Hi all,
I have a 98, 5.9L Durango and there’s two things going on. First she doesn’t blow hot air, but once in a great while yet it seems like the face, feet, and defrost modes sound like they change. I even hear what sounds like a door flipping when changing from defrost to feet setting but no heat is coming out. My temp gage stays berried low and might only clime to the first line then fall back to below.
second issue the cruise control pops out for what seems like no reason by me. Everything good, going down road at speed, then outta nowhere cruise control stops. Some times she surges but maybe only 2 times. Or cruising find, no change in speed yet the speed-o-meter drops then goes to right speed.

could these issues be due to some vacuum loss or could they be linked???? Idk what to do and need to figure out the problem b4 my husband starts throwing parts at her. Any suggestions and help is much appreciated.
happy holidays,

 

I'd still replace the brake light switch. On the heat, how many miles on the engine? 120,000 or more? The first thing I would check there is the thermostat. If it's too low or someone put a "FailSafe" unit in it, it may be stuck open. Every single Failsafe I've worked on failed open. I will NOT use them and if I get a car that has one that hasn't failed, I'll replace it as a matter of course. You want a 195 degree stock type unit.

I'll also bet your fuel mileage is pretty poor. A cool thermostat is an old hot rodders trick to get a denser fuel/air mixture. It works fine on carburetors with a choke. It does NOT work on fuel injected engines. Since you don't have a choke, the computer goes into "open loop" mode. It injectes more fuel to warm the engine up. Since it never warms up, it stays in that mode and sucks the fuel. You'll get no heat and eventually burn your catalytic converter out as it melts from catalyzing the fuel.

If you have the proper thermostat, and you have 120K or more on the engine. Your timing chain may be worn out. Since you have a first generation Magnum, it's not too bad. A decent mechanic can replace it in an afternoon. If it's worn out and you put a new one in there, you may be amazed at the difference in the heat, the better fuel economy and generally happier engine. It's like a new engine compared to the old chain. Replace the hoses and water pump while you have everything apart and you'll be roasting this winter.

 

so every failsafe thermostat you have worked on functioned as designed!!! Thats why they call them failsafe, so im curious why you hate them other than they fail in a mode that doesnt damage the engine? (yes if left unnoticed it can cause other problems, but with modern computer controlled cars they SHOULD throw a code that they take too long to warm up LONG before that happens)

 

An old style thermostat will last for years. I usually replace them after 5 years or so when I flush the coolant. Failsafe units often don't last a month. I put one in when they first came out and it failed within 2 weeks. When I got paid to work on other peoples equipment, I don't know how many "no heat" problems were just a failed Failsafe. The owners were happy with the lower cost but they shouldn't have had to have the repair in the first place.

 

i have the same issue. But I have a 1998 5.2 I think I forget sorry. But I replaced the water pump and thermostat. And coolant. Still no heat. My temp gauge reads only the first line. If I sit for about 30 min the cab gets hot from engine heat I believe. But sometimes if I’m on the highway the temp gauge works as normal. Only sometimes. My clutch fan is somewhat hard to turn and stays turning when motor is running. I thought it stopped spinning till the motor warmed up. I’m not sure. But it never over heats or any check engine lights.

 

Like I asked the OP, how many miles on your engine? The first generation Magnum engines are like the LA engines they are based on in that the timing chain will stretch over time. I've seen them stretch at 120,000 miles and I've seen some go 170,000 miles. However, when they stretch, the valves aren't quite in time and a lot of your heat is going out the exhaust. Plus you're using a LOT more fuel than necessary for two reasons. One, the fuel is burning while it's not properly compressed so you aren't going as far and, two, with no choke, the computer goes into "open loop" mode. It injects extra fuel to warm the engine up. When it never warms up, it continues to shoot more fuel than you need. In the short term, you don't get much heat and waste fuel. In the long term, you're cooking the catalytic converter and it will melt inside. Any decent mechanic can replace the timing chain set. On my own vehicles, and I would recommend to customers when I ran a shop, I would replace the thermostat, water pump, the hoses, (especially that pesky bypass hose), as well as flush and put fresh coolant in the vehicle. It can be replaced without dropping the oil pan but I always did just to run an eyeball over the bottom end and to clean any junk or, in older engines, sludge out of the pan. Trust me, if your chain is worn, when you freshen the engine up with a new one, you'll think you have a new motor.

 

I didnt have to but did on my 2000. Cam was broken valve seat was broken oil was thick sludge. Its my exs truck to drive. I have the 98. So i did a topend rebuild. With all the names issues it still ran. If i had video to back it i would. It was very quiet also. After i broke it down and found all the issues i was shocked.

 

OP, had similar issue that I fixed last month with no heat and engine temp reading on a 99 5.2 that I picked up last year. Temp gauge would stay very low and no heat from hvac.

My case was rust in cooling system and after flushing system, I was still getting no heat. So had to flush further and took the hoses that lead to the heater core off, and would alternate jetting water from a garden hose in the inlet and outlet hoses until I cleared a pretty serious heater core clog. After that still not getting too much heat and changed out the thermostat, which was stuck in the open position, and now temp readings are where they should be and I have heat works.

have no idea about your blend door noise though as well as your issue with the cruise control.

 

yes. I changed the stat first. Then used the hose to clear both core lines. New coolant new water pump. Still no heat. Also not sure but my clutch fan is hard to turn but it turns with slight force. Only option now would be chain

 

Does the engine get up to temp? Are both heater hoses hot? Is your blend door electric? Vacuum? Cable operated??