Drop in RPM when accelerating: APPS or Fuel Pump?

[LostAfrican]

Please help me identify which component to replace?

01 LX 470, 140k miles

Sometimes, when I want to accelerate and I push the accelerator pedal, instead of seeing a (very slow ) increase in RPM, the needle will fall down to the 1k rpm range. To recover I need to pump the accelerator pedal one or two times, then it comes back and I can accelerate.

It doesn't always happen, and it seems to happen only when I am already driving quite fast (so engine is already in the 2400 rpm range). It happens when climbing and it happens when its flat. It may have happened in the past when starting from a stand still, but that hasn't happened in a while and my memory is fuzzy. Its possible that a MAF/throttle clean fixed the low speed case.

I don't think its the transmission - the rpms drop resulting in a loss of speed. It doesn't shift gear then slow down.

My research has led me to either the accelerator pedal position sensor or the fuel pump. Which is it do you think? Or, what else might it be?

Its an 01, so the APPS is up on the throttle body and based on MUD there were issues with these. I have already replaced the TPS (low hanging-cheap-er fruit) but that didn't change things. Also cleaned the MAF and throttle body (multiple times).

I am inclined to think its the APPS because if I was having fuel pump issues, I think it could happen any time, not just when I push the accelerator harder. I haven't noticed a temperature or tank level dependence. Could it be the Fuel Pressure Regulator?

Does the fuel pump maintain a constant pressure in the fuel lines, does it actually pump based on the accelerator pedal position, i.e. increase its output, or, just pump at the same rate all the time?

Appreciate your experience and insight!

 

[LostAfrican]

Any suggestions?
Can anyone answer the fuel pump question? Does it's flow rate vary?

 

[elk]

Have your fuel pressure tested. Below 40psi indicates a failing fuel pump. I'm inclined to think APPS.

 

[LostAfrican]

Have your fuel pressure tested. Below 40psi indicates a failing fuel pump. I'm inclined to think APPS.

Thanks Elk!
That’s a good idea to test the fuel line pressure. I’m going to see if Autozone has a gauge to borrow.

 

[gwatts]

@LostAfrican What did you find out? I'm having similar issues. Cleaned the MAF/TB and no difference. I really only get the RPM drop when it is near freezing in the mornings. I figure its the TPS...

 

[nissanh]

WHEN TPS goes bad the RPM will fluctuate + or - 100 may be from idle RPM. If you have 200+ miles, try replacing the TPS as it is simple and not too expensive. Stay with OEM.

 

[LostAfrican]

@gwatts your timing is pretty good. Day before yesterday I replaced my APPS aka Throttle Body Lever Sensor.

To recap, other things I have done/tried so far...

fuel filter (part of baseline, sometime ago).
replaced TPS - no change
cleaned (and I mean really cleaned by removing the throttle body and using steel wool!) throttle plate - no change
cleaned MAF and reset ECU (many times) - no change
pinch test on fuel return line - no change to IDLE (basic test for fuel pressure)
replaced PCV hose and grommet (needed to anyway)* - no change
replaced ventilation hose (other V-bank) - no change
>>APPS/Throttle Body Level Sensor - IDLE in Drive is tighter, so far no RPM drops.

* LX now has 145k miles, is an 01...so its old but not a lot of miles. Both the PCV hose, its gromet and the ventilation hose were hard and easy to crack. So sometimes its just age and not miles! Gromet was so hard it crumbled while trying to remove it.

Honestly though I have not driven it enough since replacing the APPS and under all the conditions to be sure I have fixed it finally, however, several other things seem to be much improved so I am hopeful. I will give it more time and post back.

I have noticed that my IDLE rpm in Drive is more steady. Before it would vary quite a lot I'd say between 550 and 750 rpm (in Drive with foot on brake). Now with the new APPS (and a full ECU reset (20 min battery disconnect)) I see 600 rpm +/-10 at most in Drive with foot on break. Also when I turn on the AC it bumps up to 750 I think, like it should. Before it didn't adjust up when AC was on.

I think my UltraGauge is showing more "levels" of absolute throttle percent. It gives a throttle position in % reading - basically reading the TPS I think.

I have tried to get the rpm to drop when pushing down hard on the pedal, but so far without luck since I replaced the APPS. We recently did a longish roadtrip into Mexico and after all of the fixes (listed below) except the APPS, I thought I had it fixed...but nope...it actually happened several times. Push the pedal down to accelerate, rpm's drop to ~1k, let go pedal, push, sometimes a few times, and then it comes back.

During this trip I used my UltraGauge to help debug. Remember I was trying to figure out if it was fuel pump or APPS. So I set it up to monitor long and short fuel trim and throttle position. When the drop in RPM happened I noticed that the TPS % on UG stayed very low...like 14%, it didn't adjust up into the 50-60% range until I had pumped the pedal a few times. This gave me confidence that I should replace the APPS and not the fuel pump. Fuel trim numbers all looked pretty reasonable, before, during and after.

I don't know for sure yet that I have got it fixed, but I am hopeful. There's not a lot left to try that I know of.

Finally I have to 'fess up that I bought my new APPS from Amazon. I was shocked to get the latest dealer pricing. I had expected ~$230 based on what I have read here, but even with appropriate TLA/CSC discounts it was still above $300 almost $400. I found a new one in Lexus/Toyota packaging on Amazon for $158 incl. shipping. Based on my inspection and comparing to the original, its new and genuine. And I think it works better than my old one.

 

[eddoc4fun]

Thanks for your post, I have the exact same problem 01' LX470 about 87k and I did nearly everything that you did to try to troubleshoot it ( even replace spark plugs, new air filter, cleaned out intake) except for replacing the APPS and I did not check the fuel pressure yet. Have you had any problems after replacing the APPS. My issue is very random, but only happens went it is warm and I stomp on the throttle and it does nothing. If I let off and press again it will accelerate. If I hold it down to the floor it contines to do just idle...It is an easy job to replace APPS?? Thanks.