I had a high idle since I got the car, and it stuck. So I replaced the Throttle Body. Since then it no longer stuck, but the idle was still high. I replaced the TPS and that seemed to do nothing. I also replaced the IAC. The problem is when I start the car and go for a drive, about 5 minutes into it, the car will bog extremely bad. If I clutch it, the rpms go way down and it will fluctuate up and down a small amount. But attempting to move only causes the car to jerk back and forth in a very unpleasing way. Also, I can keep the rpms up and go WOT with no bogging problems, but anything less than WOT and it has trouble. It sounds as if its misfiring or something of the sort and a buddy behind me said he noticed an abnormally large amount of black smoke from my exhaust. The car had never smoked before. I'm stumped. Don't know what to do. I've read some things about adjusting idle but I'm not really clear about it. Also, I've heard of a difference in the cold start idle and running temp idle? I don't know. I'm asking for help and opinions. Thanks ahead of time guys.

-Tried replacing MAF with one from my friends IROC, no change. 02 sensor was replaced during header install 3 months ago. I don't know what to do. I really need this fixed ASAP. I would appreciate ANY opinions.

 

Sounds like an extreme rich condition... How's your ignition? Plugs, wires, cap/rotor? Have you done a spark test on your ignition coil? Another possibility, which happens to suck, is that you might have a broken spark plug or too. My stang used to bog down to 3000 and then spin the tires from 3000 to 5500...turns out I had 2 or 3 broken fuel injectors. What's your fuel pressure at?

Edit:
If the LB9 is a carbed motor, ignore my fuel injector things! Can't help ya if it is..I dont know diddly squat about carbs!

 

The coil, cap, rotor, wires, and plugs were all changed about a month ago.

But, how do you go about testing and finding out if a fuel injector is bad?

 

Do you have a fuel pressure gauge? If you do....hook it up to the Schrader valve and start the car. Let it idle for a minute and watch the fuel pressure. Then, turn the car off and watch the gauge carefully. If it starts dropping at any decent rate of speed, that means you've got a leak somewhere that's bleeding off pressure from the fuel lines...which COULD indicate a bad injector.

 

Oh, you really don't want just ANY opinions. Rodney Dangerfield is likely to be out there somewhere...

Your symptoms sound like a common lean throttle tip-in condition. There are a few things that can be done to minimize that.

But first, there is a difference in target idle RPM depending on the CTS input and a couple of cold- or hot-start timers. IIRC, cold-start is 300 seconds, hot-start is more like 40 seconds. It varies a little from program to program, but most factory PROMS are about the same in that respect. The target RPM at startup will always be higher than the hot idle target RPM. That's why the idle is high at startup and slowly decreases, no matter what the starting temperature might be. Final target idle RPM is usually between 600-700RPM in drive. The AC compressor being on will add 50 RPM. Hopefully, that covers that part of the question.

Next, operating at WOT invokes open-loop mode, so the mixture is automatically going to enrich. If you were operating at a slightly lean mixture in closed-loop, that would cure some of the problems associated with that, but still doesn't address the idle problems. WOT also necessarily lowers the intake vacuum, which would raise the fuel pressure on a TPI that is otherwise functioning normally. Again, if you were operating a bit lean, that would help offset some of that.

The keys are that the rest of the system must be functioning normally. That means a reliable ignition system, correct fuel pressure, and good mechanical condition of the cylinders and valves. If you're sure about compression, timing, and spark energy, you can proceed. If not, you need to verify those first.

One significant point you made os that after about 5 minutes (300 seconds) the engine starts to run poorly. Several things happen after 300 seconds, including the expectation of closed-loop mode by the ECM. If the ECM does not see correct signal inputs by them, it will revert to Backup Fuel and Spark Mode, which is typically rich and timing retarded. If the oxygen sensor isn't active by then, it could cause that symptom. Since you have a relatively new O², you may want to presume it's operating properly, but a mV test of the sensor would remove any doubt.

Also, after 300 seconds, the EGR and EVAP canister purge solenoids can become active. If the vacuum source for the EGR and EVAP is not the timed vacuum port on the underside of the TB, the systems might be operating when they should be inactive. If the EGR were just simply stuck open, you'd probably be getting poor operation as soon as you turned the key, not 5 minutes later.

A sluggish throttle response at tip-in can be caused by a lean condition. There are a couple things you can do to minimize the effect. Nothing is better than a MAF for measuring the intake air and allowing the most accurate fuel metering. However, for that to occur, all intake air must flow through the MAF. Any intake duct work damage or leaks, vacuum leaks, or PCV system leaks can cause a lean condition by allowing unmetered air to enter the intake. One of the biggest mistakes is to install a breather type oil filler cap on a MAF engine. The extra intake air usually isn't enough to cause an error code, but will lean out the mixture. A leaking EGR valve can similarly cause a lean mixture as well as very poor vacuum and idle/low RPM operation. And an often overlooked vacuum leak is the power brake booster. They don't fail often, but when they do it can be a real head-scratcher.

If you've addressed all the potential vacuum leaks, the TPS is a good thing to check. The ECM meters fuel partly based on the RPM and TPS inputs, and factors that by CTS and MAF inputs. If the TPS is set too low, the mixture could be lean. The acceptable range is from 0.47-0.61 VDC. the middle of that range is usually most desirable (0.54VDC) but you can increase it to 0.61VDC and still be within specifications. This can get the ECM to add fuel sooner due to an increased voltage a throttle tip-in. The increase isn't major, but every little bit can help.

Another cause of sluggish acceleration can be the position of the throttle plates when "closed". At the idle position, the IAC controls the idle air rate. The ECM can adapt the IAC to a pretty wide range of conditions and still maintain control. If the throttle plates are truly fully closed - tightly against the throttle bores - no air is passing around the plates. All idle air would be passing through the IAC air passages. The IAC can easily open enough to admit the necessary amount of air, so idle speed wouldn't be affected, and would still be under full ECM control. However, when the throttles are opened, the air just in front of and behind the plates would be completely static. There would be a lag in getting flow established around the plates, as well as some "internal struggle" over which side of the plates would be the predominant side for air flow. That's usually the trailing edge, but that would take more time to establish from a fully sealed condition. To counteract that tendency, we can adjust the position of the throttle plates at idle. As long as the throttles are opened to a point that doesn't admit all the necessary idle air flow, the IAC is still used to add air and have control over the final idle RPM. If the throttle plates are opened too far, the ECM will shut down the IAC completely in an attempt to lower the idle RPM. The key is to have some air flowing around the throttle plates at all times to prevent the stagnant air situation. Through the Throttle Minimum Air Position adjustment procedure, you can adjust the throttle stop screw to a point just below the target idle RPM, thus allowing ECM control of the idle speed but minimizing the dead air. That will help reduce more of the lag at throttle tip-in as well.

I wouldn't be too concerned about a failed MAF at this point, you've nearly proven its reliability by swapping, and if you had a MAF circuit or connection problem, you'd already know it. The ECM will lock in an error code for the MAF in 600 mS or less when the signal is out of range - even if it corrects itself. That's why the "tap-test" works on a MAF.

I hope some of that will help.