coryc85

jstluise - There's a lot of information here that I only skimmed, so I might not have it all quite right, but here's one theory that points to your IACV...I think it could be stuck open. So my theory is that is got stuck open, which was letting in extra uncompensated air at engine warm. You played around with some things, installed the new plugs, etc and set your idle warm (and at this point the extra air was coming in). Then the next morning, you blocked off the extra air, and now at cold, the engine won't hardly run. At cold, the cold start injector is dumping extra fuel, so you really need some more air. At warm, the cold start injector is off, so you were running fine again. I personally think you should pull the IACV off there and test it. Throw it in the freezer for a bit and it should open up, stick near a heater and it should close up. You can look thru the hole and see that happen.

I'm assuming too that this is the older style engine with the separate IACV...the newer style have it integrated into the Throttle body I believe.

Anyway, it makes sense as I type it, but there may be pieces of the story that don't quite fit.

jstluise

Thanks for the response, Cory. It is obvious that the IACV is causing issues...haven't quite figured out why but since removing it from the system the engine is running better...almost normal. One issue with your theory:

The cold start injector is only active when the key is in the START position. As soon as your are done cranking, the cold start injector stops dumping fuel (you may see threads that state otherwise, but it is obvious if you look at the wiring diagram). The reason that I have problems starting/running cold (with the IACV removed) is because that is precisely the job of the IACV: to let more air in to raise the idle when cold. Currently my foot has replaced the job of the IACV and I just have to keep on the throttle to manually raise the idle until it is warm.

Once the engine gets to operating temp, the truck idles fine because no extra air is needed from the IACV. And in my current configuration (IACV removed) it is as if the IACV is closed like it should be.

I believe before that the IACV was working intermittently. Like I said, I could go days without any surging and everything seemed normal. Then other days it would surge like hell. However, I still haven't quite figured out how the IACV could cause surging and stalling around idle (usually with a bad IACV the surging is well above idle speed).

Anyways, I plan to tear into everything this weekend and check out the IACV. Hopefully I'll have good things to report. Otherwise, if the IACV seems to be operating properly and I am still having issues I'll have to look into the temp sending unit, the AFM, or possibly the EGR system (an EGR valve opening and closing for whatever reason can cause stalling).

***Oh, and this is the version with the separate IACV. It wasn't until later ('90?) that they switched over to the integrated IACV.

Odin

I decided to investigate my TPS a little today. I didn't touch it with the multimeter though. I found the Idle Stop Screw was adjusted pretty far off and was holding the throttle plate open too far throwing the TPS off. After I backed it off it gained alot of bottom end and after a short drive it felt like it started to gain power almost everywhere in the rpm range, even the idle started to clean up a little with less vibration and random puff's.

It sure felt better but it didn't last. After about a 15 mile drive it started to feel somewhat like before. I think if it were bad it wouldn't have made that amazing recovery so I'm thinking maybe the TPS is sticking and needs to be removed and cleaned.

At least I didn't purchase a bunch of unnecessary parts.

92ehatch

Is easy to check this. I even made a quick write up on how i fixed mine.

https://www.yotatech.com/forums/f116...e2-fix-286302/

jstluise

Odin, definitely explore your TPS more to make sure it is adjusted and functioning correctly. Follow the link that 92ehatch posted (specifically the one posted on 4crawler). Once everything looks good with the TPS, then you can move onto the next problem, if it exists.

I tore into my engine a bit today. Removed the TB and the IACV. First, I checked out all the ports in the TB and upper plenum. All were clear and free flowing, but I still cleaned all the ports in the TB. Also, the coolant lines were all free and clear, specifically the port coming off near the water neck that people report getting plugged up.

I opened up the IACV and it was pretty damn clean inside. When it was cold (40F) the rotor was turned to let air through. I let it soak in my oven at 190F for a bit and checked it; the rotor turned to close off the air but not as much I was thought. Here are a couple pictures:

Cold (40F):


Hot (190F):


I haven't done any research on the IACV as far as how much it is suppose to turn. But regardless, I don't think it is my problem (explained below). I ran out of time today to work on it, so I completely removed the IACV by bypassing the coolant line (running a coolant line directly from the TB to the block) and blocking off the ports on the TB and upper plenum. At least I can drive it at the expense of difficult starting while cold.

I removed the temperature sending unit and tested it. Heated it up in water from 40F to 190F while monitoring resistance and it stayed within the acceptable range specified in the FSM. I don't think it is my issue.

Driving it today after removing the IACV I was still experiencing the RPMs dropping below idle before recovering when I let off the throttle. I also, a few times I still noticed surging. For example, I was idling at a light and I gave it a rev to see how the throttle dropped. It dropped and then recovered, but then started to surge like before.

So I'm confident my problem isn't with the IACV. Though it may not be operating correctly I proved it is not the issue by completely eliminating it from the system. The surging almost seems like a vacuum leak that comes and goes, stalling the engine when it is present. Other sources of a vacuum leak would be the EGR system, brake booster, power steering, and PCV system. The PCV valved checked out fine and the problem occurs regardless of brakes being on or off so I don't think it is the brake booster.

I still have the AFM that didn't check out, so it could be that. I guess I could throw a new AFM on it and see what happens. Otherwise, next I think I would look at the EGR valve...maybe it is pulsing for some reason causing the idle to fall and rise.

Odin

That's the write up I was following because everyone said it's about the best they've seen.

At least you've eliminated one of the things that could be causing your surging problem. I'll enjoy following your process, always interesting to see problems resolved.

jstluise

I was thinking about it some more last night and I'm thinking it is either the AFM or the EGR system. Since I got a code for the AFM and it didn't pass one of the resistance tests, I'm banking on it being the AFM.

For completeness, the idle can be raised by directing air from a monitored source (intake, downstream of the AFM, upstream of the TB) into the upper plenum (downstream of TB). This essentially leans out the mixture, but the AFM senses the increases air flow and the ECU compensates by introducing more fuel, thus raising the idle. This is how the IACV and the power steering air ACV raise the idle; the valves are placed between the intake (before TB) and the upper plenum. When the valves open, a monitored vacuum leak is present and the idle is raised how I explained before.

So, there are two ways I can see to get my stalling/surging idle: one would be the EGR valve opening (this is essentially a unmonitored vacuum leak) or the AFM isn't correctly sensing the air flow and the ECU can't get the right mixture at idle, which will cause surging/stalling.

I'm going to double check my AFM again but I think in the end I'll be getting a new sensor. Depending on what test fails I might be able to adjust it...we'll see. For more information on the AFM and its theory of operation, see: http://www.4crawler.com/4x4/CheapTricks/AFM/index.shtml

coryc85

I think you've done a very thorough job of troubleshooting so far. I guess my theory about the IACV was not the problem.

I guess since the AFM has failed a test, that is a strong indication of where the trouble may be. Keep us posted, I think you are close.

bswarm

Did you try adjusting the trim screw at the AFM yet?

jstluise

Not yet. I'm going to pull it off tonight and do a more thorough job of testing it.

bswarm

If you've ruled out vacuum leaks, then remove the little silver plug on top of the AFM and adjust the screw under it. It works wonders, especially if it's still factory sealed. Think of it as an idle mixture screw on a carb.

jstluise

Yeah, I was reading about that. We'll see what test fails (I can't remember from before) and if it can be brought back into spec with the adjustment screw. Better than dropping $140 on a new AFM!

bswarm

Our shop used to charge 20 minutes labor to remove the seal and adjust, on top of diagnostic time. With a drill and small slide hammer with a screw tip it takes about 2 minutes to unseal and adjust.

Odin

You mentioned you suspected the EGR a couple times.
I generally hear people complain about pinging when EGR problems arise. Had you heard any pinging?

jstluise

No pinging. The only reason I mentioned the EGR is that is the only thing that I haven't tested and I know if the EGR valve opens too far (especially at idle) it can cause the engine to stall. However, at idle there might not be enough vacuum to even operate the EGR, let alone cause it to open and close making it surge. At this point I would put money on the AFM rather than the EGR.

bswarm

The EGR should never open at idle. If it does then the EGR is stuck open or the EGR control valve is bad. There should be 0 vacuum at the EGR vacuum hose at idle.

jstluise

Right. I didn't think about this much until my last post. And even if it did open from a bad control valve I would expect it to stall the engine and kill it, rather than have any sort of surging.

bswarm, do you have any information on the IACV as far as adjusting it and/or how much it should be opening/closing at different temperatures? The FSM doesn't offer anything. Figured while I have it off I should make sure it is adjusted correctly.

bswarm

No, sorry. I never had one go bad on a Toyota in the shop. The GM ones frequently used to go bad but that's a different flavor.

jstluise

I couldn't anything online about adjusting it, either. Seems like you really only need to adjust it if the cold idle is super high or you can't get the idle down using the idle air adjustment screw after everything has warmed up (granted, that is assuming everything else is working correctly).

Mine definitely turns when warmed up, I was just curious how much is necessary and if air should only be flowing through the small hole when it is warm, whereas right now I still get air flowing through the larger opening (see my 190F image).

I'll keep my IACV off right now until I fix the AFM problem. Once everything is operating nominally then I'll add the IACV back in to see what happens.

bswarm

I had one on a Cadillac that operates similar to the Toyota, as the engine warms up it closes. This one wouldn't open at all when cold, so it would stall in drive unless you held the throttle open until it warmed up. If there's no spec in the FSM for how far it's supposed to open or close, then it's probably a "it works unless it doesn't" test.