hard hot start, throwing O2 codes

[MatSLO]

1997 Tahoe with 5.7 Vortec - trouble starting when warm (starts when I fully depress the gas pedal on startup). When cold, it starts immediately, "just show it the key".
Otherwise no issues, truck runs good, always idles smoothly and performs fine.

Checked coolant temp sensor, seems to be working fine.
However, the scanner showed following fault codes: P0141, P1133, P1153 and P0161 (in other words, all four O2 sensors)... BTW, CEL is not on (yes the bulb works)

Before that, I thought the issue might be the fuel pressure regulator, but perhaps I need to replace O2 sensors? Or do O2 sensor readings get messed up because of the problem with the fuel pressure regulator?
Or is it something else?....

And if replacing the fuel pressure regulator, would it make sense to replace the whole spider assembly (with the new MFI spider) while I'm at it?

 

[RawbDidIt]

1997 Tahoe with 5.7 Vortec - trouble starting when warm (starts when I fully depress the gas pedal on startup). When cold, it starts immediately, "just show it the key".
Otherwise no issues, truck runs good, always idles smoothly and performs fine.

Checked coolant temp sensor, seems to be working fine.
However, the scanner showed following fault codes: P0141, P1133, P1153 and P0161 (in other words, all four O2 sensors)... BTW, CEL is not on (yes the bulb works)

Before that, I thought the issue might be the fuel pressure regulator, but perhaps I need to replace O2 sensors? Or do O2 sensor readings get messed up because of the problem with the fuel pressure regulator?
Or is it something else?....

And if replacing the fuel pressure regulator, would it make sense to replace the whole spider assembly (with the new MFI spider) while I'm at it?

You're going to need more information in order to proceed. It very well could be the fuel pressure that is causing your hard start issue, or it could be the EGR system. More tests would tell you which way to go. Start with a scanner, that will tell you whether your O2 sensors are actually bad, or whether you're just seeing old stored codes. All 4 of those codes should throw the dummy light, so the fact that they aren't means the bulb is burned out (you stated it isn't) or the problem cleared, but nobody removed the code from memory.

From here forward is pure speculation: I think you have old codes in your system, and your O2 sensors are either fine, or reaching end of life and working ok, but slow to react causing codes to be thrown sporadically. Even if you test it and the O2 sensors are bad, they don't affect startup warm or cold, they only affect performance after the system goes into closed loop once the O2 sensors warm up. For your problem, that pretty much eliminates them as a suspect. That said, you'll need to test more to see what the issue is. It could be the fuel pressure regulator as you mentioned, could be the EGR system. Vacuum system is somewhat tied to both, so you could look there a well. If you have to replace the FPR, there's no reason to replace the spider if it's working fine unless you want to. It's readily available if you're looking to upgrade, but not necessary if it's working fine for you.

Sent from my SM-N976V using Tapatalk

 

[HotWheelsBurban]

Don't know if this will help, but here's my experience with something like this. 99 Suburban 5.7 vortec (the truck in my picture). In January it began to be hard to start when hot. In greater Houston, Texas, January is cool, 40s and 50s farenheit. When making short trips for work, only sitting for 30 minutes or less, it wouldn't want to start. Longer time sitting, it was fine. Finally refused to start for 3 hours. When the wrecker gets there, guess what happens: fires right up!
The starter is 3 years old, battery replaced last April. I always use a bigger ( more cold cranking amperage than required)battery. Replaced the left front O2 sensor a few weeks earlier, both rear ones are less than 2 years old.
Took it apart, noticed the starter was dirty. Oily dirty. Cleaned it up, took to the parts store to have it checked. Works fine! Also noticed the battery cables are a little corroded. On these trucks there's 2 positive cables, one from battery to starter and the other from battery to main computer on left side of engine compartment( inner fender panel). This is the one that goes across the fan shroud, in the little notch. These cables are very expensive in the USA, can't imagine what you'd have to pay in Europe. I found a battery shop that could make cables; they checked my cables out and made a new one for the one that was bad. It was the 8 foot long one that goes from the battery to the main computer. When I reassembled everything, I saw that the small terminal on the starter solenoid was loose. That goes to the ignition and it activates the solenoid to make the starter drive engage. Tightened that up, put truck back together, no more problems. The fuel injection issue may be part of it ,but I think it is more of an electrical problem. Good luck and post up what finally fixes it.

 

[HotWheelsBurban]

Just read Rawb's post; I agree with him on the codes. Oxygen sensors usually go out gradually, and when they do malfunction, the computer will usually throw a code. That's how I knew my truck's were going out. The rear ones are no big deal to change, so I replaced them once they'd coded a couple of times. The ones in the front are another matter though, on accessibility. I ended up taking the truck to a local shop and had them change it. I couldn't get the right angle for leverage on it under the truck. And it was the original one, so it was pretty stuck!
You need a good scan tool with these trucks; so many of their problems are computer related.

 

[stutaeng]

I doubt the O2 sensors would cause the no-start when warm, as they are not really needed to start an engine.

I'm assuming you have checked for spark when the symptoms occur? I would start there.

It may be a fuel pressure regulator. Do you have a fuel pressure gauge? They are around $33 USD. Hook it up and note fuel pressure with key on (engine off) and let it sit for 15 minutes. The pressure should hold within a few psi.

Could also be a injector(s) stuck open, flooding cylinder with fuel after warm shutoff. Or they die... Watch this video:

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Somewhere on the minute 32 he shows what a good injector should do...


The O2 sensor codes may or may not be related, at least some of them.

 

[MatSLO]

Firstly, I apreciate the replies so far, thank you for taking your time.

I doubt the O2 sensors would cause the no-start when warm, as they are not really needed to start an engine.

I'm assuming you have checked for spark when the symptoms occur? I would start there.

It may be a fuel pressure regulator. Do you have a fuel pressure gauge? They are around $33 USD. Hook it up and note fuel pressure with key on (engine off) and let it sit for 15 minutes. The pressure should hold within a few psi.

I guess I have spark - when I depress the gas pedal to the floor, the engine starts fine.
I've read somewhere that (in cars with computer fuel injection), flooring the gas pedal on startup actually cuts fuel to cylinders, to help start a "flooded" engine. I don't know if it's true or not, but it works...
BTW, the problem is more pronounced in warmer weather. I started noticing it this spring when it got warmer. Later, we got a cold spell and the problem almost went away. Now it's back.
No issue with startup in the morning or when going home from work. Also starts (but sometimes hesitantly) if I start the engine up soon after shutoff (up to 10-15 minutes after). But if I drive somewhere and return to the car after 30 minutes, or an hour or two (for example, grocery shopping), I have to press the pedal to the floor to get it started.

O2 sensor codes are probably not directly related (perhaps more as a consequence, not the cause).


Ignition was overhauled a few years ago (new spark plugs, wires, ignition coil and distributor cap); in autumn of 2019 starter was replaced, and also the battery.


Although I'll have it checked by my mechanic when I take the truck to him for oil change, I'm suspecting fuel pressure regulator or injectors, possibly both. I guess I'll have to order the whole (upgraded) spider assembly - if my mechanic has to take the upper engine apart to access the fuel pressure regulator, might as well install the upgraded injector spider while he's at it...

 

[east302]

I've read somewhere that (in cars with computer fuel injection), flooring the gas pedal on startup actually cuts fuel to cylinders, to help start a "flooded" engine. I don't know if it's true or not, but it works...

Yes, that is correct. You mentioned that you checked it, but do you have access to a scanner that can read the coolant temperature sensor output?

Hard hot starts are a common symptom of a flaky CTS that is reporting a wrong temperature.

A fuel pressure test may be in order to see how long it holds pressure once the ignition is turned off (leaking injector).

 

[MatSLO]

Yes, last week I took the Tahoe to an A/C shop (condenser needs replaced) and the very helpful guy who works there has a ~$1500 scanner which showed the o2 codes.
But the first thing we checked was actually the coolant temperature sensor which appears to be working fine - it indicated the expected temperature, and the reading was consistent with the dash gauge (which, as I understand, gets its reading from a separate sensor)