t2d2

iTrader: (8)

No, I haven't tested the coils, although I've seen a few suggestions similar to yours and had that on my list of things to check. I even mentioned it to my mechanic yesterday, and he agreed with me that it had to be running on full power on the post-suspension test ride. Wouldn't a bad coil have shown in the compression test? We tested them last month and all eight were between 235 and 245 psi. I suppose this problem could have started since then, but I think it's been there all along just masked by other stuff.

Also, I thought coil failures are supposed to mostly manifest themselves under load? This is pretty much the opposite of that... It's great at near-WOT, but backing off to ~20% throttle is what triggers it. We were thinking the fuel pump ECU is the most likely culprit (cutting fuel at partial throttle), since it's already on my Most Wanted list for contributing to the start-up hesitation, so I drove around yesterday with the diagnostic box jumpered to bypass it, and the car was still bucking. We were going to test fuel pressure to the engine, but those factory fittings weld themselves together over the years. The flare nut wrench eventually snapped before that thing would come loose, so we gave up on that before getting to the same point as the fuel filter two months ago, which was so impossible to remove and put the new one in that it eventually required patching in a new fuel line with clean fittings.

But if I were to test the coils... I'll have to look up which wire exactly that is I'm looking for. I know what the coils look like, so I'm assuming it'll be pretty obvious with the plastic trim bits taken off. I've read that they'll zap you pretty good if you don't wear thick gloves? And that that test could still indicate a distributor problem, so it requires also pulling the plug wires?

t2d2

iTrader: (8)

http://www.aa1car.com/library/ignition_coils.htm

Ali SC3

iTrader: (10)

compression test has nothing to do with the coil pack, that has to do with health of the motor and its ability to hold compression. also those numbers seem too high, but as long as they are all about the same it should be alright must just be the tester used.

the coil failure can cause all sorts of symptoms like that, it actually is the least noticeable at WOT.
another thing yours could be related to is the tps.

I never said to remove the high voltage part, that would be the big lead coming off the top of it.
There is also a 2 wire connector on it, which is low voltage as it is what powers the coil, and I said to disconnect that. there is a difference yes. You can also disconnect it with the car off and try and start it, if it starts and runs the same as before when doing one side then you know one of them is bad, and its the one which when disconnecting it causes the change.
You can also not do it and get a spark plug tester if you like, seems you know how to do it I am not going to argue with you about which method to use.

Instead of diagnosing evrythin else based on a theory on detecting a coil failure at certain loads, you could also just quicky test it as its the most common problem that causes that known on here by experience. 2nd most common is a TPS. less common things are a plugged cat but thats rare. knowing the mileage and service history can also help diagnose issues. normally the plug wires on these cars last the life of the vehicle, but sometimes people spill things or pinch things and then you can have issues there as well, again alot less common.

the distributor can only have an issue with a really old rotor, go ahead and change both if its been 60k and you are not sure they are not changed before. sometimes these break apart inside the distributor and can cause similar issues to the coil, but a vast majority of the time its the coil, the one on the drivers side.

its so common I started a poll for which one failed on your sc400.
https://www.clublexus.com/forums/per...ason-poll.html

Its not the fuel ecu where do you come up with this stuff, when that goes the car shuts off there is no in between, it would just not start.
what causes a rought start is only running on 4 cylinders and flooding the other 4 with fuel .
did your MPG do down alot recently? do you have the press the throttle near to the floor just to get up to 50?
is your cat converter glowing after a long drive and ticking twice as much as it used to? that is that extra fuel dumping in the exhaust right now.
Its not the fuel ecu leave the poor fuel ecu alone its not safe to bypass it for no reason on a stock car.

t2d2

iTrader: (8)

I was thinking the coils would need to fire to provide spark for the compression test, but it's not actually hooked up that way now that I think about it... It's the starter doing all the work. We were shocked by the ~240 numbers, too. So we looked around online and then called the Lexus service department to make sure we weren't seeing something out of whack. They confirmed that the minimum acceptable reading is 142 lb, 178+ is good, and the 240 range is great. (I think I saw similar min/good numbers in the factory service manual.) I've seen people in the forums report readings of ~175, which probably means their engines aren't running nearly as strong as they think.

Hmm, okay. That's counter-intuitive. TPS is one of the things I was going to tackle this week.

No, not at all. I get myself in trouble every time I work on electrical stuff without a clear understanding of what I'm doing, so I was asking questions so I'm not confused about what you were describing. That explanation makes more sense. I just need to disassemble stuff and find the 2-wire connector you're talking about.

The car has 189,000 miles on it and I changed the plugs/wires last month. I hadn't heard previously that the plugs last the life of the car, but at close to 200k, assuming they were original, they may have been due anyway.

Yeah, I don't know the service history of anything between 74,000 miles and now... Guess I'll add that to my to-do list.

Dream36

I've had to follow every step on the how to on fixing hesitations. I've replaced coils every few months, fuel ECU mod, cats delete, new caps and rotors, new wires, fuel filter and only when I got a rebuilt ECU did I fix it permanently. But I also discovered that running low coolant can also create these problems. I suggest to anyone stsrting to search these problems start with the coolant as it's free this time of year. Currently I have a vac leak that I'll fix this weekend. I've been surprised at how often I need to open the hood on this car. I imagine if my car had the splash shield still (didn't come with mine) I wouldn't go through coils so fast.

t2d2

iTrader: (8)

I didn't see the questions you added in while I was responding...

1) It wasn't just me, the mechanic also thought the fuel pump ECU could be the culprit if it was incorrectly cutting fuel under partial throttle. Aren't most of the fuel pump ECU bypass mods done to help one that's performing poorly, rather than one that doesn't work at all? That seems to be the gist of all the bypass recommendations, at least.

2) What do you mean by a rough start? Mine is a lengthier cranking time on cold starts than on warm starts, but both seem reasonably quick with the fuel pump ECU bypassed.

3) I've only had the car a little over two months, so not much mpg history. I lost mileage tracking the first time when I disconnected the battery and lost the trip odometer value ... I'm used to analog clusters. Before that I had only done partial fill-ups, with fuel filter changes and fuel cleaner partial tank requirements. So, I just got my first full tank reading last week and it was 17.6 mpg with a mix of city and highway. Probably a bit low, although I might have been driving more aggressively around town while testing stuff out.

4) Don't know. I haven't tried looking at the cats after driving, and I don't know how much ticking is normal. Is it even possible to see the cats at ground level without jacking the car up? I guess if they're glowing, you might see that stand out.

5) It's not for no reason. It starts quickly every time with the fuel pump ECU bypassed, and more like 50/50 without. It's been a pretty dramatic difference after a few days of testing.

t2d2

iTrader: (8)

I pulled the 2-wire plug on the driver's side coil and it died instantly. I haven't pulled it on the passenger side yet. It looks like the only way to get to the plug on that one is by removing the wire tray and cover, and the spark plug covers? Even so, the low voltage connector is still tucked in below the high voltage one, making it tough to get to. Is there a better means of access?

There's a 2-wire connector that sits loosely right below the passenger side coil, but it looks like it drops down below for something else and I think one of the wire colors is different. It could be significant, though, because the mechanic noticed last month that its clip is broken so the plug isn't held in firmly. I'll need to pick up a donor plug at some point and try swapping the vampire taps. I'm pretty sure it's not for the coil, though.

Also, Ali, I found this thread where your thoughts for the same symptoms leaned more toward IACV and O2 sensors:

https://www.clublexus.com/forums/per...n-jerking.html

Do you happen to know the O2 part numbers you recommended there? That question never got answered.

t2d2

iTrader: (8)

Ok, so I combined a couple of suggestions and tested the passenger side coil by disconnecting the high voltage plug. The car started up briefly and died. I then did the same thing with the driver's side and got the same result. Now, that's sort of comparing apples to oranges (inputs vs outputs), but combined with the previous test of the driver's side coil disconnection while running, it would seem to confirm that the coils are okay.

I saw this suggestion on the Cadillac forums and gave it a try:

http://www.cadillacforums.com/forums...esitation.html

1) disconnected TPS -> no bucking
2) reconnected TPS -> bucking returned
3) disconnected TPS -> no bucking

That seems pretty definitive that the TPS is to blame, although it could be misleading, if disconnecting it simply changed the shift points relative to throttle (tranny shifts much more aggressively/abruptly with the TPS disconnected). It's definitely worth running through the TPS calibration, though, to see if it's simply off a tiny bit. Do the TPS units go bad on these cars?

Ali SC3

iTrader: (10)

I would suggest to check the rotors and finish with the ignition system, then go on to the other stuff. if you have checked the coils and replaced the plugs (which you should do every 60k), you missed one of the most important parts and that is the rotors. so with potentially 180k on your rotors, I would be surprised if they are not misfiring already or in very poor shape. to put it in perspective, they usually fail before 150k, on my 300 it failed at 130k and left me stranded, and who knows if it was even the original or not.
basically your ignition service was incomplete, I would start there and then go forward.


I think the the 12v ecu mod and plugging and unplugging the tps can be masking the problem and just seems like it runs better. There is no reason to supply 12V worth of fuel to the car at low speeds you are just returning a bunch of extra fuel via the fuel rails back to the tank or covering the real problem, and I understand this is just my opinion you can do the 12v mod if you like and see how it goes you can always undo it.

For the TPS, they can fail with age, but usually if not messed with they hold up pretty well. sometimes the connectors crack and any cracks in a connector that will let the pins move even slightly will cause a bad connection on these cars with the heat in the engine bay. I had a cracked tps connector that would give an erratic tps signal. one time I had a bad tps that was just locked at like 40% or something all the time, but its not as common as the other issues. I think you can do a voltage check on the signal out and see what its reading to calibrate it.

another quick test for the tps is to mark its original position, and then loosen the screws and turn it all the way from one side ot the other and back again. if the car does not react at all, then the tps is likely shot and giving it just one reading. if the car idles up and down or changes in sound, and really it should stumble for a second half way through the sweep in one direction, then you know its getting input from the tps I want to say, and at that point its a matter of setting it in the right position, which should be the original position if you have never changed it.

unplugging and plugging it in what happens its hard to tell cause the ecu goes into a different sort of backup mode every time you unplug one of its inupts, so maybe on a caddilac but not sure that works on our cars.

Also the car should really run with just one coilpack connected, not just start and die again. It makes me think there is some type of ignition problem, other sensor problem or air fuel ratio problem so yes all of those things you mentioned are worth checking but start with the rotors and get your igintion system back to healthy. there is no point in having good plugs and wires when the spark that comes across the rotor is weakened. I don't even typically do the wires, normally just plugs and rotors will get you back in good shape.

*Edit* I just remembered, it could very well also just be a bad ecu, these are more and more common every year. check around the passenger floorboard for any signs it may have gotten wet before or possibly even open it up if all else fails and take a look.

Also about the o2's if yours were really bad that could affect it. you mentioned resetting it and it drove well, did it drive really well? if so you can also disconnect the plugs for the main o2's and reset again. this time the car will not learn off the bad o2's. if the problem does not repeat you prob need to change your o2's and at 182k its likely time to do so anyways. these have a service interval of around 100k give or take, so its up to you to check everything the previous owner simply skipped doing, which is often alot on sc400's.
I don't know the specific part number but its best to look at your old one. its either a 2 bolt type or a screw on type, and then count the number of wires. knowing those 2 things you can easily order the denso universal replacement for it, or you can use the model or vin number to get the real plug and play part.

Also some sc400 owners dont think they need to change a timing belt before 200k so double check your timing belts when buying any sc400 and try and get documentation on it, I would do it every 120k at the least and every 100k if it was a 95+ motor which can have top end damage from throwing the timing belt.

t2d2

iTrader: (8)

Lots of good info there, thanks. On day 2 of driving with the TPS disconnected (I had to make a trip to the store to pick up the right sized angle screwdriver to do the TPS adjustments), I got three CEL codes:

14 (ignition signal), 55 (knock sensor #2), 71 (EGR system malfunction)

It happened again after restart, but didn't come back with the TPS reconnected (but the bucking hesitation returned), so that fits what you said about confusing the ECU with its various inputs changed. I've had the fuel pump ECU jumpering removed the past day or two, BTW.

Are the rotors something that can be checked/serviced, or is it something that just needs replacing if you're unsure? The car was meticulously serviced by the dealer through 74,000 miles, then there's no history for the next 114,000. Tough to say what was and wasn't done during that time. The plugs and wires are brand new. I don't see a date anywhere on when the timing belt was done, but it looks to have been done quite recently. Either 60k or 90k service intervals would put it at 180,000, so shortly before I bought it, hopefully.

I did open up the ECU last week to look for leaking capacitors and looked good as new inside. I didn't specifically look for water around that area, but everything was dry and I didn't notice any stains or rust. It would be nice to drop a borrowed ECU in for a quick test, just in case mine is glitching without the typical capacitor issue, but I'm not sure it's worth buying a used one as a long shot like that.

Yeah, after resetting the ECU (pulling the main EFI fuse), it drove like a new car. That sounds like a worthwhile test on the O2 sensors. I looked around this morning and found lots of supposed parts matches for $20-30 ea, which fits what you said in the other thread. The "main" ones are upstream from the cats, right?

t2d2

iTrader: (8)

Got that check done. The idle changes throughout the sweep, and it does stumble a bit in the middle of the range. Interestingly, that middle section is pretty much spot on with where it was set. Once I get my PC connector shaped to fit the port so I can do the calibration, I'll be curious to see where it rests compared to the previous setting.

I just found my angled screwdriver of the right size -- it was in with the tools in my other car -- so I could have done it yesterday instead of setting off all the CELs...

t2d2

iTrader: (8)

Done with the TPS settings, and fingers crossed, breath held .... so far so good! My throttle set screw was way out of whack. I couldn't find any spot where the TPS setting passed both the 0.16mm and 0.26mm feeler gauge continuity test. Then I noticed how far the throttle had to be pulled to get continuity back on one of the tests, so I played with the set screw and went from backed almost all the way out to screwed almost all the way in. That allowed it to pass the continuity tests, and driving around for half an hour just now, none of the bucking hesitation returned.

My idle is drooping a bit low at times, so I'll need to adjust that further. I'm guessing someone butchered the TPS setting previously in trying to account for the idle. Hopefully, I can find a happy medium between the two.

All in all, I rotated the TPS ~1/8" counter-clockwise from the initial setting. I also replaced the hard to reach Phillips screw (which was almost unsalvageable after a couple of turns; it's obviously been worked on before) with a high strength allen head which should be way easier to work with in the future. Good thing I picked up a couple of them, because the first one fell down into engine...