Here's my story:

I have an 89 IROC 305 TBI. When the car is idling, it usually sits at about 750 (what is the proper idle rpm for the 305 TBI?) but is goes in cycles, it sounds like the engine struggles to idle and it will drop to 500 rpm and then back up to 700-750 rpm, and it keeps doing this as long as it idles. When cruising, it seems fine. Somethings I've tried to correct this: I found the #6 spark plug wire melted to the stupid manifold, so I replace all 8 cables with MSD 8.5mm wires, still idles erractically. I also changed the distributor cap and rotor button as they were corroded, still idling erractically. I checked the in-line fuel filter and it looks brand spankin' new and the in-tank fuel filter has already been changed a little while ago. My next step is to change out the spark plugs and see what that does. Is there anything else that could possibly cause this? Is it a fuel system problem? How about an idle ajustment, if there is such a thing on the 305 TBI?


Thanks,
IROC-Z Racing

 

the IAC valve would be what you're referring to, and if it's old/bad it can cause such problems.

unfortunetly I don't know enough about that perticular part to say anything more than to change it.

I know some people clean them(with brake fluid I believe), but that part is very sensitive and can be destroyed if you do it wrong, so you'd need to google search for more info about that.

I also know there's a way to check the voltage on it, but I don't know what the voltages should be at, nor do I actually know how to check it, I've never personally done it.

again, you can simply change it, but if I remember correctly it's like $35 and it may not fix the problem, hopefully somebody can shed a bit more detailed light on this, or you can search more specifically now.

oh, and the stock idling rpm is 600 or 650, I believe.

 

The surging you are talking about is the IAC cycling to keep the motor running. So it chasing it's tail is the result not the cause. There could be a number of reasons why you are surging.

The best wayt to trouble shoot this situation is to data log, if that's not a possibility you need to double check the basics. Check the timing (make sure balancer hasn't slipped) check your fuel pressure, check for trouble codes and so on. If you don't find anything strange you can just keep throwing parts at it or pay the cash to have a shop put a scanner on it.

 

thanks for that. I'm still open to any advise on this, as the problem will cause the motor to stall at any given time, but again, only if it is idling...

 

A few things I've picked up over the years:

Believe it or not but fireing off a clean spark at idle is one of the hardest things for your ignition to do, so a messy idle could be the first sign of ignition problems.

You could have a coolant temp sensor out to lunch, when they fail they basically fail all the way cold which will flood the engine with too much fuel.

A leaky injectors can also give you too much fuel.

A bad fuel pump can give you too little fuel.

A flaky TPS sensor can cause erratic throttle readings that can be interperted as you moving the throttle so the ECM adds pump shot and floods the engine.

These are just some of the problems I've had.....as you can imagine a LOT could be wrong, you just have to start a process of elimination to find it or wait to the part fails completely.

You can build an aldl cable for less than 5$ in parts, check out this site http://winaldl.joby.se/aldlcable.htm

 

There could be numerous things going on as BMmonteSS pointed out abd 750 for an idle doesn't seem to be out of line on a fully warmed engine. One of the things you can try is back probing the TPS with a digital volt ohm meter. Your voltage at idle should be somewhere around .5v and increase smoothly up to 4.0v as the throttle is depressed. If it doesn't gradually increase with the press of the pedal or the voltage seems to be erratic, the TPS could be suspect. Hope this helps.

 

I have an old Burn1 programmer I assume those are obsolete by now.

I just went thru the task of replacing my fuel pump old one got plugged up with the filter sock material. I've got heavy mods to the TBI unit along with h/c mods. Nothing has changed since I data logged last time at least not in the sense of modifications, only thing I need to do is change the lockup to the standard setting instead of 7000rpms which was a way of locking out the feature on the car.

The idle runs at 1500 rpms from startup and rises to 2000 5 minutes later, my father thinks its got something to do wtith no back pressure because the muffler is off and its 3" open pipe with no cat. Could it be because there is no tranny attached to the engine? less mass to rotate resulting in more RPM? I don't remember what the problems are that relate to this situation.

I seemed to have forgot most of the stuff learned about the TBI years ago. I put the project on the back burner for 4 years and haven't touched it until 2 weeks ago.

 

I've changed the TPS Sensor and the IAC Sensor. Both of which never solved the problem, It still idles wrong. It's so bad that about half the time it stalls if the the car is idling. What are other sensors that could be a problem? I watch the engine when it's idling and when the rpms drop, more fuel is sent in to compensate and the engine revs back up to around 600 rpms. Could it be a vacuum problem? I'm hoping it just needs plugs and a tune up. Could a tune-up be the answer?

 

Fouled plugs and / or a corroded distributor/ rotor will definately cause an erratic and rich idle. Check the basics first. Plugs, Wires, Rotor, Vacume leaks, coil, ect. will all cause a definate loss of power and unstale idle if not in good working condition.

 

89 IROC 305 TBI

Alright, I've replaced the spark plugs, the distributor cap and rotor button, plug wires, IAC and TPS. I got an SES light that was code 33 which I believe is the MAP sensor. Could this be the problem child of my idling?
Which as a reminder, the car idles at about 600 then chugs and drops down to around 450-500 and surges back up to around 750 sometimes 1000 then idles back down to 600 and repeats. Sometimes it will stall.

I have noticed that in the morning after the car has been sitting a while, when the engine is cool or cold, it runs fine and smooth, then when it warms up, it idles wrong and when accelerating when the engine is hot, it is very hesistant, kinda sluggish.

Also, I WAS going to check the EGR valve, but the SOB's two bolts WILL NOT break loose, and with GM's brilliant (yeah right) engineering, there is NO room to work! That pisses me off

The in-tank fuel pump has been replaced and the external fuel pump looks brand spankin' new. The throttle body and injectors have been cleaned. If I had the money, I would SCRAP the P.O.S. 305 and drop in a 427 Ram Jet and be done with it all! PLEASE HELP ME!!! I'm at the bottom of the barrel scraping for more answers!

 

Let me add that the MAP sensor's vacuum hose is good. I checked that and it holds a vacuum, so it's either the connection or the sensor itself, I'm leaning towards the sensor being bad.

Also back to the EGR valve, I was able to check it's vacuum hoses too, and they also hold a vacuum, so those are good, but I'm still pissed that I can't break loose those 2 bolts to get that little flying saucer off the intake!

 

I know you probably dont want to hear this, but without a scanner you have to rule everything out...which means testing the egr. If the egr is operating while its idleing (which it shouldn't) you will also get the condition your talking about. Throwing parts is easy but gets expensive real quick. IIWY I'd test your map sensor first, then test your EGR, then your Fuel presure. all could be suspect. You have to rule them out one by one.

 

As a follow up on my idling issue, I've FOUND IT!!! I did not see this coming at all! Can anyone guess what it is??? I bet not. It was a CRACK in the internal magnet in the distributor . Holy smokes, didn't see that comin'
I'm just glad it isn't stalling on me anymore because of the issue. I'm so happy I could just... !!!!!!!

- IROC-Z Racing

 

For future reference I wonder if Timing Light would pick this up - jumpy timing at idle with by pass set open.

//RF

 

most likely it would look like a cylinder would stop firing, as in steady spark flash then an extra long "off" time for the light during the stumble/stall then a sudden increase and fast light flash cycle

glad you found the problem!

 

that may be, all i know is that not all the time, but it would stumble so bad, that the IAC (it was reading an 8 or 9 on the scanner when it should be reading around 20 to 25) could not pick up the rpm quick enough and the car would stall. See, that is why I absolutely hate sensors, expecially during the 80s and 90s because everything was linked together. One sensor goes broke, they all act up, thus confusing the crap out of you cuz you can't pinpoint what sensor has gone south since everything else has gone south with it. Stupid B.S. engineering on GM's part!
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Like rocko said, I'm AM glad I found the problem and got it fixed!!! Phew, I'm done, lol yea... right. Thanks for all ya'lls input on what the problem could be, even though it was something totally different, "Whooda thunk it?"

 

of the many problems with with early ecms were that the were reactive in relation to engine tune. the couldn't keep up with the state of internal condition change in the engine and roughly guessed at best with what would happen next.

the balance was very "delictae" to put it another way. not like now (96' and up)where you can unplug a maf and see if the engine runs better with a non code setting stumble "event".