2001 Monte Carlo SS 3.8L

So last Friday I went to leave a friends house and my check engine light stayed on when I left. Had the codes read the next day and it was two codes: P0128 and P0420. I looked them up on an OBD website and the common denominator was engine coolant temp sensor which I was all prepared to buy. Stopped by Autozone and the dude told me the P0128 is almost always a thermostat. So I bought the thermostat instead and it seems to have fixed the problem. I disconnected the neg. battery terminal and replaced the thermostat and the light is out but I'm not sure if its just because I disconnected the neg. battery terminal? Car also seems to heat up MUCH better then before.

Some other symptoms I have had which may or may not be related are: Low coolant level light has been coming on in the information system. The overall fluid level is down at times (so apparently I have a leak somewhere I suspect the plastic intake manifold). The car didn't seem to get quite as hot as it should have and took a long time to heat up. The fans stayed running the other day after I shut off the car, I had to disconnect the neg. battery terminal to kill them. The radiator cap doesn't seem to get hot even after I have driven it for a good long time.

So I'm thinking I probably need a coolant level sensor at a minimum? But what about a temp sensor? I also know I need to get that damn Dexcool crap outta there as I know it gums up the system.

Recommendations? Suggestions?

 

Sounds like the thermostat was the issue. Iv also replaced mine. Next thing to replace is coolant elbows. Most likely your leak source. Make sure you replace them with aluminum ones.

 

First, DEX is not going to damage anything. It needs flushed regularly (like most people neglect to do with the green stuff).

P0420 is normally a code related to the rear O2 sensor and the health of the cat. Granted, it may also depend on engine temp and if the t-stat was messing with the engine temp, it could have caused the PCM to believe their was an issue.

Personally, I would wait to see if it trips a code again. Until then, problem solved.

 

Cool I was thinking the intake manifold but I just read your write up too on the elbows so I'll consider those as well.

Yeah I'll wait to see if that CEL comes back on.

Now I always thought DexCool was "corrosive" after like 100K? Just what I had read on the 'net and of course I know that isn't always gospel. Or is that only if you don't flush it?

Thanks guys appreciate the info.

 

Dex cool likes to eat nylon after about a 100k but if its not well taken care of it can decay even faster. I think it really depends on the heat of your engine how long dex cool will last. Just keep up the maintenance such as coolant flushes and the like to keep the engine running well. The nylon elbows contantly exposed to 180 dergees plus dex cool was a bad design flaw. If they haven't been replaced im 90% sure that's your problem as I find it to be the most common issue on our cars.

 

Is there a video or something around here or on Youtube about replacing these elbows? I see the diagram in another post about replacing them but I keep reading mention of a "video"?

I found this video on Youtube concerning a Pontiac GP 3.8L looks to be the same engine right?

 

Yeah, same engine, same procedure. Just make sure you get all the little bits of plastic out.

 

DEX has been the "public scape goat". Here's some quick info:
- Yes, GM was sued over it
- Yes, studies found the original DEX formula from the 90's and GM had re-formulated it (as a result of the class action suite).
- Yes, the '00+ Montes came factory with the new version of DEX.
- Yes, DEX is not maintained is is a problem (allegedly hired acid properties, sludges up more). Flush it periodically like you would the green stuff. Truly, 100,000 miles is WAY too long IMO.

Now, here's the fun part. People claim DEX is what destroys the LIM gaskets. I've worked on cars, early 90's 3100's that had nylon gaskets and NEVER a drop of Dex (not even from the factory, it was always the classic good ol' green). Yet, LIM failures anywhere form 80-120K. I changed the LIMs twice in the ownership of my '94 Grand Am (the second time was as I was doing head gaskets, but when I took the LIM apart, yeah, they were a hair from failure). That car NEVER ran DEX.

People overlook, Nylon cannot handle the the high coolant temps and pressure of the cooling system and that is why they fail. GM has used nylon LIM gaskets in the 3100 and Series I 3800 and all had failures with or without Dex.

And the nylon coolant elbow failure is another example of nylon being a bad idea.

 

imho, most of the intake gasket failures in the 3x00 is due to over tightening the outer bolts in the intake. hence the reason why the updated gaskets have a metal "button" on them to help avoid over tightening.

the second reason, imho, for failure in the 3x00 is from not using a thread sealer on those intake bolts. thus allowing the bolt to loosen over time and allowing the gasket to "flex" and crack the plastic/nylon in the gasket.

i dont think dex caused the issues with the gaskets. i do think installation error and simple maintenance (which you alluded to) is the problem.

 

That's a good thought. I'd definitely agree, over tightening will definitely cause problems (probably crap the LIM gasket during re-install).

On my '94 Grand Am, the factory LIM gasket failed at 90K.... When I did the LIM gasket replacement, I torqued to factory spec (per the GM shop book), but I don't recall using a threadlocker (but the book did call for it). The next LIM failure was at about 180K. Not a bad run (about average) in my case (I can't speak for all other LIM issues).