Temp related loss of fuel?

Hey all, got a 99 taco with a 5vz-fe

truck starts up fine when cold however 15 or so minutes after it reaches operating temp it starts bogging aggressively and dies.

start it back up and it’ll struggle and die. Giving it plenty of throttle, nothing happens it’s as if it’s not getting any fuel.

After 5 mins or so I’ll start it and it holds a super rough idle, giving it any throttle only makes it worse, it just falls on its face.

once the truck is cold again it’s fine.

Check engine light came on a day prior with “system too lean” code. I drove it with the tank pretty empty so that’s what I thought triggered it. Filled up, cleared it and went on..

I started with cleaning the maf, it did have a soot film over it. I cleaned it well and thought I fixed it till after a 20 min drive it happened again.

only check code is “system too lean, bank 1”

 

You cleaned the MAF with MAF cleaner only, right?

Is it too lean at idle or under load? Need real time OBD2 data to see when the fuel trim is the highest positive number. At least I think that makes sense?

 

Maf with Brake clean,
Can feel it cut under load
After being at idle temp for a bit it cuts out and feels like there’s little-no fuel

 

Have you replaced the fuel filter?

 

Temp sensor,

not good to use brake cleaner on MAF
https://enginerevup.com/can-i-use-brake-cleaner-to-clean-my-maf-sensor/

 

Without data from a scan tool you are just going top be throwing parts at it. Get a scan tool that can read live data and see if the engine coolant temp sensor (ECT) is truly reading bad. It could be a combination of problems so dont get frustrated if the first repair doesnt fix it. As a curiosity, measure the resistance of a fuel injector cold and then do it again when it is hot.

 

Will get a proper scan tool today, also going to pickup new pump and filter, probably about time I change them anyway.

I used brake clean since it doesn’t leave any residue behind, I believe the maf is fine but I’ll update after I go over live data..

 

Get a new Denso MAF... and in the future, use electronic safe contact cleaner, or MAF cleaner on it. Brake Cleaner is not electronic safe.

 

If it happens eventually at idle, that eliminates a fuel delivery problem, I think? So if it's air related, maybe the MAF, but before buying a new one I would do more basic checks, like check all the vacuum/air hoses carefully, everything without a clamp. Things can be carbon clogged everywhere around the throttle body too.

The live air flow rate from the MAF sensor might also be revealing.

 

Should probably be checking fuel pressure too when the problem is happening

 

My guess is that code was P0172 or bc of 2 banks on a 3.5L a P0171

 

Those reasons are where I would begin

maybe go down a rabbit hole when u get time and search ‘hot no start condition’

 

Money on bad MAF or AFR sensor. Fuel system is good if it drives ok for the first 15 minutes. Good chance its MAF considering cleaning it seemed to fix it for a bit. Since its throwing lean codes the AFR sensor is probably ok. I'd throw a denso maf at it. Only use denso.

 

Coolant temp sensor is suspect. He says it happens reliably at a spot in the temp range. I'd be curious to graph the signal on a scan tool and see if it drops out somewhere while warming up

 

I think he means as soon as it heats up. "after it reaches operating temp it starts bogging aggressively and dies" Seems like as soon as it heats up and reaches closed loop.

 

Try unplugging the front oxygen sensors and warming it up

 

When you get your scan tool from a cold start you are going to want to monitor ECT, long and short term fuel trims and the MAF in g/sec.
ECT warmed up should be 180 - 190 ish
Fuel trims should be +- 0 to 10% is acceptable
MAF should be 3.2 to 3.6 g/sec at idle rpm

I am leaning towards your injectors as as they age the resistance increases and this seems to effect a warm engine more than a cold one.

 

Didn’t get as much data as I wanted today but thought I’d do an update

Looked at the data and ect + maf seem to be good, maf was at 9g cold but as it warmed and idle rpm went down it stuck around 3.8-4

under load the maf appeared to be consistent with readings, however fuel trim appears to be high (40%)

I’ve gone over vac lines and haven’t spotted anything other than the intake tube with some wrinkling/cracks, though I don’t think it is torn all the way through

I wasn’t able to get the truck in the shittin bricks stage cause my time was a little tight today, I’ll update tomorrow with more readings

 

If there are ANY splits, cracks or other false air after the MAF but before the throttle the MAF will give an incorrect input to the PCM. I will explain. With the throttle closed ALL vacuum in the intake manifold is on the engine side of the throttle blade, as you open the throttle the vacuum, i.e. negative pressure, starts to equalize with atmospheric pressure. As this lower pressure “travels” up the tube it will pull unmetered air into the tube and the PCM does not know how to compensate for this. The front 02 sensor reads this lean condition and the PCM compensates by changing the frequency of the injectors to allow a greater open time, hence your 40% reading… anything above a 25% rich or lean condition will set a CEL. It is my understanding that this is what is refereed to as a bank fire injection, that is ALL 6 injectors duty cycle is increased.

Pull the air flow tube and replace it, they arent that expensive. Certain model years require additional tubes/bits because of the new Universal design air tube. Your Toyota parts guy should know this. When the new tube is installed run the tests again.

 

I’ve ordered a new tube in, took another reading today.

MAF warmed up, in closed loop is 3.5 - 4g/sec

Short fuel trim is 19.5%
Long is 40%

I seem to get maybe 5 mins of closed loop status before it begins acting up, it starts heavily missing and gradually gets worse till it reaches a point where it will no longer run until colder.

The live feed reads similarly when rough to when it’s idling smoothly.

I’m gunna read injector resistance tomorrow, if I’m not mistaken it should read somewhere near 13.8ohms

I used to get more time out of it till it would start acting up, probably an injector gradually dying?