coupeguy2001

I went from a 48 to a 52mm throttle body, and reset the TPS and min idle. '86 coupe........auto
zz9 cam, 383, ported factory exhaust manifolds, true dual exhaust with dual cats, BBK TB, gasket matched intake plenum, runners and base manifold. Initial timing set at 11 degrees BTC, Bosch III injectors, pertronix coil, 88 corvette prom, new plugs and plug wires. EGR is connected and A.I.R system disconnected. stock 91 firebird 700R4 and firebird converter.
radiator is a dual aluminum core with a 170 thermostat, the engine on the freeway runs 180, and in town, stop and go, 195-200
Engine seems to run great until I come to a stop light and expect it to idle for more than 30 seconds, then after a little bit, it just lopes and jerks around till I put it in neutral, then unloaded, it is better.
I reset the idle TPS to .437 in order to lean it a little. It's better, but the same thing is happening.
The engine runs really good up to that point.
My next move is to change the O2 sensor.
Mileage readout claims 19.5 avg.
With the 350, it was 24.6 avg. with the same sensors.
I put the fuel gauge on it, and it was 50 at idle,vacuum connected, so I turned it down just a little bit to 45 with vacuum, should I get it down to 42 with vacuum?
The problem as I see it, is it gets stinky rich at idle after a little bit.

The car is driven daily in traffic and on the freeway 50 miles a day total.

383vett

If it's smelling rich, it must be rich. Turn down the fuel pressure. Maybe you'll get a better idle. I have mine running at 39 with vacuum and around 43 without. You're probably loading up your plugs after 30 seconds of idling causing your stumble. Willie

0Paul Ruggeri

You may want to get a wideband O2 so you can see what your car is doing. Have you retuned the ECM? My 86 with an LT4 Hotcam used to surge at idle until I redid the low RPM timing tables. That calmed it down a lot.

383vett

Paul, he doesn't need a wideband if he has a nose.

neverendingprojects

Best answer so far in this post!

rodj

just did a HSR / 219 cam /L98 setup ( MAP ).
Rich , surging idle as soon as it went into closed loop until the low end tables adjusted ,
now it just purrs down low

neverendingprojects

Some of you guys seem to be speaking about things related to bigger cams,,, The ZZ9 combined with a 383, and MAF should not really get crazy at idle like the OP stated.. Regardless of tune... He mentions problems since the upgrade of a bigger TB..

coupeguy2001

The idle sounds really good at the back just before it falls off it's regular idle. Like a nice better than stock factory cam. Sort of how a early corvette with a .030-.030 cam used to sound like......or a 70 Z28 Lt-1 with factory mufflers. sort of lopey but not rumpity rump.

The engine does not surge and hunt, it just loads up, and I guess the O2 sensor is telling the computer to cut back. When it does, it's like it cuts off the fuel all together, then turns it back on.
I turned down the fuel pressure to 40, and it's a bit better.

Paul, did you retard the timing tables or advance them?

My "retune" thus far was to get an eprom that was closer to what the engine is. Originally, the engine was a flat topped piston 350 with 76 cc iron heads with shim type head gaskets. I went to 128 heads on the 350 with the flat tops, and .040" head gaskets and retarded the timing. Now I have a 383 with SRP 14cc dish pistons and 113 heads that have been pocket ported, and milled .008. They have been relieved around the intake valve area about .100" to enhance any swirl motion that might be present. I'm using .040" thick head gaskets, oh, and 1.6 roller rockers.
So now I have an eprom for an '88 with aluminum heads and dished pistons. My compression is probably somewhere around 10.2

rons85

My guess is that you are going to need to dial up the ignition advance in the low load/low rpm region. Where it's running at idle (in or out of gear). Not knowing where you are now I'll say that you probably Should have a Minimum advance of something like 20 deg. Maybe as much as 25.

Not the "offset", or the correction for where the distributor is installed, but the lowest advance number anywhere in the table.

Try just advancing the distributor itself, make a healthy swing at it, and see if you can "cure" the idle problem by doing so. You'll find out if that will fix it..

Also - what size (lb/Hr rating) are the Bosh III's? Is the Rest of the fuel curve something close to correct? There are tables in the chip for temp and voltage corrections to injector pulse width, also one for low pulse width correction. That "low pulse width" correction can be a pain - if the "correction" is larger than the pulse width itself the ECM will Never be able to get the flow rate right in that range.

I took that entire "low pulse width" correction table to Zeros in my stuff, finally. Just to get it out of the way.

PLRX

Has this issue with my 95. It only liked 1+ PSI.