1998 P0171 recurring

Ok, Getting grey hairs. Damnable P0171.
1998 4Runner, Limited, 3.4L, 2WD, Automatic, 363K miles, Wonderful vechicle.
Random P0171 code, starts great cold, randmonly hard to start hot.
When hot & not wanting to start, it will turn over, backfire & stumble, little to no throttle response, bogs down when starting but if i can get it up to 1500 to 2000 rpms it'll settle down and run great. If I shut it down 10 minutes later, it might start perfectly. It might not.

Always starts great cold.

This has been going on for months but it is getting worse.

Strong vacuum, 20+ lbs at idle hot or cold. Replaced Mass Air Flow, O2 sensor, Idle Control Sensor, cremoved and cleaned throtten body, resealed plenium, replaced Engine Coolant Temp Sensor at front of engine. (The ECT fell apart in my hand when I replaced it, I was sure I'd found the problem... not!)

No change.

Suggestions?

 

Did you use TOYOTA parts? Aftermarket MAF sensors, especially DO NOT WORK. Things are so bad, many mechanics won't even consider anything but OEM sensors. Test MAF: Did you do the brake cleaner test? (spray brake\tb\maf cleaner or starting fluid around vacuum lines, etc looking for a leak and listening for a change in vehicle RPM.)

What are your fuel trims when cold? When hot?

Did you check your catalytic converter to see if it is functioning? (https://youtu.be/9VZ5K8n5jj0?t=386)

 

There are no vacuum leaks, tested via guage & throttle cleaner. (Brake cleaner can damage engine sensors if there is a vacuum leak) I pull 22" of vacuum at idle. The sensors are known good. I do not have ability to pull fuel trim. A bad converter would show issues hot & cold.

 

I’ve had the exhaust gaskets blown before the O2 sensors and get lean codes. Stuck CPC solenoids will cause a nasty lean condition.
Definitely spray brake or carb cleaner over the vacuum lines, and any gaskets around the intake manifold. Be careful not to hit the exhaust and get a flare up.

 

CPC Solenoid? Im not familar with that. No vacuum leaks engine hot or cold, confirmed by vacuum gauge. My problem seems to be heat related. Engine cold she starts great. Engine hot she intermittenly has issues starting, but not consistently. I'll have to repeatedly ccyle the ignition fully. If it starts, it might only stumble and pop at a few hundred rpms with no throttle response. After a few tries I can get the throttle to responde, and if I can get the rpms up to 1500 and hold them for 30 seconds, she'll settle down and run perfectly.
What would cause a throttle stumble or non-response with a warm engine? A stumble that can be pushed through to normal operation?

 

Ive also pulled and tested the fuel pressure regulator, it tested good but since I had a new one I replaced it anyway. But kept the old one just in case.

 

When something like this happens to me. I pray to kill eveyone on earth and it usually gets better.

 

I don't agree. On a cold start the car should start in open loop mode. The ECM doesn't look at the O2 sensors so it's just running without them.

When it's hot, it starts and goes straight into closed loop mode which means that the O2 sensors are now in play.

A bad cat shouldn't affect cold running. But will cause issues when the system is hot.

 

Ok. A bad cat could cause problems with engine hot & not cold. Would these problems clear up once yout increase rpms? When it does spuuter and pop with engine hot, I can eventually get it to settle down but running rpms up to 1500(ish) and letting it run. Then it does great, no problems.

 

My ffuel trims are showing Short 4.7% and Long 32.8 %
Im also pulling a P0133 code, 02 Slow response bank 1 sensor 1
& a P0130 code, 02 sensor circuit bank 1 sensor 1
Of course the 98 only has two O2 sensors, an upstream & downstream.
I replaced the upstream 6 months ago when I started having issues. I could pull and clean it & see it that helps.