What causes engine to stumble on tip in.

[nsxxtreme]

My 383 with tune runs really great with a minor issue.

Every once in a while not always it will stumble when you get on the throttle from idle. I have a 54mm TBI.

What I have read and seen is its not enough air getting past the butterfly valve at idle. You can crack that valve open some to get more air by but then run into problems with idle surge because the idle valve isn't able to keep up.

This doesn't occur all the time the majority of the time things work perfectly. But it does suck sitting at a light and then stepping on it and have the engine die especially when pulling a load. Smash the throttle more and it wakes right up. Any other things I might try?

 

[zombiyjay76]

Good thing I saw your post, I'm going through exactly the same thing. And I was going to start a post my self.

 

[eagle mark]

To answer the title question: AE = Acceleration Enrichment. It is the pump shot equivalent of a carb in EFI.

Bigger TB equals more air which needs more AE. Why yours is only sometimes? Just barely enough and engine gets hotter at the time it happens? Battery voltage a little low from idle when it happens? Voltage = fuel pressure.

With a TBI it is always good to have throttle blades open as much as possible to mix air and the fuel coming in above the blades. More so on modiifed engines. Trick is still keeping a few IAC counts counts to control idle. Stock setting is to high IAC counts and as wear happens over time throttle body blades close more = even more IAC counts/air through IAC not throttle blades.

Look at IAC counts when engine is fully warmed up and driven a little. If above 20 then open throttle blades 1/4 turn on the stop screw. Turn engine off for 30 seconds and restart to let ECM reset TPS% back to 0%. Rev engine for a minute and recheck. 5 to 10 is optimum. Get this right before doing any tuning adjustments. Will be more or less at other conditions like cold...

 

[nsxxtreme]

To answer the title question: AE = Acceleration Enrichment. It is the pump shot equivalent of a carb in EFI.

Bigger TB equals more air which needs more AE. Why yours is only sometimes? Just barely enough and engine gets hotter at the time it happens? Battery voltage a little low from idle when it happens? Voltage = fuel pressure.

With a TBI it is always good to have throttle blades open as much as possible to mix air and the fuel coming in above the blades. More so on modiifed engines. Trick is still keeping a few IAC counts counts to control idle. Stock setting is to high IAC counts and as wear happens over time throttle body blades close more = even more IAC counts/air through IAC not throttle blades.

Look at IAC counts when engine is fully warmed up and driven a little. If above 20 then open throttle blades 1/4 turn on the stop screw. Turn engine off for 30 seconds and restart to let ECM reset TPS% back to 0%. Rev engine for a minute and recheck. 5 to 10 is optimum. Get this right before doing any tuning adjustments. Will be more or less at other conditions like cold...

I think I have played with the AE. Doesn't help. If I open the throttle blade up to where it no longer stumbles on tip in then the IAC valve isn't able to control the idle. It seems I can't get enough air going over the throttle valve at idle. I'll check the other stuff you recomended. I was curious if a hotter then stock ignition might improve things?

 

[eagle mark]

I was curious if a hotter then stock ignition might improve things?

Oh yeah, I assumed you had some tuning already with the 383. Also assuming it's not a stock cam adding some idle/off idle timing will help a lot. Probably need to work on the entire spark table for that motor.

 

[nsxxtreme]

Oh yeah, I assumed you had some tuning already with the 383. Also assuming it's not a stock cam adding some idle/off idle timing will help a lot. Probably need to work on the entire spark table for that motor.

I wasn't refering to the spark table that tune has been done. I was referring to the components such as the coil and distributor, producing a hotter spark. The cam is a roller lunati 60120 cam. So it has decent lift.

 

[eagle mark]

Opps. Not really if everything is working well of course.

That's a lot of cam to be trying to make run well off idle. Specs are 1600 RPM up for one I looked at so you have an area from idle to 1600 RPM that the cam is really not designed to run in.

If you have it as good as only an issue once in awhile I'd say you've done better then most.

 

[nsxxtreme]

Opps. Not really if everything is working well of course.

That's a lot of cam to be trying to make run well off idle. Specs are 1600 RPM up for one I looked at so you have an area from idle to 1600 RPM that the cam is really not designed to run in.

If you have it as good as only an issue once in awhile I'd say you've done better then most.

That's what I was afraid of.

 

[eagle mark]

What RPM does this idle at? Increasing it to whatever you can stand can be a help with lots of things. With an Auto trans having a higher stall convertor is another big help.

 

[nsxxtreme]

What RPM does this idle at? Increasing it to whatever you can stand can be a help with lots of things. With an Auto trans having a higher stall convertor is another big help.

Idles around 900 rpm. Much higher and that would cause it to fail smog test.
I'm debating on scraping the blazer project and getting a newer turbo diesel truck.

I'll probably hang onto this for another year though. The power this thing puts out is fricken sweet. My only engine complaint is this occasional stumble. So I'll probably just have to learn to live with it.

 

[63chevyll]

Did you ever get your oil.pressure issue figured out?

 

[nsxxtreme]

Did you ever get your oil.pressure issue figured out?

I don't know that I have any issue at all. I'm not a Chevy guy this is my first Chevy. I followed some advice the first time around which was to use a low volume high pressure pump. With an oil cooler and the added centrifugal force of the 383 flinging the oil out and the sloppy clearence of eagle rods, add that all together and probably not the best idea.

The new build I used a high volume pump and threw a 7qt oil pan on for good measure. The block and bottem end were replaced I wasn't taking any chances. I never drop below 10 psi per 1000 rpms. So I was probably being a little over paranoid because of what happened the first time around. I hooked my boat up and towed about 300 miles with it with no issues this last weekend. Towing was finally enjoyable

If I were to start from scratch again I'd stay away from eagle rods. There clearences are pretty sloppy.

I bought the truck originally to just thrash on and do some mild wheeling. Then ended up being a great camping rig. Then I decided to tow with it and that's when all the weaknesses started showing up. Thing went out of control from there Its a fun truck but I haven't done anywhere near as much wheeling as I thought I would. I use it mainly for towing and for that I think a diesel would work out better. I'll probably use this truck the rest of the year to tow my boat.

 

[63chevyll]

Good to know, but sounds like it hauls butt too.

I hear ya about the rods. I see no need for aftermarket rods under 400hp, then at that I use LT1 rods now.

GM nailed everything about the SB chevy except air flow

 

[nsxxtreme]

Good to know, but sounds like it hauls butt too.

I hear ya about the rods. I see no need for aftermarket rods under 400hp, then at that I use LT1 rods now.

GM nailed everything about the SB chevy except air flow

I used to hate this truck, with the bigger motor its a blast to drive. Towing is cake now, I have no problems accelerating up hill while towing my boat. When I built the motor I bought a 383 kit. So rods and crank all came balanced and ready to go. I figured it was the easiest way to go. I just made the mistake of going with eagle rods because I didn't know any better.

If I did more wheeling and I trailered it to where I wanted to go it would be a blast. But it's really hard to get used to getting bounced around on leaf springs. I've heard good and bad things against some of the newer diesel truck so who knows I might hold onto this thing beyond this year.