Hi all, long time lurker, first time poster on PT.net. I've got an issue with my 1994 Chevy ECSB. 5.7L engine, automatic. On the interstate, when it tries to down-shift into passing gear, engine stumbles then when you let off the throttle, it backfires. I've been kicking ideas around with several people, I think it's ignition related. My pop and I are both thinking Throttle Position Sensor, but at this point I don't want to start shooting in the dark. Also, truck has 285K miles. Thanks in advance. -Max

 

anyone?

 

My brother had one go bad on a Bronco (not sure which engine) that would cause stuff like that anytime you changed throttle position---up or down. Once you held steady throttle position,it would be fine.

If you have an analog ohm meter you can pull the connector and actuate the tps(WITH THE METER PROBES ON THE TPS PINS). If the needle jumps instead of moving smoothly up and down,the tps is bad.

 

OK, new development, problem no longer produces itself, didn't change a damn thing, had the thing scanned twice on two different scanners, no codes. In need of help...please.

 

It takes an act of congress to get OBDI to throw a code and when it does its like a grunt then a description about as simple as its broke and thats it. It makes it hit or miss to diagnose but at the same time I prefer it because you just don't have much trouble with it. If you haven't done a tune up (plugs, wires, cap and rotor) lately that would be a good idea, but if the fuel filter is old replace it. They are 10 bucks or less. The TBI fuel system only runs 9-11 psi and they will actually run on as little as 4 psi (I did it but not on purpose) but it will run like pure crap, have no power and drink gas. The throttle position sensor is nothing more then a reiostat. Get a schematic for it and one wire will be ground, one will be 5 volts constant voltage, the other will be varying voltage based on throttle blade angle. Closed throttle voltage should be around .4 to .7 or so, and wide open throttle voltage close to the 5 or so constant volts on the 2nd wire. Then with a volt meter hooked up slowly open the throttle watching the meter and watch for any dips or spikes in voltage. If you see anything like that your throttle position sensor is bad. Give that a try and go from there. A handy way to probe your wires without puncturing the insulation is to strip about 3 or 4 inches of a piece of scrap wire and seperate out a couple or strands, twist those and then unplug the throttle position sensor and place them in the terminals and then fold the excess wire back towards the harness and plug the connector back in. Some say this will spread the terminals but if you only use a couple strands it shouldn't and it is a simple way of probing without puncturing the harness.

 

Fuel filter is less than a month old. Cap/Rotor/Plugs/Wires are all less than a year old with less than 20K on them. I checked the plugs and wires, all are in good working order, none fouled, no burned wires. I'll test the TPS, but the more and more I read up on this, the more I think I've got a vacuum leak somewhere in the system.

 

TPS tested good, no vacuum leaks found, plugs, wires, cap, rotor...all ok. Fuel filter, replaced, old one not clogged. Problem doesn't present itself still, but I'm still concerned about this. Any thoughts???

 

bump...

 

How's your compression looking...do a compression test.

 

I would check fuel preasure and volume. I had a 97 suburban at work that would do a very similar thing. It had 58psi of fuel preausre but didn't have the volume to keep up with the motor and would back fire in the intake. good way to check is to hook up your fuel preasure gauge and run it up the the windsheild and drive it and see if your feul preasure drops off.