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-   -   How Urgent is TPS replacement? (http://www.saturnfans.com/forums/showthread.php?t=176822)

SlowRide 02-06-2012 03:17 PM

How Urgent is TPS replacement?
 
I've had my '98 SW2 for about 2 months and it seems that the TPS is failing, from what I've read here in the forums. A couple times the idle stayed high, ~1500 rpm, after driving ~45 minutes. The first time I tapped the accelerator and it went back to normal which was ~850rpm. I cleaned the TB (in place) and after that it has been idling at 800. about 6 weeks later, I had another episode of high idle, which did not go away with tapping the accelerator, but went away after turning the car off and restarting. Does this sound like a failing TPS? Can the TPS failure cause the car to be undrivable?

TXSaturn02 02-06-2012 03:33 PM

Re: How Urgent is TPS replacement?
 
I drove around with my TPS randomly giving me issues for a couple of years. I don't think it really will cause much damage - it will just cause you to burn more oil and gas than it would if it idled normally. Since I didn't spend much time at traffic lights, I let it go for a while. Finally decided to spend the $50 and get it fixed since it is such an easy job. You should get OEM for this part.

BV22 02-06-2012 03:45 PM

Re: How Urgent is TPS replacement?
 
^+1 - Drove mine that way for a couple years also. With a manual trans, it is less of an issue since either the clutch is pressed or the car is in neutral when coming to a stop. On an autotragic, you might find yourself fighting to stop the car. It is an easy job with the right tools (torx bits and a 1/4 drive ratchet) and small hands. Got my replacement at Autozone and it has worked fine for a couple years now, but OEM is likely best.

SlowRide 02-06-2012 03:47 PM

Re: How Urgent is TPS replacement?
 
how about the AC/delc0 part?

TXSaturn02 02-06-2012 03:56 PM

Re: How Urgent is TPS replacement?
 
[QUOTE=SlowRide;1841355]how about the AC/delc0 part?[/QUOTE]

I believe most people think AC/Delco is OEM most of the time. You might want to do a search. Sometimes if you ask the guy behind the counter, they'll know if it is OEM or not.

Be careful when removing the screws - if you drop it, you might have a tough time finding it. Unscrew by hand once it gets loose.

SlowRide 02-06-2012 03:59 PM

Re: How Urgent is TPS replacement?
 
rock auto lists the Delphi as OE and it's like $55, the AC part is only $30

TXSaturn02 02-06-2012 04:04 PM

Re: How Urgent is TPS replacement?
 
[QUOTE=SlowRide;1841367]rock auto lists the Delphi as OE and it's like $55, the AC part is only $30[/QUOTE]

List the part number and maybe someone else will let you know. For something like that I just go to the Chevy dealership (I paid about $55 for mine). I know some of you live in the sticks and ordering on-line is an easier option.

SlowRide 02-06-2012 04:10 PM

Re: How Urgent is TPS replacement?
 
ACDELCO Part # 21023673

price went up since i looked a few weeks ago, now $34 and 61

OldNuc 02-06-2012 04:17 PM

Re: How Urgent is TPS replacement?
 
That is the correct part number for your model year and it should function fine.

SlowRide 02-06-2012 04:27 PM

Re: How Urgent is TPS replacement?
 
Thanks to all for your replies!

RobertGary1 02-06-2012 04:53 PM

Re: How Urgent is TPS replacement?
 
I've noticed that Rockauto has started putting little hearts next to their more popular offerings for a particular part. In generally I've found it is usually a very good balance between price and quality (its rarely the least expensive).

-Robert


[QUOTE=SlowRide;1841390]Thanks to all for your replies![/QUOTE]

manualman 02-06-2012 06:38 PM

Re: How Urgent is TPS replacement?
 
I need some remedial education here. How can a bad TPS make it idle faster?

A TPS has no control over the throttle plate itself, correct? So it cannot regulate how much air flows into the intake valves. Wouldn't a faulty TPS sending a high throttle reading on low throttle condition, just make the mixture really rich? Wouldn't it actually need more air to idle faster?
Doesn't it sound like his IAC is bad rather than TPS? (Make no purchases! I'm just learning how to reason this stuff through)

Correct me where I'm wrong here. The TPS sends a signal to the PCM which also consults the MAP sensor and ECTS and then selects a pulse width to send to the injectors, which may be modified for the next round based on feedback from the O2 sensor. Still sounds like a bad TPS would just run either rich or lean until the PCM sees the disconnect between sensor inputs and sets a code.

OldNuc 02-06-2012 06:45 PM

Re: How Urgent is TPS replacement?
 
When you start the car the PCM checks the TPS and assumes that reading is closed throttle. This is why you have to keep your big foot off the throttle when turning the key to on and cranking. If the TPS is defective and decides to not return to the set value then you will be left with the rolling RPM in most cases, 1200-1400 RPM. To enable close down of the IAC the VSS has to indicate zero speed and TPS indicate throttle closed. Then the PCM will go to idle RPM.

With the 98 and newer PCM when you hit the gas the TPS rate of change provides initial enrichment. Once the manifold vacuum responds then pulse control is back with MAP as primary pulse width control modified by O2 sensor signal. The earlier cars also employ a increase in fuel pressure along with MAP and TPS. The lower idle and high vacuum condition lowers fuel pressure to allow a longer pulse time.

RobertGary1 02-06-2012 06:49 PM

Re: How Urgent is TPS replacement?
 
I agree that its never really made sense to me why the TPS would affect idle. Empirical evidence suggests that the IAC is activated for some reason based on the reading from the TPS; but why is a mystery. Moving the IAC is the only way the PCM can modify idle speed in these cars. However, from having solved this issue in two different Saturns I can assure you the solution is valid.

-Robert



[QUOTE=manualman;1841466]I need some remedial education here. How can a bad TPS make it idle faster?

A TPS has no control over the throttle plate itself, correct? So it cannot regulate how much air flows into the intake valves. Wouldn't a faulty TPS sending a high throttle reading on low throttle condition, just make the mixture really rich? Wouldn't it actually need more air to idle faster?
Doesn't it sound like his IAC is bad rather than TPS? (Make no purchases! I'm just learning how to reason this stuff through)

Correct me where I'm wrong here. The TPS sends a signal to the PCM which also consults the MAP sensor and ECTS and then selects a pulse width to send to the injectors, which may be modified for the next round based on feedback from the O2 sensor. Still sounds like a bad TPS would just run either rich or lean until the PCM sees the disconnect between sensor inputs and sets a code.[/QUOTE]

TXSaturn02 02-06-2012 07:48 PM

Re: How Urgent is TPS replacement?
 
[QUOTE=RobertGary1;1841472]However, from having solved this issue in two different Saturns I can assure you the solution is valid.

-Robert[/QUOTE]

Yepper. This is one of the few that I have actual experience with - new TPS resolved the high idle issue. :yes:

manualman 02-07-2012 11:07 AM

Re: How Urgent is TPS replacement?
 
If I understand OldNuc correctly, the bad TPS tricks the PCM into keeping the IAC at high idle when it should be telling it to go to low idle. So the PCM and IAC are still working right, but are getting bad info from the TPS.

The scary thing is that I actually understand all that alphabet soup!

OldNuc 02-07-2012 12:34 PM

Re: How Urgent is TPS replacement?
 
+1^^^That is the Cliff Notes version of what is going on. The deep logic is that the reciprocal of an OR logic gate is an AND gate.

RobertGary1 02-07-2012 02:24 PM

Re: How Urgent is TPS replacement?
 
But what is the scenario in which the car says "I think the throttle is open (because of the TPS) so I'm going to activate the IAC?". It just seems like an odd reaction for the PCM to have.

-Robert


[QUOTE=manualman;1841712]If I understand OldNuc correctly, the bad TPS tricks the PCM into keeping the IAC at high idle when it should be telling it to go to low idle. So the PCM and IAC are still working right, but are getting bad info from the TPS.

The scary thing is that I actually understand all that alphabet soup![/QUOTE]

OldNuc 02-07-2012 03:05 PM

Re: How Urgent is TPS replacement?
 
Once the car reaches operating temperature and either the throttle is opened or the VSS indicates road speed the PCM positions the IAC to the ~1200 RPM position. This is part of the emission controls. So to reset the IAC to idle you have to meet BOTH conditions, TPS full closed AND VSS zero.

RobertGary1 02-07-2012 04:09 PM

Re: How Urgent is TPS replacement?
 
[QUOTE=OldNuc;1841834]Once the car reaches operating temperature and either the throttle is opened or the VSS indicates road speed the PCM positions the IAC to the ~1200 RPM position. This is part of the emission controls. So to reset the IAC to idle you have to meet BOTH conditions, TPS full closed AND VSS zero.[/QUOTE]

Is this part of the coasting idle requirement? It makes sense that it would be concerned with the VSS indicator but why would the coasting idle be affected by the throttle position? If anything I would think it would not go into coasting mode if the throttle were pressed. Wouldn't opening the IAC in response to throttle advancement result in a jerky advancement?

-Robert


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