Erratic Timing

[clunker]

I am stumped. Trying to set ignition timing, but the mark keeps dancing around. I thought it might be the timing light, so I hooked up my meter, and the RPMs jump around a bit too. If I clamp the sensor near the distributor, it seems to be more consistent, but if I clamp near the plugs, it jumps a lot. In fact, for some reason, the meter thinks the #5 plug is at 200 higher RPMs than #1.

New distributor
Recent NGK plugs and wires
New cap and rotor
Recent TPS

The symptoms suggest that wires are crossed, but they are at stock separation in most places and even further apart everywhere else. Could there be an issue if one of the wires is too close to a grounded surface? Any other suggestions would be appreciated.

 

[nicnmike]

how does it run if you give it some gas?
Have you checked for vacuum leaks?
How old is the coil?

 

[flyinhawaiian01]

Are you using a dwell/tach gauge to check rpms per cylinder at the plugs?

Is your timing light a decent one and not one of those harbor freight lights?

Are you shorting the diagnostics plug while checking your timing?

Is your idle set at the correct rpm when testing?

Do you have any codes?


When you say "crossed", I assume you're meaning "crossing over one another". You can use a bunch of zip ties to keep them way apart from one another if you suspect the spark is jumping wires. You can also just start the truck at night, or in a very dark place, open the hood and watch the wires closely, you will be able to see the arcing if that's what's happening. You'll also get some good corona burns where that's happening, most notably around the spark plug's ceramic area.

 

[clunker]

Thanks for the input guys. Maybe a little history is in order. The 4Runner has had a hesitation issue since I bought it. It feels like someone is tapping on the brakes while you try to accelerate. Based on research and forum input, I have replaced almost everything (Fuel filter, fuel pump & filter, TPS, plugs, wires, cap, rotor, timing belt & bearings, water pump, distributor, valve shims, PCVs, intake gaskets, EGR valve, etc.). I raised an eyebrow when I saw the erratic RPMs.

The engine idles smooth with only the occasional misfire. It revs fine other than the hesitation when accelerating. The ECU has never thrown a single code even though the TPS was way out of spec.

I completely forgot to jumper the diagnostics connectors, but all that procedure does is prevent the computer from retarding the timing. Still worth seeing what happens. I am in fact using a dwell/tach meter (it is a decent quality old-school diagnostics unit). The timing light was pretty cheap from O'Reilly, but I have not had issues using it on other cars.

I'd doubt the coil has ever been replaced at 172K. The PO did very little to maintain this thing. I have tested for vacuum leaks but only found two that made zero difference when corrected. Idle is a tad high (900-1000), but I was waiting to reset it after the other bugs have been worked out.

The plug wires are not touching each other anywhere. At least 1/4" separation. There are no splits in the jackets or boots. The wires are one of the first things I replaced, so they have been removed many times. Could that cause a problem with connection to the plugs?

One more thing. This engine overheated before I bought it. Some of the wiring split loom was crispy as potato chips, and I would not be surprised if the knock sensor wiring harness is cooked. However, I have driven it with the diagnostics jumper in place, and it made zero difference.

 

[flyinhawaiian01]

Okay, there's a lot of things here that we could look at, now that we have more info on the setup.

One thing that sticks out here though, is the fact that timing without the jumper was done. It must be jumped as the ecu advances timing right off the bat, and that includes at idle. You cannot set the idle properly without the jumper in place!

Also, the idle must be set properly for timing to be adjusted correctly. You'll need to set the idle to the correct spec depending on whether its manual or auto. At 900-1000rpm idle, the engine will sometimes "hunt", meaning the idle will go up and down slightly, which could be causing the timing to jump around slightly, although I doubt that's what's causing it.

If the engine's been overheated before, I'd go ahead and do a quick compression/leakdown test to make sure you have no bad cylinders. I don't know if this is a 4 or 6 cylinder, hopefully its the 4, as they are much easier to repair, as well as cheaper. Judging by your comment about the knock sensor, I'm going to assume its the 6 cylinder.

Which brings me to the next thing. I'm sure you've read up on the issues with the knock sensor wiring on the 3VZE, so I won't cover all of that, but suffice to say, that could also be one of the issues. I wouldn't get to that right away though, work through the easier stuff first.

And as for a coil, I doubt its that, but it is possible. Weak spark can be caused by a coil going out, but after 240 something thousand miles on my truck, I still have the same coil and have beat on my truck in cold winters and desert heat of SoCal, and several years in Hawaii with the humidity, salt air and heat there too. Never been replaced and its still going strong, so to speak!

Now, the final thing for me is the fact that driving with the diagnostics jumper in place changes nothing on your vehicle! That right there tells me something is wrong, you should notice:
1. lower idle when its jumped
2. lower power when throttling up or taking off from a stop
3. with foot off the gas pedal, check engine light should be blinking

When you jump it, do you get any codes at all? Are you at least getting a steady and equal quick on-off-on-off flashing? If not, your tps is not adjusted correctly. Sometimes touching the throttle ever so slightly will get it to flash, that means your tps is adjusted too far one way and it needs to go the other way. Don't recall which way it needs to go, only that it should turn slightly more in the same direction as your throttle plate when it opens.

One other thing here. You mention the surging at speeds, if this is an auto trans, there's a completely different set for getting codes from the trans and I remember reading something about surging with an auto there were codes set to the auto ecu or something. You'll have to search for it though, unless someone remembers where that thread is.