I have a 91 Z28 witht he 350 TPI. I just replaced the plugs, wires, fuel, air filters. I reset the timing and checked for any vacuum leaks, loose electrical connections, I didn't find any. Here is what its doing, when you start it up it runs fine. Once it has ran for about 30-45 seconds, about the time the fast idle comes down, it starts to idle rough. I have also checked the tps sensor, its good to go. I have not noticed that the engine runs rough at cruising speeds or wot, just at idle. Now heres the other problem, if i shut the car off when it starts to idle rough, then restart it, it goes away for 15-20 seconds then right back to rough idle. I have not tried pulling any codes, no ses either. The ECM ws replaced about 8 months ago. I was thinking maybe the IAC or Baro sensor??? Any help would be great.

 

15-20 seconds = closed loop operation. First, clean the IAC unit. If problem persist, get a code checker (around $30 at Auto Zone a& others)could also be MAP or O2 sensor. These are ignored during first 15-20 sec.

 

Check the resistance of the injectors...should be between 11-14ohms I believe. My car is doing the EXACT same thing. Turns out, only 2 of 8 of my injectors are in spec. Just something to think about.....after you've checked everything else...good luck

 

So can I ask what finally fixed it, my 89 GTA is doing the same thing. I am about to test the injectors...

ICM,TPS,O2 are already replaced recently. Once the car is hot, I believe the injectors are changing there ohms output.

220 for 8 accel injectors from summit.

(what is the stock pressure, what should I order for a stock 350 tpi, air bgox and exhaust mods only, headers in a little while?)

thanks
Doc

 

You can get 8 brand new injectors from www.fiveomotorsport.com for $188. They are take out injectors from 04 Mustang GT's. They are rated at 19lbhr in the Mustang....but when put in the 350TPI, they are identical to the stock injectors.....plus you dont need a PROM burn to fix the new injectors. Just something to think about.....good luck.


FWIW, I just installed those injectors(BoschDB9) into my L98 about three days ago...and it runs like a brand new car now....

 

before you buy a code checker takea paper clip and with the car off, you can stick a paper clip in the two upper right hand pins of your diagnostic port, then turn the key to the on position. your service engine light will flash any trouble codes first it will flash 12 (flash, pause, flash flash, long pause) 3 times, then it will falsh your codes. if it keeps flashing 12 there are no stored codes.

 

My '89 Formula 350 does the same thing, but doesn't even idle at all untill it has run for a minute or so unless the motor is hot.

I have to hold my foot on the throttle to get it to idle. Then idles fine. No codes come up at all. The IAC, TPS, etc are pretty new.

I am stumped too.

 

Mine idles great at first then fluctuates...a mechanic said to turn down the fuel pressure and it should idle just fine. Do all of you have adjustable fuel pressures...mine is set at around 45. SDPC said i'd have to drill a hole into the throttle blade. I'm not that brave yet....

Stan

 

Drill What! I know that helps with a carb'd motor running a healthy cam but I've never herd of doing that on a Throttle Body.