Hello, more knowledgeable ones!
I have a '98 Sienna that’s still running well (271k miles) but about a year ago it started acting strange, just a rare but minor irregularity in the normally smooth engine. A few months ago I changed the fuel and air filters, but that didn’t affect it. So I tried changing the tank pick-up filter and confirming that the tank didn’t contain any gunk. Again, no change. Since then the engine light has come on, but it still runs well enough for now. My fuel pressure is solid (manual specs, 50psi I think). I’m not sure if it’s too rich or too lean, but I’m convinced it’s fuel related unless someone has a better idea.
My suspects: fuel pressure regulator (in the fuel tank), clogged fuel injector(s).
Other potential clues:
• 270,000 miles on the original engine.
• No smoke (that I’ve noticed).
• After changing the filters my fuel pressure was perfectly solid at manual recommendations (50psi if i remember correctly).
• New battery, new fuel cap (factory cap; sometimes store-bought ones don’t vent correctly).
• Sometimes the fluctuation goes away for a day or two, while other times it’s hard to accelerate uphill at highway speeds.
• Issue is not clearly affected by temperature (same when 40° or 80°).
• It was much less noticeable when I drove to Montana this winter; they have a winter blend, which might have masked the problem (or the higher altitude). It’s possible I just didn’t notice it as that drive killed my steering pump and a CV boot, so my focus was on other problems at the time.
• I found today that can I make it shudder a little at highway speeds: letting up the pedal for a moment and gently giving it a little more gas causes a gentle shudder, and very rarely what sounds like a small muffled backfire.
Does anyone think I’m on the right path or is there something else I should investigate? Any thoughts or tips are greatly appreciated. I just don’t want to drop the tank again unless it’s likely to fix it. 
Cheers!
-Spaz
Start by investigating the stored fault codes in the ECU. They’re usually a good place to start with this type of problem.
And congratulations on your success with your Sienna.
It’s true that the first thing to know is what the error codes are. Many auto parts stores (esp. the big box stores) read these for free. It’s as easy as pulling in and asking them to do it. Write down the exact codes and post them - e.g. “P0123”
And I wonder if you realize that for all of that write-up you gave, you never did actually describe the problem. “Occasional shudder” is really vague. As PvtPublic pointed out, the one description you gave about how you can make it happen does sound like TCC shudder. So - get some more specifics about this shudder.
Thanks for the insights, I’ll check the code later today. I didn’t realize how vague my description was, and I appreciate the additional areas to consider. In the past I’ve encountered problems that were never helped by a code so I tend to forget to check that out.
270K is a pretty good accomplishment. And the engine still runs pretty good from what you say. Hopefully this is a minor problem. Besides getting the diagnostic codes, what’s the status of the car’s maintenance schedule. Anything behind on the owner’s manual recommendations?
The only other thing I can think of is what you said about slow accel going uphill. That could be an exhaust restriction, like a plugged cat. Does this car have the original cat installed? If so, there’s some relatively inexpensive tests that can be done by your mechanic to determine if the cat is the cause.
Edit: I should comment on what you said about your prior experience with the CEL, that the codes were of no help to you. This is frustrating to the driver when the code says something is wrong, but that isn’t what is wrong. It actually isn’t the car’s diagnostic software’s fault. It is that the manufacturer doesn’t supply the needed information to the car owner to help the owner understand what the problem could actually be. It’s like if you have a cough, and the doctor says “well, you have a cough”. True, but not much help.
The manufacturer probably thinks to try to explain what the possibilities are would just confuse the owner, or make the owner upset. But since you paid for the car, and you paid for the diagnostic software, I think the manufacturer has an obligation to explain exactly what these codes mean. I believe there are efforts in several states along these lines.
The maintenance is fairly up to date. And I don’t really fault the diagnostic codes, I’ve just been fortunate enough not to need them until now. Besides routine maintenance, I’ve replaced a couple steering pumps, the rack & pinion, a radiator, shocks, brake pads, starter, alternator… those kinds of things that can be diagnosed without the codes. But bigger issues like scheduled replacement of the timing belt I feel more comfortable paying an experienced professional.
Now for the big reveal: P0305 and P0420. These have in common the oxygen sensors, fuel injectors, plugs/wires and the catalytic convertor. I’ll keep an eye on this and see if it generates a code for a different cylinder, but as soon as I can I’ll definitely have the catalytic converter checked. It’s the original, so you might have called it, George. It definitely has lost power in the last year, and since my exhaust was straightened a couple years ago (rear-ended) there could be other exhaust damage that wasn’t seen before. I really hope it isn’t a valve or something worse. I hope to get another hundred thousand miles out of this van.
Thanks again to everybody, and I’ll follow up when I find something new or get the exhaust checked. Fortunately I don’t have to get it smogged anytime soon.
Cheers!
-Spaz
I would not put off looking into the cylinder 5 misfire. It may very well be the basis for your shudder, but is the kind of thing that can just cause more problems if let go. And be sure to think about it holistically. It can involve spark, fuel, and/or compression.
Spark plugs!
I was about to clean the injectors, and started with the plugs to see if #5 was fouled. Turns out they were long since in need of a replacement (and not properly gapped). My wife took it in for a tune-up about about two years ago, and either they didn’t check the plugs or used some bargain ones that didn’t last. None of them were oil fouled, sooty or burned like I was worried, but #5 was definitely worn down a little more than the others. I put in a new set of the right plugs and the shudder has been gone since. I’ll still check the compression on #5 to be sure, but I’m not as worried as I was before and I haven’t seen any errors. Maybe I can wait til summer to check the injectors and the cat (when it’ll be time for another timing belt anyway). She might just make it to 400k yet!
Thanks again, everyone! I especially appreciate pointing out that I was focusing too narrowly on fuel and not considering other causes.
-Spaz