(New) Code 21 and (less new) bucking/surging at idle while stopped (related?)

Hi TVT! It's me with the 91 Previa with the engine I pulled and refreshed down to the block while replacing the head gasket.
Now I'm 8,000 miles in the future. She's been treating me very well for the most part. Still so happy to have this van back on the road.

I have two situations to tap into the collective expertise about, one new and one ongoing, and they may or may not be related to one another.

Situation #1:

This afternoon driving home (~60 min cumulative drive, mostly highway and state routes, 50-70mph, nothing unusually taxing), about halfway through the trip, I stopped for gas and filled the tank about half full.
About five minutes afterward, I noticed that my check engine light had come on, solid. For about 15 seconds, and then off. After about two or three minutes, the same. On for about 15 seconds, then off for another few minutes.
It did this the rest of the drive home. Didn't seem to correlate with engine load or RPM or uphill or downhill or anything I could feel or hear.
This is the first time the CEL has been on for non-diagnostic reasons since the rebuild.

Got out the old paperclip and jumped my TE1 and E1. Reading error code 21, which according to Tim in this post from 2014 is related to the main o2 sensor, likely a bad o2 sensor itself.
Troublecodes.net confirms this - Code 21, Heated oxygen sensor (H02S)- front – circuit.

By the way, is this diagnostic table in my factory Toyota repair manual somewhere and I'm just not looking in the right part of it? Is it in the owner's manual?
(I'd like to know because in diagnostic mode my oil level light is also flashing once every four seconds and I should also figure out what that's trying to tell me, if anything.)

I didn't get down in this evening to do a visual check on my o2 sensor to make sure nothing's frayed or loose, that's tomorrow's job.
Is there anything telltale that I should look for while I'm down there? Does anyone have recommendations from experience for or against any particular replacement o2 sensors?



Situation #2:

This one's been puzzling me but it's been minor enough that I've let it be until the weather warmed up enough for me to want to mess with diagnosing it. For the past month or two, sometimes but not always, when I'm moving very slow or stopped while the engine is running (eg. at a stoplight), I'm experiencing a sort of "surging" or "bucking" type action where the van feels like it's wanting to move forward for a moment, in conjunction with what feels like a momentary hesitation (or spike?) in the normal idling pattern. When it's happening, it will happen once every few (3, 5, 10) seconds, until I accelerate again, at which point the irregularity either goes away or becomes imperceptible, I haven't figured out which. Occasionally there is a slight hesitation accelerating again, usually there's not.

At some point I noticed that when I'm in Park instead of Drive, even though I don't get the "surging" or "bucking" action to the same extent (I assume because the irregularity isn't being transferred down the driveline in Park), if I let the engine idle for long enough on a day when it's doing it, I can feel the same periodic intermittent hesitations (or spikes) in Park. It doesn't stall or feel like it's at risk of stalling. I wish my Previa had come with a tachometer, it would be great to know what the RPMs are doing more precisely when this is happening.

I checked out the exhaust once I knew I could hear/feel it in Park. Each hesitation produces a sputter and a visible vibration from the exhaust. That's probably not surprising or illuminating. No smoke of any kind.

At some point I noticed that when I'm in Reverse, on a day when it's doing it, I can feel the hesitation happening in Reverse as well. Always intermittent, just one spike / moment of hesitation every handful of seconds.


So, that's the symptoms of my Situation #2. I ruled out a clogged air filter by getting a new air filter. Not air filter.
I ruled out spark plugs by putting in the new set I already had waiting to go in. Not spark plugs. Tested spark to each cylinder, each cylinder is getting spark.
It "feels" a little bit like an intermittent misfire feels. I haven't ruled out injectors, but the set of injectors in there are only ~8 months old. Not that new parts can't fail.
I did keep my original injectors around in case I decide to send them off to be professionally cleaned and tested. I haven't had the current ones out to give them a once-over since I installed them.

I also have not yet done a compression test. I'm not ruling out that a cylinder might be misfiring very intermittently, and I'll wind up doing a compression test as part of my spring tune-up checklist anyway, but it just seems to me like the problem would be showing up a lot more regularly if I had a bad cylinder, and it also feels like I'd feel it more than just when I'm stopped or going backward. Perhaps not.

My amateur assumption has been that it's airflow or exhaust related. I did a cursory inspection of the intake boot that connects to the throttle body and there is one very small beginning of a tear on the edge that meets the throttle body, but it doesn't look like it even makes it as far as the hose clamp, so I don't think it's actually in the sealed part. But that might be a sign that the material could be getting brittle in other, harder to see places too. This is the original 32-year-old boot. I picked up a replacement boot anyway because I'm sure that little edge tear will find its way past the clamp someday if I let it. And you never know when what parts are going to get harder to find. I haven't installed the new boot yet, pending further inspection of the current one.

Then I'm thinking, judging by the kind of situations the airflow is in when the hesitation is noticeable (very slow, stopped, reverse), maybe the airflow is not being regulated correctly? I don't know much about the intake/air control system in a practical sense. Idle Air Control Valve? Mass Airflow sensor? This is the part where my lack of experience catches up with me. Could it be an exhaust system issue and have something to do with the failing or failed o2 sensor?

Does this situation sound familiar to anyone? Does the description imply anything about potential causes to someone who's better attuned to troubleshooting these engines than me?

Would love second opinions or thoughts here. Much appreciation for reading, best wishes.

Alright, well, after getting under the car today to check out the condition of the O2 sensors and wiring, I remembered why I didn't replace them to begin with when I had the exhaust out.

The van's previous owner did some interesting welding work on the exhaust system ... where there should be flanges and a gasket between the cat assembly and the muffler pipe, he's just straight-welded the two parts of the exhaust together.

Attachment 12101

Attachment 12102

I assume at some point his flanges were failing somehow, or he replaced the muffler assembly with a used one that was cut off straight, or ... something ... since functionally the only thing this does is make the exhaust system much, much harder to remove and reinstall (...the whole thing has to be pulled out the rear over the back axle, cat assembly and all, and fed back in the same way...)

I just made peace with the situation because I didn't have the budget for a new cat assembly and muffler assembly both last year and I didn't have time to mess with trying to find or fab new flanges to weld on.


In addition, though, it looks like ... both the o2 sensor mounting flanges are ... welded to the cat assembly? Am I looking at this wrong? The front one doesn't even have its retaining nuts, but it's sure enough fixed in place.

Attachment 12103 Attachment 12104

Mike, if you're out there somewhere ... I just want to know the story ...

So, maybe this o2 sensor replacement isn't going to be as simple as I was hoping it would be when I read the code.
I should really ... probably just replace the whole exhaust system downstream of the manifold, just for future sanity and ease of maintenance's sake.
The muffler has never shown any signs of trouble though, and hasn't been eaten through anywhere, and especially if I'd just be replacing it with a used one, there'd be no real gain. But since it's welded to the cat assembly, if I keep the muffler and replace the cat assembly I'll have to find or fab a new flange to weld on to join it. Aye ...

I guess the silver lining is that I'm finally being pressed to start thinking about what I want to do with the exhaust system, since if I'm going to be cutting and welding and fabbing, I may as well explore my options.
I'm not in an EPA/emissions testing state, so I have a little bit of freedom here, but I do care at least a little bit about emissions ethics. And ... noise ethics. I do own a cutoff wheel and a welder, though, so my temporary solution doesn't necessarily have to be my permanent solution.


EDIT:
Drove a little bit today (5 min away, 5 min back) just to see how she's doing. CEL on from startup, solid for about two minutes into the drive. Off for the remaining three minutes. Parked for about 5 minutes. Started back up, on for about two minutes of the drive back, then off for the rest of it and didn't come back on while parking or idling.
It's not doing the hesitation/bucking thing today, or at least it didn't during any of the road crossings or stoplights in this 10min drive. Nothing in how it drove today felt unusual or rough, although it's possible I've just gotten used to a non-ideal normal and have forgotten how it sounds/feels when it's at peak efficiency.

If you replace the system do yourself a favor - weld in 18mm bungs instead of the flanges. Opens up a far bigger selection of O2 sensors.

This is a good idea. I picked up a bolt-in replacement for the cat assembly that has the flanges, and I have a set of new O2 sensors with the flanges now, but I'll keep this in mind for the future.


So, some good news and some bad news.

Good news:
I found a pretty decent price for a direct-fit pre-fabbed cat assembly on eBay from a reputable seller of direct-fit exhaust parts. Time will tell on build quality, but the price was good enough to pull the trigger on for the time being.
My plan is to just cut and butt-clamp to the catback pipe and keep the muffler for now. Assembly should be here by this weekend and my new O2 sensors are already here.


Bad news:
I've still got problems upstream of the exhaust.
I was curious to see how the engine would do with the cat assembly unbolted, exhaling straight out of the exhaust manifold. First of all I wanted to see if I was still getting irregularities, second of all I wanted to see how reasonable it might be to drive it un-catted and un-muffled for a short time if push comes to shove. Well, the second answer was easy: no, even if it had run perfectly, I can't ethically do that to the public, my goodness, what a noise.

Unfortunately, it looks like there's more at play than just the exhaust issues. Even with the exhaust free-flowing, I'm still getting quite a rough idle and hesitation. And now that I can hear the engine loud and clear, the intermittent sputtering and popping is much more audible.

I took a couple of videos because I figured that'd be more useful than trying to describe it in words.

https://youtu.be/A7gleAxk5a8 https://youtu.be/lq4iCDFdXfU
(These are both with the exhaust system removed past the manifold.)

So ... clearly there's something fuel, air, and/or combustion-related happening, right? It doesn't seem to have the regularity that I expect from a cylinder consistently not firing or a physical problem with valves/lifters (correct me if I'm wrong there?), but that doesn't narrow it down much. One or more of the new injectors clogged? Idle or throttle out of proper adjustment? Residual buildup from the exhaust issue? Something else? Forgive my ignorance here, my knowledge is still patchy and this situation has hit a lot of my blind spots at once.

New cat assembly is installed with new o2 sensors in place, CEL resolved immediately.

It did not seem to affect the surging/hesitation at idle and in reverse, so that's something different happening. Still getting that intermittent puffing/sputtering sound and vibration along with it, like unburnt fuel igniting somewhere it shouldn't be. Occasionally stuttering accelerating from a stop, but not always. Nothing perceptible from the cabin once I'm at speed.


Going to test and recalibrate my TPS this weekend. Reading up on throttle adjustment on the forums, I realized that I've adjusted the throttle stop screw since the last time I calibrated the TPS, which it didn't register to me at the time that that would fully throw off the sensitive tolerances I set when I initially recalibrated the TPS. There was a period last fall after putting the engine back in where I was trying to resolve a periodic idle stalling issue, and I think I've ended up with my idle mixture screw out of proper adjustment after that as well.

Not sure if either could be the source of my trouble but I'm sure it's not helping. I've found good info here on calibrating the TPS with feeler gauges and an ohmmeter. Is there a best practices walkthrough for recalibrating the idle mixture screw?

https://zenseeker.net/Previa/PreviaM...nce-Engine.htm - Zenseeker.net has archived a reply from a no longer accessible yahoo conversation that seems to be in response to a very similar situation to mine, about halfway down the page under the subheading Almost Stalls at Stop, Stutters when Accelerating when Warm, although I can't see the context of the reply.

I'm going to go down through the tests he's listed there as well, they seem simple and I may be able to narrow the problem down further. I'm waiting on a new ohmeter and I'm out of town till Sunday morning. Hopefully I can start ruling more out Sunday. Or finding more problems.

Just a small update to this thread - after some months off the road and a visit with a very friendly local family-run performance shop, we found that the MAF / air meter (the unit right above the airbox, at least on my 91) was faulty and sticking in the open position, and that my distributor cap had succumbed to the moisture/gunk buildup that Previa dist caps are notorious for. I had read about that problem on this very forum, but somehow I had failed to remember to check in there when I was going over things earlier in the year. I also failed to consider the forward air meter unit when I was going through my air intake checklist.

One cleaned distributor cap and a new air meter later and the van is alive, well, and happily back in service as my daily.

His tech handled the air meter diagnosis and replacement - after the fact, I wish I would've asked for more detail to share about the nature of the unit failure. I didn't wind up with the old unit to examine. I wonder if it was something that could've been repaired with some inspection and time on the bench, either as a spare or just for educational purposes. But the replacement was relatively inexpensive, so I understand why he went that direction.

I still have a little bit of fiddling with the throttle cable and shift cable to do, I still don't think I've hit the exact optimal positions, but I did re-calibrate my TPS and stop screw successfully. This video from the inimitable ToyotaTarago1992 YouTube channel helped me a lot there.

Anyway, I just wanted to tie this thread up in case anyone's reading in the future. Check on that distributor cap!