Rough Idle on Warm Startup

[punkrex]

I have a very annoying intermittent problem. Basically goes like this, exploder is warm and I turn it off. When I go to start it back up, rpms rise like normal, then drop below idle, to the point of almost dying, the recover to normal idle.

What I have done with no success:

Replace intake manifold gaskets.
Sprayed intake and hoses with starting fluid to try to find air leak.
Cleaned MAFS.
Cleaned throttle body.
Replaced plugs.
Swapped fuel relay with A/C relay.
Seafoamed fuel on last 5 tanks.

85k on 4L flex engine
Replaced fuel filter ~65K (not an easy job :thumbdwn
Recently blew driver's side cat into muffler, replaced both. but problem existed before bad cat...

 

[punkrex]

one thought was maybe a fuel check valve. Any help would be appreciated

 

[Rarestace]

have you check your fuel press to see if its not a fuel delivery issue cause by a weak pump

 

[adambant]

I have a very annoying intermittent problem. Basically goes like this, exploder is warm and I turn it off. When I go to start it back up, rpms rise like normal, then drop below idle, to the point of almost dying, the recover to normal idle.
QUOTE]

I have the exact same issue, and have replaced about the same as you trying to chase this issue as well as trying to smooth/steady the idle.

I have replaced:
-Plugs and wires
-ALL 4 O2 sensors
-Fuel Filter
-Air Filter
-Upper AND Lower intake manifold gaskets
-Ran seafoam in 3 tanks of gas, and added thru brake booster 2 times.

Cleaned:
-IAC
-MAF

 

[punkrex]

have you check your fuel press to see if its not a fuel delivery issue cause by a weak pump

If its a weak fuel pump then why would it start just fine when its cold?

BTW I have no CEL for this issue.

 

[punkrex]

I'm finding that this doesnt happen on a full tank of gas...

 

[adambant]

hmmm, interesting. I filled up yesterday, and it has still done it 4 out 5 warm starts between last nite and today.

 

[punkrex]

I really think its a check valve issue with the fuel pump. Simmilar to old vapor lock issue on carbed engines. I'm going to check my fuel pressure to verify my assumption. It would be an easy fix if thats the case.

 

[adambant]

I know where the schrader valve is to check the fuel pressure, but where is the check valve?

 

[punkrex]

I believe the check valve is integrated with the fuel pump