No Heat in my 91 Ex!

[lofton316]

I replaced my rad a year ago for the same reason you have now...
I replaced the heater core today and I now have heat! It's much hotter now and consistent, b4 the new core it would get warm, then cold and back to warm again. The temp gauge is still fluctuating but at less of a range; it runs almost up to the N and then down to the \ line and back up again, but slowly. When I have the heater on it fluctuates slower still, but in a smaller range, almost to the N then down shy of a quarter inch. The old heater core was the original aluminum OEM unit and although it was flowing clear hot water through it there was no heat transfer to the outside of the core. My best guess is that it was evenly coated with corrosion around the inside of the tubes that didn't block flow but did not allow the heat to transfer. I'm thinking that the temp sender and/or sensor isn't working right, I think it has one of each, I'll have to look it up in the manual. I'm not familar with the sensor, m/b it's not sending the correct info to the computer? I'm not overheating with a 195º stat in it. I had to run without one or it would overheat even when I bypassed the heater core. What I don't understand is why would it overheat with a 180° and now it's not with a 195°? No AC was running, there was no blockage in the rad, water flowed hot through all hoses, and it would overheat even on the highway at speed with a 180° stat even in 30-40° weather! The only change I made since is the heater core today and it was 43-49° all day. FYI, I am running a Stant Safe-T-Vent rad cap (13psi) and I keep my coolant overflow container filled halfway, my Ex uses some coolant while running and then put a little more than halfway back into the container after shutdown, but overnight after cooling it's back at the halfway mark again, I believe that's the way it's supposed to work. I have always used 50/50 mix Prestone/Water for coolant in all my vehicles. Tomorrow I will be driving 50 highway miles to church & back, and about 30 mles local driving, I am hoping the heat stays hot at highway speeds and m/b the fluctuation will stop, if I change the temp sender that should correct the gauge fluctuation problem, if it still exists. My next question is "If I do change the temp sensor what change would I see if the old unit was not working correctly?"...I'll give you an update Monday after driving it around all day tomorrow...
Keep me posted, m/b we'll stumble onto something by accident or someone will read the post and give us advice/help...
Thanks,
PCTechman

Maybe it has something to do with the temperature why the temp gauge is fluctuating mine just started doing this a few weeks ago.

 

[PCTechman]

Update II

Okay, so driving at expressway speeds brought different but consistent results: With an outside temperature of 37º @ 8 am, I let it run for about 5 minutes b4 driving off, the temp gauge didn't budge and the heat was lukewarm, some hoses were warm, some hot so engine wasn't up to full operating temp yet. After driving 2 miles at 35 mph I entered the expressway, gauge still not moving but heat getting warmer. After 5 minutes @ 50 mph the temp gauge went to barely touching the N on the temp gauge. It stayed there at speeds from 50-60 mph and did not fluctuate at all while heat steadily went to hot and stayed there; no fluctuation in heat or temp gauge reading with heat on or off. I used cruise control to keep speed steady (it's mostly level straight driving) without variations from human input. Keep in mind this was at a steady constant speeds of 50, 55, 60, no stopping or braking for 22+ miles. I also tried to load up the engine by using all the power sucking equipment; Max AC, rear defroster and high beam headlights, no increase in temp gauge reading. After the trip and parking I watched the temp gauge for a few minutes, it went up to past the N and then steadily decreased. Previously, even with no stat it would climb to past the L and then slowly descend. On the way home @ 10pm, same trip in reverse with colder outside temp of 33º but with traffic, braking and stopping, temp gauge is the same as b4; barely touching the N with full heat on/off gauge not fluctuating. Loaded engine again with all power equip running, all hoses, rad/heater core are equally hot but not scalding. For a vehicle that previously ran to M/A on the temp gauge without a stat and overheated with a 170º stat, that's great, but I don't think the temp gauge is correctly reporting the temp @ below N. At full running temp the N is about 20% so I'm thinking at this rate if the temp gauge could get to L on the gauge it would blow up/seize b4 getting to above Normal/overheating zone on the gauge. I'm going to change the temp sender unit, it's only like $6, but I have to find out which is the correct one for my engine, I think there are 3 with blue, black or green grommets/washers on them, that should correct the info to the dash gauge, if it's in fact wrong. I read that the temp sensor does send the info to the computer which can have an effect on gas mixture/mileage. I am currently getting 18 mpg average with a man trans. Other Ex owners tell me that's very good; most with auto trans are getting 10-12 mpg. I have 203K on it and I'm getting 18 so I don't know if changing the sensor would improve mileage or what it would correct, if anything...
Anyone have any input/experience with temp sender/sensor on Gen I Ex's?
Thanks,
PCTechman :exporange

 

[dogfriend]

IIRC, there are two coolant temp senders - one for the gauge which only has one wire (ground is to the engine) and the second one is for the PCM which has at least two wires.

 

[VG30_S12]

hey PCTechman, just a passing thought, what did you replace in your cooling system other than the HC? my other thought is that a it could possibly be a malfunctioning Clutch fan...

iirc fans go bad two ways, ether freezing up or clutch goes bad... maybe its just messing up... ether way its about $40 to find out... it would of course explain a lot for me... as said, mines doing the same but my lower rad hose is noticeably cooler(don't remember if it felt pressurized)

anyways, just a thought, my plans are to replace the sender and remove the rad for a nice "tub n tile" bath

 

[PCTechman]

Cooling System Replacement

The only problem with removing/cleaning/flushing the radiator is if it's on its way out and you loosen what's left holding it together, be ready to replace the radiator, that's how it happened to me. I only replaced the heater core & installed a 195º stat (I was running with no stat from April til present). Coolant flushed/changed 2 months ago in anticipation of winter. Fan and water pump replaced about a year ago when I did the timing chain. Fan had a crack, clutch was replaced by previous owner and is easier to change than the pump if need be. Old pump was in good shape, not leaking, but I figured I had 177k on the OEM unit so while I had it all apart, I replaced it then, rather than having to do it in mid winter. (I do all my repairs outside, no garage :thumbdwn I still don't understand why it was running hot/overheating with a bad heater core; even when I loop bypassed the heater core! I flushed/changed oil, stats, checked for blockages/leaks cracked block, replaced hoses, replaced heat selector/AC dash control, to say the least, very frustrating

Also in case you do replace your heater core: mine came with a plastic reducing bushing installed in one of the tubes to reduce flow so you do not hear noise/water slushing as the water is moving at high flow. I removed the one in mine for the most flow possible and there is no noise/water slushing through the core at low or high heat settings. I wanted the most heat possible as there are only front heat vents on these old Ex's and it gets real cold here!...:exporange

 

[PCTechman]

Sender & Sensor

IIRC, there are two coolant temp senders - one for the gauge which only has one wire (ground is to the engine) and the second one is for the PCM which has at least two wires.

The one with the single wire is listed as a Coolant Temperature Sender to the dash gauge and the one for the PCM that has 2 wires or more is listed as a Coolant Temperature Sensor. Why is it that way? :scratch: I don't know m/b to signify that there are 2 and where they belong, or that a sender sends the temp to dash gauge and sensor allows PCM to read the temp? Checkout Rockauto.com, with shipping charges they are at least 30% cheaper on most items than the auto stores locally, so I buy mostly everything I need from them...

 

[VG30_S12]

anything changed?

 

[dogfriend]

The one with the single wire is listed as a Coolant Temperature Sender to the dash gauge and the one for the PCM that has 2 wires or more is listed as a Coolant Temperature Sensor. Why is it that way? :scratch: I don't know m/b to signify that there are 2 and where they belong, or that a sender sends the temp to dash gauge and sensor allows PCM to read the temp? Checkout Rockauto.com, with shipping charges they are at least 30% cheaper on most items than the auto stores locally, so I buy mostly everything I need from them...

I don't know why they have two, but the sensor that the PCM uses is also known as ECT (engine coolant temp) and it is used for feedback to determine fuel injector pulse length, egr operation (if you have EGR, I don't think a 91 does) , evap emissions operation, etc.

 

[PCTechman]

Status Quo!

anything changed?

Everything stays the same, getting heat, but gauge on dashboard hovers between N and \ below N. Between work and being busy in general I haven't had the time to change the sensor. I noticed that although the hoses are hot and pressurized at full operating temp they are not ready to burst like before this was resolved. I guess the real test will be when the weather gets hot (90's) in the summer and I run the AC in stop n go traffic. I've tried everything to overload it and get it to overheat, 4 wheel drive (when raining), wipers on full, Max AC on, highbeams, rear electric defroster, all electrical accessories on full while idling, at slow speeds around town and at highway speeds 50-65, no change!

 

[safn1949]

Just as a side note,I am getting 18.5 mpg at 70 mph on the interstate in my 94 Explorer with 4.0,120,000 miles,auto. Drove it from WV to MN,1000+ miles. I bought it in WV to drive up here and have never tuned it ,my 2 cents