Looking for stock prom for 91z28 - Third Generation F-Body Message Boards

rdowens91z · 10-23-2009, 12:00 PM

Hey guys, I have a 91 z28 with a 5.0 5 speed and i'm looking for a stock prom for the computer. It has a hypertech thermomaster chip in it which I did not install the car came with this chip when i bought it. I am having idling issues and cant figure out what the problem is. The car idles around 600 rpm and feels really rough almost like a misfire or something. I replaced the tps, IAC valve, MAP sensor, and coolant temp. sensor and nothing seems to be helping. It was fully tuned up a few months ago with plugs, wires, cap and rotor. I'm running out of things that could be causing this. in doing some research most people seem to think these chips are junk and could be causing these exact problems im having. Any ideas on what to do or what the problem might be??
Thanx

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Drumer919 · 10-23-2009, 02:41 PM

Heres what you do, take the hypertech bin, open it up, go into the spark table and remove 2* from the top 2 columns (highest kPa/lowest Vac) and you will have a stock bin, thats all $250 buys you, I have one (didnt buy it, it came in one of my trucks.)

I really doubt the chip is the problem since that is the ONLY difference between it and the stock bin.

??? · 10-23-2009, 03:53 PM

if it has a hypertech chip. it may just be a jumper board and your stock prom would still be there, just bypassed. may be worth pulling the ecm down and looking. but I also don't think its the chip... did it always run like this? if it once ran good, but started running badly without adding any mods. its something else. if I had to guess, I'd say the o2 sensor is shot. any chance it has headers and a stock unheated o2?

rdowens91z · 10-25-2009, 10:18 PM

It ran fine about 3 weeks ago, just started idling rough and stalling once in awhile. It always starts right back up after it stalls too. It has the stock o2 sensor and stock exhaust manifolds. When it is cold before it goes into closed loop it idles fine, as soon as it warms up is when the issus are occuring. I will pull up to a stop sign or a light and the idle will drop to like 400rpm and sometimes stall. Also where would the stock prom be, bc i did open up the ecm and saw the hypertech chip. I did look at all the vacuum lines as well and they all looked fine. other then the idle the car runs great and pretty much smooth all through the rpms.

??? · 10-26-2009, 10:19 PM

Quote:

Originally Posted by rdowens91z

It ran fine about 3 weeks ago, just started idling rough and stalling once in awhile. It always starts right back up after it stalls too. It has the stock o2 sensor and stock exhaust manifolds. When it is cold before it goes into closed loop it idles fine, as soon as it warms up is when the issus are occuring. I will pull up to a stop sign or a light and the idle will drop to like 400rpm and sometimes stall. Also where would the stock prom be, bc i did open up the ecm and saw the hypertech chip. I did look at all the vacuum lines as well and they all looked fine. other then the idle the car runs great and pretty much smooth all through the rpms.

I would guess either the o2 or even the egr valve could be sticking. I had that happen once. idled like ****. as for the chip.. can you post a pic of what you got? it should be the blue part that can be unpluged from the hypertech jumper board and plugged stright into the ecm without the hyper tech part.

regal301 · 10-27-2009, 09:07 AM

http://www.larryselectricsite.com/prom.html

they sell stock proms for less than the dealer,i changed my maf car to a map using there harness(my harness was junk)they actually answer the phone and are great to deal with.

on a side note,your car may need injectors.

rdowens91z · 10-27-2009, 09:44 AM

thanks guys yea ive heard from several people that it could be the injectors or the o2. I plan on changing the o2 first but if that doesnt work i guess im gonna have to move on to the egr and injectors. Is there any way to test either of them without disassembly and looking? I know the injectors should be putting out anywhere from 13-16 ohms if im correct, but if one is not spraying enough or sticking open i wouldnt be able to tell by the ohms right? and ill see if i can get a pic of the hypertech up here soon if i can just take it out ill try that too. Also there isnt an idle relearning process for the car is there, bc ive heard some vehicles have certain things u have to do to let the computer relearn how to idle, and i have unhooked the batt several times in the last few weeks when i replaced all those sensors.

regal301 · 10-27-2009, 10:00 AM

the ohm reading wont tell you if its clogged,leaking or just spraying poorly.the idle may hunt up and down a little until it learns but it shouldent run bad because it hasent relearned.the o2 is cheap just change it.

rdowens91z · 10-29-2009, 10:13 AM

ok so i changed out the o2 sensor...what a nightmare, it had the stock one from 91 still in it so basically i got it out and the threads were pretty much destroyed in the manifold so i had to get a tap and clean them up. new one is in but it still idles rough, maybe a tad bit better tho. i also bumped up the idle to about 750 when its hot. it hasnt stalled out since ive put the o2 in tho but its only been 2 days. and something i didnt mention before is it is a reman engine in this car, apparently it only has about 15,000 miles on it. could this be something internal in the engine?? it didnt idle rough before tho so im thinking its now either the injectors or the egr.

boldguy · 10-31-2009, 01:12 AM

my original was an AXYC that i have piggybacked with the ostrich 2.0. It is a 1992 Z28 305 (now 350) 5 speed manual...