Hey guys, just picked up a 91 rs 305 TBI 700-r4 --

Had a bunch of mods done to it by previous owner:

Built 700-r4, Shift-Kit, Stall, BM Pro-Stick
Full Suspension / Full Exhaust (Shorty Headers, Cat delete, Y pipe) / Upgraded Brakes
Has an Open Element Air Intake
670 CFM Holley Tbi Unit / Intake Manifold
Worlds Products Heads
Comp Roller Cam .429"/.423" -- something like that.

Dude threw a Hypertech Stage 2 chip at it (dont know wtf that means)

Runs fairly decently, but will stall out ocassionally at a stop in gear, and will stall out when the A/C is turned on @ Idle in gear (can run A/C with a stumble in Park/Neutral). Also stalls out in reverse.

As far as I know, the guy didn't have it tuned -- he threw a hypertech chip at it and called it "tuned".

Anyway -- haven't had a TBI car in ages (first car was a 92 RS 305 - stock). Bought this car just to screw around with on the weekend & tinker with, but the idle/stumbling issues are making me 2nd guess the purchase.

Anyone have any idea on what I should to do make it more streetable? Would very much appreciate any input that you can offer.

I have been reading copious posts/threads on TBI -- sounds like I should look into doing an AFPR and getting a Tune (I dont know anyone locally that knows how to/has the equipment to do eprom tunes for TBI).

 

Welcome to TGO

Since PO did a lot of HP updates things are a bit more interesting.

1) Get on the phone with Hypertech techline and find out initial timing and minimum idle speed for their stage 2 chip. You'll probably need to read the label from EPROM before hand.

2) Perform IAC reset - see instructions here https://www.thirdgen.org/tpimod2
It is possible that IAC in the holley unit went south and ECM has noway to compensate idle speed when additional load is placed onto engine.

3) It is all too common for BBC TB to develop a vacuum leak in front of the TB flange. I am not so sure about Holley units. A TB to intake gasket replacement is a very common ritual for TBI equipped car owners.

4) Get WinALDL, ALDL cable and a laptop. You'll need to do some data logging.

5) For ease of tuning - EBL equipped ECM modification is highly recommended. EPROM chips are so 80's technology.

6) Additional CAM info would be very helpful..

//RF

 

Thanks for the quick reply!

Went through the guys receipts -- the chip turns out to actually be a Jet Performance Stage 2 Chip for a C/k 1500/2500 350cid 5.7L Auto - OD.

Intake Manifold is a Holley Pro-Jection High Rise dual plane manifold TBI unit.

Also had receipts for a Jet REGULATOR ADJ TBI ( this this a AFPR?) and this:

http://www.dmauto.com/web/dmauto/Cat...fg/NEC/XTP679/

Assume this is a reman ECU right?

Judging from the receipts this is the cam:

Brand: COMP Cams
Manufacturer's Part Number: 08-300-8
Part Type: Camshafts
Product Line: COMP Cams Computer-Controlled Camshafts
Summit Racing Part Number: CCA-08-300-8


UPC: 036584780571
Cam Style: Hydraulic roller tappet
Basic Operating RPM Range: 1,000-5,000
Intake Duration at 050 inch Lift: 206
Exhaust Duration at 050 inch Lift: 210
Duration at 050 inch Lift: 206 int./210 exh.
Advertised Intake Duration: 262
Advertised Exhaust Duration: 264
Advertised Duration: 262 int./264 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.450 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.480 in.
Valve Lift with Factory Rocker Arm Ratio: 0.450 int./0.480 exh.
Lobe Separation (degrees): 112
Computer-Controlled Compatible: Yes
Grind Number: CS 262H-R12
Valve Springs Required: Yes
Quantity: Sold individually.


As to Datalogging -- can you advise a good vendor for a ALDL cable (and which Pin config matches the car?).

For tuning -- is the EBL this :http://www.dynamicefi.com/EBL_Flash.php?



I'll try the IAC reset -- just an update on the car, its actually driveable -- the stalling in gear occurs sporadically (i can sometimes keep it in say reverse @ idle for a minute or so before it stumbles and stalls), but I noticed that on low rpm turning, the car will stall as well.

Would jacking up the Fuel pressure help to increase driveability @ low speeds? Precision parking with this car is seemingly impossible, not sure if I'd even want to attempt to parallel park it.

Also -- the AC can run (but causes idle fluctuation/stumbling) @ idle in park, but as soon as you try it in gear (drive/reverse) the car stalls out.

My main goal would be to restore short-range mobility (i.e. low speed parking manuevering), and allow the car to run A/C at idle -- think this is possible?

Really appreciate all of your help! Looking forward to any other thoughts!

 

Following up on the IAC reset -- did a quick search on TGO:

I have to adjust/reset my IAC, how can I do this?

The IAC is not adjustable. The IAC is reset upon each shutdown of the ecm so it can ensure that the IAC will be at a known location. After which, the ecm parks the TBI.


~~~~

So -- is the IAC adjustable on TBI cars? or is Idle Speed completely dictated by the ECU?

 

Yes, it is true, but ECM has no way of knowing that IAC actually moved to a park position. ECM simply commands so many steps and calls it a day... IAC reset procedure insures that IAC head is forced into a park position. Please, re-read reset procedure in the provided link. https://www.thirdgen.org/tpimod2
I would not set minimum idle speed to 450 RPM - that's too low for your cam. 550 to 600 RPM should be ok. However, after reconnecting IAC, actual idle speed is governed by the value programmed into EPROM. This why you need to find out from Jet performance what they programmed for target idle speed. You also must know the base ignition timing value stored in EPROM. This is the value that you have to set (with a timing light) when adjusting timing, EST connector open. Stock LO3 is 0 deg BTC, but performance applications can be anywhere between 4 to 10 deg so as long as it matches the value stored in EPROM! If they refuse to provide the information (Idle speed , base timing) EPROM can be pulled out from ECM socket and read with EPROM programmer - reader. But we are getting ahead of the curve here.

8-300-08 is a mild mannered cam for 305, 350 - I run its larger brother 8-304-08 in a 383. It turned out to be a very torque combination that I was looking for.


The adjustable FPR provides fixed fuel pressure adjustment.

http://www.jegs.com/i/JET-Performanc...oductId=748464

There is no vacuum reference and pressure is fixed with a set screw adjustment. TBI can idle with low fuel pressure - in a case when fuel pump is dying, but for performance I would run 14 to 17 PSI vs. factory 9 to 13 PSI.

Yes, it is a very pricey re-mfg ECM!
http://www.dmauto.com/web/dmauto/Cat...fg/NEC/XTP679/

Dynamic EFI EBL eliminates the need for other people's chips and you can make infinite number of changes to suit your engine needs. It is a Swiss knife for tuning (TBI, MPFI) sort of speak. It is a complete system and all you need is a lappy to make tuning changes. Very powerful and flexible, but be forewarned - learning curve is involved!!!!

//RF

 

Thanks RFMaster -- will definitely look into playing with the IAC mininum idle screw. (Do you advise adjusting the idle in park or in gear? -- probably in park/neutral then switch to gear to see if effective correct?)

I know very little about cam profiles/etc., however, was wondering if anyone had any ideas on whether or not the Stock ECU would be able to handle the Cam I have installed @ present (as you say it is a mild cam).

So, I guess theres no real way around the car needing a tune to be where I want it to be correct? (the whole EBL/Eprom tuning deal seems a bit daunting).

 

IAC and minimum idle speed are performed in park.

8-300-08 is probably outside of what stock EPROM can handle. Besides you have much better flowing (WORLD) heads then stock swirl head, so the timing is all together is different.

EBL is the way to go, but try to get existing combo working half decently before forging ahead into EBL

//RF

 

Thanks RF!

On a side note -- do the symptoms that I described earlier corellate to potential Fuel Filter/Fuel pump issues?

 

Stock TBI will idle at 6 PSI fuel pressure, but will stumble badly of idle. I would check fuel pressure as matter of course. Custom chips sometimes require higher than stock fuel pressure. I simply do not know what Jet has programmed into its stage 2 EPROM. This is a reason why I suggested to get as much information from Jet. Baseline fuel pressure is one of the parameter that could have been altered in Jet EPROM.

When working on EFI cars there are basic checks one has to go through to eliminate potential pitfalls - FP is one of them.

//RF

 

Reached out to JET requesting the info you specified.

Theoretically speaking, is my handicap @ this point the chip, or should I be able to tweak the existing set up so that it won't stall (by resetting the IAC and playing with the min idle air screw) -- assuming fuel pressure checks out?

 

Since it cost absolutely nothing to reset IAC and make sure that base timing matches the value programmed in the JET EPROM I would try to make existing combination at least function. Noted, fuel pressure check should be performed. FP should be where it supposed to be and that there are no other issues that might be affecting idle quality. After that you will have to seriously look into tuning - and DIY is the way to go since you have a nice cam, very decent heads on top of the 305 short block and a free flowing exhaust. All of these parts have moved volumetric efficiency of the engine away from factory baseline. I also doubt that Jet chip takes into account aforementioned parts. Most of the mail order tunes bump-up timing and increase fuel in WOT area. It may not what your engine needs. I am going to paraphrase late Grumpy who used to say: "Give your engine what it wants, not what you think it needs".

Take your time (and save some money and patience) and read EPROM tuning guide before making your next move.

//RF

 

RF -- think I may go the EBL route eventually, will focus on trying to improve driveability with what I have though.

Did more research:

The IAC reset is definitely something that I will be doing.

Read up more on the Holley 670 cfm TBI unit

The thread below is incredibly interesting.
http://www.fullsizechevy.com/forum/g...ent-tbi-4.html

Apparently, there is a design flaw in where the MAP sensor is located on the TBI unit, its located away from the manifold right next to the IAC. They recommend moving the MAP to the front side of the unit closer to the manifold (next to the PCV).

My car has the PCV blocked off, not sure what the other two vac hoses route to.

Also, the Holley unit has a AFPR, but its blocked off by a plate -- (talked to the PO -- he didnt install the JET AFPR he purchased), guess I need to drill through the brass plate so I can access the screw that adjusts fuel pressure.

Question (which way of turning the screw increases pressure? clockwise or counter clockwise?).


I think that resetting the IAC, adjusting the Fuel Pressure, and possibly reloacating the MAP might improve idle conditions.

LMK if you think my reasoning makes sense.

 

Yes, it makes sense. I would get PVC functional - you need crankcase ventilation to prevent oil crud build-up. I have never toyed around with the Holley TB unit, but I have used Holley 300-49 intake on one my conversion. It is a very nice intake.

Generally speaking MAP vacuum port is carefully chosen since resulting control signal to ECM governs both fuel and timing. In a stock application (aka TB-220) back side vacuum port is a dedicated to MAP and nothing else.

I have one of these JET adjustable FPR. IRC turning adjustment screw CW increases fuel pressure, but it has been a few years since I worked on it. I have FP gauge whenever I make adjustments.

I would take couple high quality photographs of your TB setup and post them (either here or link to another photo sharing site) - perhaps someone else with more hands on experience on Holley 670 TB can chime-in. EBL is definitely a Swiss knife solution to this problem, but you have to take care of the basics first, such as where MAP gets its vacuum signal.

//RF

 

To be honest with you. I don't think Jet changes anything in the chip but spark timing in the Upper Rpm's and your torque converter lockup.