blueman26

2004 GS430 with 120k miles. No engine mods. All service has been performed by the Lexus dealer, including having the TB cleaned. No check engine light or codes.

The problem is a hiccup/stumble on cold start where idle rpm drops from roughly 1300rpm to 700rpm and bounces back to 1300 just as quick. It will randomly repeat this over and over (makes no difference if idling in park or driving down the road) until the car reaches operating temp. It happens only during the summer months when the temp and humidity are higher in the midwest. The car runs PERFECTLY during the cooler fall, winter, spring months, not a glitch to be found.

I’ve taken the car to Lexus about this problem twice now, summer 2012 and summer 2011. The dealership says they can’t replicate the problem, so I’m either a curse to my own car or they just can’t figure it out. I’m going to try again this year, but wanted to ask here first before I spend another $140 on diagnostic fees. Any ideas?

SaiyanGS3

iTrader: (8)

maybe plugs or maf going bad?? Stumble/jerks on Nissans are typically caused by a maf going bad.

rush161

iTrader: (1)

its most likely the throttle control motor / idle air control valve ...when the throttle body is cleaned it usually mocks up the the idle control .

SaiyanGS3

iTrader: (8)

Ahh I forgot about this. this is most likely the problem, the control sensor. that's why I don't clean mine, it's running good right now.

blueman26

From reading countless other posts with somewhat similar symptoms (minus the seasonality I’m dealing with) it sounds like the only fix for that is to replace the entire TB. Is there a sensor feeding data to the TB or built into the TB that could be impacted by higher temps or humidity? I wonder if it has something to do with the vapor pressure sensor? Weird that the problem completely vanishes when the weather cools off during the fall and winter.

DosyBoy

iTrader: (1)

Man that sucks, did u at least try and reset the ecu to see what happens? Worth a shot....

blueman26

Going to schedule an appointment with Lexus.

blueman26

Lexus dealer suggested the first thing to do was clean the TB with a "new" product from a company called MOC. The drive home from the dealership was smooth, but the next morning the problem was back although it wasn't as bad. The two diagnostic techs that worked on the car recommended replacing the throttle position sensor if the cleaning didn't solve the problem. Will be taking the car back after the holiday.

Random question: why would they need to take apart the center console to diagnose this problem? When i got the car back the plastic slider that moves along with the gear selector had been reinstalled incorrectly and now the whole thing is bent up and needs to be replaced.

vijayn

The thing that controls the throttle body motor is the ECU. The main Inputs that may affect the signal sent from the ECU to the TB motor is: Pedal Position Sensor, Engine Coolant Sensor and Throttle Position Sensor.

Does that problem only happen when the engine is cold or also when the car warms up?

blueman26

It’s by far and away the worst when the engine is being started first thing in the morning. It can show up when the engine is hot, but only if I’ve been running errands all afternoon in 95+ degree summer temps. Think the coolant temp sensor could have something to do with it?

bruceb58

Had the same issue and was my MAF.

vijayn

To be honest it sounds like an intermittent connection problem somewhere on a connector/wire.

Try this test:
When the engine is cold, start it up and do the wiggle test to see if there is an intermittent connection problem. Just wiggle connectors and wires on particular sensors and actuators to try an replicate the symptom.

Do this for the pedal position sensor, throttle position sensor, coolant sensor, maf, crank sensor and the throttle motor actuator

blueman26

Yesterday I turned the ignition to “on” without starting the engine, let the brake booster run it until it stopped, then floored the accelerator pedal several times, and started the engine. The problem didn’t show itself. Tried the same thing again this morning and got the same result. It’s been a week since I drove the car but will try to go out today and see if it hiccups on the road. I have an appointment on Monday to replace the TPS and do a power steering flush.

Would be curious if anyone else having similar symptoms might have the same result?

blueman26

Got the car back on Tuesday and so far the new throttle position sensor seems to be what the doctor ordered. Thanks for all of the ideas/suggestions! Hope this helps anyone else having the same problem.

Will update in a couple weeks.

SorrGwa

iTrader: (1)

2001 gs430 w/ 137,000km

ive been developing this issue in the past week as well.

so had some time to pick my techs brain and also do some diagnosis at a friends shop.

symptoms:
- cold hard start. but doesnt pertain only to cold starts. so long as the car sits for a bit, it has a hard time starting
- battery midtronics tests good
- car cranks, but takes up to a few seconds+ to "catch" and actually fire up

so first things first, scan for codes, no CEL. but check anyhow. throttle position sensor readings arent the best, but are still within respectable range based on graph.

check for fuel pressure, no actual threaded tap, so ran off the banjo fitting. BINGO. first good diag news. when removing the nut and banjo fitting, found zero pressure. didnt even have to go as far as testing. however with ignition on, fuel pressure is there. with ignition off, fuel pressure drops.

cause, check valve! the check valve, just like with air suspension, prevents the air from going backwards. it holds it in one direction. kinda also like those quick aftermarket brake bleeder screws that russel makes. the fuel check valve is suppose to retain pressure so that the fuel doesnt drain backwards through the line. since no fuel pressure at all, the car would require a long crank to fuel pressure up.

how to verify? prime your fuel system. simply turn the key to ignition on, hear the fuel pump buzz and build pressure. hold there without cranking for a few seconds. turn key off, do the same thing again quickly again. try a third time. but this time turn the key to crank the engine over. if she starts fast and normal, then its the fuel pump check valve. it is with the fuel pump, internal, so you will need to replace the fuel pump.

I'll tell you how this goes in a couple weeks when i got time to have the new pump installed. if you haven't, its a good time to change your filter, and pick-up sock as well.