Truck idles rough at start up only.

[JACK34]

I have a 1994 with a 350 engine. I have replaced the distributer, IAC, TPS, coil, temp sensors, o2 sensor only had one and I removed the catalytic converter, tank and fuel pump are new and tested for pressure all good, fuel lines are new, cap and rotor, wires and plugs are new. Recently it started doing this. On a over night cold start it starts fine then immediately starts to idle rough for around 45 seconds then smooths out after that all is good and it drives fine. 2 days ago it did the same thing at my work only sitting for 9 hours. All the grounds have been replaced and are good. I just replaced the plugs a week ago and the old ones were around 5 months old and they all looked good. I thought it may be the valve seals so I changed the oil and put a additive in but so far with 75 miles on it nothing has changed. I also added some Sea Foam in the tank but like I said nothing has changed. Any thoughts would be appreciated. Thank you.

 

[tayto]

check/replace EGR valve

 

[Schurkey]

Connect a scan tool, look at the data stream.

 

[Sabinoerc]

so hard to troubleshoot something like this without data. I think I agree with shurkey that you’ll probably need to look at data stream to figure out what’s going on. Some people out a fuel pressure gage in-line at TBI to be able to confirm pressure remains good/is good for diagnosing stuff like this.
if you are good with a laptop, TunerPro is good, free-ish.

 

[JACK34]

check/replace EGR valve

Thank you for the advice. I purchased and installed EGR valve today. Before I did I started it up in the street and it was running rough like I said pulled it into drive way let it run for around 10 minutes. Installed EGR and had to remove a couple of vacuum lines and IAC connecter. Before I did this I unhooked the Neg post on the battery. Put everything back together and started it up. Runs great but now idles at around 1700 RPM, I let it run until it was warmed up still didn't snap out of it. Going to drive it tomorrow we will see how it starts and acts in the morning. Whatever I did changed something.

 

[Schurkey]

Why would you replace the EGR valve without testing it first?

 

[tayto]

usually after i disconnect the battery on EFI vehicles i let it warm up until it is up to operating temperature with our interfering with throttle, gear, accessories etc . make sure you have the right gasket and hoses connected properly. the map sensor is in that area it's possible you disconnected the vac line to it or broke it.

@Schurkey , i helped a friend last year that had a similar problem and ended up being the egr valve. i had been doing a "mechanical restoration" for him and had rebushed and rebuilt the throttle body, injectors professionally cleaned, new TPI fuel pump & filter, cleaned IAC&TPS and verified operation with scan tool, heated o2 sensor, removed car and new exhaust from Y pipe back, plugs/wires/cap/rotor/PCV valve, replaced vac lines, CTS, etc etc etc. The truck would run very rough intermittently, at first usually after sitting for a week+. I ended up driving it for a few weeks datalogging and troubleshooting. the truck actually came with a new EGR valve so I ended up throwing it on and it solved the problem. this is the only reason I suggested it.

 

[JACK34]

Why would you replace the EGR valve without testing it first?

Another member suggested it, it was cheap and I have replaced everything else on the truck might as well keep going now.

 

[JACK34]

usually after i disconnect the battery on EFI vehicles i let it warm up until it is up to operating temperature with our interfering with throttle, gear, accessories etc . make sure you have the right gasket and hoses connected properly. the map sensor is in that area it's possible you disconnected the vac line to it or broke it.

@Schurkey , i helped a friend last year that had a similar problem and ended up being the egr valve. i had been doing a "mechanical restoration" for him and had rebushed and rebuilt the throttle body, injectors professionally cleaned, new TPI fuel pump & filter, cleaned IAC&TPS and verified operation with scan tool, heated o2 sensor, removed car and new exhaust from Y pipe back, plugs/wires/cap/rotor/PCV valve, replaced vac lines, CTS, etc etc etc. The truck would run very rough intermittently, at first usually after sitting for a week+. I ended up driving it for a few weeks datalogging and troubleshooting. the truck actually came with a new EGR valve so I ended up throwing it on and it solved the problem. this is the only reason I suggested it.

I think you are exactly right I am going to replace all the vacuum lines next week. And map sensor and EGR solenoid. If that doesn't do it I'm not sure what else I can do. Thanks for the help.

 

[JACK34]

so hard to troubleshoot something like this without data. I think I agree with shurkey that you’ll probably need to look at data stream to figure out what’s going on. Some people out a fuel pressure gage in-line at TBI to be able to confirm pressure remains good/is good for diagnosing stuff like this.
if you are good with a laptop, TunerPro is good, free-ish.

I'm not good with a laptop. I just recently had the fuel pressure checked and it was good. It's very close to the point I may just put a Carb on it.