Hey everyone. My 1999 chevy lumina 3.1L has made it'* way miraculously back to my driveway yet again. The girl I'd sold it to over two years ago informed me yesterday that she'd never actually gotten the title properly swapped into her name a few years ago, and that the car was abandoned at her old apartment complex. She wanted nothing to do with it (she said it was "having trouble" when she parked it a year ago). Apparently, it was about to be towed which is why she gave me a polite courtesy call, meaning -I- was the responsible party. A few frantic hours later, I'd rescued the car just in the nick of time.

That'* all well and good, of course, but there'* still that "trouble" she was talking about. I took it for a drive and it manifested. The car lurches, chugs, between 20mph and 30mph if I go slowly and easy on the throttle. Past that it'* fine, below that it'* fine, from 45mph to 90mph it'* running beautifully. It'* just that 20mph-30mph window that the car feels TOTALLY wrong (steady lurching feeling from the engine, chugging - it'* definitely coming from the engine and not the transmission).

After some basic work (oil change/air filter) and getting everything in order (new turn signal switch, new battery) I'm down to two codes being thrown. P1635 (5v reference voltage circuit) and P0122 (throttle position sensor).

I went ahead and replaced the TPS just for giggles, but that didn't solve the issue. 30$ wasted, I do believe.

So now I'm down to that P1635 code, with absolutely no idea how to tackle it. I've heard I'm going to need a multimeter and check for 5v along the reference voltage circuit, but I don't have a circuit diagram and I have no idea what that voltage circuit connects to (outside of the TPS). Does anyone have any ideas that could help me track down this gremlin and get the old lumina back up and fully operational? I've heard a p1635 isn't supposed to cause any noticeable difference in driving, but there is certainly -something- happening. I'm guessing 5v isn't being passed properly and it'* effecting something along that circuit - perhaps even the new TPS.....

Thanks in advance for any assistance! I really appreciate it.

The 5v reference circuit I believe feeds the EGR, MAP and TPS sensors. The MAP and TPS have 3 wires. One is the ground, one is the 5v reference, and the last one is 5v signal return.

You can use a multimeter at the TPS and MAP sensor to determine which pin is which.

1. Set DVM to continuity test.
2. Connect DVM ground lead to engine ground
3. Connect DVM positive lead to 1 of the 3 pins on the TPS or MAP sensor electrical connector until your DVM beeps, this will indicate which pin is ground.
4. Set DVM to 20 VDC and turn vehicle power on and probe the other 2 pins to determine which one is 5v reference.
5. Turn power off. Reconnect connector.
6. Back probe(I use a bobby pin) and pierce the 3rd wire(5v reference return wire)-not the 5v reference or ground, but the return signal wire.
7. Start engine, and monitor voltage on the return wire.

On step 4, you should see about 5v.
On step 6, you should see somewhere between 5v and less. Depends on the sensor.

Thanks for taking the time to reply Mike.

I'll be whipping out the multimeter to see what I can figure out later today. I think before I do that I'll try cleaning up the EGR and MAP with some throttle body cleaner just for giggles. I appreciate the help and the step-by-step. I'll let you know how this turns out.

I just want to thank you again for the help. I used some throttle body cleaner to spruce up the MAP and EGR and like magic, the car is working perfectly. Once I knew what parts to look at I was golden.

Only other problem I'm seeing on the car now is the transmission pan (gasket around the bottom-plate of the transmission) appears to be leaking a bit. Before I go unbolting that to replace a gasket, is this as simple a job as I imagine it to be? It appears to be just bolts holding on a bottom-panel, so I feel as if I could drain the tranny, remove the bottom plate, replace gasket, and bolt everything back up. I'm not certain however, that there aren't things held in place by that pan - anything I should worry about before yanking the pan off? I'm pretty sure taking it off won't end in a showering of assorted interior transmission parts, but a quick googling doesn't have a description of the job so I figured I'd at least ask before I do something stupid .

On the same coin, should I attempt to tighten bolts around the gasket to seal it or is this smarter to simply replace it?

I would start by tightening the bolts. Gaskets compress over time which can loosen the torque.

The only tricky part about dropping a pan is most cars don't have a transmission drain plug, so all the fluid comes out with a big splash if you aren't expecting it. Do it properly and you won't have a problem. It is a very simple repair.