Heater blows cold , Suggestions ?

[Gearhead]

Before responding  ,   read this  ..........................

The vehicle , a 1988 Comanche w/ injected 2.5  4 cyl.

Recent repair / new parts  ( in the last couple of months )

    New Radiator , new thermostat , new water pump , new fan clutch , new rad hoses.


The heater was working awesome , the best vehicle heater I've ever had ! ,  then .......... without warning , the heater began blowing cold air only. The temp guage doesn't  move either. The coolant level is fine , there are no leaks.  It seems to idle a little higher than it was , noticable any time it's started , cold or hot. So far this has been a great  truck , runs real good , a bit of a gas pig but a great little rig.

What am I missing here ?  , one thing for sure is , I have little time for this right now , being in the middle of home renovations , my garage is plugged up with reno related crap.

Suggestions ? vacuum problem ?  , I was hoping one of you has seen this before and save me some diagnostic time by suggesting where to look.

[BlackYJ]

Probably the heater core.  Are the lines to and from the firewall warm/hot?

[Gearhead]

Probably the heater core.  Are the lines to and from the firewall warm/hot?

None of the hoses , heater , radiator ,.......... any of them are hot.

I have seen before , a thermostat is replaced and it's life is short , like only a couple of months . I usually buy failsafe thermostats , when they fail , it's in the open position.

I'm gonna check it out with an infrared thermometer this morning  start up / warm up ,  then again , once I'm at work

[4PLAYZJ]

Check the two heater hoses with and infer red gun.  When the vehicle is hot and your fan is on Hi.  There should be some temperature loss at one of the hoses(but just a little bit).  If there is a big temp difference or no difference at all it is probably a plugged heater core.  Before you condem it dissconect the two hoses running into the cab.  Put one into a bucket and with a garden hose flush out the other one.  This will unplugg most heater cores unless it is a really nasty plugg.   If none of your hoses are hot your best tool is the temp guage in the dash.  Check the upper rad hose and seem if it gets warm before the thermostat should open(like in a matter of minutes) it will tell you that it is stuck open or opening too soon. Good luck.

[Gearhead]

Messed with it a bit today , using an infra red thermometer ,  there isn't a great deal of heat any where ?  . About 15 degrees of temperature drop across the radiator , the radiator inlet temp is quite low.

I need to set aside some diagnostic time , I've got a cooling system pressure tester , vacuum tester and a bunch of other automotive gadgets,  ................  the last couple of months have been spent demolishing and rebuilding a bathroom from scratch lot's of tile work ! , pretty much consuming every spare moment not spent at work ,  vehicle repairs ? ....... pfffffffffff .......... Michaelangelo never dirtied his hands with greasy car problems.


I've got another thermostat or two around here , a couple of leftover temp guage senders , I need to get the temp guage working too , it stopped working the same time as the heater went cold.

[Bnine]

My xj does the same thing Jeff.

I check it off to a gremlin electric fan that always kicks in, and most likely a failed safe thermostat thats stuck.


I just havnt got around to farting with it. My gauge never gets over 40 no matter how much I romp on it.

[HIWYH8R]

If ya want to come by on sunday I'll take a look at it. Jus let me know

[redbull]

Air bubble in the system? Tried burping it?

[Gearhead]

It's fixed

Cause of problem ?

Thermostat stuck wide open , till now I had been using fail safe thermostats , sold under the "Motorad" brand name.  After experiencing two recent failures on two separate vehicles and discussing this with a few people , I've decided , failsafe thermostats are crap , I won't be buying another "Failforsure" thermostat . This one was two months old , the previous one was less than a year old.

I'm pulling the "Failforsure" out of my 88 YJ , I just had a fancy (expensive ) radiator built for it , to get control of the temps in the summer , now I can run a 195 thermostat instead of the 165 that's in it.

I can't remember if my 94 YJ has one of these junky thermostats in it or not , it's likely it does , the heater sucks in that Jeep , the windows do not defrost well at all , my sweetie drives that one , she's gratefull that I built it for her and reluctant to complain about it. Next service I'll take a look , if it's got a "failforsure" in it , it's coming out  and going in the hydraulic press.

From now on it's Stant thermostats and radiator caps only , it seem's they are harder to find , as most people prefer "budget " brands like Motorad . I generally don't go for budget parts and would rather pay a little more to have better quality , sadly  , most auto parts stores don't stock Stant brand products , they will order these parts for you , usually they are not on the shelf.