94 YJ 4l Running cool and showing code 17 - JeepForum.com

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post #1 of 20 Old 02-09-2017, 01:51 PM Thread Starter
Tony01
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94 YJ 4l Running cool and showing code 17

Hi all
well it's getting a bit chilly here in the UK and my jeep has started to run cool, too cool in fact with the temp gauge only showing one or two marks, not much heat from the heaters and it now shows code 17 (engine running too cool.

So I back flushed the heater core, some crud came out but not much.
removed and checked the thermostat it worked correctly. 2yrs old
re installed the Tstat with burp hole facing the top and facing the correct way
Radiator is clear and the water pump is only a few years old and pumps very well
refilled with antifreeze
ran the engine up on ramps to burp the system and still getting code 17

this is doing my head in, any ideas as to where the problem may be as I am at a loss
regards TC

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post #2 of 20 Old 02-09-2017, 02:22 PM
Luuca
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Quote:
Originally Posted by Tony01 View Post
Hi all
well it's getting a bit chilly here in the UK and my jeep has started to run cool, too cool in fact with the temp gauge only showing one or two marks, not much heat from the heaters and it now shows code 17 (engine running too cool.

So I back flushed the heater core, some crud came out but not much.
removed and checked the thermostat it worked correctly. 2yrs old
re installed the Tstat with burp hole facing the top and facing the correct way
Radiator is clear and the water pump is only a few years old and pumps very well
refilled with antifreeze
ran the engine up on ramps to burp the system and still getting code 17

this is doing my head in, any ideas as to where the problem may be as I am at a loss
regards TC

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I have absolutely NO EXPERIENCE trying to get a YJ to run hotter.


That said, a simple piece of cardboard over the lower 1/3rd to 1/2 of the radiator would probably cut enough airflow, temporarily, to help the engine obtain the desired 210 F you are looking for. And don't give me any of that Celcius crap either.

But seriously, go get some of that plasticized cardboard people use to make signs and cut it to cover the lower 1/3rd of your radiator. Place it in front of the radiator and punch some small holes in the corners and zip tie it in place. If you need more heat, then try 1/2 of the rad from the bottom.

91 YJ "Alice" 4.0L, AX-15, NP 231 Flange, Adams Driveshafts F/R, 2.5" OME 36Rs, 5/8" /f12 Booms, 33" Yoko ATs, 4.10 D30/35 Aussie Lockers F/R
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post #3 of 20 Old 02-09-2017, 05:03 PM
Chrisnvegas
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If it's not warming up (and not just the gauge) the thermostat isn't doing it's job.
I won't use an aftermarket t-stat. Mopar t-stats are the only ones I have had good results with.
They have a check valve in the upper portion that will allow air to pass but not much coolant.

When I bleed air in a CJ or YJ, I don't jack it up.
I remove the highest heater core hose from the front of the engine and slowly fill.
When anti-freeze comes out I replace the hose and overfill the coolant reservoir.
Over a few heat cool cycles the system will bleed the remaining air out and the level in the reservoir will stabilize.

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post #4 of 20 Old 02-10-2017, 07:40 AM Thread Starter
Tony01
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Thanks chrisnvegas
I have covered most things trying to improve the heat problem, but now it is throwing up code 17 i think it may be the Tstat.
I will change it and see what happens
Thanks once again for your input
Tony.
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post #5 of 20 Old 02-10-2017, 08:53 AM
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Quote:
Originally Posted by Tony01 View Post
Thanks chrisnvegas
I have covered most things trying to improve the heat problem, but now it is throwing up code 17 i think it may be the Tstat.
I will change it and see what happens
Thanks once again for your input
Tony.
See if you can find a Mopar with the check valve.

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post #6 of 20 Old 02-10-2017, 10:59 AM
Luuca
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Quote:
Originally Posted by Tony01 View Post
removed and checked the thermostat it worked correctly.
How did you check it? I agree ot may be the tstat, but your statement said it worked.

91 YJ "Alice" 4.0L, AX-15, NP 231 Flange, Adams Driveshafts F/R, 2.5" OME 36Rs, 5/8" /f12 Booms, 33" Yoko ATs, 4.10 D30/35 Aussie Lockers F/R
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post #7 of 20 Old 02-10-2017, 12:03 PM
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Are you going by the temp gauge in the dash? The last time mine was on the road the gauge read only 100 degree f and that was with the radiator busted and blown head gasket, so it was either the gauge or the sensors not working.
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post #8 of 20 Old 02-10-2017, 02:36 PM Thread Starter
Tony01
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hi labhound, I removed Tstat and put it in a pan of water and boiled it up with a thermometer in it and noted the temperature at which it opened.
around 88 degrees C which is 190.5 F.
The inlet hose to the heater core gets hot but the return is only warm, indicating no flow of water through the heater core, yet when i flush the heater core the water flows well. The only thing I can think of is an airlock in the system, but I will put a new T stat in and a new radiator cap, other than this I am at a loss,
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post #9 of 20 Old 02-10-2017, 04:19 PM
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Quote:
Originally Posted by Tony01 View Post
hi labhound, I removed Tstat and put it in a pan of water and boiled it up with a thermometer in it and noted the temperature at which it opened.
around 88 degrees C which is 190.5 F.
The inlet hose to the heater core gets hot but the return is only warm, indicating no flow of water through the heater core, yet when i flush the heater core the water flows well. The only thing I can think of is an airlock in the system, but I will put a new T stat in and a new radiator cap, other than this I am at a loss,
Traditionally, an air-lock would cause an overheating situation.

If you're getting no heat inside the cab that's probably a blocked heater core. A code 17, I'd test your front temp sender - the one on the tstat housing to ensure it's feeding your computer the correct temps.

No matter what, the engine should get to 210 after idling for some time.

Here's what I would do.

1. get a laser thermal reader from Harbor Freight
2. idle the engine for about 15-20 minutes until at running temp (hot rad cap to the touch)
3. measure the temp with the laser temp gun at the thermostat housing. shoot from the driver's side at a surface not being hit directly by the fan blowby for best measurements.
4. Record 3 separate measurements at the same location

What to do:

If the temp is 195 - 210 at the tstat housing, you have a bad temp sending unit in the tstat housing and it's giving the ECM a bad reading and throwing a false code.

If the temp is really cold - use cardboard to block flow and run the test again. if with the cardboard blocking airflow the engine gets up to 195-210 F then you know you have the most efficient cooling system on a YJ EVER. Summer in the sahara should be a breeze.

IF your engine is running at 195 F PLUS and you are not getting any heat then check the following

.1 the blend door and blend door actuator
2. vacuum lines to HVAC controls
3. replace heater core.
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post #10 of 20 Old 02-11-2017, 07:41 AM Thread Starter
Tony01
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Cheers Luuca,
lots to work on, thanks for your input I will keep you informed on what i find when I start to work on it next weekend
regards Tony
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post #11 of 20 Old 02-11-2017, 08:01 PM
Drednot
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One more thing that may cause overcooling...a defective fan clutch.

If it fails in the locked up mode, it will way overcool the engine in winter.

Quick check would be to try to spin it by hand. It should give slight resistance. If it is very hard to turn, or doesn't turn at all, that's the trouble.
Also, if the fan "roars" noticeably , that's another indication that its overcooling.

You should get an IR thermo gun to test the system looking for cold spots that may indicate other issues.

I don't think you have harbor freight stores in the UK...Machine Mart would be your equivalent. Not sure which is closer to you, Swindon, or Luton
https://www.machinemart.co.uk/p/clar...rmometer-irt1/

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post #12 of 20 Old 02-12-2017, 10:00 AM
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You should also let the engine get to full temp, 30 mins of idle will do, and then test the resistance or OHMs of your Coolant Temp Sensor. Use a multimeter set to OHMS and run the leads one on either side of the pins of the CTS after removing the connector. Should be around 750 OHMs. It sounds like the Fan clutch that @Drednot mentioned may also be worthy of a test and that you may solve this issue soon.
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91 YJ "Alice" 4.0L, AX-15, NP 231 Flange, Adams Driveshafts F/R, 2.5" OME 36Rs, 5/8" /f12 Booms, 33" Yoko ATs, 4.10 D30/35 Aussie Lockers F/R
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post #13 of 20 Old 02-13-2017, 12:37 PM Thread Starter
Tony01
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Hi Drednot, thanks for the thought,
I tested the fan clutch today and it is ok there is a little resistance when you turn it but it can be turned freely by hand

I am wondering if this problem is all my fault because I remember now that I drilled an air bleed hole in the Tstat and i think it was bigger than it should have been, it should just let any air through to let the air burp out of the system but not allow too much water through and I am now thinking that the hole I drilled may be letting too much water through, essentially bypassing the Tstat and not heating up enough to open the Tstat, I have ordered a new Tstat and I will look at the hole size in the new one and compare it with one fitted, (but I think the one fitted has a too large hole).
watch this space
regards Tony
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post #14 of 20 Old 02-13-2017, 12:58 PM
Luuca
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no matter if you had no tstat in the vehicle, eventually, the water temp would reach 210 eventually.

If the ambient air temp is keeping the YJ from reaching 210 at idle, then you are living in siberia or something.

In my opinion you have two different issues:

1. Heater not blowing hot
2. Computer thinks the engine is running too cold for too long.

Follow the tests I listed before and post the results.
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post #15 of 20 Old 02-13-2017, 01:01 PM
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Thermostat genuine OEM Mopar part #83501426AD

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