Struggles to start, struggles to run, dies quickly

So far what we have done:
- Replaced Fuel Pump
- Replaces Fuel Filter
- Replaced Spark Plugs
- Replaced Starter
- Replaced Fuel Injectors
- Replaced Throttle Body
- Replaced Timing Belt
- Replaced Water Pump
- Replaced Mass Flow Sensor
- Replaced Idle Air Sensor
- Checked Fuses
- Removed Aftermarket Alarm
- Checked Cables
- Replaced Terminal Ends
- Checked oil levels - never low, doesn't leak
*Not in this order

History: Bought 2001 Tundra around 2015-2016 from 2nd hand dealer. Worked great!
Some things have been repaired other than what's listed above, but shouldn't have to do with current issue.

Starting issue: Truck started to become hard to crank - get in, turn the key, engine wants to turn over, but won' start. After several tries engine starts and runs.
Husband seemed to think that cold temp starting has an effect on it cranking right up, but if its warm, either sitting in sun or after driving somewhere and getting back in to crank, it wasn't to act up. I disagree, to me it didn't matter if it was cold or hot.

Alarm issue: Had an aftermarket alarm that was wired into the ignition. It got to where the alarm would just go crazy. it began to arm itself, but could still be turned off. After awhile it got harder and harder to turn off. Finally, one day it wouldn't turn off. had to disconnect the battery to get it to shut up until my husband could get there to completely remove it.

After the alarm was removed it was still having starting issue, but the starting issue seemed to get worse and go downhill fast.

One day, sitting in the drive through at Burger King at idle and waiting on food, engine oil pressure dipped and truck died. Hard as hell to get it to crank again, but got it cranked after about an hour and got it to my mom's house and it set for several hours. Got in it to come home (45 miles away) started no problem and got it home. As long as you are giving it gas, oil pressure stays good. If you come to a stop after its been running oil pressure stays good. Sitting at a dead stop for a bit, oil pressure want to dip.

New issue: now when you get in it and try to crank it, it still struggles, but if you don't
catch the right time and right pressure to give it gas without flooding the engine, if it cranks, it will sound like its back firing and die. However, if you are able to get it going and in gear it will run. Stop lights, stop signs, slowing down to idle in traffic will kill it, good luck getting it started again in the middle of the road - won't even make it to the road side. In this instance oil pressure is good.

I don't know if the above issues are separate issues or are related issues, but it's what I've noticed, in the order I've noticed them. We are just about ready to give up the ghost on this truck, but I love this truck. I want it back on the road. any help would be appreciated.

Husband is a diesel tech and not familiar with Toyota trucks, but can do general auto repair (mostly focused on Firebirds)... I'm a web dev and don't know what the hell I'm doing in an engine bay, so please be kind.

 

Check your cam and crank position sensors. Have you installed a mechanical oil pressure gauge to see if its actually dropping pressure?

 

An engine coolant temp sensor that is reading incorrectly could be the culprit. Comparing the reading with a scan tool and an infrared temp gun would be an easy check.

I’d going to assume there is no check engine light on, and it comes on for self-check at start-up. If this isn’t correct, please post any trouble codes it has or had.

Has a compression test been done?

 

Crank position sensor was also on my list. Malfunctioning crank position sensor would cause fuel pump to stop running. It's not uncommon with the 2UZ for the serpentine belt to cut into the wire loom for the crank position sensor after a timing belt change if the mechanic didn't properly tuck the wire up. There was another member whose truck wouldn't turn over at all due to faulty crank position sensor, and it started fine and ran after crank position sensor was replaced. You'll want to use an OEM part here...

But I'd also test the Fuel Pump Resistor. On OP's truck, that's a silver finned thing on the top of the driver fender between the brake booster and fuse/relay box. More info on location here, along with test instructions. It's required to keep the fuel pump in low pressure mode (i.e. idle) ... see my extra note on this two replies down.

Important to note, there are two coolant temp sensors, side by side on the V8. One is for the dash cluster gauge, the other is for the ECM. Vehicle apparently won't crank if the one for the ECM is not sending any reading. That could be the culprit, maybe, if faulty, or a non-OEM part was used.

and @KarenK it's important to know whether you used OEM (Denso/Aisin/Toyota) parts for all the repairs or not.
Also important to know if you bought any parts from scAmazon, fleaBay, or WallyMart, since all three are havens for counterfeit automotive & appliance parts.

 

You can almost right off the oil pressure as its rarely a problem and the gauge tends to run lower than people are comfortable with. Normal for these. I say almost, because it still may be worth checking with a separate gauge to confirm your gauge is working correctly, I presume it is.

The aftermarket alarm is a problem. As they age, they get less reliable and wonky. Removing it was wise, just need to make sure everything is back to factory setup. Merely unplugging the box will not eliminate the problem. Make sure all of the additional wiring and modules are removed.

All of the additional things you replaced could very well be factoring in to why your truck struggles to start. "New parts" are no guarantee of good performance. Lots of poorly rebuilt items, lots of counterfeit items available unfortunately. If the parts you used weren't OEM, then issues can arise. These trucks do not respond well to aftermarket parts, as many are poorly designed knock-offs.

You describe hard starts and dying at idle. Most likely culprits in my book would be the new throttle body, MAF sensor, fuel pump, vacuum hoses, injectors, plugs, and possibly the aforementioned coolant sensor or crank sensor post timing belt job. The crank sensor sometimes gets improperly routed after TB replacement and the accessory belt will eventually chew into it. I didn't see any mention of a CEL, worth checking with a code reader even if there's not one, 2001 tundra has all incandescent dash lighting that is subject to failure. Good luck with your search and repairs!!

 

One more point on this - you can also use a jumper to bypass the fuel resistor. It'll force the fuel pump into high pressure mode. This is a test only, not a perma-fix.

Also worth mentioning, @KarenK are you in an area where rodents are common? They like to nest on top of the gas tank and chew up fuel pump wires. They also like to get up under the intake manifold and chew up the starter wiring, and knock sensor wiring. Both could be the source of all the symptoms you cite. They like the soy-based wire casings and warmth offered by both spots.

 

No relation.