Hello Everyone,
I have an 89 formula 350. Lately (over the past month) it has developed an eratic idle. Here is what I have tested so far:

1) Fuel Pressure. Well within specs. 40 psi when running and stays at that pressure after shutoff without dropping, ruling out fuel pump, leaking injectors, etc.
2) Vacuum test. Hooked a vacuum gauge and it is at 19 which is considered normal so no vacuum leaks.
3) Checked all MAF connections, hoses, etc and all are tight. did the screwdriver tap and nothing changed the idle so I think I am good on MAF.

Now the car needs plugs, wires, cap, rotor which I am going to change out but my question is could an ignition coil or ignition module cause this rough idle? Not sure since I just bought the plugs, wires, cap and rotor but nothing else.

Car runs strong while driving....any other thoughts?

Thanks.

 

IAC motor (by throttle body)... probly got oil/dirt on it. my 89 was doing that 2 months ago..just suddenly had a bad idle and it stalled on me a few times. driving normal tho and at WOT, it was fine.

my IAC was covered in dirt. took it out, cleaned it up, and put it back in. simple quick thing to do and now the car runs like a champ. no more idle problems

 

Thanks...how do you clean the IAC? ALso I rechecked my injectors when they were way hot. They all read 17.2 except one that read 3.2. I am guessing that is my issue. When i unplugged that one it ran alot smoother.

 

that does sound like it could be an issue.

the IAC is pretty easy to clean. you unscrew it from the throttle body, then wiggle out the shaft. once its apart, i used carb spray cleaner to get the oil and dirt out. then wiggle the shaft all the way back in and place it back on the throttle body to proper specs. then your suppose to connect the A and B pins of the ALDL port and turn the key on but dont start the car. the IAC motor will reset itself to proper specs. should start now and idle normal

heres a thread on it
https://www.thirdgen.org/forums/tpi/...ght=Lt1+intake

or the tech link
https://www.thirdgen.org/tpimod2

 

Thanks Orr89RocZ!

 

Hi Everyone,
I have another question. Since I tested all my injectors (hot) and they were all 16.9 or 17.2 and only one was 3, I am going to replace the one bad. I really cannot afford to do all the others right now and since after disconnecting the one injector lead to the 3 ohm injector the car ran so much better, I am confident that is the problem. My question is what else will I need since I am keeping the other 7? like new o-rings, seals, etc. I also know I will need new fuel line o-rings and plenum gaskets. Just want to make sure I have everything I need prior to starting.

THanks!

 

Spray the inside of that Throttle Body good too - I had all KINDS of gunk run out of there cleaning mine and I still have to tap the pedal to get it to start. I think my IAC just doesn't work too well.

I was about where you were - I've replaced the plugs, wires - didn't do cap or ignition module yet... I had to put an O2 sensor in too. But it idles a LOT smoother now - but still rough on warm up. On occasion. My wife says it's just a temperamental car - I can't really disagree, lol

I think I'll just go buy a good IAC and maybe just pull that throttle body off for a real good cleaning. But - hell, if I do that, I may as well polish up the Plenum and Intake too..

And of course, while I'm down that far... (maybe I should just clean the TB, lol)