99 Tacoma 2.7 runs TERRIBLE!!

Hi all, long time viewer first time poster on here. I finally broke down and made an account on here to pick some people's brains on my Taco that's been giving me a massive headache for a few months now. I've spent countless hours looking at just about every forum on the internet regarding this issue and came up empty handed.

About 4 months ago my truck started misfiring very badly out of nowhere when driving home, so bad that if I gave it gas it would literally just die out and come to a stop on the road, and if I stabbed the throttle to get it to rev up in the slightest it backfires and pretty much refuses to do anything. It'll start and run great for about 2-3 minutes when cold, then it goes into this "mode" where it'll barely and I mean BARELY idle at like 150-300 rpm and will eventually stall. Then it will bounce back up to normal rpms and run perfect for a second if it doesn't stall, then the loop continues. And as mentioned if I give it any gas (unless I open throttle very quickly and rapidly, but still barely revs at all) it dies also.

I'm fairly mechanically inclined; this truck was my first car and I've done tons of work to it over the many years I've had it, however I get stumped on occasion with electrical issues on some cars, like this one. I finally got it to throw some codes, p0100 and p0110. About 2 months ago I bought a new oem MAF after cleaning the old one didn't do anything, didn't change anything. I've also attached another ground wire to the motor, cleaned maf connector, inspected harness that plugs into maf, and some more I can't think of atm. I thought about possibly a bad crank or cam pos sensor, but haven't looked much further into that. Definitely seems electrical to me as it runs great when it's cold (for a min) and when the idle bounces back up.

Any help is greatly appreciated, I have other cars to drive for now but I haven't been able to drive my Taco for about 5 months now and it's making me kinda sad ngl, I miss it a lot.

 

Check for pending codes and look at the freeze frame data.

 

Yeah I’d do the basic troubleshooting first. Change the easy things - plugs wires etc. while doing the plugs do a compression test. Collect that data then report back. No fluid loss anywhere? Check the live data for things like the MAF, IAC, fuel trims all that fun.

The more data the better!

 

So when I look at live data and freeze frame, what should I look for? Not really sure what the MAF values are supposed to be, and if I remember correctly from last time I looked it was adding like 10% to the fuel trims but I'll check again and report back. I know compression is good and plugs/wires are new about 15-20k miles ago, but why would an ignition issue be causing MAF codes? I haven't verified yet but it's very likely that I'm not getting any power to the MAF based on the codes and running condition but I'm not sure of all the reasons that could be. Does anyone know which wires in the connector for MAF I should check with multimeter? I don't have a wiring diagram

 

Welcome to TW. Hope you get your Tacoma running right.

 

Here is a link to start with.

https://charm.li/Toyota Truck/1999/Tacoma Extra Cab 4WD L4-2.7L (3RZ-FE)/Repair and Diagnosis/Powertrain Management/Computers and Control Systems/Air Flow Meter/Sensor/Testing and Inspection/


https://charm.li/Toyota Truck/1997/...rain Management/Diagrams/Electrical Diagrams/

 

The MAF is a brand new OEM Toyota MAF, and as far as I know the old one is still good since it runs the same with either... I was more looking to test the wiring on the actual truck that plugs into the MAF, I'm thinking it's not getting power there but don't know which terminal is which on the connector

 

Any updates?

 

I would look for vacuum leaks.....they can be insidious....hard to find.

Even the break booster, pcv hose/grommet, or evap lines are suspect.

Bad injector seal.....ect........ 4bangers have cups for injector seats that can crack out.


Need to run live data to watch fuel trims to see if its adding a bunch of fuel for an air leak....

Other than that the maf and the front O2 sensor run the show....if there is a problem with either of those.......again would show up under a live scan.

First thing a $160/hr mechanic does.......is hook up his high dollar scanner and peak at a running engine.

 

Sorry I've had a lot going on lately and getting ready to leave for vacation, I'll see if I can run a live data scan again tomorrow and refresh myself on what it said. I know it was adding a bunch of fuel, but not constantly, only when it starts running really bad. Here's a video I took of what it's doing from a few months back

https://photos.app.goo.gl/BtmHNtP1Zzbvdde29

 

Sounds like it fails when it goes from open loop to closed loop. Check the temp sensor for proper OHM reads. Had an S-10 that did the same exact thing. Ran perfect until it went to closed loop, the temp sensor was sending faulty temp info to the computer that it was -40f and the computer was dumping fuel to the injectors to match. If you were dumb enough to shut it off, forget about restarting it until it cooled down.

PITA to figure out, cheap as heck to repair.

 

Ok I was finally able to get out and do a little more testing;

So I checked power at MAF connector, connector has power and good ground so I'm no longer worrying about that. I put a brand new oem denso coolant temp sensor in it about 3-4 years ago, but just for giggles I unplugged it when it was warmed up and it still runs the same with it unplugged. So I'll rule that out too since I know it would normally run fine with that unplugged (my old one was broken in half, was a pain to cold start that way but ran fine). As far as vacuum leaks, I can't imagine that would cause this as it's an intermittent (although persistant) problem, and I've had a vacuum leak on this engine before years ago and it ran bad all the time but nowhere near this bad.

I got some more live data info too, first two pics are when engine was cold, second two are once it was warmed up a bit

 

Well it's been a while and who knows if anyone cares anymore, but I FINALLY got the truck running and driving again, however there's a catch; it's not all the way fixed. The main issue ended up (somehow) being the IAC valve. New oem IAC and it revs up fine now and has power, but still has a random stumble here and there and a very erratic idle at times.

I discovered though, that if I unplug the coolant temp sensor, it runs perfectly just like it always did (of course hard to cold start this way, have to plug it in, then start it, then unplug it). I thought the sensor might be bad then, so I swapped it and still only runs good with it unplugged. What else could be causing an issue that appears to be related to the CTS, but isn't the CTS itself?? Just glad to be able to drive it again though finally.

 

Hmmmm
I’m not ‘big’ on realtime data(bc I’m more mechanical….(a fault of mine))
But
I may have found something

 

Brrrr Rabbits
Unk if you are in Alaska but 27 degrees is cold

guessing here
But possible that IAT(bad) is over ruling the CTS(good)???

I dunno
Just a thought
Electrical Gurus members might be able to explain

 

I forgot to mention that now with new IAC, the MAF codes are gone. Now the codes are for CTS circuit, throttle position sensor circuit, and insufficient coolant temp, the first one likely from having it unplugged. Also tested TPS with multimeter and tests fine, and CTS is brand new oem.

As far as the 27 degrees, that was acurate it was cold out that day, I live in northern Michigan. Also the IAT sensor is part of the MAF sensor, which is brand new oem.

 

Dang
A firm that