Join Date: Mar 2002
Location: Austin, TX, USA
Re: 1989 throttle problem? please help
A major vacuum leak that fools the mass air flow sensor (MAF) or the manifold abosulte pressure sensor (MAP) can cause some pretty weird behavior. Look for large vacuum lines that may have been blown off by a back-fire. The break boster and PCV hoses are suspect. Also, if you have a MAF engine and there is ductwork between the throttle body and the MAF, make sure it hasn't slipped apart or developed a crack along a seam.
I hate to see you throw so much money at it though. Most sensors can be verified before replacing them. It requires a volt-ohm meter. A couple of straight pins (like you find in new clothing) can be used to back-probe the connector in a live circuit. A couple of aligator leads help connect the back-probe pins to the meter. Most sensors have 3 wires. The outer wires are usually hot (+5V) and ground (0V) with the center being the signal wire. A couple of quick checks with your meter will help you figure out which is which. Often times starting the motor and operating the throttle by hand while looking at the meter reading (set on DC volts) will help you find a dud sensor. Voltage changes should be smooth with no abrupt changes in the reading. A MAP sensor can be operated by a hand vacuum pump (if you have one of those) while watching the meter change with vacuum level.
And finally, those 80 models were well known for bad connections - even in the 80s. Imagine what nearly 20 years has done to the metal pins of the connectors. I've fixed several issues (on those model years especially) just by brightening up the connector pins with a tiny piece of rolled-up sandpaper. A small pick probe or even a brad nail can be used to squeeze the sockets closed a bit. Finally a nice dob of dielectric grease will help keep them corrosion free and will provide some moisture resistance. That's probably the best advice of all. It's cheap, easy, and no paid mechanic is going to invest that level of care until they've racked up a huge bill in replacement parts.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI