DWNSHFT New Reader
Aug. 6, 2008 2:39 p.m.

I'm a terrible diagnostician so please help:

I have a 1999 Toyota Tacoma 4X4, five speed, canadian market with 152,xxx miles. It has developed a bad stumble/misfire. It happens under load at lower RPMs. For example, at 2000 RPMs and half throttle it bucks like a horse. There has also been a general reduction in power. Mileage has not been affected. If it matters, I short-shift a ton to try to improve mileage (I get 19-20 MPG). At light throttle it runs fine, and more revs helps the misfire go away.

Any and all help and suggestions is very much appreciated! Thank you!


Dr. Hess SuperDork
Aug. 6, 2008 2:41 p.m.

Gonna guess here: Intake leak or a bad sensor. Check for leaks first, then pull the codes from the motor (cheap OBDII readers are even at wally world and HF now.)

John Brown SuperDork
Aug. 6, 2008 2:42 p.m.

Throttle position sensor or Air Meter.


SupraWes HalfDork
Aug. 6, 2008 4:17 p.m.

Bad fuel injector(s) or clogged EGR system is my guess.

Jensenman SuperDork
Aug. 6, 2008 4:51 p.m.

I'm gonna go with ignition. First place I would look: pull the plugs and look for what has the appearance of a crack on the porcelain, but you can't catch your thumbnail in it. That's called carbon tracking and it looks sorta like a thin black lightning bolt. This is a cracked spark plug, but carbon tracking looks very similar.

If you see that, replace the wires and plugs as a set. If you don't replace both at the same time, the carbon tracks inside the old plug wire boots will make new ones on the new plugs and you are back where you started.

Also check the inside of the cap and the rotor for similar lightning bolts, they are usually gray or white and are the same thing as the spark plug carbon tracking.

Nashco Dork
Aug. 6, 2008 5:15 p.m.

If you haven't done plugs and wires (either at all, or if you have but used cheap aftermarket stuff and it's been a while), I'd start there as well. It could very well be something else, but most people forget these are wear items and tend to overlook them when diagnosing stuff and doing regular maintenance.


Aug. 6, 2008 5:16 p.m.

that new a car without coil-on-plug ignition?

Jensenman SuperDork
Aug. 6, 2008 6:58 p.m.

You know, it could be C-O-P. If so, the carbon tracking thing still holds true. It means that if there's carbon tracking he'd need to replace the coil and/or that spark plug boot if available separately from the coil.

FWIW, a quick search shows plug wire sets being available for that truck with a 4 banger.

Nashco Dork
Aug. 6, 2008 7:24 p.m.
belteshazzar wrote: that new a car without coil-on-plug ignition?

It's a decade old econotruck...you expect COP? What trucks that old have COP? I'm genuinely interested, I can't think of any off the top of my head.


DWNSHFT New Reader
Aug. 6, 2008 8:58 p.m.

I'll check and let you know. I don't think there's a distributor, though.


m4ff3w Dork
Aug. 6, 2008 9:20 p.m.
Nashco wrote:
belteshazzar wrote: that new a car without coil-on-plug ignition?

It's a decade old econotruck...you expect COP? What trucks that old have COP? I'm genuinely interested, I can't think of any off the top of my head.


95 Toyota T-100 with the V6 did.

dan_efi New Reader
Aug. 6, 2008 9:25 p.m.
Nashco wrote:
belteshazzar wrote: that new a car without coil-on-plug ignition?

It's a decade old econotruck...you expect COP? What trucks that old have COP? I'm genuinely interested, I can't think of any off the top of my head.


Any Ford F-series trucks from 97+ with the 5.4L will have COP.

Back to the misfiring. Anytime I've had misfiring, it's always been the plug wire boot. Watch at night while you give the engine a WOT rev, you'll probably see sparks down near the plugs or around the dizzy cap. Temp solution is to wrap electrical tape very tightly around the boot, and put a bit of dielectric grease around the ribbed part of the spark plug.

RussellH New Reader
Aug. 6, 2008 11:36 p.m.

I'm with those that said it's ignition related; igniter, C-o-P or wires, coil etc.

Nashco Dork
Aug. 7, 2008 3:08 a.m.
dan_efi wrote: Any Ford F-series trucks from 97+ with the 5.4L will have COP.

That would make sense I guess, I forgot about those. I knew the older Ford V10 had them, but obviously not an "econo truck" if it's got a V10...however, the older 5.4 F150 would be more in econotruck terrority.

m4ff3w wrote: 95 Toyota T-100 with the V6 did.

I thought those were the 3 spark plug wires for a V6 jobbies...no? Toyota has been silly like that on a bunch of their V6 engines.


Jensenman SuperDork
Aug. 7, 2008 6:35 a.m.

In 1995, the '95 1/2' Rodeo came with C-O-P ignition. So I suppose the 'Yota could have as well.

wreckerboy SuperDork
Aug. 7, 2008 6:48 a.m.

All good points, but unless I missed it the OP hasn't told us about a baseline on the truck. We have no idea what state of tune the thing is in, or what condition the plugs and ignition system and even the air and fuel filters. Before you can go chasing sensors and metering devices you have to make sure the basics are in place and serviceable.

spitfirebill HalfDork
Aug. 7, 2008 7:50 a.m.

99 4runner with 3.4 V6 is COP

Dr. Hess SuperDork
Aug. 7, 2008 8:41 a.m.

The V6 will have CoP, the 4 cylinder will have waste spark.

iceracer New Reader
Aug. 7, 2008 9:30 a.m.

As has been said. Do the basics first, Whether or not it has COP is kind of a moot point and is very easy to identify.

SoloSonett New Reader
Aug. 7, 2008 1:58 p.m.

my '93 4Runner started that. Turned out to be fuel issues. Clogged filter, low pressure fuel pump..and after F$^*k'n with it; a cracked fuel line comin outta the tank.

I got Real good at dropping the tank!

But after all that , it runs like a champ now. 150XXX miles and towin strong

DWNSHFT New Reader
Aug. 7, 2008 3:33 p.m.

I have good news! I think I fixed it. Here's how:

First, I pulled the Chiltons manual. That didn't help a whole lot. The PCV valve tested good. After about thirty minutes of head-scratching, I found (what I think is) the EGR valve (thanks, Chiltons) , but it looked a PITA to get to so I let it lie for now. I started pulling spark plugs. This V-6 is a distributor-less ignition (coil-on-plug) with a twist: it has only three coil packs, with spark plug wires running to the other three spark plugs and utilizes a wasted-spark set-up. OK, fine. I had to pull a few things to get them out of the way, but getting to the plugs is WAAAY EASIER than on my old SBC. Wow, am I glad I sold the Suburban! LOL

Plugs had very little torque on them; three were snugged tight (barely), two were marginally finger tight and one was not even snugged. You know those little metal doodads that screw onto the threaded portion on the top of the spark plug? Three of those were loose, one had a little corrosion and one had a lot of corossion. Note, all the worst examples were on the driver's side cylinder bank. That's the side with spark plug wires and traditional spark plug boots, which snapped on with some positive feel but then could wiggle up and down some. Andthe worst corroded doodad was on the loosest spark plug. Hmmm.

None of this looked conclusive to me, but then, I dont' trust ignition systems for nuthin'. I replaced the two corroded doodads with new ones, tightened all six doodads, then goobered them with dielectric grease. Torqued the plugs about 1/8 turn past snug (Chilton's said 11-15 lb-ft if no crush washer, which these plugs had) and reassembled. Also, Chilton's was showing a dual-electrode plug for the V-6 and mine are single electrode Bosch platinum. Hmmm.

I had minimal hopes that it was fixed. Air filter was a little dirty so I figured I'd replace it just to eliminate it as a possibility. I was thinking I'm going to have to dig back to the EGR valve. So I hop in the truck to drive to NAPA to get the new air filter and ask to look at an EGR valve so I hopefully take apart the right thing and I can feel it just off idle, it feels better: more responsive and better torque. The misfire had been worst when lugging the engine so I lug it in second, third and fourth gears and it feels fine! It was just around the block so I'll take a little longer drive later today but I think it's fixed.

So, what was the real culprit? Ignition, obviously, but what specifically? You tell me. I think the plugs weren't torqued enough but I think the real problem was the corrosion on the spark plug doodads. What caused the corrosion? Bad connection, I guess, but one would think that the spark plug wire to spark plug doodad connection would be solid, since it's engineered to be a push-on connection. But like I said, I didn't trust it. Maybe just cleaning up the doodad and adding dielectric grease would have been enough but I just happened to have a few spare plugs for the shifter kart so I liberated a couple doodads for the truck.

Thanks to all who offered suggestions. I was thinking PCV or EGR but all you who focused on the ignition were spot-on. Way to go!

I love GRM!

David, Happy in Phoenix

PS - too all of you in the rest of the country, yes, I was forced to work on my truck in August in Phoenix! Be glad you're somewhere else right now where it isn't 101 degrees with a "real feel" of 110.

spitfirebill HalfDork
Aug. 7, 2008 3:47 p.m.

There is nothing better that fixing something by goobering your doodad.

Jensenman SuperDork
Aug. 7, 2008 4:26 p.m.

Yessir, getting my doodads goobered always relaxes me.

I'm gonna say high resistance from the little nut (the 'doodad') being loose on the plug combined with corrosion.

FWIW, I have * never * had good results with Bosch platinums. I have seen them induce cold start problems, it even happened to me once. I have used probably every other kind of platinum available and they have been fine. The regular Bosch 'Supers' have never given me a problem. I have a set in the J-H.

DWNSHFT New Reader
Aug. 8, 2008 10:29 a.m.

Well, I celebrated too soon. The misfire is still there, although it is somewhat improved. Today I will replace the air filter, but I'm thinking either EGR valve or fuel delivery; maybe a plugged fuel filter?

Also, it's OBDII (1999) so why isn't it throwing a check engine light? I'd gladly invest in a scan tool if the CEL would come on. What do you think?

Anyone have any ideas? How about a way to determine the real problem? I don't want to just start throwing parts at it.



jwx New Reader
Aug. 8, 2008 11:35 a.m.

Does the CEL ever come on? like with the key on but engine off?

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