I have a 1997 Tacoma 4 wheel drive, with the 3.4 engine, 5 speed, 48K miles I bought about a year ago. It's had a rough idle ever since I've had it. I performed the timing belt, idler pulleys, tensioner and water pump repair which needed to be done because of a noise in that area of the engine which the repair fixed but it did not stop this issue. So I took it to a reputable local shop who informed me of two things. 1) It is not throwing any codes and 2) He is not able to receive any live data from the diagnostic port. "Without this I am left to guessing about what the problem is, and I won't do that" he said. The things he would want to obtain from the system were it working would be things like "short and long term fuel trim, grams/idle and full throttle per second (MAF), calculated load (under full throttle)." He said to possibly take it to a Toyota dealer because they may have something he doesn't have or know something he isn't aware of. I appreciate his honesty and humility. So my question is, is this a common enough issue that I could be informed on some things to take a look at. Thanks for any help.

 

Was he not able to connect at all, as in there is a possible wiring problem with the obd2 port? There are only ~4 wires on the OBD2 port, you could start by testing these with a digital multimeter. Now your truck is a different year than my engine was when I did my swap, so colors, pins could be different and what they connect to or location wont match exactly. But just incase they are not here is the info I had for my port to get you started. I would at least try to confirm your getting 12v power, ground and a signal.


1999 4Runner 5vz to 88 Pickup 3vz OBD2 Port wiring

 

I would also pull the ECU out from behind the glove box and check for water damage, and since it is out maybe swap a donor in and see if that changes anything.

 

I just talked to the tech from the private shop and he said that no "Check Engine Light" was coming on. And that if there were no Check Engine Light coming on, that no codes would come up. And so he went in the direction of attempting to get live data to begin to diagnose but that aspect was not operative. I noticed that he had a laptop in trying to get info.

 

A dealership should be able to connect to the diagnostic port under the hood (not the OBDII port),
using special equipment to figure out what is wrong.

 

You should get yourself a basic OBD code reader, you don't have to have a Check engine light on to be able to connect to the ECU, and you could also have pending codes without the Check engine light on, but yes usually no check engine light means the ECU has not flagged a problem. So either the ECU doesn't recognize the issue as a problem, or you can't retrieve any info if the OBD plug or associated wires and connections between OBD and ECU are not connected or are damaged, or the ECU is faulty. It is possible to have a leak around the windshield that could drip onto the ECU, it is also possible that the ECU has leaking capacitors that damaged the ECU, maybe this vehicle was fished out of the lake or a flooded area, that would explain the low milage aswell, but I'm stretching my guesses now.

The Rough idle I would suspect injector or associated issues, I have seen this with no check engine light, but usually it flags a miss fire after driving it a few days.

The under hood port was to test ABS, Fuel Pump, and timing advance, among other items, it is not a 2nd OBD port but can be connected to retrieve codes.


Does the check engine light go on when you start the vehicle momentarily with all the other dummies lights.

 

 

The videos on the previous 2 posts relate to pre-OBDII vehicles (built before 1996).
This technique may not work on OBDII vehicles.

Basic OBDII code readers can be bought for less than $50. All DIY'ers should have one.

 

Are you sure, maybe this video is of a obd1 motor in a obd2 car, but I don't think so, I realize some countries don't use obd2 and this vid is of a swap, but if the diagnostic port exists than it should be able to pull the codes. The PO says his OBD2 port is not communicating so $500 scanner won't work if thats the case. But I regress as I have never actually tried this method because as you pointed out a $50 scanner works fine on my OBD2 ports. Perhaps the PO misunderstood the Tech.
.

 

Sorry its been so long to get back to everyone with an update. You guys are great. I went to a buddy of mine who owns an auto A/C business and in small talk mentioned my issue. He said, "Well let me throw my reader on it and take a look." He breaks out this Snap-On reader, plugs it in and it gave every bit of info. possible. I don't know why the other shops big expensive one wouldn't but, oh well.
Any how, no history of codes and none presently. He, like several other have said, "You should do a compression test." I will but, it just turned over 49,000 miles. It runs great just off of 750 RPM and above. But it's not totally smooth at 740-750 RPM. I think to myself, "If I was back in 1997 and considering purchasing a Tacoma, I know that it would be an issue in buying one if it idled like mine is. That scenario of comparing how a new one would idle and the way mine idles is my benchmark. Doesn't seem out of the question to me. When the A/C compressor kicks in and the RPM kicks up to around 780ish, it's smooth.

 

My 96 had the same issue in that it would only work with some readers, and not others. Tested and found pin 5 wasn't a ground when it should be. I removed the 2 bolts hold the obd II port in place for easier access. After making it a ground again all was well.
Post 2 in this thread is a good reference.