GSGALLANT

Hi there,

My 2002 4Runner, 3.4L, V6, Auto, developped a slight surge under load today. Here are the specifics:

1. It only happens between 2000 and 2700 rpm. Above and below that range, it runs fine.

2. It only happens when the engine is loaded, like during acceleration on a flat road, or while going uphill (it is worse while going uphill).

3. It is a very slight cyclical surge (rpm goes up-down-up-down-up-down repeatedly by a couple hundred rpm at a frequency of approximately 1 Hz - or 1 pulse per second) anytime I'm in the situations from #1 and #2 above. Speed does not seem to be a factor... just rpm range. (It's like I'm pressing down slighly on the accelerator, then releasing, over and over.)

4. It does not surge with the tranny in P or N (in this case, it idles very well throughout the whole rpm range).

5. Turning off O/D or turning on ECT on the tranny has no effect on the surging, also it does not matter what gear the tranny is in.

6. Engine temperature does not seem to have an effect on the surging.

There are no codes stored in the system. Here is what I've tried to date:

1. Cleaned the MAF.
2. Checked PCV. Air comes out (vacuum on hose), but does not go in (pressurize hose). OK.
3. Tested coil packs and plug wires as per FSM. OK.
4. Pulled and checked all spark plugs (all cylinders look like they are firing fine).
5. Ohmed out crankshaft position sensor as per FSM. OK.
6. Ohmed out TPS and pedal position sensor as per FSM. OK.
7. Checked inside of throttle body. Nice and clean.
8. Checked for vacuum leaks as best as I could (inspected hoses from engine compartment right back to charcoal cannister, and sprayed TB cleaner all around engine compartment while engine was running).
9. Inspected exhaust. No signs of any leaks.

The TPS and pedal position sensor tests as per FSM were very basic. Just a resistance check across two of the terminals with throttle closed. I haven't found a procedure in the FSM that takes readings during a throttle plate sweep like I've seen before for earlier models. Not sure if there is anything like that available for the 2002 fly-by-wire system.

At this point, I've run out of ideas. I haven't tried swapping out the O2 sensors, because I don't have any spares (it would take a few days for them to come in after ordering). Anything you can suggest to try would be appreciated.

GSGALLANT

So a bit more info. I tested the truck out this morning after it sat all night. First two to three minutes of driving, the problem was not there. After three minutes, problem is back, so I'm thinking the problem is due to fuel management in closed loop. To test it, I got the truck up to normal temperature (problem was there), then disconnected the A/F sensor, and took it for a drive. Check engine light came on, of course, but the surging problem was not there. I cleared the code, re-connected the A/F sensor, took it for a drive, and the problem had returned.

My diagnosis seems to make sense, but now to confirm which sensor is causing the trouble. I'm thinking A/F sensor at this point, however, I replaced it two and a half years ago, at 115000 miles, with a new OEM one from Dallas Toyota. Two and a half years later, and now I have 140000 miles. I suppose it could have gone bad after 25000 miles...

I'm assuming that by disconnecting the A/F sensor, it forces the truck to go into open loop, meaning that the problem could actually be with any of the sensors that come into play for fuel management in closed loop. Does anybody have a complete list of all those sensors?

skjos

You are awesome at explaining the situation and tracking down the root cause of your problem.
I have no information, I just wanted to say nice post. I hope someone can help you out with the information you need.

vital22re

iTrader: (2)

first thing that comes to mind is a loose crank pulley. it throws the timing around. Those crank bolts are known to wander off.

GSGALLANT

Thanks skjos. I appreciate the compliment.

vital22re: I had thought about the crank pulley as well, but no wobbling, and the bolt is still tight. When I did the timing belt a little while back, I torqued the crank bolt to spec, with blue loctite.

Anyways, I've got it figured out. I had saved the old A/F sensor from when I changed it 2.5 years ago. It was still working fine when I changed it, but my gas mileage had gone down a bit, so I changed it as preventive maintenance to try to regain the couple of MPG that I had lost.

Yesterday, I picked up a new sensor gasket and nuts, and installed my old A/F sensor back in the truck. It runs like new again, so that was the problem. I'll see what the mileage is like, and decide if it's worth picking up a new one now for $160 + shipping. I'll have to check back in my mileage database, but if I recall correctly, the new sensor that I installed a couple of years back hadn't improved the mileage very much.

I knew the ECM was smart enough to compensate for sensors that are starting to foul slightly, but I always thought that the ECM would throw a code for the A/F sensor before one would notice any drivability problems on the vehicle. I guess not in every case.

Champion

I had a bit of a similar problem when I first bought my 4runner. It would buck and accelerate when I hit the gas exactly in the situations you described. Turned out to be the O2 sensor. Glad you got it figured out.

runnermedic

maybe same problem here,mine is 98 limited with 4.56 gears no cel light,the only issue is that im not able to read codes because the fuel pump keeps priming and not turning off in a few seconds,well i figured out testing with another ecm and the fuel pump keeps priming,so maybe the fuel pump is not making enough pressure. the surging is present only going uphill around 2000-2500 rpms and take off seems to be a little sluggish sometimes. could it be possible that the fuel pump is going south? the PO installed a 4 cylinder fuel pump assembly (a tacoma one for sure fuel sender is different)

maybe the 4cyl fuel pump is weaker?
clogged cat converter?
maf cleaned
tb cleaned
no vac leaks
injectors ok
wiring and relays ok
new plugs and wires (oem)
seafoam treated twice
new fuel filter
coils ok