Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, and is not responsible for any consequences. Please proceed at your own risk.
This page covers fuel-injected cars.
Idling and driveability problems can be caused by a variety of issues. Sometimes, the computer can tell you what is wrong [instructions].
On cars with carburetors (phased out through the 1980s), the most common problems include the distributor cap and rotor, vacuum hoses (which tend to go bad when they get older, but which are cheap and easy to replace), spark plugs, carburetor tuning - which most mechanics do not do well - and the carburetor itself. Cars with automatic feedback carburetors (mid-1970s to 1980s) can also have problems with sensors and wires.
On cars with distributors (Chrysler phased out distributors in the mid-1990s), common idle and stalling problems involve the distributor cap and rotor, MAP (manifold air pressure) sensor, spark plugs, and Hall effect sensor. These are all easy and cheap to replace. Typical prices are about $14 for the distributor cap, $8 for the rotor, $35 for the MAP sensor, $2 each for spark plugs.
With or without a distributor, erratic idle (including high idle speeds) can be caused by a faulty throttle position sensor (TPS) - a common problem (see diagnostics below). Likewise, the idle speed motor can go bad. Leaking fuel injectors can cause a lumpy idle (along with high fuel use, lower power, and higher regulated emissions). More problems include...
My 1994 Town & Country (3.8 liter V6) fell flat on acceleration. No code was set, but she ran terribly, with no power, major misfires, and an occasional backfire. Then, on the same trip, she started immediately, with no problems. The next morning, she was a little hesitant on start-up, but ran rough with no stalling — and no Check Engine light. Then, after work, she stalled again, and after a restart ran acceptably, with a little miss and surge.
Perplexed, I took her to my local shop (also one of my customers, I work for NAPA Auto Parts), and had him plug in a scan tool. The only code that was present was the EGR Code (32). The EGR code is set by my free-flowing exhaust, not enough back-pressure make the valve sensor happy (the valve is six months old and works just fine). He cleared the code then started (attempted to) the van. No start, just crank, rev, backfire, die. On the scan tool screen he had highlighted the MAP sensor reading. Way off, but not enough to trip the code. He said that it was most likely going to fail out-right in a day or two.
Drove straight back to my store and looked at the MAP sensor on the shelf, felt cheap, looked cheap, just like the NAPA unit I had previously installed (Made in Taiwan printed on side). I had no choice with the NAPA sensor the first time due to money issues. (It still cost me $50, should have paid the high stealer price and done it right the first time)
Now this (newer) NAPA sensor has failed, so I ordered a brand new OEM sensor from Timberline Dodge in Portland, Oregon, so I know I am getting a quality part. Part Number is 5234024. Comes with a 12 month, 12,000 mile warranty. Shortly after I finally got the MAP sensor code and check engine light. I made it home, with restarts at every stoplight as well as a major surge on acceleration. Suprisingly once the light comes on, she runs marginally better (doesn't die at red lights) — the computer apparently ignores certain sensors when it detects a fault. But this does not cause limp mode, unlike the crank sensor.
The sensor (on my van) is screwed into the the rear, right side of the upper intake plenum next to the alternator (actually about six inches away). It is black plastic and has a 10MM bolt head cast into it. Simply unscrew it from the plenum, and screw the new one in until tight. There is printed on one side (the flat side) of the sensor, THIS SIDE UP. That side must be up for the sensor to work correctly. In all it takes about five minutes to change the sensor. The appearance and location of MAP sensors varies wildly depending on the vehicle.
Abnormally high idle implies a cracked/open vacuum hose to the MAP sensor. (The location of the MAP sensor and its appearance depends on the vehicle; these instructions are for a LeBaron GTS.) The MAP sensor's probably located by the firewall on the vehicle's right side (passenger side). It'll be be a small rectangular box with a three-wires connector going to it and a vacuum hose connector. The vacuum hose goes to the intake manifold via the barometric pressure reading solenoid ("baro read" solenoid) or, just directly to the intake manifold (depends on the car). The wires on the MAP sensor are: signal ground (usually, black -- this is a direct connection to the computer and may serve as a signal ground for the TPS, also), reference voltage (usually, red -- 5.0 volts reference signal), and signal output (probably green). Diagnosis is as follows:
The throttle body sensor (TPS) may be bad. To test it, Brian Minnebo suggested the following (which requires the TPS to be off the car):
First thing is pull diagnostic trouble codes (DTCs). [Click here for instructions] If you want details on troubleshooting that code, you'll either need extensive Chrysler experience, the "Drivability Manual," or equivalent information.
Suppose the DTC is "MAP pneumatic circuit problem." This indicates that the powertrain control module did not see a drop in manifold pressure AFTER the engine started. The most likely culprit is a cracked/loose vacuum hose running to the MAP sensor. The computer now sees a wide-open throttle power demand and proceeds to cheerfully flood the engine while you're idling.
The next thing I'd look at is system fuel pressure. For the 2.2/2.5 liters TBI engines, the throttle body intake fuel rail pressure better be 14.5 psi +/- 1 psi. If it's too high, you'll flood the engine (especially at idle). For multipoint injected engines, the fuel pressure at idle better be much lower than pressure at wide-open throttle. If the pressure in a multipoint injected engine doesn't change with power demand, see if the vacuum hose to the pressure regulator fell off.
Now, with the engine off, see if the fuel pressure drops like a rock. If it does, then you have leaking fuel injectors (if you have engine run-on, then you certainly have leaking injectors).
Next, look at the quality of the ignition system. With the engine running, taking a plant misting bottle (the type you can buy at any hardware store), fill it with water, and mist the ignition wires. If you see sparks flying all over the place, then you have wires where the high voltage has punched through the insulation. Replace ALL the wires -- I recommend Chrysler wires, NOT aftermarket (experience, here).
Now, examine the spark plugs. The 2.2/2.5 liter engines (except Turbo III) use Champion RN12YCs gapped to 0.035". Be sure that this is what you have and the gap is correct. Now, examine the cap and rotor -- if the amount of copper on the rotor's tip is small, replace BOTH cap and rotor. The best aftermarket cap/rotor kit I've run across is made by Wells. The Chrysler one isn't that expensive.
Ed Treijs wrote: The wires going to the round connector which lies on the intake manifold behind the throttle body had brittle insulation and were showing bare spots when I got the car. A subsequent examination of the FSM reveals that the two ground wires for the logic module pass through this connector. [This may explain the idle problems, including both high and irregular idle, he experienced.
Matt Miles wrote: "Was cruising tonite on the highway at 65 mph in cruise, and occasionally the motor would stumble. It didn't do it at first; maybe after 20 miles or so. It was sporadic; not rythmic at all...Well, my Horizon had the exhaust innards collapse on me causing the same stuff... a J.C. Whitney universal cat and stainless check-valve setup set me back $85 and the lifetime Auto Zone muffler set me back a whopping $19.04. I have no problems... occasionally I experience a mild surge at cruise, but I think it's my throttle position sensor. Other than that she's like new :)
We strive for accuracy but we are not necessarily experts or authorities on the subject. Neither the author nor Allpar.com / Allpar, LLC may be held responsible for the use of the information or advice, implied or otherwise, on this site. This page is offered “AS IS” and without warranties. By reading further, you release the author and Allpar, LLC from any liability.
All Mopar Car and Truck News