'94 Previa LE S/C unusual acceleration problem

[jmrodz0]

I have a '94 Previa LE S/C that I have been refurbishing. The list of repairs made is extensive and includes replacing the head gasket. While everything was apart, I cleaned up the EGR valve, the engine throttle body, PCV, replaced the vacuum lines on the vapor canister, replaced the belts, distributor cap, rotor, wires, plugs, coil, transmission filter, O2 sensors, fuel injector O-rings/insulators, and fuel filter. This set of repairs followed replacing the engine wiring harness two years ago.

Aside from an ongoing vibration at idle while in drive (suspected SADS coupling issue), the van is running better than ever. The only problem I have now is that under full acceleration, the van feels like it is getting choked out and loses power. Gradual acceleration does not exhibit any problems until the van gets to 75MPH when it again starts to shudder and "misfire" (for lack of a better word). There is also a faint smell of gas in the cabin on occasion.

I have had the van at two different dealers and neither has been able to diagnose the problem. I have ordered a Throttle Position Sensor in hopes that a miracle will occur upon replacement, but thought I'd give a shout out for help to see if this sounds like a problem anyone has already encountered. Again, there are no check engine lights and the van runs well other than this full acceleration problem.

Thanks in advance!
Jim

[TheMAN]

check for boost leaks in the charge pipes? what are the condition of the spark plugs? which brand and # are they?

[jmrodz0]

check for boost leaks in the charge pipes? what are the condition of the spark plugs? which brand and # are they?

Thanks for the reply!

The spark plugs were new in October. They are NGK plugs, but I will have to pull one to check the number since I'm not sure if they were Iridium.

I am in the process of replacing the SAD couplings and was going to take off the belt to the supercharger to see if that would isolate the problem. There isn't much information available on testing/servicing the supercharger, so it's a black box to me, but I'll check the charge pipes...as soon as I learn about what charge pipes are.

Jim

[jmrodz0]

check for boost leaks in the charge pipes? what are the condition of the spark plugs? which brand and # are they?

The plugs are NGK IR IFR5T. No obvious vacuum leaks. No change in performance after changing the throttle position sensor, but the van still seems to be running rich.

Thanks
Jim

[TheMAN]

those plugs are too hot... they are for the non-S/C engine... it will cause preignition issues.... ignition timing maybe pulled and cause richness
you need to get IFR6T-11, or BKR6EP-11 (factory equivalent)

or get Denso PK20R11 (same as factory), or SK20R11 (iridium type)

[jmrodz0]

those plugs are too hot... they are for the non-S/C engine... it will cause preignition issues.... ignition timing maybe pulled and cause richness
you need to get IFR6T-11, or BKR6EP-11 (factory equivalent)

or get Denso PK20R11 (same as factory), or SK20R11 (iridium type)

Damn those Autozone folks! LOL
The plugs were pretty dirty when I pulled them so that makes a lot of sense. I'll change the plugs to the Denso PK20R11.

Thanks!
Jim

[TheMAN]

you can't really fault them too much... the NGK catalog lists the 94 previa as using IFR5T-11, and autozone usually use catalog data from the parts makers... but you must remember 94 was an odd year for the previa... they came out with the S/C model mid year... so for all intents and purposes, whenever you're buying parts for your van at the parts store, just say it's a 95 and you should have a better chance of getting the right parts

[jmrodz0]

you can't really fault them too much... the NGK catalog lists the 94 previa as using IFR5T-11, and autozone usually use catalog data from the parts makers... but you must remember 94 was an odd year for the previa... they came out with the S/C model mid year... so for all intents and purposes, whenever you're buying parts for your van at the parts store, just say it's a 95 and you should have a better chance of getting the right parts

I installed a new Throttle Position Sensor and the Denso plugs yesterday but haven't done much driving with it yet. The hesitation/shuddering at full throttle is still there, but I'm planning a little jaunt this afternoon which hopefully will get the timing self adjusted. Regardless, the gas smell in the exhaust is now gone and the van is running a bit smoother which is a big step forward.

Thanks for the advice on both the plugs and for future parts ordering!

[jmrodz0]

The plugs resolved some of the issues with general performance, but I've still got a problem with the shuddering at full acceleration. If I ever find the cause I'll post it here in case anyone else runs into the problem.

[jmrodz0]

Update: I've taken a couple of 200+ mile trips since replacing the TPS and the SAD couplings with no significant improvement. On my trip home the other day I got a check engine light! Normally this would be bad news, but I've been waiting for some electronic indication that something wasn't right, so it was a bit of a relief to see it. Turns out it was two codes: P0505 and P0773. I have seen the P0773 before but the P0505 was new. Seems it's related to the Idle Air Control (or the throttle body) which would make some sense given the issue at hand.

I haven't done anything about it yet, but I plan to pull the throttle body and clean the IAC next week.

I did have a couple of epiphanies along the way so I thought I'd share those in hopes it might help someone in the future. The first was that the charcoal canister was connected incorrectly. The vacuum line to the canister was attached to the wrong connecter at the purge solenoid...and the worst part is that I connected it. Thing was, that both hoses were disconnected before I started working on the van, so I just connected the short hose to the canister because it wouldn't have reached to the intake pipe. Then I had my epiphany when I looked under the hood and saw the vacuum diagram....which I had been searching for months to locate.

After finding the diagram, I had my second epiphany when I saw the procedure for resetting the timing was also under the hood. Go figure. Since this is a '94 SC, the Haynes manual had been no use for this problem since it talks about rotating the distributor which is not possible on this vehicle. With great trepidation, I followed the procedure of a short circuit between the TE1 and E1 terminals and the engine smoothed out...a bit.

My next step will be to service the throttle body (which had been cleaned up a bit months ago), but I never took off the IAC, so this time it gets the full monty. It also wasn't clear from the Haynes manual that there are two throttle bodies: one that has the TPS in the engine bay and the other that connects the air intake to the intake manifold. Course, they may not both be called throttle bodies per se, but since the procedure to clean the carbon gunk out of the "throttle body" that affects the EGR system is the one under the driver's seat and the other "throttle body" in the engine is where the TPS connects, I'll just assert there are two until someone corrects me.

No idea on the P0773 code, but the Toyota dealer wasn't too concerned with it.

[TheMAN]

there is only one throttle body and one throttle body only.... and that is under the hood on the S/C
the one under the driver seat is where the N/A one would go... it's just an adapter/coupler that fits a "square peg in a round hole"... this is just a carry over legacy from the N/A engine, despite the fact that the S/C intake manifold uses a different part number... I bet you it's all the same except for some minor differences, thus they saved money by not making a new mould for casting a completely different intake\

be sure to get yourself a JIS P2 screwdriver before taking off the IAC.... you'll save lots of headaches this way.... you can find them easily on ebay and other places online

and the P0773 code seems to be a shift solenoid code... if your trans hasn't been shifting right, then that will explain why

[jmrodz0]

there is only one throttle body and one throttle body only.... and that is under the hood on the S/C
the one under the driver seat is where the N/A one would go... it's just an adapter/coupler that fits a "square peg in a round hole"... this is just a carry over legacy from the N/A engine, despite the fact that the S/C intake manifold uses a different part number... I bet you it's all the same except for some minor differences, thus they saved money by not making a new mould for casting a completely different intake\

be sure to get yourself a JIS P2 screwdriver before taking off the IAC.... you'll save lots of headaches this way.... you can find them easily on ebay and other places online

and the P0773 code seems to be a shift solenoid code... if your trans hasn't been shifting right, then that will explain why

Thanks for the info on the throttle body. After getting check engine codes for the EGR system and cleaning the throttle body with no success, I eventually removed the other thingy and the air line to the EGR valve was completely choked with carbon deposits. A can of brake cleaner later and the engine code hasn't came back...yet.

The transmission has been good. The fluid and filter was changed recently and with the new SAD couplings it has been driving very good. Now that I'm thinking about it, there may be an issue under full acceleration when then transmission (I assume) tries to downshift, but since the acceleration problem exists in Park it may just be coincidental.

Good tip on the screwdriver (no pun intended). Any idea what the thread specs are on the screws? I'll probably to replace them with bolts or hex screws. Too inconvenient a location for a screw.

[TheMAN]

There's no need to replace the screws as long as you have the right screwdriver... they are supposed to be very tight and the JIS cuts are at an angle where cam out is basically impossible... using anything else is asking for over torquing and stripping the threads from the aluminum throttle body

[jmrodz0]

Good point!

[tbkilb01]

I'm looking to clean my EGR, passage, and pipe and wondering what size wrench you used to remove the pipe nut? I read two other postings and each differ on this subject... my van is a 92 previa. the vans in other postings were 95 S/C and another toy van of yesteryear....

[jmrodz0]

I'm looking to clean my EGR, passage, and pipe and wondering what size wrench you used to remove the pipe nut? I read two other postings and each differ on this subject... my van is a 92 previa. the vans in other postings were 95 S/C and another toy van of yesteryear....

I don't know the technical size in metric, but I used a 15/16" open end wrench and it fit like a glove. I can also attest that a crescent won't work.

You might also want to consider getting a 12mm/14mm offset wrench if you don't already have one. I found one at NAPA and it has many applications for this van.

UPDATE: I just looked at the photo you supplied and it said 15/16" too. LOL

[tbkilb01]

hey thanks, and sorry, I attached wrong picture...DOH..
and I thought it was a 15/16 its just the other picture ( of S/C) uses 1" crowfoot
but I will go with 15/16 and check into the off sets....

[TheMAN]

15/16" should be a little more than 23mm, but not quite 24mm (that's close to 1")

[jmrodz0]

I heard a hissing noise coming from the engine compartment and it appears to be coming from a little thingamajig attached to the throttle body. I haven't been able to find a part number for it, nor do I have any idea exactly what it does. It has a little vacuum actuated plunger on the bottom that acts as a stop for the throttle, so I'm guessing it regulates the throttle somehow and in turn affects the supercharger operation (I think).
P080215_13.34.jpg

Anyway, if anybody has any idea exactly what this part does and how to get a replacement (short of buying a new throttle body) please let me know.

Thanks
Jim

[TheMAN]

EGR valve?