MAIS0N

Found this: http://www.tacomaterritory.com/forum....php?p=1824733
Sounds similar. Coil packs were culprit.

MAIS0N

More searching. COIL PACKS!!!!

https://www.yotatech.com/forums/f2/c...roblem-132395/
http://www.los-gatos.ca.us/davidbu/toyota.html

#2 is almost dead-on to what I have been experiencing, especially the explanation for the backfire through the intake manifold. It's hard to tell for sure with the blower on there but it sure sounded like that and I would occasionally see a jump on the boost gauge needle. I'm almost convinced. Another big $$ trip to the dealership is in my near future. Fingers crossed...

brian2sun

Coil packs can be tested to see if they're in spec before you go shell out $200+.

MAIS0N

Good point. From what I am seeing the problem is not so much with the actual coils themselves but with the boots at the spark plug end. Based on the theory from link #2 above when the plugs get too worn the spark begins to take an alternate route along the boot interior past the plug insulator to the plug base. This definitely applies in my case as the passenger side plugs were pretty well worn. Luckily this is the wire side rather than the coil pack side so hopefully at worst I am looking at new plug wires.

I expect that the arcing would create a carbon trail on the interior of the boot. New plugs would reduce the problem but not eliminate it entirely as long as the carbon trail is still there. That all fits my present circumstances perfectly. I'm thinking that some work on the interior of the boots combined with dielectric grease might fix it without any new parts at all. I'm going out to test the theory now. We'll see...

1990yota-pickup

check for acring while its dark...

Daemon

I pulled all the plugs and they looked perfect.
Brushed em clean anyway and reinstalled, no difference.

Ended up replacing the 3 plug wires (but not the coils)
and that seems to have cleared up the problem.
It's no longer bucking if I start in 2nd, and it pulls cleanly
at WOT through 2-4k rpm.

I still would have expected the symptoms due to bad wires to
cause it to throw some misfire codes, and it never did, but if
the problem is gone then I'm not complaining.

Thanks for the suggestions. Hope this is useful to someone
else someday.

ian

MAIS0N

Yup, I'm fixed too! I found a carbon trail on the inside of the boot for the middle cylinder on the passenger side. I scraped that out and cleaned the insides of all the other boots, added dielectric grease to the insulators on the plugs and done!

The reason that you don't get misfire codes is that the ECU only watches the spark cycle to determine misfires. In this case the spark pulse is taking an alternate route around the spark plug gap but the ECU only sees that the cycle completed and the spark pulse went to ground. The arcing is not visible in the dark because it is all taking place inside the plug wire boot around the ceramic spark plug insulator.

In retrospect and for the record I would add one other telltale symptom. I don't know if you noticed this also Ian, but after driving the truck and shutting it down I was noticing an unusual amount of the "tink-tink" cooling noises coming from the exhaust pipes. I had recently done some exhaust work so I just figured it was new parts settling in. Apparently when the spark pulse gets diverted and the fuel charge doesn't fire one of two things happens: Either the fuel charge gets ignited on the exhaust stroke by the "wasted spark" resulting in a backfire into the intake manifold or the unburnt fuel makes it out of the cylinder and into the exhaust system. If it makes it into the exhaust system it burns off there causing the exhaust overheating and unusual cooling noises.

Thanks Ian for starting the thread. It helped me quite a bit to find that someone else was having the exact same problem on an engine that didn't have the blower, piggyback, etc. This could have taken a lot longer without those things eliminated. I was starting to doubt my trusty SMT-6 after 6 years of faithful service.

1990yota-pickup

job well done.

Daemon

I did not know that the misfire logic was that simple. I assumed
it worked like my other cars.
In both my previous '98 VW GTI and '04 R32, the misfire logic is based
on the engine speed sensor wheel. It watches as every tooth in the
wheel comes around and calculates when the next should arrive
based on average rpm plus the natural pulse of each cylinder firing.
Any time there's a misfire, the next tooth passes the hall sensor
slightly delayed and after a few of those it starts throwing misfire codes.

ian

mt_goat

I think that's right in our trucks too.

MAIS0N

That's interesting. If that system is in place it would appear that it is not very operational. I have seen that type of system cited in Honda applications but have not seen it referenced in the Toyota trucks. I have seen reference to the spark-event reporting signal provided by the ignitor to the ECU.

mt_goat

This 2000 FSM page has a good discription on the misfire detection process.
http://www.ncttora.com/fsm/2000/Repa...n/cip0300p.pdf

MAIS0N

You're right. There it is!
So why no MIL light in this situation?
Perhaps an intermittent misfire is not consistent enough to meet the threshold?

Daemon

Yep, that's the detection algo I'm familiar with, and on my VWs
it would start throwing misfire codes (before it blinks the MIL) long
before you really notice any significant deterioration in
performance. On my truck, it was bucking really hard at low
rpms, and bogging badly at mid rpms (up to 4k). Both of which
should have easily triggered even the most conservative
misfire detection logic. I can't quite deduce from that pdf
whether the 200 and 1000 rev values are the total # of misfires
needed to throw a code, or the they are the rev interval over which
some unspecified # of misfires must occur before it throws a code.

ian

1990yota-pickup

i hope you have a nice heated garage.

MAIS0N

Pole barn. Wood stove.

mariogingras

ive been getting some similar problems with my 3.4l.

put scanning tool on codes are misfiring in 3rd cylinder , air/fuel mixture too lean.

truck lacks power, sputters at low rpms, like you said chomps at high rpms, poorest gas mileage, bought the manual so i got some light reading to do.

any suggestions would help.

MAIS0N

Sounds like a bad injector...

1990yota-pickup

take it to the dealership..

buy a new engine..

trade in, your ride.. for a new one

sell it and regret it, for as long as you live.

man up, and find the probelm and fix it your self and drive it like you stole it.