Intermittent stumble/miss/misfire on 2000 LC - no codes

[re_guderian]

I am having a hard time diagnosing a intermittent stumble/miss/missfire that give an ever-so-brief drop in the RPM's, but throws no codes. It happens when in P or D, but not usually in N. Makes me think TC, but I don't know how to troubleshoot that.

History - first noticed it last summer, and I thought it was a coil pack on the fritz, and sure enough, a few weeks later one died. PO replaced coil pack #4, I replace #1 and #5 when they failed last year. After #1 failed, I had to drive a while to get a part (was on the outer banks in NC). I thought DS catalytic converter might be clogged, but IR temp gun showed exactly the same temps between the 2 sides above and below the Cats.

I recently bought two new coil packs, and went to town swapping them out with remaining 5 that hadn't been changed, using the old ones if the problem didn't go away. I ended up swapping them around until all had been replace, including #4, and replaced all 8 plugs with NGK's. I even seafoamed the intake. Intake has been cleaned. New, upstream O2/oxygen sensors 2 weeks ago too. I thought the problem was fixed, but about a week later, it's back.

So... new/good coil packs and plugs all around. Clean TB. Cats equal temps both sides. In P or D with foot on brake can feel the idle hiccup, but doesn't throw any codes. Not a grind/rumble/boom, definitely a hesitation. AC always on (c'mon, it's AZ). Doesn't happen long enough to even register on an RPM graph in Torque Pro. MAF clean. TB clean.

Otherwise, car runs OK. Power is OK. Had 8 people and towed an enclosed 12 foot trailer up to the mountains last weekend fine. Gas mileage sucked, but I attributed that to the trailer and the speed/hills (~8 MPG), lots of time with overdrive off. Usual lately is about 12 in town. I'm on 295's, stock gears.

Any ideas? vacuum leak after MAF? injectors? torque converter? Anyone else have similar issues?

 

[re_guderian]

Finally threw a 0304 code tonight after idling unusually rough. At least I know which one is out.

 

[Trunk Monkey]

Yeah, probably hadn't happened enough or "correctly" for the ECU to turn on the light.

 

[re_guderian]

It's baaaaack. It's not plugs (8 new ones), coil packs (4 new w/in a year), upstream O2 sensors (2 new densos), air filter (new), or TB dirty (cleaner than my plate). I've seen Slee reference injectors "leaking" and the engine flooding, and it does "seem" that if I hold WOT on a hot start it doesn't crop up. How does one go about figuring out which injectors might be bad absent a code?

 

[re_guderian]

Hmmmm... Premium gas seems to help. I've also dumped some Chevron fuel system cleaner in the tanks with good results. Wife is out on a road trip, and reported that with a tank of regular, the stumble would come back every so often. Anyone else have this sensivity to octane rating? What would cause this? It's not a "knock", but a stumble/hesitations for a split second. Injectors fouled? I've got a new fuel filter waiting to go in, but it won't happen for a few weeks until I catch up with the truck.

 

[Gridy]

Any updates?

 

[TTO5]

throttle position sensor on the side of the throttle body could be going. I am a master certfied toyota service consultant with Toyota for 15 years and the owner of a 2000 l/c with 205k on it. Be careful, some dealers will tell you to change the whole throttle body but the throttle position sensor is avaialble seperatly. I had one go out on mine but it failed more dramatically in that once it failed it stayed failed until I replaced it. Maybe this will help?

 

[re_guderian]

Any updates?

Nope. Still does it. I've thought of TPS, but have not decided to fork out the $ on an off-chance.

 

[TeCKis300]

Sounds like you have the diagnostics pretty well covered, but this one seems tricky. Couple more ideas:

- Check the short term and long term fuel trims the ECU is reporting to see if there's any significant skew.
- Check the ground wires to the motor

 

[lexi4darin]

PCV valve? I'm not sure it would matter if the trans was N-D-or-X but it's a cheap part to do and 10 minute job to replace.

 

[re_guderian]

PCV valve? I'm not sure it would matter if the trans was N-D-or-X but it's a cheap part to do and 10 minute job to replace.

Already done, no change. I'll douible check all the grounds, as well as log the fuel trims, but honestly I don't know what those numbers mean or what I should be looking for.

 

[lexi4darin]

My guess then would be vac leak.

 

[re_guderian]

My guess then would be vac leak.

I've even tried cracking open the little propane tank and waving around all the usual suspects looking for idle-up. No luck. I did some reading about the fuel trim, and will log it. Ground points seems good, but I can't see the ones at the back of the heads. Could be a vac leak in a wierd place.

 

[TeCKis300]

You don't need to log the fuel trims per say, just watch them for a bit to see if there are substantial trims. You may want to drive around while you watch them.

What it will tell you is if there's anything amiss or leaking in the intake, exhaust, or fueling system. Look for whether there is a steady bias in the LT fuel trims or if there's a transient condition that leads to spikes in the short term fuel trim.

 

[re_guderian]

You don't need to log the fuel trims per say, just watch them for a bit to see if there are substantial trims. You may want to drive around while you watch them.

What it will tell you is if there's anything amiss or leaking in the intake, exhaust, or fueling system. Look for whether there is a steady bias in the LT fuel trims or if there's a transient condition that leads to spikes in the short term fuel trim.

OK, I just idled it in the garage. Long Term FT's started out at 4.6% for both banks, and worked their way down simultaneously (more or less) to 1.6% over about 10 minutes. The Short Term fuel trims fluctuated from -3.6% to 0% up to about 1.8%. They didn't always match bank 1 to bank 2, but they were never more than 4-5% different. I shifted into P with the brake on, and the stutter happened. No correlation with FT numbers. AC on (c'mon, it's still roasting here). BTW, it switches into closed loop mode VERY fast, like when the coolant temp was less than 120, at least according to the Torque app...

So, I wondering if this could be Torque Converter lockup? I have double checked my tranny fluid level twice, and even done a drain and refill (Mobil 1). I check the tranny temp via the OBD II, making sure it's good and hot after driving around, and using the shift down to L and back to P check and add fluid procedure and my level is right on at Hot. Interestingly enough, when I first did this about a year and a half ago (before the issue cropped up) it was about 1qt low. What would be the symptoms of too much tranny fluid? Could it cause the tranny to hiccup and feel like an idle "stutter"? Should I run it 1 qt. low and see if it goes away? risk to that?

 

[TeCKis300]

The fuel trims sound completely reasonable. Though I'd recommend you drive it around a tad to see what the trims are in other parts of the powerband.

Everytime the problem smooths out for a bit, have you reset the ECU in some manner? Conversely, reset the ECU and see if the symptoms change.

Sounds like the motor is very sensitive to octane and it could be the result of carbon buildup or some other hotspot in a cylinder?

 

[lexi4darin]

Tec- hot spot in a cyl? I'm thinking that would lead to a noticeable knock?

 

[arbetrader]

With those symptoms, it would seem to me it likely would not be an electrical issue. Perhaps injector/fuel filter or fuel system related?

 

[TeCKis300]

Tec- hot spot in a cyl? I'm thinking that would lead to a noticeable knock?

It definitely can. Just curious as the OP has said the symptom changes with octane.

 

[re_guderian]

So to update, stumble/misfire gone for over a week now after taking it into Camelback toyota here in Phoenix (shameless plug for MUD member Murf, who is the service manager there, and takes great care of Copper State Cruisers with service and parts ). This thing NEVER threw a code, but they were able to diagnose it and found several spark plugs loose, that were allowing combustion gasses out and messing with the coil. They said one coil had gotten so hot that the boot was stuck to the plug. Anyway, I was hesistant at first, because I had just taken it another local mechanic a few weeks earlier who swore it was just a bad coil pack on #4, but that fix didn't last for more than the trip home (I didn't have a way to get the rig to CBT at the time). So far, so good!

I was the last one to mess with the plugs, replacing all 8 with OEM NGK's. I torqued to spec, but didn't ever recheck. Lesson learned. This has been bugging me for a LONG time, finally appears to be gone for good. I know several others on here recently were complaining about similar symptoms. hope this might help them,too.