Lvangundy

(long post)

I had original TPS replaced last year with a new one (made a post about it). I thought it seemed to solve some of my problems with engine RPM dropping too quickly and not having smooth engine acceleration/deceleration.

In the last 15,000 or 20,000 miles I started noticing a problem where the RPM would go DOWN after I was giving it gas. You push the pedal down slightly and the RPM moves up a bit. Then, while keeping the pedal steady the RPM drops - without me letting off on the gas. I replaced the TPS AGAIN yesterday and the problem remains, although general engine smoothness is slightly better. I'm very sensitive to these things and know how my car operates so I know when something is different even to something small like this.

My conclusions from this RPM speed change problem are:

1. It's not a driving issue. The same problem happens in park or neutral.
2. It can cause some sort of transmission slip or T/C lockup/unlock when driving causing the car to suddently jerk into decelerate mode. (I think this is more closely related to TPS calibration AND what the other problem is)
3. The car will still if the A/C is on and you attempt to recreate the RPM drop. (I made a video of this and posted, but didn't further explain the RPM drop part) https://www.clublexus.com/forums/sho...threadid=94422

About #3, let me give a more technical/detailed example. I'll put the car in park with the A/C on (to force the car to stall out.) I'll rev to 1,400RPM and hold for approximately 1 second. The RPM will drop to 1,200 RPM(remember, without letting off the pedal.) Now at this exact moment if I let off the gas completely the car will stall out. Why? because some sort of electronic signal 'thinks' that the RPM has remained at 1,400 RPM, but the reality is the engine has dropped 200 RPM. When I let off the gas the engine speed slows down rapidly expecting itself to idle back to 600 but DROPS to 400 or below causing the stall.

To put this into some sort of analogy it's like if you were standing on a ledge and had 5 ft of space behind you before the ledge ended and went into a bottomless pit. You 'know' you have 5 steps of room left to walk freely on before reaching the ledge but somehow the 5ft of space is removed without your knowledge. You go to take a step back and there is no ledge, then you plummet to your demise (stall)

Anyone get what I'm saying?

One more point about the TPS and taking the next step to fix this problem-

I replaced the TPS again spending $90 really hoping it would fix this issue. I poped it in but did not follow the exact calibration guide but had an idea where the position it should be is. One thing I noticed is that for the first 2-3 miles I was driving the issue with the RPM's dropping didn't seem to be happening. Maybe I was imagining it wasn't but another 5 miles into it it was still happening. (*%^&!!) I also had the same pedal hesitation when driving on the freeway. I could still get the car to idle really low and stall with the A/C on. The smoothness of the engine speed was slightly better which brings me to my conclusion about the TPS's life.

The TPS is a coil-spring based, moving part. It will wear out - yes it needs to be replaced. When? It depends on how 'enthusastic' you are about using the gas pedal. If you tend to push the pedal in for a short burst of acceleration, then let off and contiunue this process rapidly (i know a few people who do this myself included) the spring will loose it's tension faster. The average drivier may not notice any difference in how smooth the RPM movement is when it's worn out or until it completely breaks. The TPS is probably not a 100,000 mile replacement part. It's more reasonably like a 50,000 part or 75,000 mile part if you are a soft gentle driver. In my case I noticed a difference in about 30,000 miles.

Now, if I'm completly wrong, there is a flaw in either the placement of the TPS in the first gen LS,(heat problem) or there is a flaw in the design of the toyota/denso TPS unit. I really doubt it's a design problem with the TPS since the same part is used on MANY different cars. I can't blame toyota for this and don't want to go there.

So what's the REAL problem? My next idea (from what I've gathered from everyone here) is that it's related to the IAC (idle air control) valve or one of it's siblings. Anyone bother to read through this?

hyperopt

Sorry to hear of your problems but I doubt that they are caused by TPS since you're on your second "new" TPS. Your Lexus TPS is of high quality when compared to other TPS I ran across. Yes they do wear out but mines are still within specs (checked with both multimeter & scope) at 150K+. Make sure to correctly set the TPS, and, if you still have the same problems, then look elsewhere for the culprit(s).

RA40

iTrader: (6)

Just something I'll throw out.

When working on some of the older Toyota FI things, we'd disconnect the fuse to the FI pooter. This would reset to factory programming.

I agree with hyperopt that this probably ins't a TPS problem, I suspect a MAF error.

NucLex

Lvangundy,
I have a "similar problem" and was told the idle air control motor was toast. I haven't found a new or used part yet, but will change it out when I find one.
Keep in touch and we'll share progress reports till we get the problem solved.
Regards,
LD Stricker
Goodyear, AZ

Lvangundy

I doubt I will spend the $600 for a new air control valve. I hope you have luck with this.

By the way - it's not a MAF problem. At least on my car.

Faceless1

Sounds much like a bad MAF sensor. When they go bad they send poor signals to the ECU and in turn the ECU can't determine the correct idle speed or RPM's when driving. The system is constantly trying to compensate for too much air or not enough air. Check the resistance and voltage of the MAF and go from there.