97 vortec 305 intermittent low power, poor mpg rough idle in park.

[McSweeney]

I have a 97 c1500 5.0 vortec it has weak acceleration, poor idle, and horrible mpg. Ive replaced a lot on this engine including all ignition except for the distrutor, and upgraded the injectors to mfi. Still feels the same. Once in a while it will feel better like it has good power again, but usually only after it sits for a day or two. My MAF seems to be working properly, about 5-6g at idle and when I accelerated up a hill at 2k it went up to 78g. I unplugged the maf while it was running and nothing changed at all. I thought maybe a vacuum leak but i dont know how to start with checking for one, and if it was a vacuum leak its not like it would be from pulling the upper intake because the truck had the same power before and after. I could really use some help

 

[WhiteUCF]

There's a couple of ways to go about checking for vacuum leaks. A smoke machine or a can of carb cleaner. A smoke machine will fill the system with smoke and you'd hopefully be able to see where the smoke is coming from. The normal method is spraying carb cleaner at the vacuum lines while the truck is running and listening for the idle to pick up.

With your coolant issue, it may go deeper. Could be a head gasket. Might try a compression test.

It could be anything from the maf to a clogged catalytic converter. Coolant temp sensor can cause dumb issues like this too.
Here's a link with the resistance values for the temp sensor.

ECT sensor

Im having some cold starting issues with my 95 2500 5.7. When the ambient temp is low roughly 20 f or lower it has to run for roughly 3-5 minutes before it will go into high idle. Every other EFI engine Ive had when you start it when its cold it will idle around 1200ish rpm. This one runs...

www.gmt400.com

Any check engine light/ codes? It's tough to diagnose stuff on the internet. Hopefully you can get this figured out.

 

[McSweeney]

There's a couple of ways to go about checking for vacuum leaks. A smoke machine or a can of carb cleaner. A smoke machine will fill the system with smoke and you'd hopefully be able to see where the smoke is coming from. The normal method is spraying carb cleaner at the vacuum lines while the truck is running and listening for the idle to pick up.

With your coolant issue, it may go deeper. Could be a head gasket. Might try a compression test.

It could be anything from the maf to a clogged catalytic converter. Coolant temp sensor can cause dumb issues like this too.
Here's a link with the resistance values for the temp sensor.

ECT sensor

Im having some cold starting issues with my 95 2500 5.7. When the ambient temp is low roughly 20 f or lower it has to run for roughly 3-5 minutes before it will go into high idle. Every other EFI engine Ive had when you start it when its cold it will idle around 1200ish rpm. This one runs...

www.gmt400.com

Any check engine light/ codes? It's tough to diagnose stuff on the internet. Hopefully you can get this figured out.

Haha yea it is hard to diagnose on a forum. Probably is a headgasket failing, guess ill just get to that when it totally fails. It has a new cat, and a new temp sensor. I might be chasing a couple different issues that i think are related which i know better than to do. This truck is great but annoys the hell out of me. Im going to check fuel pressure tomorrow maybe the pump is having a hard time. But I doubt it since it starts up on a dime. Like i said some days it runs like its brand new and others it has no guts. Ive also got that jounce in the suspension that someguys get because the frame is poorly designed so idk what to do about that. I appreciate the response!

 

[WhiteUCF]

You're welcome! If I can think of anything else tomorrow when my brain is actually working, I'll let ya know.

 

[Schurkey]

Verify fuel pressure.

Connect a scan tool, verify all sensors, and computer outputs. Check the misfire counts. Check for codes.