brdhntr Posted January 28, 2009 Share Posted January 28, 2009 the truck runs at a good temp, no underwarming or over. the heat works almost too well (such a small cab) when driving. the problem is, if i stop and let it idle, the air starts to blow cold. i start driving again and it's fine. i don't get any fogging or fluid leak like a bad heater core. i know heater cores can go bad, but i can't understand why it would heat fine when driving. the coolant is full, but i did have to add some coolant (just a little) last week. i just can't figure out what it would be since it works fine when driving. i'm sure i'm just missing something simple. :dunce: Link to comment Share on other sites More sharing options...
Randy in Maine Posted January 28, 2009 Share Posted January 28, 2009 You may have an air bubble in the cooling system and it will need to be bled correctly would be my guess. Where was the coolant leaking from when you had to fill it? Link to comment Share on other sites More sharing options...
brdhntr Posted January 29, 2009 Author Share Posted January 29, 2009 looks like there is a little leak on the lower left side. it's not a big one and will be an easy fix when the weather warms up. the guy i got it from put one of those prestone flush kits in, the ones you hook up to your hose. is there anything different for bleeding the system on one of those? i thought i had it bled out pretty good. i'll do it over though. Link to comment Share on other sites More sharing options...
Pete M Posted January 29, 2009 Share Posted January 29, 2009 Is it a 4.0 or 2.5? Link to comment Share on other sites More sharing options...
brdhntr Posted January 29, 2009 Author Share Posted January 29, 2009 2.5 Link to comment Share on other sites More sharing options...
Pete M Posted January 29, 2009 Share Posted January 29, 2009 I know there are some tricks for bleeding the 4.0, but I don't think they apply to the 2.5. :( Link to comment Share on other sites More sharing options...
jtdesigns Posted January 29, 2009 Share Posted January 29, 2009 http://gojeep.willyshotrod.com/HowtoRadiatorRestrictor.htm Try this. Link to comment Share on other sites More sharing options...
maddzz1 Posted January 29, 2009 Share Posted January 29, 2009 I don't think it is air in the system because I had this problem before on a toyota camry (when driving the heat would blow nice and hot, as soon as stopped, the air would start blowing cold. As soon as I applied the gas again it would go back to hot :???: ) I never did figure out why it did it. :dunno: Link to comment Share on other sites More sharing options...
Pete M Posted January 29, 2009 Share Posted January 29, 2009 Is the water pump failing? Link to comment Share on other sites More sharing options...
BLHTAZ Posted January 30, 2009 Share Posted January 30, 2009 Is the water pump failing?That's what I was thinking too. If the pump is old, the blades can actually be rotting away and not able to move enough water at idle... :dunno: The only other thought I had was that there is a minimal vacuum leak somewhere that is only showing effects at low RPM right now. Link to comment Share on other sites More sharing options...
JOMJ87 Posted January 30, 2009 Share Posted January 30, 2009 It makes me think water pump. Just cutting the water at speed then at idle it chugs it around enough to work? They arent to expensive like 30 bucks so you could do that swap and see what it looks like. Cole Link to comment Share on other sites More sharing options...
aemsee Posted January 30, 2009 Share Posted January 30, 2009 Poor circulation would definitly cause this type of a concern. Let it sit at idle til you lose heat. Pop the hood and feel the hoses. Are they both cold? If so, raise the idle and see if they heat up. Link to comment Share on other sites More sharing options...
aemsee Posted January 30, 2009 Share Posted January 30, 2009 [quote="BLHTAZ The only other thought I had was that there is a minimal vacuum leak somewhere that is only showing effects at low RPM right now. That would actually act the other way. Higher vacuum at idle, lower at higher RPM's Link to comment Share on other sites More sharing options...
BLHTAZ Posted January 30, 2009 Share Posted January 30, 2009 The only other thought I had was that there is a minimal vacuum leak somewhere that is only showing effects at low RPM right now. That would actually act the other way. Higher vacuum at idle, lower at higher RPM's :hmm: OK...I'm intrigued. Are you saying that there is stronger vacuum at idle than there is at say 2500 RPM? :dunno: Link to comment Share on other sites More sharing options...
HOrnbrod Posted January 30, 2009 Share Posted January 30, 2009 From the FSM: With a vacuum gauge hooked up on the intake on a warm engine, a healthy engine at idle vacuum should read steady 15 to 20 in Hg. At 2000 rpm it should read steady 19 to 21 in Hg. At WOT it should read close to 0 in Hg. A decelerating engine reading should jump to 21 to 27 in Hg as open throttle released. But I don't think it's a vacuum problem; I think the vanes on the water pump impeller have corroded to about 1/2 the original size and coolant circulation through the heater core is poor at idle, especially if the core is semi-clogged. :cheers: Link to comment Share on other sites More sharing options...
jtdesigns Posted January 30, 2009 Share Posted January 30, 2009 The only other thought I had was that there is a minimal vacuum leak somewhere that is only showing effects at low RPM right now. That would actually act the other way. Higher vacuum at idle, lower at higher RPM's :hmm: OK...I'm intrigued. Are you saying that there is stronger vacuum at idle than there is at say 2500 RPM? :dunno: Yes sir.. let me explain why... Your engine is basically a big 'ol pump right.. so air coming in the throttle body,, through head,, and out the tailpipe.. OK, so when your at idle your throttle is what?? Closed right,, but your engine is still sucking air, but from where??? Well it'll come in at the CCV, AIC, Brake booster fitting, and the Vac port.. So when the throttle closed the engine is still sucking causing vacuum. :clapping: Same thing if your running high RPM and WFO then let off the throttle alot of vacuum is produced. Link to comment Share on other sites More sharing options...
BLHTAZ Posted January 30, 2009 Share Posted January 30, 2009 OK...I understand the lack of vacuum under acceleration / deceleration, but at 2000 - 2500 RPM, according to Don's FAM info, it would be stronger than at idler as I suspected. The difference is not significant enough to really have the effect I was thinking anyway so it really is a mute point ;) . I still believe it's the pump. Thanks for the education though :clapping: Link to comment Share on other sites More sharing options...
jtdesigns Posted January 30, 2009 Share Posted January 30, 2009 OK...I understand the lack of vacuum under acceleration / deceleration, but at 2000 - 2500 RPM, according to Don's FAM info, it would be stronger than at idler as I suspected. The difference is not significant enough to really have the effect I was thinking anyway so it really is a mute point ;) . I still believe it's the pump. Thanks for the education though :clapping: I would think that it'd be lower at 2500.... Maybe because theres some sort of "sweet spot" between the throttle being slightly open and still pulling air on the plenum through the engine.... :hmm: Link to comment Share on other sites More sharing options...
aemsee Posted January 30, 2009 Share Posted January 30, 2009 OK...I understand the lack of vacuum under acceleration / deceleration, but at 2000 - 2500 RPM, according to Don's FAM info, it would be stronger than at idler as I suspected. The difference is not significant enough to really have the effect I was thinking anyway so it really is a mute point ;) . I still believe it's the pump. Thanks for the education though :clapping: I would think that it'd be lower at 2500.... Maybe because theres some sort of "sweet spot" between the throttle being slightly open and still pulling air on the plenum through the engine.... :hmm: Not to :hijack: too bad but, it is dependent on throttle angle. 2500 RPM sitting still is totally different than 2,500 going down the highway. Especially on a lil 2.5 Link to comment Share on other sites More sharing options...
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