Stubborn engine idle problems when hot

Hey fellow Previa owners.
I purchased a used previa in 2002 it has served our famly very well with all trac, it's a great snow machine....and when the odometer hit about 230k, the head gasket went, this was ok, because we had already pulled a motor from the junkyard for a rebuild. The rebuild would be attempt to try to make small changes....9.5:1 compression pistons, ported head, gasket matching all throughout, and N/A Toyota cams and dual inlet air box.

Almost a year later we are still diagnosing a engine stumble at idle in gear or when putting in gear when hot. The engine occasionally dies. Our A/F gauge also shows a dramatic jump in fuel vs. air, when the stumble occurs. The engine runs great the rest of the time. We have looked at, cleaned, electrical tested- cleaned and replaced all suspect ground connections, load tested the electrical system, inspected and/or replaced all vacuum lines, tested fuel pressure....a partial list of other areas we looked at include IAC, temp sensors, plugs, wires, rotor cap, coil, ICM - ignitor, vacuum lines etc. Even our local Toyota dealer had it for several days and could not figure it out!

When working on the van this past weekend, I smelled fuel. I did a quick interweb search and came up with injector seals as a possible cause.

Looking for ideas from the more experienced mechanics on this forum, are we barking up the right tree, something else we should look at?

I suspect spark timing has already been tested, correct? Best to eliminate low hanging fruit. Bad timing can give you low torque at idle, giving you some bad idle symptoms under load. A car needs 3 things to run well: fuel, spark, and air. I have always approached each car problem with that mindset. Spark is the easiest to eliminate, air is next, and fuel is the most difficult to diagnose. Spark is easy. Check your timing, inspect your plugs, heck for spark. You may have done those. Air is IAC, MAF, and EGR. Bad idle when hot under load makes me suspect the egr and iac. How did you evaluate the iac? Did you clean it? With what chemical? And your EGR, though I have not done work on an egr, I would suggest doing some research. EGR only function when the car is hot, so I would look into cleaning/checking that.

I think your leaking fuel injector seal is a possibility. Did you smell fuel when the car was hot, cold, or, after sitting? Cold fuel smell is normal, because the engine will run rich. Hot fuel smell is a problem. After sitting is even worse. I haven't fixed injector issues myself, so I cant chime in on how to diagnose these , but maybe research and other's input can!

Thanks for responding. Much Appreciated.
The 1994 SC Previa's timing is controlled by the ECU. We have checked and/or replaced Spark plugs, wires, coil, rotor, cap, distributor, coil, ICM.
Also we have cleaned and actuated the IAC on the bench, replaced temp sensors, crank sensors, egr, checked intregrity of vacuum lines, wiring , paying a lot of attention to ground connections, replaced the battery. On the fuel side, we have ohm'd the injectors, checked for spray pattern, check fuel pressure, replace fuel filter, etc. etc. We know it has air. Recent replacement of fuel injector seals unfortunately did not solve the problem.

The only recent developments has shown the problem happening more often and not always at full heat soak...we hope this manifest to allow us to catch the problem...

This weekend we will replace the torque converter with a higher stall speed unit. This will allow the engine to idle with less load - maybe not seen as a direct approach...but we have almost exhausted all other options....

Still welcome more ideas. We don't want to give up on this thing....face it, the previa is not the coolest looking machine out there, ( that would be a 55 bel air ) but it is so dam practical, and you wouldn't believe how this thing corners!

Update:
Although the Previa is not a popular platform for engine mods and piggyback computers, we found the solution on other Toyota ( Lexus ) and other forums. The Toyota ECU fuel control is a batch fired system, and this forum had the solution: Still tuning but this solved the biggest problem

Oh by the way our high stall speed converter seemed to help with acceleration and the price was around 300$

http://www.clublexus.com/forums/perf...gte-sc400.html

My 97 alltrac is having the same problem.

• Drove it 7 miles, running great (normal).
• Stopped to eat.
• Started the van - idle was very rough.
• Error code: P0401
• Cleaned throttle body.
• Replaced plugs and wires.
• Replaced VSV.
• Checked EGR modulator.

I'm guessing the EGR may be at fault, but it seems like a problem that would take some time to manifest itself. I only run premium gas and change the oil religiously. I am hoping not to see the build up of crap inside when I pull it off.

Another follow up.
Although our rewiring fixed the major problems, we have still been experiencing stumbling at idle- sounds like a misfire. One issue my son found on the Inter-Web is that sometimes the sensor in the distributor fails intermittently under heat soak. The replacement aftermarket unit didn't cure the problem, so we ordered a brand new distributor from Toyota ~450bucks ...Yikes! We have not been able to install it yet, because a lifted 4x4 modified the front end of the van and I am waiting to get it back from the body shop. I will post the result once we install the new distributor. Yours sounds more like a plugged up EGR.

Any updates on this? My 1996 SC 2wd is having the same problem. Everything is smooth and nice, but after its been driving for a while, it starts to rough idle (side mirror on driver side shakes a lot!). Starting up (cold engine) has no problems at all. When Im at a stop light (engine at operating temp), I put it into neutral and the shaking/rough-idle stops. Red light turns green, I put it into Drive and the shaking/rough-idle starts until I get it to about 5mph and above then everything is smooth again. Thanks all!