Troubleshooting TBI

[kennyw]

1991 Chevy K2500 w/ 350 TBI and 4L80E (motor vin K). It is definitely running rich, my engine temp gauge quit working after I bought it, but it had the check engine light already stuck on when I bought it.

I get codes 34 and 45 and found this:

CODE 34: MAP sensor signal voltage was too low (high vacuum) when engine speed was under 1200 RPM or the engine speed was over 1200 RPM with a throttle position angle above 21%.

CODE 45: Oxygen sensor voltage was over 0.7 volts for 30 seconds of closed loop operation with a throttle angle between 2% and 20%.

I'm guessing the code 45 is a result of the problem causing the first one... but any idea's on how to start the troubleshooting on this? I know the Map sensor has been replaced already by the dealership. The truck sat for some period of time with a bad heater core before being used as a trade in there.

 

[dyeager535]

First things first, remove the ECM fuse for 10 seconds.

Start the truck back up, see if/when the check engine comes on with it idling. If nothing, hit the throttle and see if you can duplicate the above statements.

I'm thinking your TPS may be bad, but I wouldn't replace it yet. It may even be adjustable, but from what I understand, most of the TBI ones aren't.

Closed loop may take awhile, or it may NEVER go closed loop at idle/park, so you may need to take it on a cruise.

Also, if you ever want to meet up again, I've got an OBD1 scanner that will work on that thing.

 

[kennyw]

I reset it after finding the Map sensor plug not completely clicked on and at idle it ran fine... the check engine light didn't come back on until I was driving down the highway. I caused a code 22 on it as well, but that was only because I unpluged the TPS. I'm guessing the TPS maybe the culprit as well since it seems be be related to the over 1200 RPM part of the 34 trouble code.

 

[dyeager535]

TPS is easy to check with a scanner, but it can be done with a multimeter as well.

If it's adjustable, it may be as simple as that. If it's not adjustable, it may not be that simple, and it may not be the TPS. Some of the setups used "reset" the TPS at shutdown, so wherever the throttle is when it is started, is "idle".

 

[kennyw]

It wasn't the TPS... it was even simpler than that. I started messing around with the TPS and noticed the plug on the MAP sensor didn't look like it was plugged in again (I'm standing on the passenger side with most of the connector and sensor blocked by the bracket). After taking a closer look I realized it came unplugged again on the last test drive because of this...

So with a $0.10 fix I have no more trouble codes...

I'll have to find a new connector for a real fix, but at least for now I can keep it out of open loop and get reasonable gas mileage out of the truck.

 

[dyeager535]

Yeah, that should probably help things out.

Good eye.