sparkyguy

'93 SC400 V8
111k miles, previously running fine, engine tight.
Suddenly, engine has NO power. It runs, hits on all 8 cylinders, but there is suddenly nothing in the way of power behind it.
It idles "basically" ok (maybe slightly rougher than normal) but takes a LOT of pedal to get up to speed. Shifts somewhat rougher than normal. Loss of power is NOT due to missing cylinders because it idles far too smoothly. A vacuum gauge is solid as a rock (NO bouncing at all) but is way down around 15" at idle. When engine speed is raised to 2500 RPM or so, vacuum will ultimately drop back to 15" after a couple of seconds. I originally suspected a clogged Cat Converter but this was eliminated by observation (both sides are completely clear). The next thing we suspected was insufficient fuel delivery so the fuel filter was changed. No effect either. Even if it was getting insufficient fuel, I wouldn't expect it would have LOW vacuum or that the gauge would be rock-steady. Engine seems to idle too well for it to be bad valve timing, and on this engine I would think it would run like crap if the valve-train was mis-timed. I can't hear any MAJOR vacuum leaks, but that might also be considered. I've also noted that the tachometer will frequently drop to zero when accelerating and will rise when you decelerate. The tachometer also seems to be erratic and sluggish (it doesn't respond as fast as what the actual engine RPM is). I don't know if this is related or not to the other problems but I noticed this beginning at about the same time that the engine suddenly lost power Any suggestions for issues that might be common for the SC, or items to consider?

sparkyguy

Mage,
Thanks for the reply.
After a ton of digging, I also started suspecting that as a strong possibility. Whatever is wrong seems to affect operation of the ENTIRE engine. THAT is what led me to begin to suspect the ECU. If the timing belt slipped, I would expect a vacuum gauge needle to jump and it is rock solid (but reading very low). I suspect the ECU is actually now in "Limp Mode".

Since I don't have access to a 2nd unit to swap, are there any "forum tricks" to determine it is actually bad (other than a physical swap)? I found a number of ebay sites with units around $75-$150, or optionally companies that claim to "rebuild" them. Are these still the best option? It is actually my son's car and his budget is VERY tight. A new unit from mfr is obviously out of the question.

SCereal

iTrader: (4)

You can try pulling the ECU out and taking the cover off and looking at the capacitors. If they're leaking or distended/bloated, they're bad and ECU either needs rebuilt or replaced.

sparkyguy

Thanks for the suggestion SCerial,

I'm a retired Electrical Engineer and I was wondering if they were visible inside an ECU.
I've never physically opened one up to look (yet) and thought the interior might be encapsulated or potted with epoxy. If they are standard electrolytic capacitors (which is what they probably are if they go bad with age), they usually split and/o leak when they go bad. I will certainly take a look. Certainly got nothing to lose.

soarer93

You do have the benefit of using the Onboard Diagnostics -

Once a few springs ago, pulling car out of storage... it had some of the same symptoms - including the tach going to zero while driving (usually while braking actually, or putting on the 4ways or something that drew a lot of power) as the batt terminals were in need of a replacement.. car felt like it was bogged down from ovefueling or something when accelerating, but only occasionally.

but new terminals and ultimately a new battery was all I needed to fix my issue.

Ali SC3

iTrader: (10)

Moved to maintenance.
If you are an EE it will be fairly obvious when you open it up. They will leak the electrolytic material all over the green board and it will be a brown crispy color. The toyota test for the ecu besides opening it up is to take a hairdryer to it, that speeds up the warming up of the cap's and hence the problem since the material has leaked out. there is also a humidity test of using a hose to spray water over the windshield or car but if the area around the ecu is dry already I wouldn't bother with that.

Usually these ecu issues start out small and grow with time, so I am not sure it is that but worth a look.

What I think your problem is is that you lost a coilpack. is it idling a bit lower than normal?
Its really hard to tell when those v8's are running on 4, as its not just one bank that goes out but 2 cylinders on each bank when you loose a coil, and it runs like a 4 cylinder, basically sounding fine but no power.
Also the cat's will start to get hot from the unburnt fuel.
ITs worth a test most sc400's loose at least one coilpack in the first 200k.
disconnect the 2 wire lead going to one of the coils. if there is no change that coil is bad, if it changes or stalls out then that coil was good. reconnect it and test the other coil. let us know how it goes.

These cars rarely skip any timing unless its been 200k since the last timing belt change. generally its the last thing to go out or if it is out its cause someone did the timing belt job wrong. I would look at fuel and ignition.

Another neglected item in the igntion department is the rotor's in the 2 distributor caps. often owners never change them and eventually the tips break off and drop a certain amount of cylinders or the just disintegrate completely and also leave you running on 4. Everyone claims that its still running on 8 because they never thought a v8 running on 4 would be so smooth, but a 1uz running on 4 is still a pretty smooth running motor.

SC400slide

iTrader: (11)

Sounds like one of your two coils took a dump. Unplug one and run the vehicle.

sparkyguy

OP back again.
I wasn't certain which forum to originally post in.
Is a schematic, wiring diagram or block-diagram available somewhere that shows how the ignition on these things is actually laid out? I'm used to ignition on typical V8's (even old systems with points) but not with dual distributors, etc.. We've only owned the vehicle for a few months and haven't dug into the engine a lot yet.

sparkyguy

I found a diagram and understand how spark & ignition is wired. VERY interesting layout (for a guy brought up on small-block Chevy's). One distributor serves 1467 and the other serves 2358 and firing order alternates distributors LRLRLRLR.with individual coil for each distributor. Electrically, it's a pretty straightforward arrangement and will be easy to disable one side in order to check the other. Will post back with findings in daylight tomorrow.

Ali SC3

iTrader: (10)

yeah just pull the coil and you will hear the change or not hear the change. Each distributor has its own coil and ignitor. the only point of the 1467 and the 2358 is its designed so that when one goes out you don't feel a lumpy running motor, it still have a workable runnable firing order on just one distributor cause it ties into 2 on each bank. You can think of it like one distributor controls the 4 main cylinders to run the motor, and the other distributor carries its companion cylinders. The reason they are non-adjustable is because when they divide it into 2 like that they don't want you to change the timing on just 4 cylinders, so the UZ has 2 fixed distributors with the ecu controlling the final timing, whereas the 2jzge has 1 distributor for 6 cylinders which has a manual baseline adjustment and the ecu controlling from there which would be alot more familiar to other ignition systems.

The 1uz is special in alot of ways, but that's why they are known to pretty much run even in the worst of conditions, even if it is on 4 you will just have no power and burn through the cats eventually.
Far from the domestic V8 firing order which use a single distributor.

sparkyguy

FIXED !
Ali SC3,
The diagram you posted was the exact one I had already found last night. In any case, it helped a lot. Surprisingly, the firing order is the same as the old small-block Chevy's I grew up with as a kid (1-8-4-3-6-5-7-2).

Turns out that one of the ignition coils WAS bad.
I've been back-yard wrenching for over 45 years, rebuilt several engines and probably owned a half-dozen Toyota's. However, until last night, I didn't know these engines had 2 coils and 2 distributors. I would have NEVER believed it would actually run as good as it actually did on only 4 cylinders or that a vacuum gauge would be as steady as it was while running that way.

Thanks for the suggestions.
Much appreciated..

Ali SC3

iTrader: (10)

Glad it was the coil which is relatively simple to fix. It happens alot, shuld be back to full power now.
please take the poll if you have time.
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