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I have a 1996 Nissan Altima. I had to change my radiator recently because it developed some cracks on the top of the tank and I would have to refill the tank every few days.

When I changed the radiator I also changed the thermostat and the two radiator hoses.

After changing the radiator my car started to misfire or die for a split second and come back when I would accelerate while the engine was still cold. This would happen all the way up to until it was at normal operating temperatures. When the tempature gauge needle is around the middle it stops misfiring during acceleration. The engine also takes a bit longer to warm up now in general.

I think i possibly failed CA smog because of this problem too. Since my car is misfiring possibly because of high chamber temperatures which increase my NOx. I specifically failed because of the NO test. These were my results:

CO2    O2   HC   CO    NO
14.7   0.1  49   0.23  950  15mph 
14.6   0.1  37   0.18  700  25mph 

Anyways, what do you think what went wrong after I changed my radiator? Because it did not do this before the change only after. Any tips advice on what to do next?

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What ignition related areas have you checked so far for possible faults? –  Timo Geusch Jan 8 '13 at 0:48
    
@TimoGeusch I haven't checked any because I didnt have this problem until after I changed my radiator. So wouldn't the problem be related to the radiator? –  Nils R Jan 8 '13 at 1:14
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What you're describing sounds like an ignition issue. I'd check if the plugs and wires got splashed with coolant and then have a look at the rest of the ignition system. Does the check engine light come on? –  Timo Geusch Jan 8 '13 at 1:38
    
Ill check the plugs, the wires, and distributor as well. No check engine light, but a few days after changing the radiator a code popped up. It is no longer in the system and I forgot the exact code, but the code basically meant that the engine was taking too long to heat up. I resealed the thermostat correctly and the code went away. –  Nils R Jan 8 '13 at 2:07
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2 Answers

Did the car ever overheat? If so, look at your oil. The head gasket could have gone bad, and the crack was easier for the leak to use than the head. Also, you should pressure test the system and see if it hold for more than 15-30 minutes.

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It never overheated before the change, it just ran through coolant very fast because of the cracks in the top of the old radiator tank. The oil doesn't have any coolant mixed in either. Ill try to pressure test the coolant system in a bit –  Nils R Jan 8 '13 at 19:32
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Do you have a temp sensor mounted in the radiator? If the temp sensor is faulty it may be sending a "warmed up" signal to the engine control module. This will cancel the "run rich" I am cold signal. The result could be rough idle when cold. A possible clue that this is the case is if the electric fans start immediately even when the engine is cold,this could also explain the longer warmup times. If the radiator has a metal tank where the sensor mounts it may supply the ground for the sensor. Over application of teflon/threadsealer tape could open this ground.

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