jtalics

Here is a description of a subtle idle problem in my 5VX-FE 290k miles 2000 4Runner 3.4L. Essentially, there is a quick "stumble" in the RPMs at idle. The stumble is no longer than 1.0 seconds and consists of a drop in RPM and a quick recovery. At idle with the transmision in Park at full operating temperatures, the RPM is a super smooth 700 rpm idle, but once every few minutes there is soft stumble. In contrast, at idle with the transmission in Drive (with parking brake on) at full operating temperatures, the RRM is a smooth 600 rpm idle, but harder stumbles occur generally periodically every 10-20 seconds. These annoying stumbles are very noticeable when standing in traffic. They don't feel like the car is going to stall, but more like someone tapped or bumped the car slightly.

To solve this problem, I studied it as follows:
Purchased ELM327 Android OBD-II scan tool: Studied theory of sensors: and here: http://www.mr2.com/files/mr2/techinf...a/3vzVS5vz.pdf
as well as many other resources about things such as nominal sensor ranges.

Then I sat in my car and stared at the ODB-II raw sensor output for a half-hour as it warmed up, looking for patterns in the raw sensor data. All sensor values were within nominal values. MAF=5g/sec, IAT=79F, 0.0V<Oxygen>1.0V, Trim Short and Longs near 0%, TPS ok, timing advance 8-12 degrees. Also, compression on Cyl 2 is 150psi, and new spark plugs.

So far so good. Then the stumbling started. The RPM would drop to 525-550 for an instant but then pop back up. The only perfect correlation in the sensor data that I could see is the Timing Advance. The angle hovered tightly around 10+/-2 degrees but exactly at each stumble, then the angle would spike to 18 degrees and almost instantly drop to 6-7 degrees and then almost instantly recover to 10 degrees. All within 1.0 sec. The short term trim varied; I tried to see if it was correcting a lean vs. rich mixture, but there was no obvious pattern. No pattern detected in the oxygen sensor values or any other one... but that does not mean there is not one.

My "take" on the situation is as follows. The computer directly controls the Timing Advance angle. If the sensor values land in a bad parameter space, I am sure that the computer algorithm could adjust the Timing Advance if it wants to do so. But I am seeing that the raw sensor data looks ok! So I believe that this suggests that the RPM stumble is being cause NOT by the computer+sensors being faulty but rather something funny going on with the actual physics/chemistry of the engine. In other words, my ECM and sensors are working OK.

If I shift the gear back to Park and the RPM goes back up to 700, then the stumbles almost completely disappear. If I shift it back to Drive (with parking brake) the RPM drops to 600 and the generally periodic stumbles come back. it is reproduceable. What is it about applying a slight load to the engine that makes this happen? Vaccum? Drifting air/fuel mixture? Any ideas or hypotheses I can test? Thank you for your ideas.

Pacman88

I know that this is a month old, but you may still be fighting this.

I had an odd one like this that took about a months time to figure out at the dealership on an Avalon. During idle, the car would randomly feel like a miss and eventually shut down. What was happening is the canister was filled with liquid fuel (yes, it can actually happen) and when the purge valve would start purging, the engine would go richa nd stumble, pick it up an dlean out to run normally. No codes or anything funny in the data.

If it's not something like that, Id get the ol' ocilloscope out (we had the PicoScope but those are stupid expensive, try looking on eBay) and start tracing you fuel injectors, ignition, and CKP sensors. The OBD isn't going to be accurate or fast enough to see issues like this.

jtalics

Thank you for the help Pacman, the best engineers live in Dayton, and I mean that.

My 4Runner is driveable but I am still annoyed when it "bumps" hard after it is warmed up and I am at a stop light. I feels like the car behind me just tapped my bumper.

I think you may be on to something. As I stare at the OBD2 realtime, I notice that the air fuel mixture is deviating (not sure which way - lean or rich) then I get the stumble.
Pardon my ignorance, but what is the canister? Is it in the emission control system? How do I check it?
I already own an oscilloscope! I will research youtube on how to use it on a car diagnosis. (wow, cool!)

Here are additional observations.
1. Not temperature dependent.
2. I can reproduce a Check Engine light. Floor it going uphill, maxing out engine load. Throws P0300 codes, but I can' t feel it. Check Engine light eventually goest out. (Maybe a separate problem...)

Pacman88

Jtalics,

There are quite a great engineers/inventors out here and Dayton is rich in history. It's a little flat (transplant from Northern CA) but it really isn't a bad place to live and provide yourself a good quality of life.

Now, yes, the charcoal canister, right near you brake booster that holds the fuel vapor. Make sure it's not purging fuel/vapors when it's not supposed to. It'll usually throw a CEL, but in the case that I've seen, it didn't do anything. Overfilling your tank a lot can put liquid fuel in there. It's not an old wives tale, I've seen it first hand which caused the car to fail CA state smog check.

Another thing to look at is the TPS. On on my way to work right now so I can't reread your issues, but based on memory it said something like just off idle went away. There may be a TPS calibration issue at the idle spot in the sensor, like it's momentarily dropping off signal (where the o-scope comes in handy) .

And of course, check your basics. Make sure the wires are in good condition and not carbon tracking going down the boots. No cracked/loose/damaged spark plugs. What's the leak down on the cylinders? All things to verify before getting the o-scope out but may be necessary.

Hopefully this leads you in the right direction. I'll keep thinking on it! GL!

jtalics

Pacman, I am going to approach the canister/purge valve the following way, do you concur?

*** Block the vaccum line between the intake manifold and the purge valve. It might throw a CEL (Check Engine Light) but if my problem goes away, then I *know* it's the canister/purge valve assembly, right? In other words, the canister/purge valve does not need to be connected to have normal working engine... the canister is there just for the emissions, right?

For others, I found a good orientation video:
I'll check the TPS (Throttle Position Sensor) if the canister passes.

Thanks for the advice!

jtalics

For others with this problem, hope this help: Info on the canister system is on page DI-73 of FSF (Factory Service Manual) available in various places on the net. Here is one:
https://drive.google.com/file/d/0B4-...WNLpTf_ZldgUAw
I think another name for the purge valve is VSV (Vaccum Switch Valve)

Pacman88

Cap off the purge VSV (purge valve) where it plugs into the so you don't have a vacuum leak and run it. It will eventually cause a incorrect flow error which is self induced. If it goes away, you know that its in that system, yes. There are tests that can be done to check that VSV.

If you get to the TPS, the FSM should have the readings it's supposed to be. The O-scope can graph it and show any dips in the voltage.

jtalics

Pacman, you got me going in the right direction and I solved it!

I went to do that experiment we discussed above by blocking the vacuum line between the canister and intake manifold, and discovered the vac line was was disconnected. I reattached it and my problem went away. I am surprised that no CEL was thrown.

Thank you thank you thank you!!! I thought my admiration of Dayton engineers could not go higher but it just has!

Looking forward to having my 4Runner in perfect conditon at 300,000 miles, just 10k away!! Well except for that oil leak....