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I can't figure this one out. You tell me what you think.

1989 XJ, 4.0, AW4, 2WD. I bought it about 10 months ago from a fella who said it was overheating and he couldn't fix it. After driving and checking I decided it had either a leaking head gasket or a cracked head. Removed head. Gasket looked like it may have been leaking. Couldn't tell for sure. Replaced it. Cleaned, magnafluxed head. No cracks. Left rear freeze plug showed seepage. Replaced all freeze plugs. Checked thermostat. OK. Checked temp gauge and sending unit. OK. Flushed block, radiator and heater core. Got a coffee can full of crap out of block. Put everything back together, burped system, went for test drive. Went 5 miles and was overheating so bad had to pull over and let cool down. Got it home and checked everything, NADA. Burped system. Topped off coolant. This went on for a month. I would drive it to town, 14 miles and it was steaming. Let it cool down, top off coolant and drive back home. It would be steaming. Like I said, this went on for a month. Then about a month ago I went to town. Temp gauge went up to 190 and stayed there. Ive put about 7-8 hundred miles on it with absolutely no problem. Temp goes up to 190. Stays there. I give up. Mileage is terrible, 15mpg, but it runs great. Any ideas? Jim :dunno:

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Oxygen sensor? It may have been running lean, and then the O2 sensor finally failed and dumped the ECU into "limp home" mode -- which for the Renix I believe is the same as warm-up, open loop mode and defaults to a rich fuel mix.

OS tested good, both resistance, 5ohms and voltage .5VDC.

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Sounds a little like my '88 Pioneer (in the sense that a problem mysteriously appears and disappears without warning) with 4.0L and AW4. I've replaced the radiator, waterpump, thermostat, fan clutch and temp sender. No leaks in or around expansion bottle. I can drive it for the first 10-15 minutes and it stays around 200-210 degrees. Then out of nowhere, the temps start rising and it will stay hot (gauge is reading right before the RED zone) and nothing I do will cool it off. No evidence of coolant leaking into oil and vice versa. No idea what my MPGs are. If you can figure this one out, let me know. I attribute my problem to an incredibly finnicky Renix closed cooling system...

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Does it blow hot air ALL the time? If not water is not getting by the thermostat. The mileage is probably O2 related, I had a grand prix that ran rich because the temp would not let it go into closed loop so it ran rich like mentioned above. If the pressure cap isn't holding it will overheat very easy, do you have a pressure tester for the cap and pressure bottle? The water pump may not be flowing water good enough or there is some blockage, you should notice with irregular hot and cold air through the heater.

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