Some please help-Very bad idle, running rich porblems
I have 350 TPI/L98 sbc( 1990 speed density 5.7 ltr)
Motor was customly put into 1995 chevrolet Z28 convertible, pain in the *** to do. Had to get custom motor brackets done
performance specs: Everything new
stage 2 nitrous 150-250 range(custom welded into after market bigger intake tubes)
custom grind 230/230 cam advert:286/286 .566/.566 intake;exhuast lift 112 lope from comp cam.
computer controlled HEI big cap with after market in-cap coil
MSD 8.5mm wires from Summitracing
Spark plugs gap NGK brand .45
cloyes timing chain all set at 0 dot to dot( distributor put in on compression stroke)
bored 40 over
SRP forged dome tops( bumping compression up to 12.7)
JE moly rings properly end gaped
1.52 compcam roller rockers, screw in studs
ARP head bolds
race ported corvette aluminum heads 210-215cc(resurfaced/new valve seals)
ported intake plenum
ported throttle body 58mm
1227730 standalone ecm and harness
custom chip from HarrisPerformance from ebay
heated 3-wire 02 sensor
true dual exhuast for now
h-beam 5.7 lenght eagle rods
forged 1988 chevy pickup truck crankshaft 1pc. rear main seal(balanced and rotated/lightened by professional performance shop Matt Johnsons in Saint Louis,MO)
700r4 tranny..1300 invested alone with every high performance part in it(shift kit, 13 vane pump, 5 planetary gears, superhold servo, lastest clutch plates inside, wider band,hardend sun shell, etc..)
stock rear next to be changed asap
I set valve lash/ valve train accordingly
I have doubled checked tps, knock sensor,MAP sensor, distributor, Coolant temp sensor, O2 sensor, ground and power wires.
MY motor timing fluctuates at idle from 8advance to zero on the timing tab located on the timing cover. Runs so rich after warm up my eyes water like onions
When I increase throttle by hand the power stumbles and drops, but it will pick up and sound mean . Im also running 110 racing fuel to keep from knock and pinging due to my calculated compression ratio. Lastly when given throttle at say 1500 r 2000 rpm the balancer mark show 8 degrees retarded. My HEI distributor in advanced just alittle bit but i know i did something wrong with the timing. What should I do?
Havent even drove the car out my garage yet untill i get the timing right.
Last edited by 68NovaSS; 03-10-2010 at 06:56 PM. Reason: Other.
The stock GM TPI ECM will not run with a big cam.
The reduced intake manifold vacuum at idle created by the camshaft you have fools the electronics into thinking the engine is under more of a load than it is, therefore it dumps in more fuel than the motor needs to idle.
A major remapping of the ECM fuel and spark maps calibration is nessessary.
That cam will want increased spark timing at idle (20+deg).
Plan on doing some chassis dyno testing and tuning if you ever want it to run correctly.
You may find some expertise on the TPI forum at
Last edited by F-BIRD'88; 03-10-2010 at 06:34 PM.
idle with your cam you need about 25 degrees advance. Are you using
the stock computer?
All of the people around here that run EFI on GM engines are using the engine
management systems from the LS series engines. Even with the older vortec
engines they swap out the computer with the newer stuff. These computers
are much easier to tune. As for the old TPI set up its not worth the effort.
moneymanshawty, its time to upgrade.
You need to talk with the people at TPIS not some squid on Ebay. TPIS has done a lot with the L98, if anybody can lead you out of the wilderness it's them.
Twenty years for a production fuel injection system is an eternity. Can you
make these antique systems work for a high performance application? Yes;
but its just not worth the effort.
the learning curve in computer tuning is steep no matter which system you go with.
part of the reason the newer OBD2 computer systems are more forgiving with a tune that isn't dead on because of the MAF sensor.
they also have better tables than the older systems.
there are people on thirdgen that have bigger cams than you have and they were able to tune the factory computer to work just fine.
i believe most of them tuned the ECM to run in open loop.
the 7730 ECM and 8D mask that you have is an old system, but its only been recently that the aftermarket has really caught up with it.
the 7730 and 8D are one of the best hacked DYI factory systems out there and there is a ton of info about it at thirdgen in the stickies in DYI PROM.
some people got together and assembled a much better than stock mask call S_AUJP_V4.
with it, you can set it to idle in open loop and go into closed loop off idle. it has a hi-res extended VE table that you can use.
there are also a number of other enhancements to it too.
there are also some people working on code for forced induction for the 7730 ECM.
to tune the 7730 ECM, if you already have a laptop your looking at around $150.00 going bare bones.
with DYI computer tuning, a good wideband O2 sensor is a big plus regardless of the system you go with.
if you decide to get into computer tuning and which ever system you go with, be prepared to do a lot of reading.
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