What could cause multiple TPS's to go bad? (S4 n/a)

[need RX7]

It all started when my FC got very hard to start, and when it did it could barely idle. I checked the resistance on the TPS and it was out of spec, and I couldn't adjust it back into spec so I decided to get a new one. I went and picked one up from a junkyard ($230 from mazdatrix? F that) I installed it and it also wouldn't get in spec. It didn't strike me as odd though, since it was sitting out in the open on a car with no hood for who knows how long. So I picked up another one from a different junkyard, long story short, another dud.
I finally went and bought one from a (supposedly) running car off a guy on RX7club. After receiving it, I was sure it would be the solution to all my problems. I was wrong once again, with all the adjustmen in the world, I can't get it under 2k ohms closed, and it immediately jumps to 8k at WOT (just like all the other ones). It does start though, but it has an odd pulsating idle and doesn't like to rev higher than 3k rpm.
I'm testing with a standard multimeter, with the leads stuck in the Black and Green leads on the TPS plug, with the TPS unplugged and a jumper wire in the initial set coupler.
So am I just having bad luck with buying parts, or is there something else wrong somewhere else? Or am I doing something wrong? I'm open to any suggestions, I just want to get this thing running correctly before the major snow comes

[need RX7]

One thing I'm starting to suspect is the BAC. I read somewhere that the idle should drop if you unplug the BAC (makes sense.) I went and unplugged mine while the car was doing its pulsing idle thing and nothing happened, absolutely nothing (no drop in revs, didn't skip a beat in its pulsing).

Also, while the car was running, I unplugged the boost sensor. Nothing happened when I unplugged it, but when I went to plug it back in, the idle would smooth out for 2-3 seconds as I inserted the plug, but would go right back to pulsing.

Interestingly, the dealership tested one of my junkyard ones and said was good... I wish I knew someone else around here with an S4 FC to try it out on.

I'll try to get a video of my problem tomorrow (I'm sure the neighbors wouldn't like it if I went out right now and started up the FC with no exhaust connected )

[NoDOHC]

I don't know if the idle problem is your TPS or not, but the voltage should run about 0.6-0.9V from ground to the center pin (I forget what color it is and I have no stock s4 cars to look at anymore) when the throttle is closed.

EDIT: This is done with the TPS plugged in and the ignition switch in Run (with eingine running or not).

By the way, I would suspect a vacuum leak for the poor idle. If it is idling at 3,000 rpm, the BAC should be shut (trying to idle it down).

It really is a pity that you don't live a little closer.

[need RX7]

Originally Posted by NoDOHC

By the way, I would suspect a vacuum leak for the poor idle. If it is idling at 3,000 rpm, the BAC should be shut (trying to idle it down).

I'm pretty sure there's no vacuum leaks, I've replaced all the intake gaskets, injectors cleaned with new grommets and o-rings and replaced a bunch of vacuum lines after the rat's nest removal. I think the problem is purely electronic.


Also, I realize the first posts are kind of confusing, so the main questions is, is there anything that can cause a TPS to be out of spec if the TPS itself isn't bad? I'm pretty sure this TPS I have now is good, but I can't get it below 2k ohms closed and it always goes to 8k at WOT. One of the other TPS's I had that was 'bad' tested good at a dealership. I'm pretty stumped over here.

[need RX7]

So I guess I just ended up with another bad TPS?

[My5ABaby]

Originally Posted by need RX7

So I guess I just ended up with another bad TPS?

I had to replace mine a while back and went through a couple bad ones.

[need RX7]

Dang. I guess I'm off to look for another...

[NoDOHC]

I don't know about you, but I distrust multimeter resistance readings. I would rather watch the voltage sweep and verify that it is linear than attempt to compute the resistive dividing ratio of the sensor (I do understand that the factory service manual lists a resistance spec, but I also know that the ECU reads voltage, not resistance).

If you like, I can try to test the TPS voltage on my bone-stock '90 GXL (which purrs like a kitten on one rotor) right now.

I know you said that the TPS was unplugged, are you absolutely sure that you disconnected all wires to the TPS and that you had a good strong connection with your multimeter leads?(contact resistance is the reason that I mistrust resistance readings).

[need RX7]

I'll try it again tomorrow and double check everything, but when I adjusted my first TPS with the same multimeter it worked fine, until the TPS crapped out for no reason a while later. The part I find weird is that 1) my first TPS went bad for absolutely no reason and 2) one of the TPS's that wouldn't work on my car tested good at the dealer. It just seems fishy to me. I appreciate all your help, can you elaborate on the voltage method so I can try that out too?

[NoDOHC]

First of all, this is an s4 engine, right? The s5s have a full range TPS.

I ought to look around, I have a stock TPS somewhere that is slightly scalded (engine fire due to PD leak) but worked fine on my '86 until I installed the Haltech, at which point I removed it. (If I can find it, you can have it).

I tested the voltage between each of the pins and ground. (With the connector plugged in).

I found that one of them was 0 Volts (sensor ground) one was 5 Volts (Sensor supply) and one was variable from 0.6V to about 4V based on the position of the plunger. Setting the TPS such that this one ran around 0.6-0.9 Volts at idle made for optimal operation.

I am not sure that the sensor is your problem. The dealer would have nothing to gain by finding your sensor to be ok. (They won't sell you a part). I am guessing that it is ok and your meter is lying to you.

I really wish that I could remember exactly what the values were for the TPS, but it has probably been 3 years since I adjusted an s4 TPS.

If I were looking at it, I would go by the factory manual. Check timing then TPS, then idle adjustment with the test jumper installed the timing again. If you can't notice a change when you adjust the TPS throughout the entire range, chances are that something else is wrong.

If all other ideas fail, you may check your CAS pickup gap and your idle mixture adjustment screw (which is hopefully capped).

It is late now, hopefully tomorrow I can dig that sensor up for you.

[need RX7]

I went out and started up the car today. It starts pretty easy, but whenever I give it any throttle it just dies. I haven't checked the TPS yet, but I'm guessing it's just getting progressively worse....

[Max777]

Again, this is EXACTLY the problem I have with mine, I've been trying to figure it out...

[need RX7]

Well I guess this TPS is bad too. I took it off the car to test it, pushing hte plunger all the way in gets me 1.5k ohms and full open is 8k.

[Max777]

you know what's funny? I took the TPS that I got with my new TB, and tested it, and it was way out of spec on my old (modded) TB, however, my old TPS that I had which used to give a weird reading after 3.9K ohms worked just fine when re-tested on the old TB.

I think that my old TPS wasnt the problem after all, so i dunno I guess you could get a trashed TB, remove the thermowax and test all your TPS sensors to see if they are witin spec?