Rough idle in closed loop
 

Here is a stumper.

93 suburban 350, TBI. When cold/closed loop the idle is terrible.. Rough, idle drops, then speeds up but almost never dies. Sounds a lot like a cam with a lot of overlap.

Once it goes into open loop it smooths out and runs fine. No issues at all, just in closed loop.

High mileage engine but everything is in very good shape, I have kept in maintained. Cold air intake but otherwise stock. Checked the obvious things. Just put new plugs in and the old ones looked good. Grounds look good. No vacuum leak. Coolant temp registers correctly (using a computer to read). IAC working. Fuel pressure is 12 lbs. Data logging nothing stands out.

I have searched and find this is not an uncommon issue but have yet to find an answer to why. Vacuum leak is more common issue. I used starting fluid around all the gaskets, pulled and checked the hoses.

It used to run great but has slowly started the rough idle over the last year. Much worse in cold weather but again, once it warms up and goes to open loop it is just fine.

Totally stumped at this point.

Re: Rough idle in closed loop
 

You have the open and closed loop backwards.

The system is in open loop when started cold and does not go into closed loop until specific criteria (oxygen sensor hot enough, timer expired) are reached.

When running in open loop the system is very dependent on the coolant temperature sensor reading, TPS, and the MAP (Manifold absolute Pressure) to calculate the fuel needed to run well.

By what you described, the mixture is off. It may be running to rich when cold. Here are some things that can cause that.

IAC not increasing the engine speed enough- bad IAC, IAC connections, RPM reading (reference pulse error)
MAP reading low vacuum making the mixture rich.
You ruled out - to high of fuel pressure.
Injectors to big for the engine or leaking.

With a cold air intake, if it has no heated air intake, ice can form in the TBI up to 51 degrees. Above that it can't be the problem.

If you can post what you have data logged I would look it over. A log of a hot idle where it is running well will tell me the most.

Re: Rough idle in closed loop
 

Never can keep it straight because it seems backwards to me.

Running rich makes a lot of sense because the exhaust smell is terrible.

IAC may be the issue. It works in that turning on AC or turning the steering wheel will adjust the idle when it drops. But the cold idle speed does appear to low. It is an aftermarket unit, the original has been replaced.

Injectors are rebuilt replacement units. I had one clog and cause a really lean condition. As far as I know they are the same size that was in it. They have been in there at least a couple of years now. I have not looked at the spray pattern since I replaced them so a leak is possible. Will look at that.

Don't have a good data log recorded, but that is not a problem to record it. I was going to do that today anyway.

Re: Rough idle in closed loop
 

Maybe have found it, IAC connection. Pulled the plug just look at the pins. Nothing obvious wrong. So plugged it back and wiggled it a little since my contact cleaner was in the basement.

Fire it up and idle barely varied. 650-675 rpms. Noticed the IAC position is changing too. Before it would show 150 and never change. Now it hovers around 38. Turn on AC and it bumps to 56 and then drops back down to low 40's. AC off back to 38. I was assuming it couldn't read the data.

I will check it again tomorrow in the morning to be sure. But looks like the IAC just didn't have a good signal.

Re: Rough idle in closed loop
 

Sure sounds like the IAC connector was not making a good connection.

With some drive time the computer will relearn IAC position and the count may come down a little more.

Re: Rough idle in closed loop
 

Can you read Tuner Pro files? I have a data log of the hot idle.

Cranked this morning and it was the same ole thing. I didn't have the computer hooked up this morning to verify if the IAC was working. I will do that again just to make sure it is not the issue.

Re: Rough idle in closed loop
 

I don't have Tuner Pro software.

Often people post a data set on a spreed sheet layout which works for me.

Re: Rough idle in closed loop
 

As far as I know there is no way to convert the data to a spreadsheet. No idea what program other folks are using

Re: Rough idle in closed loop
 

Other people export it to excel and post a link to the spreadsheet. That is all I know about that.

If you have any way I can see the data and labels for what the data is I will try to make sense of it.

Re: Rough idle in closed loop
 

Can't find any way to export data to a spreadsheet.

What I can tell so far is that IAC is working. Max's out at around 145-150 when it is cold and idle is loping around. Drops way down once it starts to smooth out.

MAP sensor with the engine shut off reading matches the air pressure for our elevation.

Don't have another tach to check the RPMS but the computer and car match up. Numbers seem close at idle, just can't verify.

Need to pull the air cleaner and see if there is any dripping fuel.

Temp sensor appears correct. Did not get my IR thermometer but it shows very close to outside air temp at start up. I have checked it hot a while back and it was right on the money.

Re: Rough idle in closed loop
 

With an IAC count of 145 to 150 it should have a very fast idle. It should not be slow and loping.

The PCM sends out pulses for the IAC to open as the count is going up to 150. The PCM does not know what the real position of the IAC is it just tracks the pulses sent out to it.

Make sure the IAC passage is not blocked with muck and build up. It kinda sounds like that is the problem. The IAC may be moving but there is no increase in air getting though it.

Maybe the wrong IAC is in it. The pintles have different shapes on some of them.

Re: Rough idle in closed loop
 

Do you have any codes set?
Check 02 cross counts.
Disconnect the battery for 1 hr to reset the computer.

Re: Rough idle in closed loop
 

A little update, it started running fine now. No idea why since I have done nothing but drive it.

I was going to check the IAC passage but it wasn't a priority since it was just when it was cold, so I put it off. Needed to get the steering box and pitman replaced and now with all that done I decided to take a look and realized that it hasn't been acting up in weeks.

Of course it will probably start up again but we will see.