Neon fuel or cooling issue:(

I've finally gotten my 2.4 swapped Neon together enough to drive to work, and its been fine for those 10-15 minute drives.

Drove it across town Thursday night, and after about 30 minutes of steady state driving on the interstate, the coolant gauge started climbing from about 1/3 of the way up to 2/3 to a 1/2, depending on how long I let it climb before kicking the heat on full blast to bring it down. When I reached my destination, I let it set long enough to cool, and checked, no loss of coolant as far as I can tell. On the trip home at night, it did the same thing. Fine, and at one temp for 30 or so minutes, then started climbing. When I got off the highway, the check engine light came on. Checked it, and it was a code for the upstream O2 sensor being lean.

The next day after work, I confirmed there does not seem to be any way outside air is getting to the sensor after the combustion chambered, the vac lines all look fine, and the relatively new spark plus only have a little bit of light tan buildup, no white, or anything that makes me think the car is running particularly lean. I pulled and replaced the O2 sensor, which was very crudded up, but haven't had a chance to go out and test this, since I killed the battery leaving the dome light on.

Here's my setup 98 Neon 2.4 engine swapped from a Dodge Stratus 24 lb injectors (stock is 19) Mopar Performance ECU (more timing and a richer fuel map)

When I test drive the car I'll try to get my wife to ride with me, and watch fuel, O2 sensor, and coolant readings on the scanner, but I'm still not sure what is going on. Here's my theory, though: I suspect after a lot of starts and stops during the build, the O2 sensor got a lot of buildup and is going bad. It reads everything as lean, and is dumping a lot of extra fuel in the combustion chamber, eventually heating things up too much. Another possibility is it is actually lean, but there's no pinging going up hills or anything. But what doesn't make sense, is why does the coolant temp stay fine for 30 minutes, then start to rise. Any ideas, or opposing theories, or thing you think I should check?

This project has been maddening. Despite being a well worn path, I seem to have done a great job of running into issues no one else has before.

also, radiator cap/lower radiator hose failing due to heat soak?

Air dams and radiator shrouds all intact and in good shape?

Cap and hoses seem fine, both were replaced while everything was apart over the winter.

No air dam (none stock), and the fans are shrouded. With how long it runs fine, I'm thinking its probably not an airflow issue, but I could be wrong. Outside temps were considerably lower on the drive home, but it did the exact same thing. Tempted to wire up the secondary (AC) fan to a switch, but that's just a crutch. I'd like to figure out the real issue.

If it were me, i'd take out the thermostat. If it still does it, it's either water pump or radiator flow problems. If it doesnt do it, put a new thermostat back in it and see what happens.

Sounds like a weak head gasket/ warped head too me. Have you done a leak down test?

eastsidemav wrote: No air dam (none stock)

Are you sure? The one my dad has one came with one, the weight on the front motor mount goes though the middle of it. I doubt thats the problem though either way. The only times it has ever had overheating problems are the radiator or water pump leaking, other than that idk.

In reply to Travis_K:

Hmm, just assumed it didn't have one, since I got it hthat way, and a few others I've seen are the same. Maybe I'll try to build a quickly removable one in the future.

No leaks in coolant as far as I can tell. Coolant also looks good, but I have not done a leak down test. Oil on the dipstick looks clean, though, so I out its a bad head gasket.

Vigo, thermostat is new, I assume it could be bad from the factory, but again, with it running fine at a steady temp for half an hour, then heating up, just seems weird. If I remove it, should I put an old thermostat so there is still a flow restriction? Otherwise I have doubts about it hitting operating temp.

sounds like a coolant flow problem. collapsed hose, dirty radiator, water pump ? , radiator cap?

eastsidemav wrote: No air dam (none stock)

The neon doesn't have a ducted radiator, so the stock airdam is actually pretty important in that it directs air up into the radiator.

Where i live, a car with no thermostat has no problem hitting operating temperature, it just takes longer. Not sure what the weather is like in your area. It was 96 in the shade outside my kitchen door a minute ago.

Is this always a long highway cruise when it overheats? If so, check for cold spots on the rad. Prolly clogged.

Airdam on the front is for directing air below engne for better undercarriage air flow. Very much doubt it has anything to do with the fuel.

I'll work on fabbing up an airdam soon. I can't guarantee the radiator isn't clogged, but it looked fairly new when I bought the car last year, and the coolant I drained from it when I did the engine swap wasn't too bad. I'l have to see about getting a hold of an infrared thermometer to find out if there are any cold spots. The upper rad hose has no problem heating up, though.

Just in case it helps, here's a pic of my O2 sensor. I'm pretty sure when I swapped it into the exhaust manifold I'm using, it had a little black soot on it, but none of this white stuff. I've read this could be from silcone RTV, but when putting together anything that could end up in the combustion chamber, I was very careful not to use anything that wasn't labeled as "sensor-safe". Anything else this could be, or am I likely to ruin another sensor once I get the car running again (the battery is still on the float charger)?

somebody use leaded gas ? Just a thought.

O2 sensors are wear items, don't think too hard about it.

iceracer wrote: somebody use leaded gas ? Just a thought.

Well, some of the gas in the car was "winter" mix from last fall. I had added several gallons of fresh high octane earlier on Thursday, but that damage could have been done by then.

Knurled wrote: O2 sensors are wear items, don't think too hard about it.

Not too worried if its just a bad O2 sensor, just worried about the possibility its something else, with the weirdly-timed coolant heating.

Does the fan actually come on?

Knurled wrote: Does the fan actually come on?

Don't know for sure. It has in the past idling in the driveway, but at highway speeds, I haven't been able to tell due to the road noise. Out of curiousity, on the return trip, I turned on the AC, so the auxilary fan would come on. Didn't make a difference.

I know bleeding the cooling system can be a pain, despite how deceptively simple it is.

Infrared? I just use my hand, doesn't hurt that much. They tend to clog on the drivers side, where there is little coolantflow. Horribly desiged radiator. Just tr feeling the far drivers side top tank, then bottom, then middle. Make sue the fans arnt on:-P