[deledink]

Got an 86 with the 2.5 that is misfiring at idle and hesitating upon acceleration when warm. Upon cold starting, it seems to run fairly smooth for the first minute or two and then starts running worse and if driven, will hesitate when slowly accelerating from a dead stop. Did have a bad TPS when I bought it and threw a code 26 for that, but the sensor has been replaced and there are currently no codes and haven't been for a few months. The car sat for several years and probably had old gas in the tank. If I warm the car up, shut it off and disconnect the O2 sensor, it seems to run smoother when started back up. I took a voltage reading of the O2 sensor (with the lead going to the computer disconnected) and it seemed to stay around 0.9V. Is that normal or could that be causing the problem I'm having? The cts seems to show fairly appropriate resistance values when cold and hot - about 13k cold and a little over 800 when warm. I took videos of the engine starting up when cold, when hot and one of the voltmeter reading at the O2 sensor when warm with the engine idling and above idle. I also have a vacuum gauge hooked up in the videos. Will try to post them or link to them if it would be helpful in diagnosing the problem. Thanks in advance.

 

[ImperialCrown]

The reason that it might run fairly well at first start is that the O2 sensor isn't used as an input until the engine warms and goes into closed loop operation (using the O2 sensor signal to help determine the correct air/fuel mixture).
The O2 sensor should never stay still but should always be swinging high (0.9v) and low (0.2 v). It should never 'peg' high or low either.
If no fuel injector (rich) or vacuum leak (lean) problems seem to be present, the O2 sensor may need replacement.
Always use an OEM-style replacement for best results. I have had problems with Bosch sensors right out of the box. You may see an imprint of the manufacturer's name on the old sensor, like NTK or Denso.

 

[Bob Lincoln]

How are you measuring the O2 sensor voltage? You can't use a digital multimeter, because the voltage changes faster than the meter can update and display it. An analog voltmeter is slightly better, but a scope is best.

Code 26 is for a bad fuel injector; so maybe it was a code 24 that you had earlier?

Before replacing the sensor, check the wiring harness on the backside of the valve cover for cracked or missing insulation, and shorted wires. Also check for an intact ground at the driver's side rear corner of the block. The sensor ground is at that lug, and the wire is often frayed or broken because the factory made it a little too short.

 

[Doug D]

Back in the day I had an '86 GTS with the 2.5L. There were times when the TPS sensor was failing and it did not always throw the code (24), but it would have drivibility issues (hesitation, etc). As Bob mentioned, check the wiring, connections and insulation. It's dang near 30 years old and the wiring is bound to have issues. I agree with IC - OEM sensors seem to have fewer issues - MoPar's and Bosch don't always work well together. I also had good service from NAPA's in house brand - forget the name.

 

[deledink]

Thanks all for your input, suggestions and tips! Yes, sorry, the code was a 24, not a 26 (typo). I was measuring the O2 output with a digital multimeter, so I checked it this morning with an analog scope. Once it fully warmed up, I saw pretty choppy waveform on the screen, though it appeared to be fairly uniform, ranging from 0-1V. Also ran a min/max/avg test with my multimeter and it averaged at 0.69V, with 0.95V max and 0.43V min...not sure if that info really helps. The connector on the wire going to the O2 sensor appears to have gotten really hot at some point as the connector's insulation slides right off. Not sure if it's the original or not.

I inspected all the vacuum lines with my vacuum pump and they all hold 25 in Hg without leaking. Checked the two wiring connectors near the throttle body (large circular plug and flat 3-wire plug) and the terminals are clean and connections are tight. Insulation looks fine. I removed the firewall (on both sides of the ground strap..firewall and intake manifold) and engine block ground wires on driver's side rear of the block and polished each connector and their mating surface and covered them with dialectric grease. They all seem to have great continuity.

Checked ignition timing with the CTS disconnected and it jumps between 10 and 12 degrees, everytime it misfires. When I got the car, it had this same misfire, so I changed the usual ignition items like the plugs (Champion), wires (Bosch), cap and rotor, but that didn't seem to help. Speaking of the CTS, I noticed that I can rotate the plastic connector back and forth along with the two terminals...not sure if that's attributing to the problem. I did disconnect the O2 again this morning, once warmed up, but this time the engine started chugging at idle and running really rough...like the choke was on. Thought that was strange since it seemed to run better yesterday with it disconnected. I let it idle for a few minutes and it eventually stalled. Measured the resistance across the CTS and it was 1100 ohms. I reconnected the O2 sensor, started the engine and after a few minutes, the chugging stopped, but the miss returned.

I had thought that maybe there was a problem with the fuel injector, as I was seeing small droplets of fuel beading up on the throttle plate and running off, so I replaced it with a new Mopar (Bosch), part #C3940292. I don't see the droplets anymore, though I can still see fuel running on the throttle plate, but the misfiring symptom remained the same.

Not really sure what else to check at this point. Any ideas? Thanks for reading.

 

[Bob Lincoln]

You should not be able to rotate the CTS connector. Sounds like it's damaged and maybe intermittent. It's in series with the signal from the HEP, so any quirks with the CTS will affect the ignition signal and timing. Try wiggling the connector with it plugged in and engine running and see if you can cause a misfire or other issue.

If the O2 sensor wiring is damaged, I'd change the sensor out.

 

[ImperialCrown]

A cylinder misfire will wreak havoc with an O2 sensor as raw, unburned oxygen is passing by the sensor tip. The sensor sees oxygen in the exhaust as a lean mixture and the computer will drive the injector richer. Plug tips may get carbon fouled (black) from an overly rich mixture and fuel economy will suffer.
You need to diagnose the misfire.
If the O2 sensor is swinging high/low, it sounds like it is working, but may possibly be failing.

 

[deledink]

I replaced the CTS and now the engine runs pretty smoothly at start up; even the cooling fan seems to come on when it's supposed to. But once it gets warmed up and the idle speed comes down, it starts missing. Had to go under the car to diagnose the loud exhaust and ended up patching the muffler. That helped the noise, but it still sounded loud near the front of the car. While underneath, I noted that the connecting pipe from the convertor to the manifold was pretty loose. I could easily move it from side to side. Has a pair of springs with bolts going through them to secure it to the manifold. How tight is this connection supposed to be? The O2 sensor is right near this connection and I'm wondering if some outside air is getting in there and messing with the sensor? Another thing I noted was that if you blip the throttle when warm, the engine will hunt at for a min or so before it returns to an idle state. If I blip the throttle while the engine is cold, there's no hunting at all, and the return to idle is smooth.

 

[Bob Lincoln]

Good catch. There are two spring-loaded bolts which hold the manifold to the down pipe, and there is a donut gasket at the joint. You should not be able to do more than slightly flex that joint. The springs may be relaxed too much to keep a good seal, or a bolt may be broken where you can't see it. You can get a new spring and bolt kit for about $15 and a donut gasket is about $7. To remove the bolts, you can try to put a box wrench on the nuts and a socket on the bolts, but often they're so rusted that you're better off using a Dremel tool with a cutoff disk to slice the bolts off. Watch out for hot spring fragments falling on you as you cut. Generally you have to cut some coils away to get at the bolt.

I think you're on to it, that an air leak may be throwing the oxygen sensor off. During the first two minutes of cold operation, the sensor is ignored, so an air leak would have no effect on it.

 

[deledink]

Fixed the hesitation and the rough idle. The O2 sensor was cheap and I wasn't sure when (if ever) it was last replaced, so I tried that but the symptoms remained. Turned out to be a bad EGR valve that was leaking around the valve stem that connects to the diaphragm. I replaced that, the transducer and the gaskets and it runs great now. Even sounds quieter too