98 XJ Heater blowing cold

IslanderOffRoad · Dec 10, 2012

98 XJ 4.0, 5spd. Cooling system recently flushed, new t-stat, new 3-core rad.

Heater blows cold to lukewarm air, lukewarm when its mid day and warm outside. Unfortunately I go to work at 4am so it's a cold ride in during the winter.

I did a lot of searching on this topic, and found that most 98's have an electronically controlled blend door, which is pretty extensively documented on diagnosis and repair procedures. Mine however is an early 98 with the older, cable actuated blend door. I've unhooked the cable and it seems like everything is moving and working, just no heat.

Am I looking at a heater core replacement, or is this still most likely a blend door issue?

Ecomike · Dec 10, 2012

Does the 98 have a heater valve under the hood? Try back flushing the heater core, reverse flow with VERY LIGHT pressure from a garden hose.

IslanderOffRoad · Dec 10, 2012

no heater valve. I have backflushed the core.

IslanderOffRoad · Dec 10, 2012

Pulled the temp control cable off the bottom of the blend door last night, manually shifted it to "hot" heard the door moving, left it there this am, still cold.

Gonna try the CLR trick on Wednesday when I'm off work. Gotta be a clogged core at this point.

birchlakeXJ · Dec 11, 2012

IslanderOffRoad said:
Gonna try the CLR trick on Wednesday when I'm off work. Gotta be a clogged core at this point.

I have had some luck with this CLR technique.

http://www.youtube.com/watch?v=dcL_0TWeZJY&feature=channel

Good luck and keep us updated!

Ecomike · Dec 11, 2012

IslanderOffRoad said:
no heater valve. I have backflushed the core.

Is the return heater hose cold, or hot like the feed hose?

CobraMarty · Dec 11, 2012

I had the same and just back flushed the core with hot water and then forward flush until got a great flow and now I have great heat. I turned the pressure up (started slow until clear) then turned up to full when back flushed with the hot water and didn't bust a leak thank goodness. Stepped on the hose a few times to 'pulse' it and it seemed to dislodge some junk.

IslanderOffRoad · Dec 11, 2012

Ecomike said:
Is the return heater hose cold, or hot like the feed hose?

Sometimes. After the last flush it was, now it isn't. Seems to be common that it works better for a bit after a water flush

IslanderOffRoad · Dec 11, 2012

Anyone know the inner diameter of the heater hoses? I'd like to use some clear flex tube for the flush to get them out of the engine bay

Ecomike · Dec 11, 2012

IslanderOffRoad said:
Anyone know the inner diameter of the heater hoses? I'd like to use some clear flex tube for the flush to get them out of the engine bay

IIRC there are three sizes on the Renix, and it changes at the T fittings, closed renix bottle-heater valve style. I think 5/8 or 3/4 near the heater core itself.

DanMan2k06 · Dec 12, 2012

If your heater core is clogged, you have bigger problems as well. Your entire block is corroding from the inside, and will require a constant flushing effort. I had to eventually acid-soak and rebuilt my motor because of it.

Flushing the core should bring your heat back, but also realize that too much pressure can potentially make it leak inside your cabin. And if it does work, it may only be temporary.

IslanderOffRoad · Dec 12, 2012

Flushed and backflushed once again until 100% clear water came out both directions, put in some CLR and let it sit for 10 min or so, then repeated the flush with water.

Getting 120 degrees at the vents, hoses are both hot. Real test will be Friday morning when I drive into work at 4am!

birchlakeXJ · Dec 12, 2012

Keep us updated on how the heat works after your CLR procedure....

CobraMarty · Dec 13, 2012

That made me check mine after I flushed it last week. Tonight 40*out and was blowing 125-129* IR at the center duct. Toasty.

2001JeepXJNS · Dec 15, 2012

Just got finished using the CLR and it worked great.
Used to have warm air now hot. Blew the core out with the air compressor attachment a few times. Then filled with CLR let it set about 30 mins then ran a bunch of water and compressed air through it in both directions until it started to look clear. That thing was super dirty at first, looked like sludge.

birchlakeXJ · Dec 16, 2012

The CLR technique is worth a try. If it doesn't work, the heater core has to come out.

So what do you have to lose?? It really is a last ditch technique.

Glad to hear you have had luck with it.

Ecomike · Dec 16, 2012

That stuff is way too aggressive for my tastes, but maybe if diluted first.

http://emergencyservices.catawbacountync.gov/esapps/emsref/MSDS/CLR.pdf

pH = 0.7-0.9 (that is very LOW!!!)

I would try vinegar first myself.

OR use about 2% solution of the CLR.

IslanderOffRoad · Dec 16, 2012

I diluted it to 50/50 and left it in less than 15 minutes.

I bet had I done it longer my heat would be even better, but I'm in Texas, not Alaska.

cdrake261 · Dec 17, 2012

I know what I'm going to try soon...stupid question which hose goes towards the heater core? It would make sense to back flush it first.

IslanderOffRoad · Dec 17, 2012

Whichever one is cold will be your return line if you're clogged.

I don't think the direction you flush first will make a huge difference. I did it both directions till only clear water came out

98 XJ Heater blowing cold

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