-board diagnostics that can be down loaded either thru the "Check Engine Light" or to a PC with a diagnostic program loaded on it, or to a "Reader". Many better parts stores will read it out for you as they'd like to sell you parts so they'll do some diagnostics for you. If the problem is sensor related, the computer should have set the "Check Engine Light" which tells you for sure the computer knows something isn't right and will point at some link in the chain of sensors, it isn't always the one at fault, but it'll be in that logic chain.
Rich running can be the temp sensor, which is separate from your gauge sensor, is off and is keeping the engine in warm up mode although I'd be surprised that cold start is so rich that you're gagging on the exhaust. Vacuum leaks into the intake and even exhaust leaks will mess with several sensors including the O2. If it even thinks the engine is lean, the signal to the computer will cause it to richen the mixture. A failed pressure regulator or plugged bypass ,if used on his model, can increase system fuel pressure which will always over fuel the engine. In fact this would be one of the first things I'd look for because this can drive the mixture so rich the smell in the exhaust is going to be really bad. Other related problems can be a failed or out of synch vacuum transducer, this is reading manifold pressure, which the computer uses along with throttle position sensing and if so equipped a Mass Air Flow (MAF) sensor to determine the fuel and ignition advance values. Problems can also be from simply bad connections or voltage problems that are too high, too low, or wander because of battery, regulator or alternator problems.
So while everybody wants to immediately go to the computer and the expensive stuff, take the time to look for bad vacuum hoses, leaks in the exhaust ahead of the O2 sensor, bad wiring connections, over or under volt conditions in the electrical system before throwing expensive parts at it. Check the fuel pressure, then go to sensors. Just start simple, especially if it isn't throwing codes. If there's no error codes, it's a fairly good sign that the problem isn't a sensor or wiring between them and the computer except for maybe a physical adjustment which some sensors require like the throttle position sensor for alignment, but rather some old fashion problem like a leaking vacuum hose or a blown head gasket. Things like that change the values the sensors are looking for and when they get out of whack with what they're expecting to see or out of synch with each other, the computer gets confused and like HAL, does the best it can to handle conflicting commands. So while it may put you on the side of the highway at least it won't put you into the cold of inter planetary space without life support.
To convert back to a carb presents a sizeable effort as the computer runs the transmission, the instruments, anti skid, anti theft, etc. It needs inputs from the engine sensors to do this, so just pulling connector plugs isn't going to get ya there.