coolguy2424

1988 Jeep Cherokee XJ Limited. It has the 5,000 lb towing package, so I'm not sure if it has a 180 or a 195 tstat. When someone takes off the housing is there a way to tell?
The Jeep temperature gauge has always run halfway between the second line, and 210, I guestimate at about 180-185, maybe 190. And I have had this Jeep since 1993. I got it five years old with 75,000 miles on it, and it now has 175,000 miles on it. I have never changed the thermostat to my recollection, in 31 years, and rather doubt it was changed in the first five years right?
( https://www.cherokeeforum.com/f2/the...2/#post3143595 "They also made a 180* extreme duty(towing) thermostat")
( https://www.cherokeeforum.com/f2/bes...7/#post3576294 "180 is the minimum you should go, some XJs with the tow package came with one from the factory")

When I start the engine, and get right on the freeway, the temperature climbs over several minutes until it reaches about 180, then quickly drops down to 155-160, the second line on the temperature gauge. I presume that's the thermostat opening. It then stays running cold, and rich, and when I smog tested, it had high CO, but the smog guy thought it was the Map sensor he thought it was a "fuel control problem" as the CO seemed to be oscillating "car thinks it's under load." I tested the Map sensor and got 4.8 with key on engine off, and 1.8 with engine on, and as I revved the engine this sweeped down to 1.3 or even lower, so I figured it was okay.

A month earlier I had smoged it and gotten high NOx, and low CO, and looked under the vehicle and saw big holes in the front exhaust pipe with hot air coming out. I had another mechanic replace the front exhaust pipe a month later, and the next day went to smog again, and this. I don't remember if it was running cold during the month before I replaced the exhaust pipe, I actually only noticed that after the second smog. I subsequently replaced the Oxygen sensor, and tested the coolant temperature sensor, (and replaced the knock sensor as it was broken). And I tested the TPS sensor and found it good too.

So I was wondering if it is likely the thermostat, and if so, I presume I should stick with the 180, or if this could be caused by a cracked exhaust manifold, which connects to the front exhaust pipe, right where they replaced it, and the new oxygen sensor sits right at the end of the exhaust manifold, right before the pipe connection. When I suggested this to the smog guy, he seemed to think this was unlikely, and started talking about in that case, the vehicle has some sort of restriction or something blah blah you wouldn't see smog results like this.

The thing is, the thermostat can't be stuck open, as the car's temperature repeatedly goes up to 180 from cold, and then immediately falls down to 155-160, as the thermostat presumably opens. Could it still be broken though?


Synopsis:
When I start the engine, and get on the freeway, the temperature climbs until it reaches 180, then quickly drops down to 155-160. Stays running cold at this temperature after that. Won't pass smog as has high CO (and high HC, and low NOx). Should I change the thermostat, or before I do this, can you think of anything other cause of this??
I also unplugged the battery for five minutes to clear the ECU after I:
Changed Oxygen Sensor & Knock Sensor, and cause of the new pipe, and,
Checked Map Sensor and TPS Sensor, and Coolant Temperature Sensor

Can the thermostat be broken, and still open and close some? Or is there something else I could check first? Doing unnecessary work is annoying.

Would buying the Red Brick MT2500 to hook up tell me anything? NickInTimeFilms basically shows you everything it could show me, from 7:25-12:52: www.youtube.com/watch?v=qPV5pEGUHWw&t=445s Do I need this?

exasemech

Wow, that's quite a post.
I think what I got out of it was:

You want it to pass California smog. Did it always pass California smog before???

It passed HC before you fixed the exhaust. That hopefully means the catalyst is ok.

In 35 years the thermostat has never been replaced. Replace it with a good 195 thermostat.stop overthinking the thermostat.

If the thermostat was never replaced, how is the rest of the maintenance? Plugs, wires, cap,rotor, air filter
When you get it smogged again make sure it's well warmed up.

Saudade

Welcome to the forum. I also have an '88 and often have difficulty passing CA smog.

First, the proper thermostat is a 195 degree. Normal temp should run around the 210 mark. Once fully warmed my temp needle hovers just below the 210 mark depending on the outside air temp and my speed. I would suggest you verify the temps using an IR thermometer at the outlet of the thermostat. The temp gauge uses the sensor located at the back of the head. Make sure the contact is clean. The engine coolant temp is in the side of the block below the manifolds. This one feeds the ECU. The engine needs to get to proper temp before it starts using input from the O2 sensor and goes into closed loop. Running cold keeps it in open loop and can result in running rich which could account for the high CO and HC.

The ECU does not store any info/settings so disconnecting the battery does not clear anything.

While the MT2500 or NickinTime simplify troubleshooting and diagnostics, you can do everything with a multimeter.

If you haven't found it yet, go to Cruiser54's web site here: https://cruiser54.com/?page_id=75. Do tips 1-5 to start and work your way through the others.

You'll find some useful technical materials there too.

coolguy2424

Yes, l actually with flying colors two years ago. As the CO and HC was very near the bottom this first time, with the high NOx before the pipe change and the second smog.

So you think it is just the thermostat? Running cold is what I theorize is causing the high CO and HC, that is, running rich.

I was concerned it might be a cracked exhaust manifold. Could a cracked exhaust manifold cause running cold, and fluctuating CO? Could a bad thermostat cause oscillating CO? Or it causes running cold, and the result is oscillating CO, from other good sensors etc?
The air filter is good, I checked. It had new spark plugs several years ago. I could check the plugs, wires, cap and rotor, whatever that amounts to, I'd have to look into it.

It just seems really unlikely to me, that something just went bad within one month. Though it's possible. The pipe was connected to the bottom of the exhaust manifold, and that's what I'm worried about. Though other things are possible.

The Map sensor's probably good as I checked it?