I've read a lot of references here about cleaning/adjusting the IAC on our 3.1 V6 engines. You can read about some trouble I'm having here https://www.thirdgen.org/forums/v6/6...esitation.html . It seem like I might benefit from IAC cleaning. What symptoms indicate IAC trouble? What is the best way to clean it? How is the IAC adjusted?

You just need to take it apart and carefully clean out the passages with a spray cleaner. There is no adjustment. It's computer controlled. If that doesn't fix an erratic idle then you'll need more wide ranging diagnosis.

There are people who will "adjust" their idle if they're having problems, but I would advise against it. If your car wont idle properly there's something wrong. There is a plug on the TB that can be knocked out and a screw adjusted, but its plugged for a reason..... not to be messed with. If all sensors/computer controlled components are operating properly, you shouldn't have an idle issue (maybe a vacuum leak).
Have you ever seen the check engine light come on, or checked for codes? What is your fuel pressure at when cold? Have you only checked it at idle or under load too?

See the first three posts in this thread:

https://www.thirdgen.org/forums/v6/1...dle-learn.html

Talks about how to do an idle learn and cleaning the throttle body (TB). The EGR dumps in right behind the TB which coats it and the throttle blade in carbon.

RBob.

I removed the IAC and found that it had a lot of soot on it. I cleaned it off and squirted a bunch of carburetor cleaner in the hole. I did learn a lesson while messing around with it. I plugged the IAC in without installing it in the intake. I cycled the key on/off to look for movement. It moved just fine and the plunger shot out of it to parts unknown. Luckily, I found it in about 20 minutes and put it back in. I mistakenly thought that it worked like a simple solenoid but it actually has a small motor that moves the plunger via threads on the plunger. It was a real pain to get it put back together and I'm not sure that it is right. I do know that the valve moves though. The IAC is reinstalled with no noticeable change in how the engine runs.

There has been no check engine light. I did check codes and got 13 Ox sensor, 33 MAP, 35 Idle speed control, 44 lean exhaust. I cleared the codes and took the car for a ride. No new codes were recorded. I have checked for a vacuum leak that best I can and found no problems. The fuel pressure was checked with a warm engine at about 45psi with the engine stopped and idling. I did rev the engine up a few times and noticed that there was a momentary increase in fuel pressure. This was likely due to less vacuum at the pressure regulator. I also pulled a plug on #6. The plug looks normal not rich or lean. While I had a plug out I checked compression on that cylinder and found 115 psi warm.

Have you checked the injectors and fuel pressure regulator? If they're not good you're mostly wasting your time with other stuff. At least check the resistance of the injectors and check for gas smell in FPR vac line. However, if they are original they almost certainly need to be replaced.

Since it is maintaining the prescribed fuel pressure I assumed that the pressure regulator was working properly. I did not check injectors yet but I can. The injectors were replaced about 3 yrs ago and probably have 7000 to 10000 mile on them. I used aftermarket Bosch injectors. Thanks for your answers.

The diaphragm in the regulator gets old and can crack and allow gas to get through to the vacuum side. During normal running you may not notice it but starting can get difficult and idle may not be great. Pull the vac line from the fpr and smell for gas in the line. But remember the stuff is over 20 years old so may be worth replacing anyway when you take the plenum off to check the injectors. I had a new injector go bad in less than a year.

I pulled the vac line from the FPS and it is dry on the inside. The injectors all check out between 15.5-16 ohms. I've been studying my Haynes book (yes I know Haynes sucks) and they suggest that a leaking EGR could be that problem. Haynes says that to much exhaust gas can cause troubles like I have. I'm planning to block off the EGR to see what difference that makes. I also put my meter on the Coolant Temp Sensor. I expected to see X number of ohms but got an open circuit. I also noticed that the inside of the intake runners was wet and smelled like gasoline.

The line might look dry but smell it too. You could also try a vacuum pump with gauge on it and see if it holds. As you have the plenum off (yes?) I'd replace the diaphragm anyway.

Yes I have the plenum off right now. I'm also looking for info on checking the ECT sensor while I have the plenum off. I see posts that talk about checking it but no one says how. Should it have a certain amount of resistance across the terminals?

The resistance changes based on temperature. You'd have to heat it up and measure the resistance at different temps. Can't remember off hand what the numbers should be. But it's pretty cheap to replace if you're not sure.

Thanks Base. I'm thinking that I should never see an open circuit between the terminals. I may not have good contact so will check again. If it were an open circuit then I think that I should get a "Check Engine" light. I'm not opposed to changing parts or spending a little money but I really want to understand what is going on first.

Update, I tried to check the Engine Coolant Temp Sensor ECT / CTS with a multimeter expecting to get a resistance reading that corresponds to the temperature. I could not get a reading on my meter. I noticed that the sensor in the air cleaner, Manifold Air Temp (MAT), is the same type of thermistor sensor as the ECT. I tried testing the MAT with my meter and got the same result. I'm sure that they are not both bad so it appears that using a multimeter to check the ECT & MAT does not work.

I found a note with the following which may be the relevant numbers. Not sure where I got it.
temp (f ?) resistance (ohms)
194 241
158 467
77 2796
68 3520

Make sure your meter is set to the right scale.

I may have had the meter on to low of a scale. It was only about 40 in my shop when I checked the MAT.

Yesterday I pulled the EGR to have a look. All 3 valves were closed and move freely when I pressed on them with a small screwdriver. Just for fun I blocked off the EGR at the plenum to see what would happen. I drove the car to the store about 20min away and it ran great. After about 20min I started the car to leave. It ran very rough and stalled 2 times trying to back out of my parking space. I stopped at the store's lot entrance about 1/4 mile away and it stalled again. All 3 of these restarts took a lot more cranking than usual. Drove about 10 minutes and stopped for a meeting for about an hour. Restart ran terrible just like at the store. There was one more stop before coming home and it pulled the same crap as the other two stops. It did finally throw and EGR code about 10min from home. Today I found a video link on these boards about checking the FPR. I checked the regulator by removing the vac line and running the engine. There was never a hint of fuel coming out of the regulator and I ran the engine for at least 10 minutes. The EGR blockoff was removed while I played with the FPR. I hooked up a fuel pressure gauge and the pressure was normal with the vac plugged in and a steady 50 psi without the vac line. Another test I did was tor run the engine for awhile with the vac unplugged from the FPR then stopped the engine for about 15 minutes. I did a warm restart with the vac line still unplugged and it stuttered and stalled just like last night. I have read several other threads with people having similar trouble but none of them seemed to have a solution in the end. What do you suggest that I look at next?

Did you plug the vac line while it was disconnected from the fpr? Also, checking the resistance of the injectors is a first step and is only definitive one way. If the resistance is way off it's bad but just because the resistance is good doesn't mean the injector is good. It could still be gummed up and not flowing properly. How old are they? You may need to remove them and have them flow tested or replaced. Did you test the fpr with a vac pump and gauge? I'd replace the diaphragm anyway if it's original.

I did plug the vacuum line while it was disconnected. I don't have a vacuum pump/gauge but I did have a fuel pressure gauge hooked up. I did note that fuel press is a steady 50 psi with the vacuum disconnected from the FPR regardless of RPM. With the vac connected the pressure drops to 45PSI and varies with change of throttle. The FPR does not leak and appears to function properly with change of RPM. I put in a set of yellow Bosch injectors about 3 years ago probably not much more than 10,000 miles. They all read to within .5ohm. I bought them from ebay and they are probably from Southbay but I really don't remember. BTW Base91, thanks for hanging in there with me on this one.

When I found the youtube video of the mechanic testing the FPR it thought that I was on to something. I think that he was working on a 3800 v6 in a Buick but it is a very similar regulator. The way that it would leak into the vacuum line seemed to answer the problems I was having. The engine was hard to start and smelled very rich. It made sense that it could be flooded from gas leaked into the vacuum line. I tested the regulator and did not find it to be leaking. I may replace it anyway. The fuel pressure regulator is about $70 in local parts stores. This seems kind of high. Is there a source to get a good deal on a FPR?

You can buy just the diaphragm. You separate the top/bottom of the fpr, replace and screw back together. Just make sure it all goes back the way it came out. I think I got it from Rockauto. Mine was really bad. Very hard to start and gas actually dropped from the vac line that goes down to the AIR pump.

I'm have trouble finding the diaphragm as a separate part. Any tips for searching? I was honestly hoping that mine would have significant leak because it would explain a lot of the symptoms my engine has.

Mine for sure was letting gas past the diaphragm but when I examined it it wasn't obviously bad. A little brittle and what looked like tiny surface cracking but no clear holes or tears. I'll see if I can dig up where I got mine.

Base, if you ran the engine with the vacuum unplugged from the FRP would gas squirt out of the FRP?

I feel silly for not thinking of this. My meter was on the wrong scale. I set it on the 200K scale and got 8.8-8.9 on both of them at 35 degrees F.

Absolutely not. Inside the fpr is a diaphragm with the gas on one side and a spring on the other. The vacuum pulls on the diaphragm to adjust the pressure. When the diaphragm gets old it gets brittle and can crack or tear and let gas through to the vacuum side where it can drop down to the AIR pump and also get sucked in to the intake which screw up the idle, restarting and fuel pressure. Has to be replaced.

Not sure about the temp sensor resistance. It's a pretty cheap part to replace just to be sure.

Your regulator never leaked like the one in the video on post #17 in the "3.1 Idling Issues" ? I suppose that I could rig some clear plastic hose to the vacuum line to watch for gas coming through while the engine is running. Hopefully, I can find the diaphragm and replace it.