first off breif description of car. the car is a 1985 chevy silverado with a speed density tpi swap. truck ran good for 3-4 months then one day it got a case of the wandering idle. my fuel pressure read 44 psi running at idle, dropped down 2-3 psi when engine not running. do not have the gauge anymore need to get one and check again. i have replaced the runner and throttle body gaskets, fuel pressure regulator, tps and aic. i have reset the aic too many time to think of but still it surges by about 500 rpm and it sometimes stalls. it seems to do it in closed loop more. according to my scanner my tps is .054v, my learned iac is 40 but the counts of the valve itself never get below 56. it throws no codes even after stalling. i tried unplugging the iac when the idle hit a flat spot and the surging went away. the only thing i know of that is wrong is a skip that was traced back to the distributor so i need to replace it. as far as the idle goes i'm lost any help would be greatly appreciated. the truck also has full length headers, true duals with no cats and a heated o2 sensor.

thanks,
chris

 

any body have any suggestions?

 

Well, would seem to me if you unplug the IAC, and the idle becomes smooth, then either the IAC has issues, or the parameters that the IAC uses to set counts has issues.

You may need to post all your scan data, note what flucuates and by how much...

i/e, don't post "O2 mv is 300"...but note that it swings from 100-800mv.

Not being familar with SD...I would however want to know the CTS reading and the MAP readings.

Have you pulled the IAC and cleaned it up, as well as dissassemble the TB and cl;eaned all passages from all the gunk? That could helkp you out as well.

 

Remove and clean the pintal and hole on the IAC. Check for vacuum leak to MAP sensor.

 

I have just been able to fool with the truck today. Changed the distributor out, idles better in open loop but still hunts in closed loop. Changed out the o2 sensor, it was black like I've been running rich but still no help. Here is some more scanner data.

map when idle hits smooth spot 9.0-9.5 "HG, 1.08-1.13 V.

block learn stayed around 156, would drop to 154 at times

o2 cycled from 232-801 mv

My cts looked kinda strange. It would read as steady climbing untill it got around 120-130 degrees then it would jump to 231-240 which I thought was strange because the truck would only run for about 5 mins. when this would happen. The sensor in there is about 4 months old. I replaced all sensors when I did the conversion. I also went and bought a new sensor and tried another one I had laying around and both of them did the same thing. Mabey ecm or wiring? Any help would be greatly appreciated. I've been puting way too many miles on the firebird. Thanks guys.

 

Forgot to mention the throttle body is a brand new BBK and it still looks brand new on the inside. I pulled apart after problems appeared and there wasn't a lick of carbon. I have also reset min. air flow way too many times to think about and neither of these things helped out.

 

Definitely look into the CTS wiring, could be a poor connection at the CTS...I don't think it's ECM...ECM just reports what it sees...but who knows?

Maybe a bad ground? Could be after the swap, you had a new/painted engine? Gotta have a clean ground for the ECM wiring.

BLM's show lean, but back to your original post..."unplugged the IAC" and it smoothed out?

Leads back to IAC control (one of which is temp) and CTS may well be your problem.

 

id also check into the cts reading seems that somthing must be up to get that jumpy reading.i'd also open up the throttle blades a little and move the iac counts down some around 20 and see if anything changes.whats it normally idle at? also the cts controls idle but it would most likely lower idle as temp go's up and not by 500 rpm so it may be some of your issue but i doubt it's all the problem
pete

 

Well guys I fixed it today. It was spark plugs! The truck had a holley carb on it before I swapped it over and I got plugs for a tpi motor a few months later and put them in. This morning while dodging three phone calls to call me into work, I was looking for something in my toolbox and there were the old plugs. I said what the heck it can't hurt so in they went and 10 mins. later the truck was running just like before. I never would of thought plugs would affect it in closed loop only. Thanks for the help and ideas on my problem.