Daemon

I've spent some time searching through "bucking" and "misfire" threads
and haven't find anything which quite matches my symptoms.

It's a '95 4runner with a '98 3.4L V6.
All emissions gear in place, OBDII sensors working fine.
When I bought it a couple years ago it pulled like a freight train
at all rpms all speeds, and would pull from a stop in 2nd gear like.. well, like a truck.

In the last few months it started to buck at low rpms (idling along in
2nd, get on it, and it bucks hard). If I get on it hard at WOT
in any low gear, it bogs and sputters through the mid
rpms 2500-3500, then pulls strong again above 3500.

Because it still pulls hard at high rpms and high speeds, I don't
think it's a fuel pump or fuel filter problem as that should hurt
most at high rpms. Same with TPS. WOT works fine at high rpms
and not mid.
It idles perfectly. It threw a purge valve code a while
back, so I replaced that, and it hasn't changed anything.
Not currently throwing any codes, even after it's been bogging
repeatedly.

I've replaced the plugs and it helped for a while, then got worse
than ever. Coils or wires seem a possibility, but not clear why
that'd only cause a problem at WOT mid rpms and not high, and why
I wouldn't get any misfire codes. Anything else obvious I'm overlooking?
I suppose MAF could do this.

ian

1990yota-pickup

check the disburator, for corrison in it. check the gap in disburator with a brass feeler gauge. also check your fuel pressure. make sure the fuel tank is not sucking air into the tank. it would cause rough rpm
change your fuel filter if it had never been changed.
check you air filter.. a clogged air filter is a engines worst enemy.
thrown a timing light on the engine and see what is going on..

Daemon

'98 motor is electronic ignition. No distributor and electronic timing.
See above for why I don't think it's a fueling issue. Air filter
is new, but if it were clogged, would limit my top end power,
which is not happening.

BTW one other odd symptom. It almost always runs
perfectly for about 30 seconds after I start it up even on a warm re-start.
Pulls hard from idle, doesn't bog. By the time I've made it to the end of the
block though, it's sputtering and bucking again.

ian

1990yota-pickup

check to make sure you dont have a cracked or broken vacum line... spary some contact cleaner on the engine while its running if it revs up More than likly its a lose or broke vacum line...

you may have a a clogged egr valve too.

make sure you are getting good flow thorugh the exhaust system a clogged cat convert can cause some bad stuff.

check the throttle position sensor too, do a ohm's test on it..

Daemon

I'm not finding any vacuum leaks and it idles perfectly.

I've ruled out the MAF and TPS by replacing them.
They've actually made the problem even more noticeable because
at the rpms where it was ok before (high rpms or high gears at full
throttle) it now is smoother and pulls harder, but in low gears, low/mid
rpms it just keeps bucking when I get on the gas. 2nd gear, 1500 rpms,
only have to touch the throttle to make it buck, where it used to
just *go*. The fact that it runs great at high rpms and full throttle
should rule out stuff like clogged injectors, fuel filter, or cat, all of which
normally kill the top end. I've got the code reader plugged in all the
time and check it often and it has yet to throw a single code. I see a
lot of misfire threads here, and they all seem to involve throwing actual
misfire codes. I'm mystified and frustrated.

I'll check the EGR next then go after coils and wires (which should
be causing misfires if they're bad).

ian

1990yota-pickup

just start replacing evrything.. its the parts changer way of fixxing

brian2sun

Daemon, Check your oil and coolant for milkshake. I would go ahead and test your coil packs, pull the new plugs and have a look - see if they are wet or anything.

1990yota-pickup

i think i would have just paided thee 75.00 by now to have toyota figure out what was wrong with.. and go from there... then fix it yourself!

Daemon

With it not throwing any codes, it's not clear how the dealer
would be able to make a definitive diagnosis as easily as you imply.
I've also had poor luck with dealers in the recent past. Last year, I
brought a recently purchased used vehicle in to a dealer, and said
"It developed a throttle tip-in rattle a couple hundred miles after
I bought it. Sounds like rod knock to me. What do you think? "
They said, "That sound is normal, ignore it."
Rod was spouting from the side of the block less than 1500 miles later.
Bit too late to go back to seller at that point.

My intention is not to just replace everything. That's why I'm here,
but it sounds like my problem is more or less unique.
I did the MAF and TPS because they were offered to me wholesale
and the MAF was probably due anyway.

I'll check plugs/wires/coils next.

ian

Ocean Racer

This is a Carb motor?

If its fuel injected, like mine, it could be a similar problem that I just had, Check the Ignition system, plugs, plug wires, coils, etc.

If it has a Distributer, replace the cap and so forth, could be moisture in there that I have seen cause those symtoms.

OH and if what the dealer says is true, you MUST use Genuine Toyota OEM parts on the ignition side. I think this is a longivity issue, but I can't be for sure. Unlike most plug wires and plugs on cars/trucks, they ones on the 3.4l actually seem to go bad from time to time. Its rare that this happens on anything else.

1990yota-pickup

did u see if you had a bad plug wire.. plug one off at a time while the engine is running .. it should let you figure out where the problem is

MAIS0N

My 98 Tacoma recently started doing the same thing. Feels like pre-ignition. It can buck pretty hard like it's going to break a rod. I was also getting codes occasionally for the knock sensor circuit. I replaced the knock sensors & wire harness but didn't fix it. That made me think that it was a hot spot in the cylinder. I already have 2-stage colder plugs for the S/C so I pulled the heads. No carbon hot spots. No burnt or sticky valves. Head gaskets were seeping a little coolant but otherwise looked okay. Valve clearance is fine.
I have a wideband O2 meter. It doesn't seem to be leaning out. I replaced the fuel pressure regulator anyway. Seems a little better but I don't think it's fixed. At the moment I'm running about 12 degrees spark retard at WOT and it's driveable. I don't see any sparks jumping between spark plug wires but it sure feels like it's lighting off the fuel mixture WAY before the piston hits TDC. If you don't have a supercharger or piggyback computer I can probably rule out most of my mods.

I'll go back to my old tuning settings later and confirm that the FPR didn't fix it but I'm pretty sure the problem is still there.

That leaves the ignition system and ECU. Did the ECU you are using come with that engine?

brian2sun

That's not ok, it's probably the root of your problems - coolant getting into the combustion chamber.

Daemon

I've yet to check the ignition side, and I will do so, but
mention of the FPR opens up another interesting possibility since
it varies the fuel pressure with manifold vacuum which in turn
varies with rpm and load, which is more consistent with my
variable symptoms.

ian

Evilmunkey

I would check the plugs, wires, and coil packs. Also some vacuum leaks aren't always easy to find but it sounds more like a misfire issue, but kind of odd w/ the PCM not throwing a mis-fire code. Also I would make sure there isn't a leak in the Intake manifold gasket and/or the throttle body gasket. (if it were a MAF/MAP sensor the engine would spit, sputter and die, but that's from my own experience w/ the '96 taco I have w/ the 3.4). And if your head gasket is blown you might as well hope the coolant leaking into the cylnders hasn't warped them in anyway or else you need a new motor.

1990yota-pickup

i willing to say that you have a clogged injector... start unplugging them while you have the truck running it let u know which injector is clogged up

MAIS0N

Daemon,

One quick thing to check. The hose in the picture can be a vacuum leak source. The end shown is likely not an issue but the OTHER end where it connects to the valve cover does not have a hose clamp and tends to get loose. Same deal with the PCV valve grommet and the rubber seal where the dipstick tube connects. Just a thought.

Also, if you disconnect the battery at any point remember to run it for a day or so before you make any decisions on changes you have made. If the ECU loses power it resets and has to re-learn its fuelling settings. It can make troubleshooting confusing.

mt_goat

Also check the grounding of the igniter, where it bolts to the fender should be clean metal (maybe a little dielectric grease).

MAIS0N

Well, I went back to my previous tuning settings and it looks like the problem is still there. Adding a bunch of ignition retard seems to take the edge off but doesn't completely resolve the issue either. Assuming we're both having the same issue, I guess we can rule out plugs, sticky valves, head gasket, cylinder carbon hot-spots & FPR. My wideband doesn't read each cylinder individually so there could still be a single-cylinder fuelling issue but then increasing the retard should have more effect on it, I think.
Overall I think everything is pointing at the ignition system or ECU.

I'm going with Dale's suggestion and starting with the ignitor.

1990yota-pickup

well just pull your ecu out and try it out on your parts truck...

do u have a buddy that has the same truck. do a ecu swap...

maybe you need to test drive yota for a few days.. from a crummie car lot