Perplexing engine issue…

The Deal:
There’s a pretty unique issue that I’m having with my 1995 Taco, and I’m a little stumpeded on this one, so I thought I’d throw it out there to see if anyone has any ideas.

The Truck:
1995 Toyota Tacoma, V6, 4WD, no engine mods, 243K on the ticker, well maintained.

The Lowdown:
A week ago today, my truck started choking and burping on the freeway and the check engine light came on. I made it home, hooked up the OBDII reader, and got a P0303- number 3 cylinder misfire. I figured I was due anyways, so I replaced the plugs and wires; while doing so, I also checked the compression, and all 6 cylinders are good, and within a couple PSI of each other. The #3 plug was cracked, and I figured that I had the problem solved.

After installing new plugs and wires, the problem went away for the next couple of days… until I filled up the gas tank again. I’d say about 30 miles from a full tank, the problem started to reoccur, although to a lesser degree than before, and the check engine light never came on. I was on my way to Seattle from Portland, and had to get there that day, so I just drove light and took my chances. The problem occurred intermittently until I got down to about half a tank of gas, and then it went away.

I drove in Seattle for a couple of days after I arrived, and it ran fine. I ran the tank until it was practically dry. For the trip home I filled up the tank, and started heading south. Once again, after about 30 miles the problem came back, and once again it went away after I got down below about half a tank of gas. Like the first time the problem occured after replacing the plugs, the check engine light never came on again.

When I returned home I plugged in the OBDII reader, and there are no DTC’s.

My thoughts:
I’ve ruled out the coil pack since the problem only occurs when the fuel tank is above the halfway mark. I’ve also ruled out a burnt valve or damaged seat, as well as rings, due to the good compression.

Could it be the fuel cap? Maybe it’s not venting right, and there is too much backpressure from the tank for the return to work properly? Once there is a sufficient amount of air in the tank, perhaps the pressure is decreased a bit?

Additional information that might be useful:
1) When the problem happens I have noticed that if I “gun it” (read: floored!), it goes away for that episode, but may return 15 seconds after returning to a normal cruising RPM, or it may return 10 minutes later. Also, if I just let it do its thing, it will go away after between 3 and 15 seconds.

2) The issue is intermittent, and is nondiscriminate to driving conditions/hills/rpm range (except when I floor it, it does not occur and/or goes away).

3) I have now run three full tanks of gas (from “really empty” to full) through the truck since the original occurrence.

4) The problem seems to occur 30 miles or so from a topped off tank, down to about ½ tank, and does not occur below ½ tank of gas.


I have searched high and low for folks having similar problems, but have so far come up empty-handed. So… if anyone has some insights as to what might be going on, I’d be grateful if you could share them with me.



:lurk:
 
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njtaco

Explorer
Could the charcoal canister be clogged? Perhaps the tank is not venting in "replacement air" fast enough? Just guessing out loud...

Check the fuel pressure regulator. Is it vacuum regulated? That is, is there a vac line controlling it? That could help explain #1 and #2, maybe?
 
njtaco said:
Could the charcoal canister be clogged? Perhaps the tank is not venting in "replacement air" fast enough? Just guessing out loud...

Check the fuel pressure regulator. Is it vacuum regulated? That is, is there a vac line controlling it? That could help explain #1 and #2, maybe?


That's kind of what I was thinking... OUCH though! there are 4 that fit that year and engine (depending on VIN), the cheapest being +/- $250. Pretty expensive... especially if its not certain that the charcoal canister is the problem.
 

Photog

Explorer
I was thinking the fuel pressure regulator also. The vacuum line can be plugged for testing (regulator will maintain max pressure), and you can see if the problem persists.

The other problem I have had, with high mileage Toyota V6, is the fuel injectors need to be removed, cleaned and flow-balanced, about as often as the timing belt replacement cycle. An injector that is running lean (clogged), can make a cylinder run hot enough to ruin a spark plug and other things.

Since it seems to be consistant down to 1/2 tank, it could easily be the charcoal canister and/or the gas cap. When you fill up, it is not a good idea to top-off the tank. Fill until the pump shuts off, maybe press the fuel lever one extra time, and be done. There are supposed to be check valves to prevent problems; but too much fuel in the tank, has been known to cause trouble for the charcoal canisters. With many miles and years, the check valves may not work right anymore.

With this many miles, it could be a little of all these things.
 
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Kermit said:
Might be an injector.

When was the last time you put a new fuel filter on?

Fuel filter somewhere between 10 and 20k ago. I don't think that it's an injector though. The reason for that is because of how the problem never occurs below 1/2 tank of gas. If the problem occurred through the full tank range, a faulty/clogged injector would be my prime suspect.
 
Photog said:
I was thinking the fuel pressure regulator also. The vacuum line can be plugged for testing (regulator will maintain max pressure), and you can see if the problem persists.

The other problem I have had, with high mileage Toyota V6, is the fuel injectors need to be removed, cleaned and flow-balanced, about as often as the timing belt replacement cycle. An injector that is running lean (clogged), can make a cylinder run hot enough to ruin a spark plug and other things.

Since it seems to be consistant down to 1/2 tank, it could easily be the charcoal canister and/or the gas cap. When you fill up, it is not a good idea to top-off the tank. Fill until the pump shuts off, maybe press the fuel lever one extra time, and be done. There are supposed to be check valves to prevent problems; but too much fuel in the tank, has been known to cause trouble for the charcoal canisters. With many miles and years, the check valves may not work right anymore.

With this many miles, it could be a little of all these things.
Brian-

Agreed, there's potential that it could be a number of little issues adding up to be a greater issue. I know about not topping off the tanks, and always tell the pump-jockey to not top it off (I live in Oregon, and as rediculous as it is, we're not allowed to fill our own gas tanks here... all pumps are "full serve").

Which vacuum line might I plug for testing purposes?
 

ntsqd

Heretic Car Camper
There should be a vacuum hose right at the top of the FPR. Disconnect there & plug.

That cracked plug sounds like a lean condition is very possible. With a high res DVOM you might see an O2 sensor voltage spike/dip that would indicate this.
 

Nullifier

Expedition Leader
What you describe happened to me once. It was as sticky valve. Apparently the intake valve of the 3rd cyl had "seated" itself slightly into the head. When it was shut it would stick just long enough to be late for the injector spray. Hence I would get a sputter like I had a bad plug or something.

I had to replace the head, so naturally I just replaced the whole motor. Since doing a head job on a high mileage motor is rarely a successful venture, when I do one half I do both. Just my personal rule.
 
The darn issue came back today, this time with about 1/4 tank, and the check engine light came back on... grrrrr. I'm wondering if it was coincidence that it was happening when the tank was 1/2 full or more, or if it really did have something to do with this, or perhaps another issue. As of right now, I have made no changes to the vehicle other than the new plugs and wires from a couple of weeks ago.

Either way, I plugged in the ODB II reader, and got code: P0303 (#3 cylinder misfire)…again.

Time of DTC (freeze frame) Details:

Fuel Sys 1 ............CL
Calc Load (%) .......68.6
ECT (ºF) ..............197
STFT B1 (%) .........9.3
LTFT B1 (%) .........3.9
Eng RPM ..............2670
Veh Speed ...........47
IAT (ºF) ..............84

I’m not sure what all of this means, but I cleared the code so that I can see if the next one comes up similar. If there is some sort of pattern, I’d like to know what it is. I also noticed that as soon as the "check engine" light came on, it flashed 12 times... does this mean anything?

From the previous posts I see that Kermit seems to think that it might be an injector, and at this point, I too am speculating that I may have either a bad fuel injector, or perhaps an electrical issue to the #3 injector. However, I doubt that it’s simply a clog. My reasoning is that I have seen plenty of sporadic electrical issues with a vehicle, but have never experienced a clogged injector that sometimes works, and sometimes doesn’t. Thoughts folks?

Nullifier suggests a valve that may have seated into the head… anyone else have experiences/thoughts on this? I don’t think that I’ve heard about that issue before on this engine, but I guess it is pretty high mileage at this point.

Here’s another thought… at what point does one say “forget this old truck… why am I driving a 250k pickup when I could just as easily get something newer?” Other than its current issue, it has been reliable for the whole time that I’ve had it. Looking back on all of my receipts since it was new, there really hasn’t been any major issues with the truck. most of the receipts are for maintenance parts (plugs, timing parts (3 times), oil/filter, fuel filter, air filter, etc), and not mechanical breakdowns. I’ve broken both rear springs twice, but that’s more my fault than the vehicle, and there was the time where on the freeway I hit a piece of angle-iron that went straight through my rear axle housing (you should have heard the "BOOM" that that made!), but again, driver error- not the truck’s fault.

Aside from this current issue, it’s really run very well its whole life… but where does one draw the line and say “okay, this thing’s got a load of miles” and go get a new rig? At what point does the truck start becoming unreliable, and have lots of little (or big) things start going wrong on a regular basis? Am I on the cusp of needing a replacement if I want a reliable vehicle that I could say, drive 40 miles into the woods without the fear of having to walk back?
 
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Martyn

Supporting Sponsor, Overland Certified OC0018
James

It sounds a lot like issues I'm having with our Subaru.

I've changed out the plugs and wires plus the coil block. No change. We switched the injectors around but still had the misfire on #3.

Next step is to check the fuel pressure and put it on a scope.

I sometimes get a misfire on 1 & 3 so I'm hoping it's not the classic valve guide that drops out intermittently. This seems to happen on older high mileage Subaru’s.

Good luck
 

Rexsname

Explorer
A new engine has got to be cheaper than a replacement truck. You say that it has been a reliable truck so far. Fix what is wrong and dont sign up for new truck payments.

My $.02

REX
 

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