GM TBI sucking air until warmed up

For everything related to using Fuel Injection in an FSJ.
User avatar
Grand_Wag_85
Posts: 1557
Joined: Sat Aug 27, 2011 8:46 am

GM TBI sucking air until warmed up

Postby Grand_Wag_85 » Tue Jul 10, 2018 10:06 pm

I am running a TBI on a 258 in my AMC Eagle. Until the car warms up it sounds and acts as if there is a HUGE vacuum leak. You can hear the moto sucking excessive air and the car does not want to idle until. Once the car is warmed up it idles perfectly and the sucking noise goes away. I thought that this was a leak between the TBI and intake manifold so I sprayed carb cleaner EVERYWHERE while the motor was cold but no surge in idle speed, except for one runner where the intake meets the head. I also went and plugged off every nonessential vacuum port but still no luck.

Any other spots on the TBI where they are known to leak? This is a "rebuilt" unit so who knows what it's been thru.
82 J10
65 J2000

You know it's bad when your car is on the EPA's 10 most wanted list!

User avatar
shimniok
Moderator
Posts: 765
Joined: Sat Jun 09, 2012 8:40 am
Location: Centennial, CO

Re: GM TBI sucking air until warmed up

Postby shimniok » Thu Jul 12, 2018 5:35 am

That is a heck of a vac leak if you can hear it. Could it be a leaking intake manifold? Are you sure it is really a vacuum leak?

What about the air cleaner housing -- snorkle valves working correctly? If the flap that closes to prevent overrun doesn't open when the engine is running it could make a racket and would block airflow preventing proper idle.
Broken Photobucket image in my post? PM me.
'86 GW "Troubled Child" tc.wagoneer.org
360, TBI, 4" Skyjacker, 33" BFG MTs, WT D44+ARB, WT AMC20 + LockRight, CB 2m 6m 70cm, K0FSJ
The world's first Robotic Full Size Jeep!

User avatar
Cecil14
Posts: 525
Joined: Mon Oct 01, 2012 6:40 am
Location: Carson City, NV

Re: GM TBI sucking air until warmed up

Postby Cecil14 » Thu Jul 12, 2018 9:14 am

Make sure the intake itself isn't coming loose. I had that happen on a 258 many many years ago, didn't end up tracking it down until it had caused some catastrophic damage. A new intake gasket isn't terribly expensive or difficult to replace on the 258, worth taking a look. If you grab the intake on the outside (near the TBI), can you wiggle it up and down on the head?


aa
Anthony

1983 J-10 - 4.6L(MPFI)/CS130D/Hydroboost/NV3550/D300/44/44/3.54/Disc-Disc/32s/42 gallon 'burb tank

User avatar
babywag
Posts: 1036
Joined: Fri Jun 08, 2012 7:29 pm
Location: Land of Fruit Loops & Coconuts

Re: GM TBI sucking air until warmed up

Postby babywag » Sat Jul 14, 2018 1:32 pm

IAC will make noise on cold startup
open element filter? first tbi?
bad fuel map?
bad IAC?
-Tony
'88 GW (aka Babywag) '90 GW (aka JunkBucket) both fuel injected.

User avatar
JeepsAndGuns
Posts: 141
Joined: Sun Jun 10, 2012 4:12 pm
Location: cullman, AL

Re: GM TBI sucking air until warmed up

Postby JeepsAndGuns » Sun Jul 15, 2018 5:38 am

If the IAC valve is pegged wide open it will make a sound like that.
Could be at cold start, it is having to peg wide open to try and keep the engine running, then once warm, it closes back off. Cold engines need more air and fuel to idle than a warm engine. Take the air filter off and do a cold start when you hear your noise. See if it is coming from the throttle body/IAC valve.
If so, my guess is the min idle speed is not set correctly. With the IAC all the way closed, the throttle blades should be open enough to let the warm engine idle at about 100 rpm below your desired (what the ecm is programmed to) rpm. If the throttle blades are closed too much, every bit of the air at idle is having to rush through the IAC valve.
79 Cherokee Cheif. MPFI 401, T18/D20, hydroboost, otherwise stock.
93 YJ. 4.0, 5 Speed, SYE, hydroboost, WJ knuckles/brakes, 8.8 rear, OME 2.5in lift, warn M10000 winch. 31x10.50's
99 WJ limited. 4.0, auto, 2wd, stock.

User avatar
Curly
Posts: 201
Joined: Fri Mar 13, 2015 11:31 pm

Re: GM TBI sucking air until warmed up

Postby Curly » Mon Jul 16, 2018 10:28 pm

Agree on the IAC being the culprit, normal sound. They can be quite noisy on cold start as the air rushes through that small opening.
I start what I finish.

77 Cherokee S
97 Jeep Wrangler

User avatar
Grand_Wag_85
Posts: 1557
Joined: Sat Aug 27, 2011 8:46 am

Re: GM TBI sucking air until warmed up

Postby Grand_Wag_85 » Sun Aug 26, 2018 7:10 pm

You guys were right, it's the IAC. Just fired it up without the air filter in place and traced the noise back to the IAC.

I'm guessing something is not set up correctly. The car is VERY hard to start and you have to keep your foot on the throttle for the first minute or so. After that it runs perfect.
82 J10
65 J2000

You know it's bad when your car is on the EPA's 10 most wanted list!

cmaje72
Posts: 193
Joined: Thu Jul 07, 2011 6:33 am
Location: Western NY

Re: GM TBI sucking air until warmed up

Postby cmaje72 » Mon Aug 27, 2018 6:29 am

Sean,
Its been a little bit since I messed with GM TBI but I think in winaldl there is a way to reset the IAC? I would have to get out the really old tuning laptop to find out.
79 Cherokee 5.3/4l65E/NP241C
79 CJ7

FSJ Guy
Posts: 1763
Joined: Fri Mar 15, 2013 3:03 pm

Re: GM TBI sucking air until warmed up

Postby FSJ Guy » Mon Aug 27, 2018 7:26 pm

Yes, there is a procedure to drive the IAC closed. then you disconnect it and adjust the idle speed. Reconnect it and restart. You should be much closes there.

IIRC, once adjusted and warm, you want the IAC to show about 50 steps when datalogging.
Ethan Brady

http://www.bigscaryjeep.com

Jeepless, but I still have a lots of old parts in my garage!

User avatar
babywag
Posts: 1036
Joined: Fri Jun 08, 2012 7:29 pm
Location: Land of Fruit Loops & Coconuts

Re: GM TBI sucking air until warmed up

Postby babywag » Tue Aug 28, 2018 11:47 am

-Tony
'88 GW (aka Babywag) '90 GW (aka JunkBucket) both fuel injected.


Return to “FSJ Fuel Injection Tech”

Who is online

Users browsing this forum: No registered users and 2 guests