Today I had a major problem with my 99 Dakota 3.9L. Truck has been working fine but today while out for a drive the CEL came on and the truck started to run hot. On average it would run around 196-202 degrees but today it ran up to 258 degrees after the CEL came on. Because of the over heating I blew out my heater core and had to bypass it to get home. After checking the codes with my scanner it kicked out the code P0171. I also noticed allot of black smoke out the tailpipe and my fuel economy went south.

Any ideas what might have caused this sudden problem? I had a new MAP and TPS installed recently after porting out my TB so I put the old ones back on and the problem is still there. I have read that it could be the O2 sensor but I am looking for advise before I walk out and spend $100 for a new sensor only to find out thats not the problem.

 

I doubt it's the 02 sensor. It could be, but the likely hood is pretty slim

Black smoke and gas mileage is because your engine is dumping fuel in the cylinder's, running VERY rich. Check for a leaking fuel injector. Did you clear the code? Do you have a scan tool? Not just a code reader, but an actual scan tool?

 

Well I cleared the code and checked everything over and found no problems. Cleaned the injectors and cleaned the O2 sensor and replaced the thermostat. Truck no longer puts the CEL on and runs smoother BUT..... Now the engine is running hotter and the temperature fluctuates from 220 - 258 degrees Fahrenheit at cruising speed 90Kph/55Mph and runs around 222 degrees Fahrenheit while idleing . What are the chances I blew a head gasket or even worse warped a head? The engine temp spiked at 268 Fahrenheit before I could get the engine shut down.

 

Is your cooling system sealed? Are you leaking coolant? Burning coolant?

To check your heads, do a compression check on all cylinder's. If your numbers are fairly even across the board, the chances of a head gasket blown or warped head are slim. If one or two cylinder's are much lower than the others, then your problem is in that head.

Also pressure check your cooling system. Make sure it holds 15 psi steady for at least 15 minutes. It shouldn't lose much pressure at all. If you do lose pressure check for the leak.
Again, with the cooling system. If these two test run perfect, then have your radiator/heater core back flushed. One or both may be blocked causing the coolant to not flow properly, making the engine run hot.

 

The heater core has been bypassed, and the Rad is new, no visible leaks, and no obvious sweet smelling antifreeze. I guess I will have to try a compression test.

 

Welcome to the forum, here is the code...

P0171 (M) 1/1 Fuel System Lean A lean air/fuel mixture has been indicated by an abnormally rich correction factor.

 

Thanks Billiam I found the problem that was causing the CEL. The truck is no longer displaying any codes but still over heating. I haven't done a compression test or pressure test the cooling system but I was checking the coolant level and making sure the system was bleed of any air and noticed the truck ran normal temp 216 with the rad cap off but as soon as I put the cap back on the temp would start to climb. I replaced the cap with a new one and it still over heats. With the rad cap off I also noticed every few seconds I would get a surge of coolant that would gush out of the rad but there are no bubbles like you would normally see if it was a head gasket. This is starting to aggravate me. I have never had this hard of a time diagnosing an over heating problem. What about the Water Pump? Is there a chance the impeller or pully could slip? The truck only has 138,000 KMs or 85750 Miles on it.

 

Burp your cooling system before you do a compression test. How long were you leaving the cap off like you described up there ^^? I think you might have a huge air bubble in your radiator. Basically, remove your rad cap, and run the truck until it gets up to temp, then run it for like 30 minutes, or until all those surges of coolant stop. If they don't stop, then look into the compression test and water pump issues.

 

I did let the truck run for almost an hour at temp 220. After every surge I would add a little more coolant to keep it topped up but it just kept surging. When you say water pump issues can you explain? I have a spare water pump that came off a working truck so I may just swap it out to see if the problem still persists.

 

Well, there's really only so much that can go wrong with a water pump. Either the seal goes, the bearings go, or the impeller gets too corroded to push coolant and doesn't work efficiently. I suppose that could cause the "coolant surge". How long has it been since the coolant had been changed? it may have caused a large amount of corrosion on the impeller blades.

If you have a spare pump that you know is good, with 120,000+ miles on the current one, it couldn't hurt to change it out.

to be quite honest, I'm not really sure where else to go with what you've already done. I'd say replace the water pump since you've already got one, and go from there.

When you bypassed the heater core did you use the proper things to make sure the pressure stays in the cooling system? it could be that the bypass is causing a leak or loss in pressure which would definitely make the truck run hot.