Thermostat Question..?

[NewGuy]

I have an old Jeep Comanche 4.0L that I would like to begin driving again. It had the thermostat (195) stuck open after years of minimal driving. I replaced the thermostat this morning and the heater began working again.

I noticed that my plugs are black and the engine just didn't seem to be idling or running correctly. After changing the thermostat, the engine seems to have more power and idles better. Am I imagining things or can a thermostat have this type of impact?

Also, I am at the beginning of an Auto-Rx cycle. Should I change the plugs now or wait until the rinse phase? Thanks for the help.

 

[Gary Allan]

Sure it makes it run better ..or at least more efficient.

I'd change the plugs now. I can't see what would be served by waiting.

 

[55Test]

You might wanna go for some fuel system cleaner. If your plugs were black you were running rich, and there's probably some hefty carbon buildup on your valves or something else along the way.

 

[1maniac]

There's a reason that the engineers want the engine at a certain temp. Now you know at least one of them!

 

[Drew99GT]

Your plugs and combustion chambers are probably pretty carboned up from the extra fuel that was dumped into the engine from it running so cool. Hotter is better, up to a point.

 

[Razl]

That was smart to do. Replace the plugs, run two bottles of Techron through the system (and/or Fuel Power). If the air filter is dirty, now would be a good time to change it. A restricted air filter can cause an engine to run rich too.

 

[TheLoneRanger]

Newguy, that ol' Commanche is equipped with a "Renix" ignition and FI system. It has an O2 sensor that controls the rich/lean injector pulse-width. Since the plugs are black, I'll bet the O2 sensor is toast. The Renix system does not have a check engine light, and so the only way to check for a bad O2 sensor is to test it for voltage switching.

It's easy to check with an analog voltmeter. There's a wiring harness that runs next to the fuel pressure regulator. Unwrap the harness, and locate the gray wire inside. Press a straight pin into the gray wire. Take the positive voltmeter probe and touch to the straight pin, while touching the negative probe to any engine ground. With the engine idleing, you should read 1v-4v, with the voltage switching once every second (for a good sensor). If you read 0v or a constant 4v, the O2 sensor is bad and causing the rich condition.

The sensor is screwed into the left side of the header pipe, visible from underneath the truck. A new (Bosch) sensor is about $49. Good luck.

 

[NewGuy]

Thanks to everyone for the good advice. I have changed the thermostat, fuel filter, air filter, changed oil w/Auto-Rx and will change the plugs this weekend. I will add Techron to the fuel tank on the next fill-up.

LoneRanger...I didn't even consider the O2 sensor and would have had no way to test it without your advice. I will test it and pick one up this weekend if needed. Will it be something fairly easy to replace? I can crawl underneath the truck without ramps, but I have them available if necessary.

I hope to get another 100K out of the old truck. They don't make them any more and I am fond of this one. I will post the results of the update. Thank you!!

 

[TheLoneRanger]

quote:

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I hope to get another 100K out of the old truck. They don't make them any more and I am fond of this one.

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You got that right, Newguy! Those Comanches are getting rare, and are a unique truck; just the right size and very reliable.

I can almost guarantee the O2 sensor is bad, because of the sooty plugs. I had to change mine recently (89 Cherokee) because it also was running rich. You can access the O2 fine without ramps, although ramps may make it more comfortable to work. The only tool needed is a 7/8" open end wrench (and possibly a hammer to tap the wrench with). Piece of cake.....

Let us know if you run into any problems.

 

[ZR2RANDO]

If it was running cool enough, the ECM may have been ignoring the O2 sensor anyway. It may have been running the cold temp mixture (default) and carboning up the cylinders/plugs...
O2 sensors do have a life though and it may be time to replace anyway.
Changing the t-stat may be most of the fix to this one- good luck!

 

[mechtech]

loneranger - Sounds like you've 'been there, done that'.
That voltage seems high for an 02 sensor, though. Decimal point?

 

[TheLoneRanger]

Mechtech, you've obviously "been there, done that" too, to question my voltages.

You're right in that most O2s read under 1v, but the Jeeps use an archaic "Titania" sensor, with a 5v reference and a signal that switches from 1v to 4v.

Here's an interesting description of the Titania that says it better than I could:
"Titania O2 sensors use a different type of ceramic and produce a different kind of signal than zirconia type O2 sensors. Instead of generating a voltage signal that changes with the air/fuel ratio, the sensor’s resistance changes and goes from low (less than 1,000 ohms) when the air/fuel ratio is rich to high (more than 20,000 ohms) when the air/fuel ratio is lean. The switching point occurs right at the ideal or stoichiometric air/fuel ratio. The engine computer supplies a base reference voltage (1 volt or 5 volts, depending on the application), and then reads the change in the sensor return voltage as the sensor’s resistance changes. Titania O2 sensors are only used on a few applications, including some older Nissans and 1987-’90 Jeep Cherokee, Wrangler and Eagle Summit."

 

[TheLoneRanger]

Since the board wouldn't let me edit my previous post, I'll clarify a few points:

First,the above quote is an excerpt from 'O2 Sensor Facts' by Larry Carley.

The 87-90 model Renix equipped Jeeps use a Titania O2 sensor that reads opposite of what is stated in the quote. A low resistance signifies a lean condition, and a high resistance signifies a rich condition. Ususally the sensor will fail by shorting, giving a low (1K ohm) resistance, and sending a steady 4v to the ECM. The ECM sees this as a lean condition, and sends a full rich signal to the injectors. This will show up as sooty plugs and exhaust pipes.

The sensor heater element is designed to bring the sensor up to operating temperature very fast, in just a few seconds. So Newguy, you don't have to warm the engine up to test the sensor, 1 minute of idleing should be enough. Dollars to doughnuts, you'll read a steady 4v on the gray wire......

 

[NewGuy]

TheLoneRanger.....guess what? You were right. I tested the sensor last night and it reads a constant 4V. I bought the O2 sensor this morning and will install it after work tomorrow. I have waited to install the plugs until after the O2 was installed. I was surprised to find the new O2 sensor comes with anti-sieze factory applied.

The Comanche runs much, much better after the fuel filter, air filter, Techron and thermostat change. I am looking forward to completing the tune-up with plugs and O2 sensor. I will let everyone know how it runs after the installation!!

 

[Gary Allan]

quote:

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TheLoneRanger.....guess what? You were right.

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You've gotta love the information age!

 

[TheLoneRanger]

Newguy, it sounds like the Comanche is about ready for the road again. With all the maintenance you've done, it should be good for many more trouble-free miles. Glad I could do my part with the O2 sensor testing.

 

[NewGuy]

I was able to leave work early enough yesterday to install the O2 sensor and the plugs before dark. The O2 was literally a few turns and plugging in the connector...piece of cake (7/8" wrench..just as predicted).

Plugs 4,5 and 6 had much more carbon build-up than the first three, but carbon had been reduced on plugs 2 and 3 since changing the thermostat.

The old Comanche runs like someone is chasing it. I took it down to Charlotte and back last night (90 miles) to blow the soot out of it. It idles much smoother and the smoke from the exhaust is not as dark on a cold morning (like today). I have about 1100 miles on the Auto-Rx cleaning phase. I might invest in a transmission flush in a few months, but I plan to drive the wheels off of it until then.

Many thanks to TheLoneRanger, Drew99GT, GaryAllan, MechTech and everyone else for their help. BITOG is a great resource.