Engine stumble returning to idle

#1

After my complete rebuild, the engine turned over and fired up just fine. It idles well, revs well, but doesn’t not return to idle brilliantly. Seems like a vacuum leak, but I’m open to other suggestions to chase this issue down. The injectors have new o-rings, I replaced a cracked vacuum line from the valve cover to the throttle body. The AFM to throttle body boot is pretty new and looks good. One part of the vacuum system I don’t understand is the venturi clap-trap on the side of the throttle body. What does that stuff do? Can I simplify that to reduce connections and failure points?

I still need to put some new gas (as opposed to 1-1/2 year old gas)before I let the engine run for more than 30 seconds and break it in. But I would really like to chase this issue down in case it’s a real problem other than just returning to idle.

#2

Great news that it is actaully running again. But anyway i would double check your valve adjustments and check for vacuum leaks. The thing on the side of your tb unit is the idle control motor. that is constantly changing to adjust idle speed. Make sure your dipstick o-ring is good, that will make it idle like crap. New fuel and a fuel filter should also help. Moisture would have settled to the bottom of the tank from sitting that long.

Taking it to a dyno and checking air-fuel and hp would help figure out if it is just an idle problem or all rpm or load issue. If its just idle, heck dont worry about it, just make sure it makes power at redline:)

Good luck,
Peter Thibault

#3

Yeah, dyno time would probably be the best way to gather information. Perhaps in a week or two…

I know what the idle control valve does, it’s the other side of the TB I’m unsure of. There are two ports, one leads to a venturi and some other things I’d like to ditch if possible.

New fuel going in soon, I’ll probably pull the x-over pipe and drain a bunch of old crappy gas out and fill up with 93 octane just to offset anything left behind (old 87 octane).

#4

It sounds like you are talking about the two aluminum ports on the tbody that connect to back to a little valve, and to the rubber elbow and also provide vac to the brake booster.

I’ve seen one car that simplified that set up with only a single hose going back from the “other side” of the tbody, but most everyone runs it as-is. What you should do tho is pull the metal ports free of the tbody on both sides, clean them, apply some JBWeld, and shove them back in. That will eliminate them as a vac leak trouble point.

I didn’t understand “to a venturi”. I know what a venturi is, but I don’t know where on the intake you mean.

#5

The first things I’d suspect would be; a) intake leaks, b) a bad TPS, c) a dirty/sticky ICV. The ICV can be cleaned with carb cleaner, the TPS tested and replaced if one of the switches is bad, and intake leaks can be ruled out by a smoke test.

#6

My first though was the ecu trying to learn the ways of a new motor, but not everyone here believes that happens. I wouldn’t turn off the kill switch unless I was on fire though.

#7

[quote=“turbo329is” post=65121]My first though was the ecu trying to learn the ways of a new motor, but not everyone here believes that happens. I wouldn’t turn off the kill switch unless I was on fire though.[/quote]What, someone doesn’t believe in long term fuel trim?

#8

People who bring half a dozen ecus to the dyno don’t get it.

#9

I consulted my friend RealOEM.com and found it’s called an air pump (11611284269), but my current AFM to TB boot doesn’t have a connection to it, so the air pump is just capped off. Might be another leak point, can I delete that guy completely?

#10

Also consulting RealOEM I discovered that I did not install the necessary o-ring(s?) on the bitch tube.

#11

That is plural. There should be an o-ring at top and bottom. See my build thread for what happens when the bottom o ring is unhappy.

#12

That is plural. There should be an o-ring at top and bottom. See my build thread for what happens when the bottom o ring is unhappy.[/quote]

The good news is that the top o-ring should be easy to get on. The bad news is that I don’t know about the bottom one. I guess I could take the intake manifold off…again.

#13

I think the intake manifold will need to come off. The trick with getting the intake manifold back on is to compress the spring on the bitch tube with two flex ties. Also, removing a stud makes this easier because you’re not forced to put both intake manifold and tstat housing into place simultaneously.

Process:

Remove top stud that fastens tstat housing. Loosen nut at bottom of tstat housing and rotate tstat housing forward so temp sensor clears fuel rail.

Put thin flex ties on bitch tube spring such that when tight they will compress spring.

Put big washer and oring on bottom of bitch tube. Put oring on top of bitch tube. Put tube into block.

Put manifold in place and fasten all the nuts. They are supposed to be 12mm nuts, not 13mm nuts. Cut flex ties on tube and guide it up into place into manifold.

Rotate tstat housing back into place and put in a long 8mm bolt to sub for the stud you removed.

#14

Yeah, I have a nice blend of 12mm and 13mm nuts for my intake manifold :dry:

Think a little RTV is worth trying on the bottom part before pulling it all apart again? Or waste of time and just a mess in the end?

#15

[quote=“OriginalSterm” post=65144]Yeah, I have a nice blend of 12mm and 13mm nuts for my intake manifold :dry:

Think a little RTV is worth trying on the bottom part before pulling it all apart again? Or waste of time and just a mess in the end?[/quote]
That would be a waste of time and is unlikely to work. Pull the intake and fix it right.

#16

[quote=“jlevie” post=65145][quote=“OriginalSterm” post=65144]Yeah, I have a nice blend of 12mm and 13mm nuts for my intake manifold :dry:

Think a little RTV is worth trying on the bottom part before pulling it all apart again? Or waste of time and just a mess in the end?[/quote]
That would be a waste of time and is unlikely to work. Pull the intake and fix it right.[/quote]

I know, I know…

I do it nice, 'cause I do it twice!

#17

Replaced some vacuum lines, o-rings on the bitch tube, but not the dip stick yet. What size are those two o-rings?

#18

Got some o-rings, swapped oil fill caps with another one that “looked better” (new one in the mail). Stalls when letting off the throttle now, not just a stumble.

Also, sounds like I need to do a valve adjustment once it got warmed up and run for a bit.

#19

If you run the oil breather to a catch can and plug the ports on the throttle body you don’t need to worry about vacuum leaks on the block.

#20

Minor valve adjustment, new oil fill cap. Idles great!!!