Backfire caused high idle when warm???

[Bakinblk]

A few weeks ago had a real good backfire on start.(yep,anticipated/let off the starter before it fired:eek )Now when its wamed up it idles at 1200-1500, the coast down idle is about 2000(was 1500) Looked for visable loose/disconnected vacuum lines. Secondary vac pump runs as it should(comes on initially and then cycles a couple seconds at 5-10 min. intervals) Have cleaned/checked/replaced:
MAT(out of spec ohm reading)
IAC ok (did try a new one, no change)
Throttle position ok
Throttle body cleaned (was real dirty)

After its warmed up if I stop, let it sit 15-20 min, on restart it idles 8-900 for a few min but always back to 12-1500 when fully warm
Is all I have left to look for under the Plenum???
I don't mind doing it, I'll just be pi$$ed if that aint it.

Also the chip does'nt have the GM code/pn I expected, its 1068008
It does have a small sticker handwritten with:
1991 ZR-1 15MARCH01 Mr
Any help/info is greatly appreciated!!
I've had the car 6 mo. and love it but its a tough troubleshooter

 

[offshore]

Was backfire through intake or exhaust?

 

[Bakinblk]

Intake and about as loud as a carb w/no aircleaner & no hood
Reminded me of some old hotrods I've had

 

[offshore]

That sucks! do you suspect a vac.leak?

 

[offshore]

If you do and have checked "all" hoses try this remove pcv valve block hose w/3/8, bolt block pcv hole, take off oil fill cap hold your palm over it if you feel a suction the intake gasket blew into lifter vally. if not than I would the maf. and that could be what caused the backfire in the first place. I had one go bad once that it caused so much fuel to dump before the car fired up that when it did it blew the valve covers off the engine! I'm not saying that is what caused your backfire but a backfire that vilonet could easily damage the maf.

 

[Tyler Townsley]

Check the back of the plenum there is a 5 in hose that blows off on backfire on early LT 5s. Goes from the map to a hose boss on the plenum.

Tyler

 

[Demps]

You have a Mark Randolph chip and that is a GOOD thing. Probably on a stock car, the fans turn on sooner, the power key is default on, and the CAGS is disabled. Also, there is probably some leaning of the fuel.
Now to your problem--There is a vacuum line, more than likely, off. Check the obvious MAP hose and the back of the plenum and under the aluminum bolted on cover. I'd bet that it is the EGR/vapor cannister hose under the throttle body. You can see this hose from the driver's side throttle body. Track the PCV/EGR hose to the throttle body. Behind it is the likely cause. Look in there with a flashlight and see if it is attached. It'll have a slip ring type clamp on it to hold it on. The IAC cannot compensate for the extra air. If you need more info, email me and I'll go take a pix.

I saw you posted on the CAC as well. If I can find the link, I'll post it there.

ted

You need to find Rob's under plenum pix. It is the hose off of the TB going rearward.