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Author Topic: P0128 Won't go away!  (Read 1543 times)

Offline Corey G.

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P0128 Won't go away!
« on: February 27, 2015, 05:26:51 PM »
2006 LJ with stock 4.0

I have a CEL that just won't go away no matter what I try. P0128 - "Coolant temp below thermostat regulating temp"
It popped up when the weather started getting colder around mid November.

Radiator cap is a couple years old.
Water pump and radiator were replaced less than a year ago with OEM Mopar parts. Also flushed the coolant at that time with correct 50/50 mix with distilled water.
I replaced the thermostat (195*) and coolant temp sensor this past weekend thinking it was stuck open but it was closed when I took it out. Also checked the coolant mixture with one of those testers with the floating balls and it was fine.

Seems like it gets up to operating temp in a reasonable time according to the stock gauge.

Theres only a couple other things I can think of...
-My fan clutch is seized so it constantly runs at engine rpm. But it's been like that for 3 years and I've never had this code come up.
-I always drill a 1/8" hole in the edge of the t-stat to make bleeding easier. I can't think it would make a difference, as some t-stats even come with a hole. The one I took out had been in there a couple years with a hole and again, never had a problem till this past November.


2006 LJ

Offline Corey G.

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Re: P0128 Won't go away!
« Reply #1 on: February 27, 2015, 05:48:44 PM »
Forgot to mention, I cleared the CEL after replacing the t-stat and temp sensor this weekend, and was all good until this afternoon the same code came up again.
2006 LJ

Offline Twisty

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Re: P0128 Won't go away!
« Reply #2 on: February 27, 2015, 10:19:10 PM »
I'm trying to figure out exactly what the CEL is telling you.  It seems like your coolant isn't getting hot enough?
88 MJ buggy, 4.0/AW4, 203/205, 3 link front/4 link rear on airshocks, 42" Iroks, full hydro

"Only wealthy men can afford to buy cheap things."

Offline Bluerocket

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Re: P0128 Won't go away!
« Reply #3 on: February 28, 2015, 07:58:38 AM »
If that's the case what about blocking the radiator or a portion of it and driving it (watching the engine temp like a hawk).    Does the jeep warm up to 210 still?
84 - 7

Offline Posimoto

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Re: P0128 Won't go away!
« Reply #4 on: February 28, 2015, 08:46:36 AM »
Have you tested the coolant sensor circuit? 

"That's the trouble with research on the internet, there's no way of verifying its truth" --Abraham Lincoln

71 CJ5 V6/T14/D18/Warn OD (from CO)
79 CJ5 304/T-150 Silver Anniversary
79 CJ5.5 360/T-18/4.1 D-300
77 CJ5 360/T-18/3.15 D-20
77 CJ7 258 Fuel Injected T-18/D-20 (from CO)

Offline Corey G.

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Re: P0128 Won't go away!
« Reply #5 on: February 28, 2015, 06:59:13 PM »
I'm trying to figure out exactly what the CEL is telling you.  It seems like your coolant isn't getting hot enough?

From what I've read online, the ECU expects the coolant to reach certain temperatures within certain intervals of time after being started. It guess the CEL is saying it's not warming up fast enough. I think it also takes input from the IAT sensor into consideration, so maybe that's another thing to consider looking at. I don't want to just keep throwing more new parts at it though.

Quote
If that's the case what about blocking the radiator or a portion of it and driving it (watching the engine temp like a hawk).    Does the jeep warm up to 210 still?

With the old t-stat & temp sensor the dash gauge was showing 210. With the new parts it's a little under 210 (maybe 195-200) but that could just be a difference in aftermarket temp sensor vs OEM.

Quote
Have you tested the coolant sensor circuit? 

I haven't. I guess I should find a wiring diagram and check it out.
2006 LJ

Offline Corey G.

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Re: P0128 Won't go away!
« Reply #6 on: March 01, 2015, 06:22:23 PM »
I'm starting to think maybe the radiator cap is stuck closed. I just let it idle for 30 minutes from a cold start, and there doesn't seem to be any change in the level in the reservoir. The top of the radiator is hot to the touch, but the cap is ice cold. The upper radiator hose seems like it's firmer than usual (more pressure) but I can't say for sure. It doesn't quite get to 210.



2006 LJ

Offline Bluerocket

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Re: P0128 Won't go away!
« Reply #7 on: March 01, 2015, 06:33:39 PM »
Any reason not to fix the stuck fan clutch?
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Offline Krome

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Re: P0128 Won't go away!
« Reply #8 on: March 04, 2015, 09:50:01 PM »
You sure you don't have a stuck thermostat, again?

SYMPTOM
P0128-THERMOSTAT RATIONALITY

WHEN MONITORED
Engine running.

SET CONDITION
The PCM predicts a coolant temperature value that it will compare to the actual coolant temperature. A significant difference results in an error. Two Trip Fault.

POSSIBLE CAUSES
Good trip equal to zero
Other possible causes
Low coolant level
Thermostat operation
Signal circuit shorted to battery voltage
Temperature sensor
Signal circuit open
(K900) Sensor ground circuit open
Signal circuit shorted to ground
Signal circuit shorted to (K900) sensor ground
PCM
« Last Edit: March 04, 2015, 09:50:59 PM by Krome »

Offline SPYDER

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Re: P0128 Won't go away!
« Reply #9 on: March 05, 2015, 08:02:17 AM »
if you give me the last 8 of your vin I can send you over a copy of the pdf of the diagnostic flow chart from the Chrysler data base. 
05 Tj d44's, 4.88s, ARB's, 37's and some bling
99 TJuggy d60,5.13s, ARB/Spool, 40's, backhalfed
06.5 GMC 2500
05 WK 2" w/32's

Offline Posimoto

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Re: P0128 Won't go away!
« Reply #10 on: March 05, 2015, 08:36:24 AM »
if you give me the last 8 of your vin I can send you over a copy of the pdf of the diagnostic flow chart from the Chrysler data base. 
That's generous, I'd be all over that.
"That's the trouble with research on the internet, there's no way of verifying its truth" --Abraham Lincoln

71 CJ5 V6/T14/D18/Warn OD (from CO)
79 CJ5 304/T-150 Silver Anniversary
79 CJ5.5 360/T-18/4.1 D-300
77 CJ5 360/T-18/3.15 D-20
77 CJ7 258 Fuel Injected T-18/D-20 (from CO)

Offline SPYDER

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Re: P0128 Won't go away!
« Reply #11 on: March 05, 2015, 01:39:01 PM »
id send you one chuck on your jeep but I don't think the database goes that far back...actually I know for a fact it doesn't cause I looked.
05 Tj d44's, 4.88s, ARB's, 37's and some bling
99 TJuggy d60,5.13s, ARB/Spool, 40's, backhalfed
06.5 GMC 2500
05 WK 2" w/32's

Offline Posimoto

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Re: P0128 Won't go away!
« Reply #12 on: March 05, 2015, 01:47:12 PM »
I knew you wouldn't have the system I'm working on because it's late 80s chevy FI. I do have all the flow charts for early Jeeps and found the late 80s chevy wiring. I do appreciate that you looked though, thanks.   
"That's the trouble with research on the internet, there's no way of verifying its truth" --Abraham Lincoln

71 CJ5 V6/T14/D18/Warn OD (from CO)
79 CJ5 304/T-150 Silver Anniversary
79 CJ5.5 360/T-18/4.1 D-300
77 CJ5 360/T-18/3.15 D-20
77 CJ7 258 Fuel Injected T-18/D-20 (from CO)

Offline SPYDER

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Re: P0128 Won't go away!
« Reply #13 on: March 05, 2015, 05:12:20 PM »
anytime...I always will try to help out if I can and have the knowledge.

corey send me your email address your vin wont help, our system has been down all day, but I was able to get it the diagnostic flow cart and email it to myself so I can just forward it to you if you want.  either just post it here or pm me it.
05 Tj d44's, 4.88s, ARB's, 37's and some bling
99 TJuggy d60,5.13s, ARB/Spool, 40's, backhalfed
06.5 GMC 2500
05 WK 2" w/32's

Offline Corey G.

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Re: P0128 Won't go away!
« Reply #14 on: March 05, 2015, 08:10:00 PM »
Any reason not to fix the stuck fan clutch?

It's definitely on my to-do list. I just can't see it causing the CEL though since it's been like that for the last 3 years.

Quote
You sure you don't have a stuck thermostat, again?

SYMPTOM
P0128-THERMOSTAT RATIONALITY

WHEN MONITORED
Engine running.

SET CONDITION
The PCM predicts a coolant temperature value that it will compare to the actual coolant temperature. A significant difference results in an error. Two Trip Fault.

POSSIBLE CAUSES
Good trip equal to zero
Other possible causes
Low coolant level
Thermostat operation
Signal circuit shorted to battery voltage
Temperature sensor
Signal circuit open
(K900) Sensor ground circuit open
Signal circuit shorted to ground
Signal circuit shorted to (K900) sensor ground
PCM

I guess it's completely possible the new one I put in was no good out of the box. For 6 bucks or whatever I might try another one.

Quote
if you give me the last 8 of your vin I can send you over a copy of the pdf of the diagnostic flow chart from the Chrysler data base.

Quote
anytime...I always will try to help out if I can and have the knowledge.

corey send me your email address your vin wont help, our system has been down all day, but I was able to get it the diagnostic flow cart and email it to myself so I can just forward it to you if you want.  either just post it here or pm me it.

Thanks! I will send you PM!
2006 LJ


 



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