rough idle after new motor
 

Cracked a head on my old motor and just dropped in a new Vortec 350 but I am having some problems. Here is the setup: Factory Vortec 350, with my original TBI with GM #12496821 intake. Everything bolted up pretty well, just had to do some plumbing for the EGR. The problem I am running into is now that I have the truck out of the shop it idles kinda rough and is a little hesitant in the A.M. Ran diagnostics and the temp sensor was not registering the correct temp, so I replaced it. I also went ahead and replaced the throttle position sensor, no help. My only thought at this point is that when I cracked the head, the TBI filled with steam from the block, and might have hosed something up. Any sugesstions?

Idle kinda rough, as in missing?
Try posting it on the 73-87 Truck/Van page on http://www.chevytalk.com also, alot more people to respond.

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Chris Lucas
1973 Chevy C-10
1975 Chevy Silverado
Captkaos Customs

Don't think it is missing. Set timing, replaced cap/rotor, plugs are new, and wires still look good, idle remains rough. Will try O2 sensor this weekend. Are there rebuild kits available for TBI's? May consider that next.

Running rough usually points to an EGR related problem. Could it be when you plumbed the system it is constantly pumping in exhaust gases which would make a poor idle and poor cold engine performance. You did not say if it smoothes out in higher rpm or not but if it does I would check if the EGR is stuck open. Check the valve, you do use a valve when retofitting a Vortec right, and see if the diaphram is opening and closing and if it is open at idle it would explain the symptoms you gave.

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69 CST 10, 402, steel crank, heavy rods, Federal Mogul Forged Pistons, 9.9:1 compression, polished ported 781 heads 2.19/1.88 manley stainless valves, Crane Powermax Hydraulic roller 288/296 and .587,.610 lift, hardened pushrods, Crane Roller Rockers, Weiand Action Plus, 750CFM Edelbrock QJET, MSD HEI, MSD 8MM Wires, should be about 500 H.P. 700R4 2500 stall, pretty nasty shift kit, 373 Auburn Pro Series Posi, buckets/console air/tilt/tach 4wheel discs
Daily Driver: 92 Chev Reg Cab LWB Silverado 4x4, loaded, 350 Accel filter/cap/rotor/coil/AC Delco RapidFire plugs, Edelbrock intake and headers, true duals 700R4 with B@M Shift Kit for now!

EGR is closed at cold idle. I am wondering if the ECM needs to be relearned for the new engine. Is there an easy way to clear the ECM of its learned parameters?

I just replaced the engine in my truck not too long ago and it did the same thing...
If yours is still idling rough you might want to check it over for vacuum leaks, mine had a leak at the base of the egr valve that when I resealed it the rough idle quit.

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86 GMC l-bed 4x4w/ 350 4-bolt main, 700-r4, Edlebrock performer intake, Edlebrock 700cfm
boublepumper, Edlebrock polished aluminum valve covers and air cleaner, MSD ignition & coil w/ splitfire dual-core wires with Hei distributer that has a new curve-clamp kit installed...
373 rear-end infront and back(by Richmond)

Replacing my factory TBI with a Holley unit really helped it. Later that week, 8-17, truck was stolen and haven't heard anything since.