Sudden hesitation started today on my daily driver: 1996 4Runner, 3.4L, A340E 2WD, 280K miles. I've noticed a few times over the past several weeks it would sort of stutter occasionally, but I thought it was the fuel pump. Occasionally it won't crank and I have to lift the back seat, pop the cover, and tap on the top of the pump, which has worked consistently to get it to crank.

Today, I started up no problem and stopped for gas. Went to leave the station, and suddenly it sounds as though it's not firing on all cylinders, but I don't get the tell-tale exhaust smell of gas going over the catalytic converter. The check-engine light came on right away, but I haven't had a chance to run by the part store to scan the codes yet.

It is idling low at operating temp, about 450RPM. If I give it decent acceleration, the problem seems to go away. If I accelerate casually, it's quite rough and very sluggish feeling.

In the past when I had plug/wire issues, I could smell it (unignited gas going past the cat). So is it maybe the injector(s) got plugged from something in the fuel I just dumped in the tank? Normal operation with moderate acceleration makes me think no.

If I sit in park, and rev quickly, it stumbles. Slow rev is rough. Moderate rev seems normal.

I "tested" the throttle position sensor by unplugging it, and the truck shut off.

Any of you veteran T4R owners know what this is, before I dig further?

I drove about 50 miles like this today, and the problem didn't get any worse along the way. When I get up to highway speed, RPMs drop and runs rough if I ease off the pedal even slightly, but just barely accelerating seems to make it stop. ???

Stalling at low idle is normally the Idle Air Control valve, or IAC. It's under the throttle body and has a coolant line going through it. It can be cleaned and replaced with a new gasket, the air passage tends to get pretty gunked up.

If you suspect plugs and wires and you haven't replaced them in a while then just buy new ones. They are regular maintenance items. Go with NGK or DENSO double ground plugs, metal type doesn't matter. Wires should also be NGK or DENSO. These are the OEM parts.

Fuel pump failure is not common but very possible. However before you toss the pump, change the fuel filter as they can become clogged a restrict flow. It's located under the driver's side of the vehicle under a plastic cover if you still have one intact. Actually, 2WD 4Runners don't have a cover so it's probably exposed and easy to see. Be sure to use a brake line wrench on the compression fittings or they will strip and you'll never get them off. Doesn't have to be Toyota filter but don't go too cheap.

You nailed it. Went by the part store after church this morning to scan the code, and it was P0505 Idle Air Control. Took it off and cleaned it out with a little carb cleaner, a toothbrush and rags, put a TINY dab of oil on the moving parts then wiped it clean again... runs like normal again.



Thanks for the help!

Well, maybe I spoke too soon. Ran fine for about a day, then started with the same symptoms, same code: P0505 (the only code). So I took the IAC back off, cleaned it to the point it looks brand new inside. Verified that it opens and closes by applying 12v to each side (B+ in the middle), put it back on.

I changed the plug wires with NGK (cylinder #6 wire was bad) and put on a new air intake hose because they were both needed anyway. I checked the plugs, and they were all pretty clean (NGK two-prong type, as spec for this motor). No evidence of arcing on the coils or boots. It does run smoother, but if I reset the code it still pops up P0505 after about 10 sec.

It throws the same code, even if I reset the code and unplug the IAC. If I understand correctly, it shouldn't throw that code with the IAC unplugged (if it's the IAC at fault).

I did the carb cleaner spray test for vacuum leaks, ALL the lines, all over the intake manifold, and couldn't find any that affected the RPMs at all. PCV valve jiggles freely. Throttle body is solid, no wear or looseness, opens and closes easy and smooth, and no air gaps around the throttle plate.

What's strange is two days ago, I stopped and put $10 worth of gas in it, and for about 2 miles to my house, it ran fine! Then, the next morning, the same problem. I just tried again today, put $10 in the tank, no change... same problem.

Here's the symptoms:

Cold start, idles low ~750RPM, and creeps up to ~850RPM as its warming up, then settles in at operating temp, idling normal ~650RPM. Before I cleaned the IAC it was idling ~450RPM at operating temp.

In park or neutral, gently revs up to ~1300RPM fine, but then starts to stumble. If I give it a bit more throttle (not wide open, but not casual either), it seems to even out. It's almost as if it's misfiring, but only after a certain point opening the throttle, and P0505 is the only code I'm getting.

In drive, it seems like the hesitation and stumbling is only occurring when I give it gentle acceleration, no matter the speed. Is it possible the throttle position sensor TPS is at fault, even though I'm not getting a TPS code?

I've got a Wix fuel filter, plan to swap it out in the morning. Just thought I'd post again in case anyone has any more ideas, and hopefully we'll solve it and it may help someone else in the future. This is a weird one, and I haven't been able to find anything exactly like this in all my forum hunting so far.

It's due for a timing belt, but could a jumped timing belt cause low idle and P0505? It seems to rev flawlessly up until about 1300RPM. The fact this happened all of a sudden makes me think it's something electrical, but... I'm stumped at this point, and I don't want to throw too much money at it with 280K miles.

Thanks for any help you could offer!

You should be idling at around 1000 rpm's when cold and in park or neutral. I would suspect a bad idle air control even though you cleaned it.

I changed the TPS and fuel filter over the weekend, and no change in symptoms, so you may be 100% correct. I went to the pull-it yard, and not a single 5VZ-FE to be found.

The only code I'm getting is P0505 (idle air control), but I didn't think the IAC had anything to do with throttle aside from idle, which is why I've been chasing down the other things (and because a new IAC is $200+ lol)

If your IAC is stuck but the ecm thinks it's open then the air/fuel calculations will be off and that may be what you're seeing with the rpm related break up. Just a theory on that part. I always recommend trouble shooting/fixing what you have the code for first before moving on to other non-dtc issues. You never know what is being caused by what with vehicles.

Well, I bit the bullet and swapped out the IAC. Still the same problem, and the same code P0505 (IAC system)

Starting with codes reset, and engine cold (ambient temp about 80F)... cranks up sort of stumbling, then idles about 800rpm (instead of ~1300rpm, which is where it was before this issue). Throws the code in about 7 seconds.

While cold, if I'm in park and rev it up, it starts to stumble after about 1300 rpm.

At operating temp and driving, it stumbles if I gently accelerate or decelerate. It seems to even out if I give it a little more aggressive throttle (not wide open, just not casual acceleration).

At operating temp, if I'm in park and rev it up, there doesn't appear to be any stumbling. At operating temp, it idles about 700rpm.

When I say stumbling, it feels like it's missing and runs rough and drops about 300rpm.

To summarize what I've tried:
-Replaced wires with NGK
-Plugs all look good and are the NGK double-contact type
-No signs of arcing on the coils, swapping them around makes no difference
-Checked for vacuum leaks all over with carb cleaner (found none)
-Several different brands of gas treatment / injector cleaner
-Tried a different gas station
-Replaced the fuel filter
-Replaced the TPS (throttle position sensor)
-Replaced the IAC (idle air control valve)

Here's what's really burning my biscuits... every now and again, at random it runs fine. Like 4 or 5 times over the last 3 weeks since this started, if I've gone somewhere like the grocery store, church, whatever, I get back in and it will fine, like there's no problem. Shut it off, start it back up, and the problem is back. The problem started suddenly, out of the blue, after stopping for gas one time.

Is the coolant temp sensor part of the EFI system on this truck? That's where I'm leaning next, and then the fuel pump (I do have to lift the back seat, pull the pump cover, and tap on the fuel pump assembly occasionally to get started... which has been happening with more frequency lately... is it possible I'm just not getting enough fuel volume?)

I really don't want to trade it in for something new and a monthly payment. I'd rather get this annoying problem resolved and at least get it past 300K miles, as any good Toyota should

The engine coolant temp sensor for the ecm is an entirely different one from the engine coolant temp sensor for the instrument cluster. I believe the ecm engine coolant temp sensor is up top right behind the upper timing belt cover, but I've never gone searching for it so I may be remembering wrong.

Check iac plug and wiring harness to the ecu for for continuity. If it is all good then throw' in a Salvage yard ecu for your year in it.

You've got it right, it's tucked right near the fuel rails close to the knock sensor harness connector.

The ECT sensor on the 2nd Gen 4Runners used to cause all sorts of goofy issues so was a common suspect. I have not seen the same on the 5VZ-FE but it's fairly cheap and easy enough to replace so it may not be a bad idea to try.

That might be next. I got a scanner and hooked it up, and there doesn't appear to be any problem with the coolant temperature sensor. I'm heading to the salvage yard tomorrow to pick up a gas tank and pump assembly. When I bought this truck 4 years ago, I discovered a HUGE amount of rust in the tank... but I cleaned it out, changed the filter (twice), and haven't had any problems since. When I changed the fuel filter again last week, no signs of rust at all.

But since I've got to drop the tank to get the pump out anyway, and the old sending unit won't put the gauge any higher than about 1/3 of a tank, I'm just going to swap it all out and put in a good pump to rule that out.

If that doesn't fix it, I think the only part left in the chain of command is ECU.

I've ruled out:
-Coils
-Wires
-Plugs
-Vacuum
-MAF
-IAC
-TPS
-Fuel filter
-ECT

May be a slight possibility of a weak O2 sensor, but I'm waiting to hear back on a different post about the normal values for the downstream sensor.

Thanks for the help and insights! I really appreciate it.

Well, after a bit of a snafu with the fuel pump, it's not the fuel pump. Ended up installing it, only to discover there was a bad connection in the pump wiring harness but fixed now. So I've had the pleasure of pulling the tank out twice in the last week, without a lift... second time with half a tank of fuel. Fun times.

In trying to pin down what's going on, I decided to try unplugging the MAF just to see what would happen. Interestingly, although it idles rough, with the MAF unplugged, I get no stumbling when revving up past 1300RPM.

However, I get no MAF code, only a P0505 code... no other codes. Any thoughts or suggestions are welcome.

Well, after quite a bit more digging, I am pretty sure it's not the IAC valve, or even the fuel/air mix at all.

I hooked up the Torque app and noticed that my timing was going way off when I'm getting the stumbling. When running right, it was about 5-10 degrees positive (I presume before TDC), but when it started stumbling, the timing was 15-20 degrees negative (I presume after TDC).

Given the weird timing, I decided to research the crankshaft position sensor, since I read somewhere that it's part of the idle air control system calculations... turns out my crank position sensor isn't measuring within the specs. It's got zero resistance (open circuit), unless I run a magnet by it, then it gives some resistance. It's supposed to be 1.6K-2.7K ohms cold, or 2.1K-3.2K ohms hot.

Strangely enough, I don't have any code for the crank sensor, just the P0505 idle air control system malfunction. I don't seem to be having any issues with the RPMs being reported by the ECU either.

I have a Denso crank sensor on order and should be here in a day or two... I'll update with my results. LOL

Hi Tim
Did you find out what was the problem? I have the same issue with my rig and it’s driving me crazy.