Hey all, my car is an 86 non-turbo. It has a kind of a weird quirk to it. When you're cruising along at real light throttle, like just barely pressing on the pedal, the car doesn't really feel like it's doing much, then you give it just a touch more throttle and it suddenly springs to life. What this causes is, when you're just cruising at a steady speed at that throttle, the car is jerking back and forth as you make minor adjustments to the throttle position. I didn't see it as much more than an annoyance until I went to smog my car today and as it turns out it happens perfecty at the point where you try to go 25 mph in 2nd gear, the speed he needed to roll the car at on the dyno for the smog test. Because of this, he wasn't able to steadily hold 25 mph and the machine couldn't finish reading so he had to abort the test. He told me the car was misfiring but I don't think that's it as it is definitely related to throttle position, and I would describe the feeling more like a "dead-spot" in the TPS.

I adjusted my TPS last week with a multimeter and was able to perfectly get it at 1k ohms fully closed and 5k ohms fully open. I also slowly opened the throttle while keeping an eye on the meter to check to see if it went open circuit at any point and it did not. It did, however, start reading at 5k ohms from about 1/3-1/2 throttle until fully open. I don't know if that's normal or abnormal. Another thing worth noting is that while changing some vacuum lines (I've actually replaced every single one in the last 2 weeks now, so that rules out vacuum leaks from hoses) the check valve that is in the vacuum line for the dual throttle broke so I ran the hose with no check valve for now, could this cause this? I have tried pinching that vacuum hose shut and driving it again and it still does the same thing, but I thought it worth mentioning.

Any ideas on what could cause this? I diligently tried searching but came up with nothing.

 

When the TPS piston fully extends the ohm reading goes to 5 and this occurs well before full throttle so if your reading maxes out at 5 as the TPS first fully extends then you are okay w/that.

And did this problem arise before you changed vacuum hoses or after?

 

Actually, you have the idea, no searching required.
Replace the check valve you broke.

I don't understand a diagnostic path that ignores an obvious broken/missing part.
I don't know if it solves the problem but you know it's wrong as is and a check valve is a cheap/easy fix, so why not try it?

Fix the basics before you look for more exotic solutions.

 

I would agree with you, but the fsm lists it as a " vacuum delay valve" not a check valve. I didn't see anything like that listed at Atkins rotary or Mazdatrix so I think I'm going to have to grab a couple next time I'm at the junk yard. I don't think that is the problem though because the car behaves the same way with that hose pinched off as it does open. I want to say the car has been doing this since I got it (2 weeks ago) but I don't remember noticing it until after changing some vacuum lines. I'm positive they are all hooked up right, or at least hooked up exactly how they were before, and I'm positive I had vacuum leaks before because I was idling around 1500, now it's normal at 750 and the car runs much better. just trying to get some more ideas on possibilities, I can pick up a one way check valve at work and try that in the line to see what happens, anybody know which direction it's supposed to face?

 

Mazdatrix sells them.

http://www.mazdatrix.com/b-egr.htm

Have you tried unplugging the TPS? Did you replace the hose to the Pressure Sensor. If so, do you realize there is a restrictor pill that is embedded in the hose which is required?

 

I saw the one-way check valves on mazdatrix, but the FSM lists this one as a "vacuum delay valve" I figured it would be different. I did replace the hose to the MAP sensor, but not until last night, so the car was doing this before replacing it, I will check and see if it had a restrictor in it, I didn't know it was supposed to have one.

I did try unplugging the TPS today and while it made the car run a lot richer, it did not do the bogging/jerking thing, so it's definitely related to the TPS. I'll try adjusting it again tomorrow and if that doesn't work I guess I'll try to find a used sensor, I'm not spending $300 on a new one when the car only cost me $500

 

Ok, today I picked up a TPS from pick n pull, went to put it on, noticed it read about 1k phms when closed but when you go to open it it gets to about 1800 and then is all open circuit after that... So I go to put my old one back in and now it's doing the same thing...

I also picked up the check valve for the dual throttle thingy, and put the restrictor pill in the MAP sensor hose. I backed out the variable resistor as lean is I could and poured a bottle of heet (methyl something or other) into about 2 gallons of gas, disconnected the TPS altogether and went for my smog check. The car did not buck and surge so he was able to complete the test. It passed the 25 mph test, literally at the maximum allowable HC's, but failed my 15 mph test by 40 HC's and CO was too high. Still running too rich. I need a new TPS but can't afford $150 for a new one...

 

Ok, I was able to get my hands on a used TPS sensor that works. Unfortunately, even with this one the car still does the same thing... That's 3 different TPS sensors I've tried where the car still does the same thing. It doesn't really do it near as much whent he TPS is unplugged but it is still there a little bit. Thoughts on where to go from here?

 

Could be a ground issue or a fuel issue among other things. It would help if you could remember if the trouble occurred before you decided to change some of the vacuum hoses or not. And did you check to see if the Pressure Sensor vacuum hose had the restrictor pill in it? And used TPS's can sometimes be junk and the best way to test it would be w/an analog meter to look for dead spots besides if it was in range relative to ohms.

 

I don't remember the issue being there before changing the vacuum hoses but it very well could have been and I just didn't notice. The restrictor pill is definitely in the pressure sensor line, I cut it out of the old vacuum hose and put it into the new one. I am testing the TPS sensors with a digital ohmmeter, I have slowly pressed the button and I do not see any areas where it goes open circuit. I also discovered I am an idiot and tested the first 2 sensors with the ohmeter set to the 2000 ohm range, so naturally it showed open circuit past 2k ohms. I clicked it over to the 20k setting and voila, I have 3 good tps sensors, so the odds are very slim that all 3 sensors I have tried would have a dead spot at the exact same throttle position.

While I had the UIM off, I cleaned the ground on top of the engine and added an extra cable from that point to the firewall. Though this should have only added a redundant ground, I'm going to try removing it just in case.

 

A digital meter is not going to be quick enough to show a dead spot.

 

You may just have the tps adjusted wrong. I've found that sometimes even with the official light that it takes a few tries to get it perfect and smooth in all driving conditions. I have certainly adjusted mine "in spec" and made things worse. You may want to start there. Also our tps isn't full range and will show wot at 1/4 - 1/3 throttle like you are seeing.

 

Yeah, I've set it to 1k ohm closed and 5k ohm open, and then tried it at several points until it was at almost 2k ohm closed and it gets slightly better but it's still definitely there. Maybe I should try going the other way with it so it's less than 1k ohm closed?

 

The TPS rod is not fully depressed to reach 1K, if that happens to be how you're setting it.

 

No, I mean it's 1k without touching it, it's at 5k when I open it full throttle, and I fully open and then close it again after each adjustment to get a true reading

 

Just to be specific, but are you setting the TPS after the engine is as hot as it can possibly get (20 minute drive)? Also, you might have a better go at it if you set the G/R TPS wire to 1 volt under the condition above plus key to on.

 

Yeah, I get the car fully warmed up first. I'll try setting it by voltage today and see what happens

 

ok, here's what I tried, all ohms readings are with the connector unplugged measuring between the top and right terminals of the tps sensor, all voltage readings are taken with the connector plugged in, key on, by backprobing the green/red wire and the negative was grounded to the alternator bolt. This is after warming the car up completely.

First: 1,000 ohms; 0.86V Problem still existed
Second 1420 ohms: 1.00V problem still existed
Third: 880 ohms: 11.35V problem still existed

At this point I took off the TPS sensor which is one I bought from a member here, and replaced it with my original sensor (which I confirmed does not go open circuit, I was merely measuring with the wrong setting before)

fourth: 1,000 ohms; 0.86V problem still existed
fifth: 1,130 ohms; 1.00V problem still existed
sixth: 1,320 ohms; 1.20V problem still existed

I also read voltage from the green 3prong connector on some of these tests but I didn't record all the measurements. For both sensors, at 1k ohms I had about 0.25V on one side, and about 10.25V on the other. On some settings, it was at 10.4V on both sides, and some of the others it was at 0.8V on both sides. After all other tests were done I hooked two ohmmeters up to it and slowly adjusted the TPS all the way from one end of the range to the other, at no point did I get 12V on one side and 0V on the other, the closest I got was 10.47V on one side with 0.13V on the other. I checked the voltage of the TPS again at this point and it was 1.08V, I adjusted it back to 1.00V and re-measured at the green connector and still have the same 10.47 and 0.13.

The problem was there under all settings, though I could tell that the higher the voltage of the green/red, the higher it happened in gas pedal travel. At 1.2V, it was pretty much off throttle was decellerating, and then you barely barely touch the pedal and the car would be accellerating. It felt best with it set to 1.0V, the car seemed to run the smoothest.

anybody wanna buy an s4?

 

are any other hoses supposed to have a restrictor pill, or just the map sensor? I might pull the UIM off again tomorrow and double check everything, remove that extra ground I added and re-do the vac lines for the oil injectors, I had to do them a little ghetto last time because the 4-into-1 connector was so brittle that it broke but I got another one to replace it with now. Any thoughts on other things to check while it's off?

Also, one of the ends of one of the vacuum switching valves on the driver's side of the motor broke off, do you think this could cause my problem? The FSM says the end is just a filter, so I figure it should still work properly with it broken off, but I thought I'd ask.

 

Which vacuum switching valve are you referring to? Is it one of the solenoids, and if so, what color marking did it have?