since i have been running this new engine i have tried everthing under the sun to get rid of a off-the-line hesitation! below is all i have done.
-set my ve table to 128
-set the spark curve to accomodate and compared to other bins.
-base timing to zero
-change all sorts of pump shot to major rich shots to lean shots(454 pumps and greater, and 350 stock pumps)
-have no vaccum leaks at all
-have no egr on the engine and chipped out the egr
-2deg spark knock retards (very conservative)
-remove wot delay
-engine goes into pe mode without a problem
-wot afr at 11.4
-changed bpw from 112 to 135 (present bpw at 118)
-changed fuel filter twice
-even used stock 454 bin
-new 3wire heated o2
-new wires,plugs, cap, rotor
note: my cam is advance 6deg for max torque low rpm

still have the hesitation with approx 200 prom burns and tuning. i got so frustrated with this i decided to install a fuel pressure gauge tonight, off the tbi. pressure is at 12-13psi. i can make the hesitation happen from neutral by just quick opening of the throttle. doing this tonight i saw the pressure drop to 9-10psi immediately. i do have dual tanks with stock pumps in both tanks and both pumps give the same gauge readings. when i am driving and give a quick throttle response it will sometimes completely die and other times just hesitate then accelerate. using the scan tool and in closed loop i do see the o2's drop to 5-10mv, yes i know lean as hell.

my next step would be to install a new 255 walpro pump and this should ensure a constant fuel pressure during max pumpshot and pe. before installing this fuel gauge i thought it was going to be worse than what i have, especially on fast throttle action. i was sure it was falling to like 4psi, not sure if putting underload will cause even further pressure drop. tomorrow i will strap the gauge to the windshield and see what happens.

other than this mild excel is just fine and even wot i it will not fall. 02 are not at constant 850 to 900mv but they are bouncing to these numbers.

unless you guys have other suggestions i have missed i sure could use them.

383 stroker, 64cc 23deg trickflow heads c/w weiand xcelerator single plane, adapter, cam is an extreme 4x4 comp. Cam. 12-235-2 grind ( .447/.462 lift, duration 210 218 lift @50, 111 lobe separation) . unit has hypr. Flattop floating pistons, eagle cast crank, 5.7eagle rods. 10.2:1 comp. Headers c/w 2.5” flowmaster exhaust. 16147060 pcm, $85def
454tbi 80lbinjectors

 

Check your TPS, I know it may not seem like the problem, but my car was doing that a while ago and I tossed on the one from my old throttle body and it curred the problem.



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1990 Iroc
Canadian Iroc (305 TBI)
-14x3 open element, K&N
-Accel: Coil, Wires, Cap&Rotor
-B&M stage 2 shift Kit
- 4 255/50/ZR16 BFGoodrich Comp TA's
-Hypertech chip, low temp thermostat and fan switch
-1.6 ratio roller rockers
-3" exuast: High flow cat, meanstreak muffler, dual 2.5" outlets
-Holley projection Manifold
-670 Holley TBI.

 

[QUOTE]Originally posted by IrocJT:
[B]Check your TPS, I know it may not seem like the problem, but my car was doing that a while ago and I tossed on the one from my old throttle body and it curred the problem.

my tbi is a brand new unit when i started this engine project. the scan tool shows position properly when i step on the peddle for all different situations.

 

The ECM does not transmit fast enuf for any TPS errors to show thru the ALDL monitor. To check it and get a definate answer you need an oscilliscope.

 

Fellow truckster, you answered your own problem...

You have a fuel delivery problem. FP drop in neutral on EFI with throttle blips is as bad as it gets. Just imagine what would happen if you were in gear and the engine was being loaded????? The FP drop will be much more drastic!!!!!!!!!!!!!!!!!!!!!!!! LEAN!!!!!!!!!!!!

Anyhow, you have an inferior fuel pump, restricted/restrictive fuel lines (3/8' ID minimum, NOT OD measured!!) are required!!!!! return line can be 5/16, or something else I cannot think of right now....

I have the Auto Meter fuel pressure gage with isolator in-cab and it NEVER moves from the set presure more than maybe .25psi. Like maybe, and the movement is esentially impercievable and it only happens at WOT 5000rpm+... It should NEVER move like your does and that fact that it does in neutral is not good, in fact it is bad.

Are you sure your pressure gage is accurate and mounted correctly??? Is the "T" for the gage causing a possible restriction and associated gage readings??? Is your pump OLD/OEM??? Is your FP an EFI pump???

 

Sounds like a fuel pressure problem. I think I have the same problem as you. The hesitation isn't ever time time but sometimes it's nasty. I've played with everything and the only way I can avoid it is if I crank the fuel pressure and run 116-118 BLM at light throttle. It's starting to **** me off and I think I HAVE to do the fuel pump. I'm making 200% more hp and 150% more torque...I just think this fuel pump is doing it's job anymore.
I'd like to know what the fuel pressure is suposed to drop too max. I'm going to remote mount a pressure gauge to my hood and watch the pressure under different runs. I've tried everything to get rid of my hesitation and I've narrowed it down to the pump. This is the reason I haven't posted any 1/4 times...I'm just waiting to afford some larger injectors and a fuel pump.

 

Yeah it does. At least for me it does. For it to be working you just need to see that it's at betwen .4 and .6 at idle and more than 4.5 at WOT. You can do this when the key is in the on position.

 

If there is a bad spot in the TPS, the only thing I've ever seen that will show where it it's bad is an oscilliscope. Most voltmeters and such usually skip over it b/c they do not scan continuously. In other word, the refresh rate is too slow.

That's how I do it



[This message has been edited by V8Astro Captain (edited November 02, 2001).]

 

i check on previous scans and my scantool will show throttle position voltage at 0.8 with no movement on the throttle and 4.45v at 100% topening.

fb, my pumps are oem stock. what pressure do you run and what aftermarket pump do you use. i am looking at the 255 walpro, to much pump for my stock reg?

i installed a new fuel fitting and 3/8tube then connented a weatherhead full bore compression tee which i then snap a gauge on a special fitting. now i can check pressure without having to leave the gauge on afterwards. it is also 4" away from the tbi.

[This message has been edited by prscarf (edited November 02, 2001).]

 

i think i woke up my 383 for the first time and fixed my problem without even installing the new pump. i changed the bpw to 142, reduce my entire ve table 12% and altered the map pumpshot table. i increased this table heavily compared to stock. this all made a big difference. i couldn't get it to hesitate at low rpm. it surprised the hell out of me, i can actually breakloose my 35" tires on blacktop for the first time (posi diff). my blm still looks not to bad but will require futher tuning.

 

I'm glad you got that worked out. I just re-read the post and before I got to your last reply I saw that you had tried changing the BPW from 112 to 135.
The more I thought about it the stranger the situation seemed. Any slight increase in BPW will cause a noticable difference and with that large of a jump there must be something major wrong like you're still far from the actual value....which you were. Glad you found the right BPW. A few guys were confused but the higher the BPW the more fuel added.
I figured part of my problem out a couple days ago. Turned out that my thermo was broken. Robertshaw thermos break in the open position and that is why I didn't think about my thermo being bad. I was installing my heating hoses and figured I might as well install my 180 thermo. WOW, what a difference, I'm really glad I checked. Now I need to double check my fuel pressure and remove this darn proflow air cleaner!

 

changing the bpw i have done before and it made no difference to the hesitation, just made everthing richer. it was the pump map that did the trick. my fuel pressure is at 12psi

 

If you have fuel pump/fuel pressure problems as you described, you are just "band-aiding" the problem by changing the PROM to enrichen everything up to make up for the lack of proper fuel pressure/delivery. FIX the problem or you will have to Re-PROM if/when you fix the fuel problem.

fuel pressure changes as you describe are very severe and should be fixed ASAP! I don't know if a Walbro 255 is to big... If your fuel problems continue, you could be in trouble/stranded later on as your fuel pump may be overworking itself!!!!

My advice would be to IMMEDIATELY set up some sort of FP gage that you can monitor while driving. Either an in-cab isolated one or mount one temporarily outside on your hood or something... Monitor FP while driving. If you can perceive a drop in FP on the gage at all, you have a problem... If you see more than a 1psi FP drop at low rpm/throttle settings, you have a SEVERE problem... You'll see HUGE FP changes (drops) at WOT if you have small ones in neutral, I can guarantee you that...

[This message has been edited by fast_broker (edited November 07, 2001).]

 

FB

Have you used one of the isolated FP guages and if so was it worth the $ spent?

Thanks

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91 RS 5.0 TBI....LT4 cam....Edelbrock headers....3"Dynomax exhaust....5spd.... 3.08.....Ultimate tbi....afpr...ZR 255-50's...Koni's