P0123 Woes

[raceskier]

Hi gang, been away for quite a while. Life has a tendency to intrude on my fun. I have a 99 with one of the original BRP Coldsides (with original PC, also have a last Beta version ELF), not yet upgraded to e-cool specs. I was going to do the upgrade this summer, when Mazda (I think, as all problems are off boost.) specific issues reared their head.

First up, an intermittent rough idle popped up. I thought, incorrectly (the first of many incorrect assumptions) Aha! coil pack. I replaced it. Rough idle cleared but now other problems.

I started getting intermittent P0123 (throttle position circuit high input) with Open Loop (ScanGuage) operation and very rich AFR's 10-11. Tried a new O2 sensor, to no effect, tried the original PC and the ELF, also cleaned the MAF sensor (used the MAF safe cleaner) along with all of the oil from the airfilter. Still nothing.

I checked the TPS and found it reading (from the factory manual C-A-B terminals) C-A 3.6K ohm to 145 ohm at full travel, C-B the opposite of that. I couldn't find any documentation as to whether that was normal, so I got a new TPS. Plugged it in and all of a sudden things were well. (I have had dielectric grease on the TPS plug contacts all along for resistance to moisture induced issues.) Or so I thought. No CEL's and running well, almost. It still would occasionally at cold idle have erratic TPS readings and go open loop. If I let it idle for 1 minute plus first, everything was fine.

Now all of a sudden on a nice warm afternoon, I fire it up, warm up and drive away to an immediate CEL (P0123 again) and Open Loop operation. Shut down, restarted and cleared the CEL and managed to get home OK.

Now I get a P0123 that re-appears immediately after clearing (less than 1 second), TPS at 99 all the time and Open Loop. Tried both my original and new TPS (both measure the same with an ohmeter) without any difference. The last thing I tried was removing the Innovate LMA-3 circuit monitoring TPS, thinking perhaps a fault here effected the TPS circuit, again no effect.

I've about come to the limits of my limited diagnostic capabilities on this one. To make matters worse, I've got our beloved CARB biennial Smog Check coming up.

Can anyone suggest anything else I can check to attempt to solve this?

Thanks guys.

[Tom @ Fast Forward]

look for a wire or wires back near the EGR that have touched it and have the insulation mented and grounded. What you have is a loss of signal from the TPS to the ECU. The original kits with Powercard and SS didn't use the TPS signal.

[raceskier]

Tom,

Thanks for the tip on where to look. It wasn't a short, but rather an open circuit. When I installed the data logger a couple years ago, I spliced into the TPS circuit to get that signal on one channel. I had a marginal solder joint, (Damn, I used to solder pretty well, back when I could F$#%ing see.) that came loose and must have been held in contact by shrink sleeving alone. Another self inflicted injury. I'm finishing up here, once it stops raining and I recharge the battery, all should be well. Thanks again.

[Tom @ Fast Forward]

At least with an 'open', you can fix it with a 'short'. :)