Truck loses throttle intermittently

Im going to start a new thread on this. A lot of information was in my thread for a high idle. I have figured out if I unhook my battery my high idle goes away only to come back. It comes back after I lose my throttle (guessing) or what I think is happening. I have replaced with OEM complete throttle body, MAF and plugs and wires. My idle goes down after reset but the minute this truck starts cutting out my high idle comes back. I have a scangauge II and watch my TPS and it seems to go down as truck starts cutting out. It’s like the truck loses the tps which basically takes power away only to come back a split second later. Does this 5-10 times and then runs great. May go several days before it does it again and may do it several times in a row. When it does this my high idle usually appears on the next startup. I can unhook the battery for 15 minutes and truck idles down until it cuts out. I have checked the ground wires in the front of engine by OBD port. Dont know a lot about electrical but have traced TPS wires about a 18” from plug and dont see any disturbance of wires. Anybody got a clue on where to even start?

 

Did you replace the tps when you did the throttle body?

 

Yes it has been replaced twice. It was the first thing i replaced. Kept doing it and then it was replaced when I purchased a new throttle body.

 

any codes either pending or current?

 

Subscribed, my 99 is doing this as well.

 

Be nice to try another ecm, maybe.

 

I got a PO120 this morning right a start up but it has never thrown a code while driving and then acting up.

 

thats what i have thought. I hate throwing parts at it because the ones i have thrown hadn’t been the problem. I’m a little out of my leage on this. There is a shop about 30 minutes from me that works on Toyota’s. I have thought about taking it in and seeing what they can come up with.

 

No experience with this but i’d chase that TPS harness to the ends. Ohm it out. Sure sounds like you got a bad connection in it.

 

Check all your ground points. Illustrated by the triangles in the photos.
https://charm.li/Toyota Truck/1998/.../Grounds/Instrument Panel and Center Console/
https://charm.li/Toyota Truck/1998/...iagnosis/Diagrams/Grounds/Engine Compartment/

 

Many of the drive by wire systems have to relearn their limits when new, parts are changed and in the case on my motorcycle, anytime the mapping is changed. Some systems even measure the idle rpm and adjust it up/down to the programmed setting. Not sure on a tacoma what has to happen to trigger that relearn event but I suspect the subsequent weirdness with the low idle after the loss of throttle has something to do with that. Spitballing here but possibly a intermittent loss of tps signal causing the system to go into it's learn mode.

 

I like to check a bunch of the live data with my scanner if there's something unusual happening like that. At least then I have a more complete picture to ask questions about.

Here's the stuff for the 5vz below.

Also it blows my mind that these ECMs can last so long. What other computer lasts 25+ years of constant use and power cycling?

 

Ewe
Since you RR’d TPS twice …..

Only TPS pigtail/harness dmgd somewhere
Or
What steves104 mentioned

 

What needs to happen now is removal of the ECM from the glove box and take a pic of the label on the top, then search that number to see if you can find a replacement one.

 

The PCM is expecting a certain voltage at certain times from the TPS. You should have a 3 wire TPS, if not that is OK to, the 4th wire is an idle switch. The PCM outputs 5 volts to the TPS and there is a resistance track inside. As the throttle is opened the wiper arm slides along a variable resistor track. As the voltage on one wire rises, the other one falls, they should be linear, i.e Circuit A is 5.0v Circut B is 1.0V, then 4.8 and 1.2 etc etc. These are not exact readings, just and explanation of what should be happeneing. If the PCM sees any change in this linear progression either up or down, a code is thrown. These checks are done at the TPS first, then the PCM by backprobing the connector. If the readings at the PCM are good then the suspect piece is the PCM.

 

Thanks for all the advice. I to have came to the conclusion its in the TPS wiring or PCM. I believe when the connection is lost the PCM is putting the car in relearn mode. Will keep this thread foing to hopefully help someone.

 

Some of these issues are real hair pullers. As a final test find a very low wattage 6v bulb and socket and wire it in the 5V reference and ground the other lead. Run the extened harness into the cab and drive around, if the bulb goes out when she acts up you know it is inside the PCM.

 

Wow ^^^^
I would need help using that trick but at least I have a 6V bulb handy from a 1970 Suzuki tc120!!!!!!

 

Ok a little more info. I have noticed that when it starts acting up the LP on my scan gauge opens for a split second then closes. This happens when its already up to temp and lp is closed. So truck starts acting up the LP opens and then closes, but each time it acts up the ICU opens. That may help.

 

Well at last the culprit has been found. Thanks for all the replies and the time it took to type those replies. After basically taking the engine wiring harness down to just the wires there was a butt connection that was eliminated and soldered. To finally be able to drive and not loose my tps actually felt good. Again thanks for your help.