Hey guys, my 2003 GS430 is acting weird lately. Randomly, it starts having sluggish throttle response. I can't seem to figure out what's triggering it. Sometimes it never happens, but more and more frequently it's happening. It seems like it's fine until I get on it for a sec to pass or something, then it's sluggish until I turn it off for the night and it's usually fine the next morning. It also not shifting right (but only while it's being sluggish) it revs much higher than normal before shifting up and sometimes can't decide which gear it wants to be in. I'll have codes in a bit once I get them pulled.

 

Ok, codes pulled:

P0300 - random multi misfire
P0302 - cylinder 2 misfire
P0304 - cylinder 4 misfire
P0308 - cylinder 8 misfire
P0325 - knock sensor 1
P0442 - evap leak detected

What's weird is that a couple of months ago, I had some other codes as well (some ABS codes, immobilizer problem, battery voltage code) and none of them showed up this time. My VSC and VSC OFF lights are on as well.

Where should I even start? I just don't want to start throwing new parts at it unless I know it's going to fix the problem.

 

Had almost the exact same problem, lagging throttle response about 3 months ago. Cleared the code and it reappeared a short while later, P0325 knock sensor 1. I have a '99 GS 400 with 215,000 miles or so.

Dealer replaced both knock sensors (it's labor intensive to do them) to the tune of $1,506. Car rides like new now. But, since you mentioned VSC and VSC off lights...those haven't come on since my throttle body needed replacing almost 10 years ago! I would take it in to the dealer and have them investigate exactly what the problem is.

 

Ya that's way too much. I'll be doing it myself. I've done the starter before on my previous 99 GS400, so I should be ok. I'm gonna start by replacing the plugs, then go from there.

 

Spark plugs won't fix the issue, although that doesn't mean they don't need to be replaced. At work, I see misfire codes several times a day and 1 in a 1000 is caused by a spark plug. You very likely have a failing knock sensor which helps to control timing advance. When you moderately accelerate, such as passing someone on the highway, the timing should advance to keep up with the engine load. With a failing knock sensor, timing stays at base instead of advancing and the driver feels poor throttle response. I would address the knock sensors (one on each bank) first.

Do you have the ability to monitor data stream? You should also check your fuel trims to see if the long terms are close to zero. If they are way off base line, then I would also address the evap code. Depending on what exactly is failing in the EVAP system, you could have a very large vacuum leak causing the engine to run very lean. A lean condition will also cause knock sensor codes to set and sluggish throttle response as you described. I'd still lean toward a knock sensor as the culprit but without monitoring data stream, your just guessing.

Diagnosing and repairing cars correctly is a hard skill to master, requiring expensive equipment and well trained technicians like myself. That's why the cost of getting it done right is what it is. If you are unable to correctly diagnose it, and start throwing parts at it, you'll likely spend more money guessing than you would have paying someone to correctly diagnose it. Take it to the dealer, pay them to tell you what's wrong, and take it home and fix it yourself. The cost of spark plugs (which will do you no good) is likely close to the cost of an hour diagnosis at the dealer.

 

I can replace any of the parts myself, but is taking it to the dealer going to tell me exactly what to replace or is it going to be a list of "could be's" based on the codes.

 

That really depends on the technician working on the car and whats actually wrong. Tests need to be done to determine exactly what is wrong. If the problem is very apparent and easy to duplicate, any good tech should be able to quickly isolate it down to a single failing component.

The problem we run in to frequently with diagnosing failures is that there are usually several things wrong with the car, but only one of those things is causing your issue of complaint. Such as in cases of customer neglect when they have issues, don't address them, and then it gets worse to the point they bring it in to me. Its very difficult to accurately diagnose a system when multiple parts in the system are worn or in need of replacement. You sometimes have to fix other things that are wrong in order to pinpoint the failure causing the worse symptom. For instance, say you have an intake gasket leaking a little, only when the engine is cold, and it causes the car to run a little funny at times, but the customer doesn't address it. They keep driving it and after a period of time, that condition causes a part to fail to the point the customer brings it in. You as the customer have 2 separate problems. A lean condition due to the leak, but running lean for a long time overloaded your ignition coils and shut down 2 cylinders. You have to replace the burnt coil first in order to correctly diagnose the lean condition. You can't always diagnose around broken components and failing subsystems.

The typical customer doesn't understand this concept at all. Most of them have no idea how their car works, and even less of an idea of what goes in to fixing it. So when the above occurs, they think they are being ripped off by the shop. Most people I hear complaining about a shop ripping them off, its the customers lack of understanding of the situation, not the shop doing any wrong. We as Tech's get a bad rap at times, some of it is deserved, some not.

That being said, your particular case sounds pretty cut and dry and easy to chase. I would first verify the fuel trim is correct to eliminate the possibility of the EVAP system causing a drive-ability issue. This is usually not the case but a possibility that MUST be eliminated. Fuel trims can be viewed on data stream, long term and short term on both banks should be withing 5 of 0 at idle with the engine at operating temp. Fuel trims can be a bit confusing to read and you need a good understanding of what they mean and how they work. I could explain further but this post may consume many pages if I tried. Just make sure they are all close to 0 which is the cars baseline fuel mixture when its idling. That will prove no EVAP issue is causing the symptom and eliminate that particular code from the list of possible causes.

Because I see multiple misfires, the possibility of a lean condition causing your issue exists. No way did you have multiple spark plugs fail at once causing all those misfire codes. Something else is failing, affecting the whole engine.

Once you disregard the EVAP and misfire codes from the list, the knock sensor, which I think is the likely culprit, is the only thing left. You can typically test the knock sensor by unplugging it and reading resistance through the sensor to ground. Compare the reading of the sensor that is flagging the code to the other sensor that is not. The resistance should match closely. If it does not, you've found the culprit. A failing knock sensor is a very common cause of your issue. They fail all the time. A lot of the time, its the wire for the sensor being damaged by heat overtime and breaking down. Or rust and corrosion where the sensor bolts to the block. Either way, if one is failing, replace them both while you have it tore down.

If you have access to a scan tool capable of reading data stream, I'll be happy to talk you through reading the info and interpreting what is going on. Good luck.

 

And just FYI, the process of diagnosis I describe above would take me all of 30 minutes, of which I would expect to be paid for an hour of diagnosis. That's how it works.