crazy427

Ok here is the rundown I have a
383 stroker,
9.5-1 compression,
F.I. compatible cam,
dart iron eagle heads,
edlebrock tbi intake
670 tbi with a spaced stock injectorpod
venom hp 88lbs injectors & vacuum ref.Fuel pressure reg
stock distributor, performance module and MSD coil
Long tube headers/dual flowmaster exhuast
02 sensor is in the collector
and a custom burned chip

Now Here is the problem. When i start it up cold it backfires thru the exhuast for the 1st 30 seconds to 1 min of running. after that it is fine.
NO check engine light
No codes
With the scaner hooked up when it is acting up , all the sensors are working properly <temp sensor with in range alon withall the others.
Fuel trim is good
fuel pressure is good
timing has been set from 0deg to 4 deg, Runs the best at 2 deg but does the backfire thing no matter what.

It isnt running rick or lean and after that 1st miniate it runs like a F%^&ing Animal.

HELP!!! im at my wits end, I even call the place that burned the chip and they never heard of anytihng like thisand couldnt steer my in any direction except to brin the truck to there shop in oregon i believe.. long hual frion Jersey.
ok at idle when acting up:
block intergrator--128
block learn-- 131
tps--.96
map vacuum---13lbs
o2 --- open loop and lean
after less than 1 miniate o2 switches to rich
abd when i bring up the rpms to 1500 it goes closed

I belive the o2 is working properly. it backfires the entire cycle of the o2.
Now get this --- while it is acting up if i go to functional tests in my scanner and kick on the back-up fuel& spark , it smooths out, runs good , no backfiring and seems to have more power.
unfortunatly i cannot select between fuel and spark to test them individually also i cannot find what the perimiters for back testing.

Any help would be appreciated,

Thanks Joe
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89 Blazer-F.I.383,Racing 700r4,np241,7in lift,ORD steering and suspension Equiped,Frt 10b locked,Rr 14bff posi,5.13s 37x12.5x16.5 ssr swampers, softtop, 9000lbs winch,dual snorkel

dimented24x7

iTrader: (2)

Some things can happen if your scanner does more then just passivly take in data, like maybe place teh ecm in ALDL mode. Id try working with the open loop afr and some other things that control fueling on startup. Sounds like it may be lean, or possibly rich. Mine used to run rich enough to blow smoke when it was cold and that with not enough timing it would spit and pop for a bit untill it warmed up.

crazy427

How do i work with the open loop air fuel ratio?
My expertise is with transmissions, differentials, suspension and carb tuning. This new engine has been kickin my but for a few weeks now and im not sure where to go next. besides my scanner, i dont have any equipment to handle this.My buddies over on the coloradok5 website told me to check out over here and see if i could get some help. Im sure with the right directon i can get this thing straitened out.

Thanks Joe

dimented24x7

iTrader: (2)

I guess the first step would be to learn the basics of ecm tuning. The stickies on the DIY-Prom board are a good place to start. After that you can get some bargain basement prom burning stuff and tinker with the fueling/added timing at cold temps to see if you can smooth it out. Itll probably be easier then the long distance mail order chip thing.

va454ss

iTrader: (1)

Define F.I. compatible cam. MAP seems low for the previous.

Sounds like you are lean and or timing is off.

Tell us about the Venom injectors.

With the engine totally cold, crank up the fp 1 or 2 lbs and try starting it. Of course this may throw off your tune in other areas, but at least you can tell your chip guy what you found.

You could also try the same thing with the timing.

Dewey316

there is a coolent temp compensation table that will add fuel, adding some fuel to the low temp areas in that table might help.

crazy427

venom HP injectors are rated at 10% over stock.I have the 80 lbs big block injectors so they are 88lbs.i dont know my exact cam specs.i tried base timing everywhere from 0 to 4 def. it is best at 2 but the backfiring is the same throughout.i have a vacuum refrenced fuel pressure regulator it is at 18 lbs at idle and moves to 20 under load. the guy who burned my chip said it should be set to 14 lbs so i am gonna to take it apart , drop it down and see where im at

va454ss

iTrader: (1)

What p/n injectors do you have?

According to this, they list the same injector for every TBI application 4.3 thru 7.4

http://www.venom-performance.com/nf/...ettruck/c2.htm

va454ss

iTrader: (1)

Something is awry there.

You should have approximately 1 lb drop in fp with every 2 inches of vac.

You have 13 in of vac at idle, so your fpr should drop from 20 with no vac to ~14 at idle.

What source are you using for your fpr?

crazy427

It is venom injector Part # hp645-800. i called them to the the specs and they said 88lbs per hour .

Now the regulator is refrenced by the throttle body, it goes in the stock location and has a tube that runs over by the injector pod. I got it from gm and believ it is off of a early 90s chevy 454ss truck

va454ss

iTrader: (1)

Did they tell you what fp that rating is at?

The reg needs a constant vac supply from either the TB or the intake.

crazy427

venom didnt tell what the fp was suppose to be for that rating. but they did say they were stock replacment so i would guess stock fuel pressure would surfise. I was goin to cut the tub on the regulator and run it to a vacuum source on the manifold but i figured it was suppose to work theway it was set up and didnt want to cuase problems by messing with it.

va454ss

iTrader: (1)

That's what you need to do.

crazy427

story of my life... when i play , i pay and when i deside not to play..... I STILL PAY!

I will lower my fuel pressure, correct the regulator and take it from there. I still want to mess around with chip burning though.....if im gonna pay i might as well play.Looks complicated though

crazy427

well..... strike 1

I installed the correct spring and the delay valve for the vr fpr. It does run a little better but still have my backfire. I have also tried to adjust the tps , clean the iac, and move the timing off the chart in both directions. It runs good and it runs bad but eitherway it still has the backfire. the only way i can get it not to backfire is to activate the back-up fuel and spark thru my scanner.

Anything other ideas

va454ss

iTrader: (1)

Hate to say it, but we're all playing a guessing game.

You're relying on long distance tuning by someone that has never seen your vehicle, so your tune is iffy, at best.

The only way it will ever perform to your expectations is to tune it yourself.

Go to the DIY Prom board and start reading

crazy427

yeah thats what i figured, i Have already begun.. Thnaks For all the help ..everyone

crazy427

DING,DING,DING,DING... We have a winner!!!!


In a last valiant effort before the big prom burning plunge i desided to swap out my distrbutor wth my trail spare.
Dropped it in set the timing to 0 deg, and fired her up. purred like a kitten. so i readjusted all the little things i have been playing with trying to figure out the problem. and made sure my new fuel regulator set -up was operating properly and took her for a run. everything looks good ,runs smooth and my cold starts are right on the money. she also idles better --a smooth 800 rpms.

Thanks again everyone for all your help.

Joe

dimented24x7

iTrader: (2)

kinda wierd that thats how the problem manifested itself. On mine with lots of dead ingition stuff it sort of ran ok at low load and startup but it broke up as soon as I hit the gas.

Cool to hear you got it fixed.

crazy427

Yeah i though it was wierd too. upon inspection everything looks good also, But you have to remember my truck is subjected to long hours on the road and trail when i go on my 4-wheeling trips along with being subjected to mud and water. I Try to kept everything very clean and well lubed with dielectric grease but shyt happens.