Thanks for getting back to me on this guys. I've been thinking along with 69 on this as it's perfectly fine when you first start it in open loop. I think it's a sensor that controls the fuel just because I know the spark and the TPS is fine, I checked that with my scanner and it's reading correct. I don't see why the IAC would work fine in open loop and then have issues in closed loop. I'm still leaning towards the O2's, as the plug wired I fixed was arcing off the header and the O2 is plugged into the header. But the readings I'm getting looks to be correct from what I've read online.

If I can't fix it by the end of Jan it's going to a shop, but man do I not want to pay someone to fix this crap.

 

lol, oops I didn't mean the IAC, I meant the IAT (intake air temp) sensor. That probe thingy that's stuck into the intake tube. The IAC, if bad, would just give you a too high or too low idle.
Sheesh, all these acronyms can sure give one a headache PDQ!

 

well hook the plug wire back up to the header,,, lol. 02s should be ok, its the sensor wire that can not have juice or ground to it. im sure a mess of plug wires have melted on headers without harm to the 02s. computer uses the temp sensors when cold, not sure of the extent the difference is, i know when the coolant sensor went in the truck it messed it up cold and hot, way worse when warmed up. mine was pegged at the coldest temp, like -40 degrees. can you see the intake air temp on your scanner, you can test it the same as the coolant sensor with a ohm meter.

 

I can see the intake temp, I don't remember that looking off, but I can check again. Anything else you can think of to check?

 

tune gone bad is one thing, not sure how it would happen in a non memcal system.

 

Sorry my post was confusing. The most difficult driveabily issue to track down is rough idle. Misses and hesitation are a piece of cake compared to “bad idle”. The problem is that almost any small issue shows up at idle, even problems that grow into easy to find issues later on can start out as slight idle issues and can be difficult to find at first.

I have had maybe 200 cars with idle issues. Our ongoing joke was “bad idle” the worst description of a problem ever. You may think that because the idle issue is only in closed loop it cannot be ignition but that is not true. In open loop the car tends to run richer and with less timing advancement. This has less stress on the ignition. Also a high resistance wires can cause static build up and cause all kinds of issue with a close loop system.

So what I was explaining was before I started the diagnosing I went through a check list. Is everything up to date maintenance wise. No one wants to spend the money for diagnosis to find out #6 plug wire is bad. The cost of figuring it out is twice what it costs to replace wires that should have been replaced anyway. If I had to guess 20-30% of the cars I had with “bad idle” my check list fixed.

A scope can pin point the issue. Is it a spark issue, or is the whole motor lean is it single cylinder that is lean. A Scope combined with a vacuum gage and a 4 gas analyzer would at the very least tell you where to go from there. Now that we know why the idle is rough the next step is to fix it. That can be harder than it sounds.

If it is computer related and not obvious I would go to the IBM CAMs system. That is a system that will not only dives into the computer and allow for data logging it has master cables that you can hook in line with say the MAF sensor. It tests each connection for resistance and watches for abnormal fluctuations in voltage.

As I said before it is pretty rare for the O2 sensors to be the cause of a rough idle but in many cases they are damaged by it. After the issue is sorted out we took the car out and ran them kind of hard to “blow” them out. We then came back in and hooked it to the gas analyzer and verified the O2 sensor were working to spec.

Truthfully even with all the equipment I have spent days figuring out issues but since I was working on newer cars there are a lot of things that can go wrong in manufacturing that should never be seen on a car that worked great for over 100k. It was very common to get every A grade tech in the shop to review a problem car in a huddle type format.

There was only one car I could not figure out as a mechanic and that was a “bad idle” car. It was a L98 89 Corvette. I could tell it was an air flow issue, GM support kept telling me it was a computer issue. 4 ECU’s later they ok’ed new heads and intake. That made no change. We shipped the car to the GM tech center and they returned it and said nothing was wrong with it and cranked up the idle to 1100 RPMs. LOL. (too bad the original customer complain was idle too high) The car became a buy back. It was the only 100% buy back I ever saw, normally they prorate based on miles. The customer proved with work orders we put more miles on the car then he did. (3 months later we got a bulletin from GM that there was a bad batch of Cams for the L98, they will have rough idle and may have been set to a higher idle at the factory to mask the issue)

 

I really don't think it's spark related, I checked all the coils and wires when looking for the missing issue that was just before this started. And it runs fine when out driving except for maybe a bump here a there, maybe twice in a 30 min drive. But when at a stop light at idle the issues come back up. It will almost die and then catch itself.

Are there any sensors that turn off in closed loop when throttle is above idle? The issue is hardly evident at all when driving as I mentioned before.

 

When it starts running ruff do the RPM fall real low? As a general rule as the RPMs drop the IAC motor should be trying to comp. Normally you end up with an up and down idle as the IAC tried to set the RPMs. If you are getting wide swing in RPMs you could have a "lazy" or slow IAC.

I have seen cars that what should have been a minor idle issue was made worse due to a lazy IAC. After the IAC was cleaned or replaced all the was left was a slight roughness but the car did not have the stall/almost stall issue. It also made finding the issue a little easier.

 

I was told to check that by one of my friends here, but wouldn't the IAC affect it in open loop as well if that was an issue? I've thought about just replacing that since it's fairly cheap.