4blizak4mn

Hi all,

I've been a lurker for a while but have run into an issue that no forum search has been able to solve. I'd love it if you all could help me hunt down an issue on my wife's 2002 SC430 (nothing better than a girl who like horsepower, right?) and promise to post photos of whatever I do as penance for my lack of contributions until now!

Background:
Recently ran into the well-documented disappearing acceleration caused by one of the trifecta of sensors on the throttle (TPS, APPS, and Throttle motor). Replaced the throttle position sensor and seemed to resolve issue. As of about a week ago, however, the car has developed a hiccup that I can't diagnose. Here's what I've recorded and done so far:

Symptoms:
-Hesitation at 1800 or 2300(?) RPM under light throttle and increasing load (often on uphills).
-Throttle down-blip of 100-200 rpm momentarily, only lasts ~1 sec
-Noticable lurch to vehicle, slows down slightly, results in 0.08G accelerometer reading when kicks back in.
-Change in engine exhaust sound as if in C-gate gears on a downhill (sounds like engine braking).
-Sometimes happens repeatedly in quick succession - up to 8 times in 10 seconds.
-Not speed dependent.
-Overall performance is fine, not sluggish or hesitating when you stomp on it.

-Intermittant issue, not replicable, happens at random.
-No error codes thrown, no CEL, no DSP lights.
-Consistent -14.7psi vacuum registering on OBDII

Actions taken:
-Recently replaced and tuned throttle position sensor
-Seafoam on a nearly empty tank for 3 mi, half-full tank for 40mi
-Italian tune-up
-Disassembled and cleaned MAF
-Cleaned air filter (K&N)
-Reset computer (battery grounded for 10 min)

So... the question is, what the heck is this? Here are a few thoughts of what to do next in order of easiest/most likely/least expensive:

-Remove ABS relay and check performance (maybe TCS is hitting brakes a little?)
-Check all vacuum lines
-Seafoam cleaning through vacuum lines
-Remove, disassemble and clean TB assembly
-Test continuity on throttle body motor
-Test continuity on VVT solenoids
-Test continuity on full range of APP Sensor
-Replace VVT filters
-Replace VVT solenoids
-Check spark plug torque?

Anybody else experience this or similar issues? Would be really grateful for any thoughts on this in advance of starting work on it and or spending a lot of money on replacing sensors. The wife would also appreciate me spending the nights I'm not at grad school until 10pm with her and not in the garage

Thank you!!

LexBrett

You mentioned a "lurch"? Could it be the transmission? I have also read that the Engine Coolant Temp sensor and Air Temp Sensor effect fuel air mixture too. How about your MAF sensor? My car was pinging occasionally and I cleaned the MAF and it stopped. Are you getting any codes?

Did you check the plugs for misfire?

You might also feed seafoam through the PCV valve to really clean the fuel system. PM me if you need instructions how to do this.

Whatever it is, it sounds very frustrating! Sorry for your woes!

By the way, what could the torque on the plugs do with this? I'm always worried that I impropperly torque my plugs

4blizak4mn

Thanks LexBrett for the quick reply. Yeah, the lurch is really slight - doesn't even feel like it's going to neutral at all, just feels like the throttle cuts for a second and literally only a second. I checked the trans fluid the other day and it was a little high if anything. I'll add a trans flush to the list of things to try as an option. Maybe an overdrive issue - it's searching when it gets load low in the gear range?

As for plugs - I set them to whatever the torque specs in the manual were - my thoughts is that maybe if one has backed out a bit, it could be affecting cylinder compression or electrical continuity and making the computer slip into limp mode for a second or two? Just wild speculation which is why it's way low on the list.

LexBrett

You might try flushing your transmission fluid.

Even if this is not the source of the problem, it helped performance in my car quite a bit. Type IV ATF is recommended, and I would also suggest putting 24 ounces of Lucas Transmission Fix. The Lucas additive makes an incredible difference in smoothness of shifts.

If you choose to drop the transmission pan, you can easily change the filter. There is also a mod to the transmission valve body, which makes shifts a but quicker. Feel free to email if you want to know how to do this.

You can also drain the ATF fluid from the drain plug on the pan (14mm socket) or the transmission return line at the bottom of the radiator on the drivers side.

-Lex

bhop

My hunch is it's a fuel pump....

4blizak4mn

Thanks Lex - I'll send you a note later today - may try Trans Fix by itself right now after siphoning off some of the trans fluid with my fluid extractor at home and then do a trans service soon.

Bhop - could you explain your hunch? Is that because it seems like it's cutting out a little every once in a while which would indicate faulty fuel delivery? Are you thinking electrical fault on fuel pump or clogged screen?

LexBrett

Sounds like a good plan. The 24 ounce bottle of Lucas Transmission FIx will definetly make your car shift smoother. Maybe if you are lucky it will solve your problem too!

bhop

A few years ago, I had similar issues with a 03 GS300 that my son drove......and after having checked everything possible, many of which are on your list, it turned out to be a bad fuel pump. Not sure if that is your problem, but it sounds a lot like what was happening to the GS....I would guess they used similar fuel pumps on the GS and SC back then...again just a hunch.

Good luck

PS - As I re-read your symptoms in your opening post, those are not only similar symptoms, those are the exact symptoms we had...Hesitation, throttle down-blip, lurching, cut-out ......

4blizak4mn

Thanks bhop - yes, GS and SCs from early 2000s share a ton of parts. I checked trans fluid again this morning and it looks low now, so going to add some of the trans fix that brett mentioned. I'll keep you posted if that fixes it, otherwise I think you're right...going to focus on fuel pump and just replace the whole thing. Of course this has to act up when the temp plunges here in MN Thanks again for the help,

UPDATE: Filled trans with ATF T-IV and drove - did the hiccup literally as I was leaving the parking lot haha! Seemed to get better on the 20mi drive to work, much less frequent with more miles. Will continue to monitor on the drive home late tonight after grad school and I'm going to do a full flush by draining/suctioning and refilling a few times this weekend and see if that continues to improve performance as fluid color is definitely not the standard clearish red/pink.

Leaning more and more toward either trans or throttle body as a solution as I can't replicate the cut-out/hiccup at idle or at partial or WOT, it only happens under very light load around 1800-2400 RPM, regardless of gear. I would think if it was fuel pump, it wouldn't happen in that specific of a range and if particulate matter in the fuel strainer or filter you'd think it would happen most under WOT when it's drawing the most fuel, right?

LexBrett

Let us know how it goes!

Glad to hear the Lucas helped, and a full flush will probably be even more helpful. I think the fluid is about 3 or 4 quarts from the drain plug on the pan. If you really want to go nuts you can drain from the transcooler return line and flush the entire system, I think about 7 quarts.

The return line is really easy to get to, let me know if you need help. I just installed a trans cooler this weekend and have been working with these lines recently.

If you change the fuel pump, are you going to do it yourself? I heard that the fuel pump is in the gas tank. I'd love to see a write up with pics on how to change the pump!

bhop

I believe LexBrett is correct that the fuel pump is in the fuel tank.....

Habious

Here's the chapter from the service manual on the fuel system.

Looks like the fuel pump is in the tank, and accessible from an access hatch under the rear seat. And it doesn't look "simple" (to me, at least) to replace, but I hope this helps.

FLYCT

I had similar issue with my 2008 SC430. With cruise set at hyway speeds 70 mph when going up slight inclines it engine felt like it was missing (bucking) and transmission would downshift. At full throttle car ran great, no missing.

I took it on to dealer under warranty. The only thing they could find was knock readings were low. They suspected bad gas. After 2 tanks of gas it got better and completely disappeared.

4blizak4mn

Well, latest update - my hopes it was the tranny fluid are dashed...still doing it. I think we're on to fuel system - Habious, those pages from TIS are a god-send - thank you! I'm going to hope it limps to work and dinner with the wife tomorrow and then start doing surgery this weekend - I will take a post pictures as I go through things piece by piece and try to diagnose this.

kjcole

Where's the fuel filter? However, if that is partially plugged you get starving on acceleration. Fuel pump, VVT filters/solendoids, are good candidates. need to proceed with a logical diagnostic procedure, which the PDF for the fuel system provides