Rig

Hey all,
I recently did a 305 Vortec engine swap into my 89' 1/2 ton truck (sorry - not a car) using a Holley adapter plate and stock TBI with the GM 4bbl Vorte EGR intake.
My problem is that I have a shuddering problem at low RPM's and a load - like at cruise on the highway.
I have messed with the timing - seems to worsen as you retard the timing. At zero degrees it runs like crap - low on power, shudder, etc. 8-10 degrees and it runs not bad, but the shudder is still there. After the shudder, a hint of black smoke exits the tailpipe. The plugs have a nice tan on all 8 cylinders. Tailpipe looks sootier than usual.
I have replaced the following:

wires
cap/rotor
plugs (#3 Delco Rapidfire) .035 gap
fuel filter
fuel pressure test (13psi steady)
air filter
EGR valve (disconnected - custom chip)
IAC valve
MAP sensor
oxygen sensor
ignition module
ignition coil
pickup coil
distributor shaft
battery
alternator
new 98' 350 exhaust from front to rear, other than cat.

I'm stumped! Any ideas?

Thanks for any assistance!

IROCketZ28

Rig,
Is your truck exhibiting the shudder when it cold too, or just when it's warm (I'm assuming it does it when it's warm since you said it does it when you are cruising down the highway)?
Have you checked for any vacuum leaks? (TB to adapter, adapter to intake, intake to heads, valve covers, vac. advance at dist., etc.)
I'm no TBI expert, so I can't offer any input on correct fuel pressure. I'm sure someone will point it out if its not in the correct range, though.
Finally, is it throwing any trouble codes?
I realize this isn't a ton of help, but hopefully it'll offer some insight to your problem. Good luck!

------------------
'87 IROC Z28
TPI 305
5-speed w/ Centerforce clutch
3 1/2" exhaust w/ Flowmaster
Clarion, Kenwood, Eclipse, Polk, MTX, Autotek 800 watt stereo system
Grant wheel
Otherwise stock, & good for a 14.68 at DeSoto Dragstrip
More to come soon...
Shooting for high 13's with bolt-on's

Rig

Thanks bud,
Yep - I've scanned the crap out of it with a propane torch, plugged off the brake booster - nothing, no visible or audible vacuum, leaks. I even replaced the intake gaskets and retorqued the manifold with GM bolts. Still seems to run like there is an intake leak.
HOWEVER - the adapter plate I'm running is a Holley version with the 2" throttle bore openings - some of the internal passages are exposed. Is this a known problem part? Is this adapter plate OK to use with the stock TBI?

I'm running a custom chip from fastchip, with the EGR circuit disconnected. The comp is not throwing any codes either.

PS: The shudder seems worse when cold, but is always there, unless the truck has been running for over an hour, where it seems to be better, but just won't run 100%.


[This message has been edited by Rig (edited April 23, 2001).]