1994 K1500 - Rough Idle, Help!

[94OldRed]

Hey Guys,

Recently, my truck has been running pretty poorly. I just rebuilt the engine about 3 years ago and it only has about 20k on the engine. A year after rebuilding it, I put a Whipple TBI supercharger kit on it. It ran fine for a while, but now it's starting to act up. On a cold start, the engine idles really rough. It stutters and wants to die. I can give it gas and keep it above 2k RPM, then let it drop down and then it wont die right away, but it will still idle rough. I've been replacing sensors left and right trying to chase it down, as none of the sensors were replaced during the engine rebuild. So far I've replaced,

• Cap & Rotor
• Spark plug wires
• Spark plugs
• Ignition control module
• Coil
• Idle air control (replaced 1 year ago)
• 02 sensor
• MAP sensor
• Coolant temp sensor

I'm not getting a check engine light, which is weird. The truck drives fine, it's just when it idles it wants to constantly die. As it warms up, it seems to idle slightly better, but not much. I know having a supercharger kit on it is complicating things, but I'm at my whits end on what else can be causing poor performance. Any help is greatly appreciated.

 

[magimerlin]

What's the fuel pressure at? Last fuel filter?

Have you checked for codes anyways for the heck of it... they can be there even with out setting a light..

sent from what use to be a great country...

 

[BoominSVX]

I can't speak for the whipple side as I've never played with one, but there are certain things that engines do which can be replicated using a scan tool. If I cracked open the EGR (provided you have one) it would do what you describe. 94's can't really see what's happening there because the OBD system isn't so much bi-directional on those trucks. You might try pulling the EGR and sealing the holes to see if you have a vac leak. I had a bad EGR once. Did the same thing. One day I was screwing around with a scanner and started opening the EGR with the truck off, then unplugging it. Each time, the pintle would hit harder and sound more solid. Eventually it sealed. The truck has been running great since 2011. The quicker way for you would be to pull it off and clean it. Also, you may have replaced the IAC, but that doesn't mean it's good now. Pull it out and see if you have any carbon on the pintle or in the bore that could stop things up. Cold engines need more fuel. A stuck IAC can play all kinds of games. If that kit came with a MAF sensor, I'd clean that too. I only do this with MAF cleaner. Gotta be very careful. I'd imagine whipple might have added one for environmental variables. Finally, spray some carb cleaner around the intake to check for vacuum leaks. Don't forget the front and back of the lower intake area. in the valley. If it idles up or dies, you have a vacuum leak. You wouldn't believe how often I come across dirty stuff that just needs to be cleaned. If none of that works, I'm out of ideas. You seem to have nailed out most of the basics. Do you smell any fuel from the exhaust when this is happening? Rich exhaust (burns your eyes) means it's got the fuel, but lacking on air. These are all very general ideas. Do with it what you will.

 

[ram1220]

I second the EGR.

 

[94OldRed]

What's the fuel pressure at? Last fuel filter?

Have you checked for codes anyways for the heck of it... they can be there even with out setting a light..

sent from what use to be a great country...

Fuel pressure is at the recommended levels per the whipple manual, I can't remember off the top of my head. A brand new fuel filter and fuel pump were put in during the supercharger install. I'll have to source a scanner and see if anything pulls up.

 

[94OldRed]

I second the EGR.

I can't speak for the whipple side as I've never played with one, but there are certain things that engines do which can be replicated using a scan tool. If I cracked open the EGR (provided you have one) it would do what you describe. 94's can't really see what's happening there because the OBD system isn't so much bi-directional on those trucks. You might try pulling the EGR and sealing the holes to see if you have a vac leak. I had a bad EGR once. Did the same thing. One day I was screwing around with a scanner and started opening the EGR with the truck off, then unplugging it. Each time, the pintle would hit harder and sound more solid. Eventually it sealed. The truck has been running great since 2011. The quicker way for you would be to pull it off and clean it. Also, you may have replaced the IAC, but that doesn't mean it's good now. Pull it out and see if you have any carbon on the pintle or in the bore that could stop things up. Cold engines need more fuel. A stuck IAC can play all kinds of games. If that kit came with a MAF sensor, I'd clean that too. I only do this with MAF cleaner. Gotta be very careful. I'd imagine whipple might have added one for environmental variables. Finally, spray some carb cleaner around the intake to check for vacuum leaks. Don't forget the front and back of the lower intake area. in the valley. If it idles up or dies, you have a vacuum leak. You wouldn't believe how often I come across dirty stuff that just needs to be cleaned. If none of that works, I'm out of ideas. You seem to have nailed out most of the basics. Do you smell any fuel from the exhaust when this is happening? Rich exhaust (burns your eyes) means it's got the fuel, but lacking on air. These are all very general ideas. Do with it what you will.

Thank you for the suggestions. I'll have to fiddle around with the EGR and see if that's the culprit. EGR was one of the few things that was replaced during the engine rebuild, though that was 3 years ago. I'll pull the IAC as well and check for any carbon.

Also, I was just troubleshooting the throttle position sensor. Would that cause it to idle rough? I'm gonna pull out my multimeter and try to test it and see if the voltages are off. I have not replaced that yet.

 

[magimerlin]

Fuel pressure is at the recommended levels per the whipple manual, I can't remember off the top of my head. A brand new fuel filter and fuel pump were put in during the supercharger install. I'll have to source a scanner and see if anything pulls up.

So you have not done a fuel filter in 2 years then? Ummm time to do one I would think....

sent from what use to be a great country...