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Temp readings all over the place. Any ideas???

J

JrodI

New member
Hello all.

I have an 05gp. Last week she starting running real rough driving on the highway, rpm fluctuation, temp gauge going crazy, intermittent power and terrible gas mileage. (I didnt pick up on the temp gauge til later) my first suspect was tranny related like the tcc or something but applying the brake slightly didnt affect anything. Rpms were still erratic.

When i discovered the wacky temp readings i changed the therm. I had planned to do so anyway because I had the therm code p0128 i think. After a few rides watching live data and trying to figure things out i got a p0118 and the temp readings were fluctuating between -38 to 192. So i changed the sensor and checked all wiring for continuity. After being satisfied with those readings i thought i was good. Put her back together and it happened again. I changed out harness etc. when i start readings look good. If i put it in gear it will drop off to -38 again and maybe come back up or fluctuate or not come back up at all. I'm confident the new ect sensor is fine. I chEcked resistance as the engine warms and it seems to be accurate. I also checked proper voltage coming from ECM and continuity from the sensor to ECM.

I've also flushed and bled system and clean throttle body and maf sensor on fairly regular intervals. I dont know what else to check. Any ideas? Could another sensor circuit cause problems with temp readings? Could it be the ECM? I'm lost. Any help would be appreciated.
 


I am waaay out on a limb here and out of my comfort zone for advice so take this with a grain of salt.

Did you change the coolant temperature sensor? My dad had a 90's Bonneville that would start really hard sometimes and would struggle to run. From what I remember our mechanic found out it was the coolant temp sensor was giving insanely low readings (like you've seen) and making fuel adjustments.

I may be way off base, and you may have already changed the sensor but just throwing it out there.
 
Also clean the TB really good, front and back with TB cleaner. Clean your MAF with MAF cleaner too. Check for vac leaks and depending on when your 05 was built, check for lim gaskets failure if it is plastic.

Edit: I failed to read.......I had an elbow crack and would allow my temp gauge to fluctuate once. Check those too
 
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Thanks for the replies.

Stocker - i did replace the sensor already. I assumed it was that right away. And what you describe is exactly whats going on.

Joe- I've cleaned the tb and maf already. I'll have to look into the gasket. Which elbow do you refer too?

the temp readings i described -38 is super high resistance or the same as it being unplugged. That's why I went nuts looking for a broken wire or bad connection. The frustrating part is it will give an accurate reading and then drop off partially or completely. I swear when I was jiggling a connector yesterday i caused the fluctuations the isolated and replaced it. Btw this is data coming from a scan tool.

Im im not sure that a malfunction in any other sensor circuit would cause false temp reading. Most other systems appear to be operating normally until the temp sensor bottoms out. And from what i understand everything starts there and the PCM makes all other adjustments based on that reading. All i have left is to assume the PCM is bad but it seems strange that of all the many things it controls there would be failure in only one system. But again I'm assuming. Maybe that occurs.
 
Those coolant temp sensors have been known to be bad right out of the box, sometimes 2 or 3 times over. Where did you source the replacement from?
 
Your temp gauge fluctuating like that makes me think it has a bad connection somewhere. Have you broken down the wire loom and traced the wires ALL the way back to the pcm? When you start the car, does your temp gauge go all the way up and back down like the others? Have you checked the wiring that goes to the cluster? I would start with that. As far as rpm fluctuation, have you checked for any vacuum leaks? The elbows that stocker is talking about are the plastic 90* elbows that are on the left side of the engine. There are 2, one is hidden behind/below the tensioner pulley.
 


I replaced my sensor before and just the other day my aeroforce was showing my temperature jumping between like 190-280 at times.
 
Yes i traced wires from sensor to PCM and i think the rpm fluctuation is all due to the faulty readings from the temp sensor. The PCM is probably moving from open to close loop etc.

the sensor is a autozone one so maybe I'll grab another. After talking to everyone i can im back to thinking it wiring/connection issue. I'm haven't checked the wiring to the gauge on the dash because I believe the fluctuation is occurring before that. If the PCM to guage wiring was bad i would understand the gauge going wacky but i dont think it would affect engine performance.

Ill start art digging again. Man i taped everything back up real nice too.
 
Update- just started fishing around again.

When end i hook up a multi meter to black wire coming from sensor and neg battery cable resistance readings go haywire when i rock the car slightly with engine/key off. I'll start searching for an issue between sensor and PCM then PCM to ground and all other grounding straps etc.. Fingers crossed!!!
 
Better update- i never took every piece of tape or wrap off every inch of wire. Well I have now and it's the original suspect. Broken wire and 2 more ready to go too. Just as the harness comes over the valve cover. 180,000 miles and 9 yrs wore through the sheath and all.

Hope this does it! I'll update and post photos of the break. Can't be the only gp this happened to.....well maybe cuz it is mine.
 


nice find. strip the wires back till you hit shinny copper wire, then if it still reaches solder and shrink tube it back together.


if need be add some new wire if you loose 3 inches to to dry wire. (that would be dull wire, keep stripping wire till its clean and new looking.)
 
Thanks Scotty. That was it. She's back to her normal self.

Photos are too big so I'll upload a link when i can.

Thanks for for all the replies!
 
Hey lucky or anyone else. What are those elbows called?

after sorting everything out and flushing the system i must have loosened some goo that was hiding a leak. I thought it was the water pump but it seems to be that elbow right above it. I need a name so I can look up the part and r&r procedure.

Also so someone here suggested changing lim gasket. How do i know if i need to? I thought this is something gm changed on the series3.
 
Those are the coolant elbows. I recommend upgrading to the aluminum ones, you can find them at your local parts store now. Remember when you install those, lube them up with some oil or something so you won't roll or tear them as you push them in. The LIM gasket was upgraded to an aluminum style in the 06 or 07+ GP's I believe, don't quote me on that. The way you can tell is by looking at the end of the gasket to see if it's plastic or aluminum. Many will say to go with the aluminum ones but when I changed mine out, I went with the plastic ones. Reason being because I was tight on cash and the original one lasted over 130k miles on it. The only reason mine went out was because I was driving the car while it was overheating on the highway and was in another city. (my temp gauge was not working at the time) so I think it may have lasted longer than that. But I ended up doing a head gasket job and worked my way up but either way, I wouldn't recommend the aluminum ones if you don't have the cash or if you don't drive very hard on it. The plastic fel-pro ones have been good to me. I haven't had a leak since I changed it out about 3 or 4 months ago. If you want a how to, let me know. I have a link that tells you what all you need and step by step instructions on how to do it with pics and everything. Probably the best one on the internet.
 


Thanks lucky. I grabbed the aluminum elbows. Changed the water pump too while I was down there. It does have the rubber gasket and it looks like there may be some minor leakage but nothing to get worked up about. Nothing wet anyway. She loses about 1/2qt of oil between changes. I figure thats normal for 180,000mi engine. She has never overheated until the other day because of the coolant leak. And she didnt go all the way. Just high enough to touch the red.

I'll reach out if i have to change the lim. Kinda happy i did this. I usually leave her to the pros except for brakes, susp, etc. tryin to save $$$ to put the finishing touches on my 70 deville. Good to see it is just an engine after u get by all the wires haha.
 
If it's leaking slightly, if it was me, i'd change it out before it got any worse just for peace of mind. How minor is the minor leakage? You definitely don't want coolant mixing with the oil. It's a fairly easy job to change them out even if you're slightly mechanically inclined especially with that write up. It doesn't take that long either. It took me a little longer than it should've because i'm sort of a neat freak and I spent more time than I should've cleaning, scraping and cleaning everything again as best as I could. Even stuff that I shouldn't have had to clean I did while I was in there. Haha. But I just looked at it as "preventative cleaning". I kinda thought of myself as forrest gump. Well since I cleaned this much, I figured why not clean this too? Hahaha.
 
I'll reach out if i have to change the lim. Kinda happy i did this. I usually leave her to the pros except for brakes, susp, etc. tryin to save $$$ to put the finishing touches on my 70 deville. Good to see it is just an engine after u get by all the wires haha.[/QUOTE]

Yea I know what you mean. The head gasket job would've been well over a grand at any shop I would think and it only cost me the cost of the gasket set and taking the heads to get resurfaced so in all I think it was a little under $250ish.
 
luckyg817 said:
If it's leaking slightly, if it was me, i'd change it out before it got any worse just for peace of mind. How minor is the minor leakage? You definitely don't want coolant mixing with the oil. It's a fairly easy job to change them out even if you're slightly mechanically inclined especially with that write up. It doesn't take that long either. It took me a little longer than it should've because i'm sort of a neat freak and I spent more time than I should've cleaning, scraping and cleaning everything again as best as I could. Even stuff that I shouldn't have had to clean I did while I was in there. Haha. But I just looked at it as "preventative cleaning". I kinda thought of myself as forrest gump. Well since I cleaned this much, I figured why not clean this too? Hahaha.
Click to expand...

Ill give it another look. I should say a slight seepage, maybe. I saw there was a small amount of gunk near one side of gasket. It wasn't wet but it had to come from somewhere
 
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