Cold start vs Hot start, is this normal? (1 Viewer)

[Mtntopper]

Well, during the lockdown this past fall I did a head gasket and valve job, replaced the engine harness with a new Toyota part as mine had damage near the EGR valve, sent the injectors to RC to have rebuilt, new fuel filter and still experience this issue. I dont think its quite as bad though. I have a theory as to what might be causing it. When the engine is cold, the ECM is in open loop mode and is operating on a set of preprogrammed parameters. For cold starting the fuel tables are probably set to run rich. As the engine temp warms the ECM sees the ECT signal and goes into closed loop at a set temperature and at that point starts accepting inputs from various sensors like oxygen sensors etc. At normal engine operating temp the ECM leans the air fuel mixture. I almost think when the stumble happens its a lean condition. If your engine has a lot of carbon on top of the pistons like mine had I think that might make the lean condition worse. I didnt replace my ECT sensor when I had the head off but I wonder if it could be that? I also know that the TPS is very critical to engine function and needs to be set accurately. I have noticed that although my hot stumble hasnt disappeared completely, it is not as prevalent as it was. Any thoughts on this?

Maybe related: I'm currently having a new Toyota short block put in my 97 LX450. The guys that are doing it in Denver have extensive experience with the 80 series. They insited that I will only be happy with new Toyota Fuel Injectors. They told me that any time they have had old injectors sonic cleaned, it has made them operate worse. Replacement with new injectors is the only way they will touch it.

 

[vegasfj40]

Maybe related: I'm currently having a new Toyota short block put in my 97 LX450. The guys that are doing it in Denver have extensive experience with the 80 series. They insited that I will only be happy with new Toyota Fuel Injectors. They told me that any time they have had old injectors sonic cleaned, it has made them operate worse. Replacement with new injectors is the only way they will touch it.

Hmm.

 

[ajax1]

Maybe related: I'm currently having a new Toyota short block put in my 97 LX450. The guys that are doing it in Denver have extensive experience with the 80 series. They insited that I will only be happy with new Toyota Fuel Injectors. They told me that any time they have had old injectors sonic cleaned, it has made them operate worse. Replacement with new injectors is the only way they will touch it.

We would not do the sonic injector clean on 1fzfe ever unless customer really wanted, seemed to always have issues after

 

[ajax1]

Interesting. I did replace the ECT sensor, so I can rule that out.

I haven't touched the TPS because I'm a little unsure of how to apply vacuum according to the FSM and to verify it is set properly. Using my ODB2 scanner the TPS value usually shows up around 11% at idle. I did find a thread here that indicated that was likely acceptable. That said, it does seem like a path to go down and verify it's set properly. I've found most things the PO did were not, so if there is a small chance it was touched, I'll assume it's not set right.

My tps is set to 7 at idle in drive per scan gauge reading

 

[Jaketh3snake]

Anybody ever find a resolution here? My 95 exhibits identical symptoms as OP. My research has lead me down the open loop at startup and preprogrammed fuel tables theory as well (how I found post).

Once started all is well. I did notice my LTFT sits around -10%… so my trucks “normal” is rich. Thinking FPR is malfunctioning . My working Theory is being when it’s hot, the preprogrammed fuel table doesn’t send enough fuel, causing the stumble. Running out of things to try