So I've had my GS350 for 3 years now (purchased certified used on New Year's Eve 2015 (12/31/15), so the 3-year warranty that came with it is about to run out in these next few days. After my dad completed an oil change yesterday, the car started hesitating for the first time ever in the 3 years I've had it, as if it is in limp mode. It is idling low (500-800 RPM, down from 1200-1500 when I leave it running to warm up) and even vibrates a little bit when in "drive" (not an issue in neutral). I see from another thread that lower idle is expected in the winter but the car feels like its about to stall/shut off when coming to a stop and it has significant hesitation when accelerating (on and off). The RPMs increase way more than necessary when simply driving down the street (the car was already at warm and at operating temperature, I never drive anywhere in winter without letting the car warm up). When I took it to the dealership yesterday, one of the service advisors that my dad knows rather well was of the opinion that the issue was very likely due to Costco gas (I've filled it with Costco 93-grade for 3 years now and never had an issue before, no issue last week or in the morning before oil change). He ignored the 3 years of problem-free Costco gas usage and stuck to the gas idea and showed a picture of what he said was Costco gas that looked like a glass of gas with a layer of some other milky/gunky fluid below/above it; he drove it as well with me in the car and I guess it affirmed his thoughts. He said that other clients had had similar issues that ended up being due to Costco gas (although those were ESes). He said if the engine or transmission were involved then I would see all kinds of warning lights on the cluster (there are still no lights). He recommended filling up the tank with Shell gas (had ~120 mile range of Costco gas before filling up) and drive it some more.
Given what I have seen in other threads and a similar experience with my sibling's old car, I get the feeling that it may be the throttle body that needs cleaning. My dad sprayed some of that Seafoam cleaner yesterday and said that it helped somewhat (I didn't check as I was busy studying). When I drove it today it feels the same as yesterday (hesitation, poor acceleration, near-stall feeling), despite a near-full tank of gas that is more Shell than Costco and the cleaner. Perhaps my dad did not spray enough of the cleaner as some suggest that you need to use a half or full bottle of the stuff. The car has 55,000 miles on it now, it is a 2013 model and it has never had this issue before (always able to give it the beans in the past with no problems). Again, there are no lights on the dash or anything indicating that something is broken.
Anyone have any other ideas of what might be the issue or is the consensus that there is some build-up due to poor quality gas that needs more Seafoam?

 

 

Those OE spark plugs will last 200,000 miles with zero difference in performance or fuel mileage.

 

Here is someone else with a Weird Idle Issue similar to yours.

 

Hmm I was wondering if they might be part of the reason but I figured something would pop-up on the dash and/or the service advisor would've mentioned it. The maintenance log doesn't mention replacing them until 60,000 miles or 72 months. Guessing there's 6 plugs since its a 6-cylinder? I figure Denso will be the best brand and a direct fit is better than universal? Just looking them up on JCWhitney now. Thanks for the prompt reply by the way.

 

Indeed I saw that when searching, just wondered if the issue was exactly the same as mine has been persistent vs intermittent and I have already tried driving it (with and without giving it the beans/WOT) after putting in Shell gas which is 2/3 of the tank now, in addition to the Seafoam treatment so I figured that there may be another underlying issue or concern that I should rule out.

 

You may want to ask your dad if he removed your engine air filter to check its condition when he did your engine oil/filter change, or if he disconnected the car's battery. Your dad must have felt there may be a problem with the throttle body since he tried to clean it. It makes me wonder why he felt it may need cleaning - could he have gotten a bit of trash in the intake when checking the air filter? Maybe he detected the poor engine idle and thought the throttle body needed cleaning.

The throttle body is a rather sensitive area - in fact oftentimes when replacing the battery it triggers the need for the engine ECU to reprogram again and quite often the ECU does not like it if the throttle body's MAF Sensor (mass air flow sensor) is dirty, which requires a thorough cleaning. Just spraying some Throttle Body/MAF Sensor cleaner into the throttle body opening does not necessarily clean the MAF Sensor itself.

 

You sure he changed the oil and not transmission fluid...

 

This video will show you what the MAF Sensor looks like and where its located in relation to the throttle body, which sits on top of the engine. It will also give you an idea of how delicate the MAF Sensor is. Anytime you change the condition of the MAF Sensor it has an effect on how the ECU adjusts its parameters to deliver the proper air/fuel mixture. And the reverse of that is also true - anytime you reset the ECU it prefers to have a clean MAF Sensor.

 

you think he put the right oil in the car? just saying I would be skeptical if you say it happened right after the oil change

 

I asked him to inspect the air filter (as advised by maintenance log) and he considered it to be clean, but I suppose it is possible that some dust/dirt may have floated into the intake/filter while we were inspecting it. We will check the MAF sensor tomorrow following that video, thanks for all the advice/help!

 

Yep, it is the Mobil-1 0W-20, same exact brand we used last year when changing the oil.

On that note, ~5 quarts of used oil were drained, while 6.5-6.7 quarts of new oil (as advised in manual) were poured in, just as last year. Is it normal for the car to have burned 1-2 quarts since the last oil change? I have not noticed any oil leaks nor any warning messages on the dash in the past year, and I do enjoy pushing the car in Sport+ mode when the opportunity/need arises, so I'm wondering if that is the reason for the difference.

 

Yes, we changed the oil filter as well.

 

Make sure any connectors around the air intake and MAF Sensor are securely connected.

Good luck with it!

Please let us know what you find the fix to be. Thanks in advance...

 

He felt it was idling poorly and potentially due to the throttle body/air flow sensor as my sister's old car had the same issue which resolved with the use of Seafoam. The air filter was inspected as part of maintenance log, not after the issue arose (although we did check to make sure it had been properly sealed back in place, after the problem started).