Shuddering Previa - getting conflicting opinions from 2 different mechanics

I’m not car fluent but not afraid to try and figure things out myself and fix if possible.
My Previa van is almost stalling out when I’m stopped in traffic, lots of shuddering when I start it. Smells like it’s burning too much gas.
I took it to my local mechanic and he told me it was the oxygen censor and told me it would cost around $1000 to fix. He didn’t give me much confidence in his diagnosis though.
A friend of mine is a mechanic so I called him and he ran a diagnostic test. His diagnosis was that there was a leak in the vacuum or intake system but didn’t have a smoke tester to check it. The notes he gave me to pass on to someone with a smoke tester say “18” vacuum little twitchy, fuel trim high at idle, High 24% 2500rpm 10-13%".
I tried to find a mechanic who would just do a smoke test on it but unfortunately faced a whole bunch of no. So I took it back to the original mechanic and told him to check the vacuum system with a smoke test. He had the car for two and a half days and still said he couldn’t diagnose the problem. He just called and still wants to replace the oxygen censor, still doesn’t sound that confident though, for some reason. Plus I went to see him and it seemed my van hadn’t even been looked at after 2 days. I have a distinct feeling he didn’t do the smoke test. He’s not a great communicator. .
I don’t really want to replace the oxygen censor if that’s not the problem, obviously.
I’m wondering if anyone can make any sense out of my issue. My mechanic friend was pretty adamant it wasn’t the oxygen censor.
In a nut shell I don’t want to get ripped off, I’m broke. Any help would be much appreciated.

you have been talking to idiots, poorly equipped mechanics and gougers

$1000 to replace an oxygen sensor sounds way out of line to me, unless there is more to this than you described. Does it also involve replacing an exhaust manifold, rusted out section of pipe, or something else?

+25% fuel trim at idle certainly indicates that the pcm is attempting to compensate for what it sees as a lean condition, such as low fuel pressure or a massive intake/exhaust leak ahead of the upstream oxygen sensor

I’m not sure if your van uses a mass airfow sensor or if it has a speed density system. If it in fact does have a mass airflow sensor, that further complicates things, as a faulty mass airflow sensor can lead to fuel trim codes, as you seem to have

If you have +25% fuel trims at idle, busting out the smoke machine and looking for leaks should be one of the first steps, in my opinion

If you took the van back to the original guy, and he straight out said he couldn’t diagnose it after 2 days, then you would be crazy to give him $1000 to replace the oxygen sensor. In my opinion, communication is an important skill. If you’re spending $1000 . . . a lot of money for most people . . . you should be told in no uncertain terms what diagnosis was performed, why the conclusion was reached that the oxygen sensor needs to be replaced, and exactly what the repair entails. And it should also include “verify the repair” . . . if the guy doesn’t understand that last concept, he’s not the guy to diagnose the van, let alone repair it

A few questions about your mechanic friend . . .

Does your mechanic work on automobiles for a living, or is this somebody who has seen a few youtube videos, performed a few pad slaps, and now calls himself a mechanic?

Does he have a fuel pressure master test kit? It would have to be a master test kit, because I believe the Previa doesn’t have a fuel pressure test port, so the gauge would have to be teed in. If he does have that kit, he could perform a fuel pressure test. Report the results, please

Any obviously missing or cracked vacuum hoses?

Any whoosing noises with the engine idling?

Any exhaust leaks ahead of the upstream oxygen sensor?

Have you visually checked out the engine air filter and the air cleaner housing?

There are ways to make your own smoke tester. There’s some info on the web, and it involves the use of baby powder. If you made such a device, maybe your friend could use it

I’m assuming your check engine light has been on for some time, probably with fuel trim codes . . .

Are you due for an emissions test soon?

Does your friend own a decent scanner, one that’s able to display live data, versus the cheap $50 code reader?


I cannot argue with a single point my good man @db4690 has said thus far. I concur. The Previa was a bit of a strange and rather expensive bird in its day. Your engine is lying at around 70 degrees on its side under your drivers seat-ish area… It has a lot of stuff attached to it as the accessories are about 3 ft forward of the engine…this includes the snorkel on the intake… plenty of places for it to get messed with and or crack… I would start there… Open that engine panel and see what you can see…the damn snorkel may be off the throttle plate or something easy to spot.

While you are at it…or someone else is at it… look at your MAF… see if its hooked up… I believe these were the days of the “vane” style MAF also…so there’s a moving part to be cognizant of as well. See if its healthy.

There are guys out there who should be able to tackle this issue for you… just ask around on some Toy forums for local “gurus” they know where they are located so this info can be very helpful. If the Toy forum guys dont outright solve this for you right quick. See what you get.


Here is the link for the Previa Forum.