Hi receiver25. Welcome to the forum. You have an unusual problem, YOU are running out of ideas, and the mechanics are idiots? I don't know of anyone who works for free including you. Well, ... Maybe me! Doctors get paid for doing diagnostics even when they don't know what's wrong. Why are mechanics held to a higher standard?
You sound like you know more about cars than the average person, but the things you mentioned shouldn't be related to the symptom of fuel smell and flooding. The only thing the IAC, (automatic idle speed motor in Chrysler terminolgy) does is adjust idle speed and provides the idle flare-up at engine startup. He ain't the problem. The TPS will cause a hesitation but not rough running. Do you mean the injectors measured 15.6 ohms? They shouldn't be the problem either unless one is stuck open. That would be REALLY weird. Injector trouble is real common on GM products but it's almost unheard of on Chryslers.
How did you check the TPS and AIS motor? Do you have a scanner? If not, do you know someone who has one you could borrow? The fuel supply system is not monitored by the Engine Computer but all of the engine sensors are. The scanner will let you read that data while the engine is running. If you're in danger of spending a lot more money on mechanics, you might be better off buying a scanner, but you will want more than a simple code reader.
Sounds like you replaced everything in the ignition system so spark should not be the problem. If the computer was not firing one coil, two cylinders would be dead all the time. Doesn't sound like that's the case either. There actually is one thing that can cause intermittent spark at specific engine speeds but it almost always results from previous service. That is the crankshaft position sensor does not have the proper air gap. I never actually replaced one on an Intrepid / New Yorker / LHS, but I've seen this happen on the minivans when the paper spacer is not installed to set the proper gap. What you might try is using an inductive pickup timing light on each plug wire, one at a time, and hold the throttle open until the misfire occurs. See if all three coils are firing. If one of them cuts out, check the crankshaft position sensor.
Since the problem is rpm-specific, I don't think fuel delivery is the issue either. A problem with volume will show up mostly during coasting from highway speeds. GM has a ton of trouble with pressure regulators leaking fuel into the vacuum hose causing a flooding condition. I've never heard of that on a Chrysler either, but to check, pull the vacuum hose off the regulator and look for any sign of wetness.
Now for the harder stuff. The MAP sensor has the biggest say in how much fuel is commanded by the computer. The acceptable range of signal voltage is.5 to 4.5 volts. The voltage can be wrong, but as long as it falls within that range, no diagnostic fault code will be set. This is where watching live data with a scanner comes in handy. These sensors caused a pile of trouble in the late '80s - early '90s, but by the time these LH cars showed up, the problem was solved. A fast and easy test is to replace it, but rather than buying a new one, search for one in a salvage yard. They are pretty reliable now so your chances of finding a bad one are real small. One clue that suggests the sensor is the problem is if the rough running occurs at different engine speeds dependent on engine load. In other words, the misfire might not be occurring at a specific engine speed; it might be occurring at a specific intake manifold vacuum. That would also mean the misfire occurs more during driving than just sitting still in park or neutral.
Another thing to look for is a vacuum leak in the intake manifold gaskets. While the engine is still cool, spray water wround the gaskets and watch for a change in speed. A leak will usually introduce extra air into one or two cylinders which can cause a misfire, then the oxygen sensor picks up the unburned oxygen in the exhaust and tells the Engine Computer to enrichen the mixture for all of the cylinders. No matter how much fuel is added, that unburned oxygen will always be detected so more fuel will always be requested. O2 sensors don't respond to fuel, just oxygen. The clue here is to view the short and long-term fuel trim numbers on the scanner. High positive numbers means the computer is adding fuel, ... Lots of it, in response to seeing too much oxygen in the exhaust. High negative numbers means there is too much unburned fuel in the exhaust and the computer sees it and is trying to do something about it by subtracting fuel from the calculated value.
Finally, here's a weird one that's real hard to find but also not real common. An exhaust leak before the catalytic converter can cause a rich exhaust smell. Between the pulses of exhaust gas, the momentum creates tiny pulses of vacuum. Those vacuum pulses can draw in outside air. The oxygen sensor picks up that oxygen and tells the computer the mixture is too lean so it adds more fuel to every cylinder. The clue here is the problem will not occur for the first few minutes after cold startup. The O2 sensors have to reach 600 degrees before they start to work. Until that happens, they won't report any extra unburned oxygen that might be present. The computer will determine fuel delivery based solely on pre-programmed values without modifying the amount as a result of the O2 sensor readings.
If it appears you have a single cylinder misfire, and you can hold engine speed long enough in the range where it occurs, you can use an infrared thermometer on the exhaust manifolds to figure out which cylinder is causing the problem. Once you determine that, you can switch injectors and spark plug with those from different cylinders to see if a different cylinder develops the problem. It will also be helpful to try to observe if you have low power at higher engine speeds when driving up a hill. When raising engine speed in a garage, a misfire that is still there can sometimes appear to be gone because the power pulses smooth out at higher speeds. What seems like smooth running at high speeds might not be so smooth under load.
As a last resort, consider using an oscilloscope to view the waveforms coming from the crankshaft position sensor and the camshaft position sensor. I am not friendly with automotive scopes. I use one from my many years as a tv repairman because that's what I'm familiar with. Also, you might have to seek out an engine performance specialist. I know there is a shop in Jolliet, IL that specializes in these kinds of problems. Their main customers are other shops. One of the owners teaches very high level classes for Carquest. Some of the things I suggested are things they have found and analyzed in the past. They even developed a strategy for determining if a misfire is being caused by lack of spark, lack of fuel, or lack of compression by just looking at the waveform coming from the oxygen sensor. I'll never be that smart!
Sunday, July 4th, 2010 AT 2:46 AM