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Old 12-05-2012, 04:15 AM   #11 (permalink)
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With the underscores. I changed the knock sensor, didn't make a difference.

Looks like I'll have to take the old engine apart to sell the heads and other good bits, nobody wants the whole thing.

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Old 11-08-2013, 07:38 PM   #12 (permalink)
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I too am interested in lowering the fan on temp. I've had excellent reliability with my '95, but in the fall, when the A/C is off, the temp gauge is very high. According to the computer, about 230 F.

The engine noise has not been an issue for either of my 3100 engines. I've also got a '96 in a Century as well. If allowed to idle for about 90 seconds, the noise goes away. I do use a synthetic oil though....

I might also add that the speedo in my car is about 5 mph faster than actual speed. However, the computer reports the actual road speed of the car accurately.

My ODB2 equipped '96 is right on the money with the speedo and computer.


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Old 11-08-2013, 09:20 PM   #13 (permalink)
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The chips used in these can be erased with UV light and reprogrammed. There's free software to edit the firmware and the files are available.

1994 and 1995 Century used a firmware unique to that model and those years. Why? Likely for no #$%# good reason at all. Something you should do on your 95 is replace the vacuum "harness", part of which plugs into a T connection on top of the intake, next to the throttle body. That T fitting hardens and shrinks, allowing unmetered air in, which will lean out the fuel mixture, helping the engine to run hot.

Hot = BAD in the 3100, especially in 1995 and earlier. 1996 got the special coating on the two most troublesome pistons. 1997 and up they put the coating on all the pistons.

It's a bandaid on a less than great design with too tight piston clearances. Can't go with slightly looser pistons because that'll set off the knock sensor which will really foul it up.

Best thing for engine longevity on these is to switch to a 180F thermostat and get the computer reprogrammed to also lower the fan switch temperatures.

Another trick is to wire both electric fans in series so they always run together, but slower. With both running they push air through more of the radiator, which is especially important if you're in stop and go or slow traffic on a hot day.

If you want to get extra fancy the fans can be rigged with relays so they kick on in series for the regular fan turn on then switch to parallel and full speed when the AC control kicks the fan on.
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Old 11-08-2013, 09:28 PM   #14 (permalink)
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the 94-95 A-body 3100 code was actually used on early 94-95 W-body 3.4 PCMs as well. later 94-95 W 3.4 changed to a different mask, but i've never seen a 94-95 A 3100 use the later mask.

a 180 thermostat is entirely unnecessary on these engines, but reprogrammed fan points are never a bad idea.
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Old 11-08-2013, 11:36 PM   #15 (permalink)
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It was on mine. Even after changing the engine with one that had only 64,000 miles it would run up to the red zone. I put a new 195F thermostat in, which I'd tested in a pot of water on my stove. It opened right at 195.

So I put the 180 thermostat in and it'd still run up too high, then it'd cool down when the fans kicked in. The thermostat would close and it'd heat up again. With the 180 thermostat and the adjusted fan temps it runs an even temperature like it ought to and cool enough so the pistons won't expand too much.

At 195F it's running at the edge of what a 16PSI cooling system can handle. What caused the overheat which messed up the original engine was the radiator cap got a little weak and I went for a drive up a mountain. Easiest thing to do is put a new radiator cap on it once a year, especially if you drive a lot.

Going to a slightly higher pressure cap might be a solution, if these early 3100 engines didn't also have the problem of blowing head gaskets.

As it is it's a marginal reliability design. GM should have designed it to run a 20PSI cooling system so there'd be some headroom between working and boiling then damaging the pistons and ruining the engine.

If you change the thermostat and the fan temps it *will not* have problems because the temperature range will be well within the capability of the system to keep it from boiling.
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Old 11-08-2013, 11:43 PM   #16 (permalink)
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the fan temp change alone did that then.... the thermostat does not change the max temp the engine will run at, only what it will regulate the coolant down to.

mine will sit at an idle for 30 minutes and never rise above ~205 in 60* weather. my fan #1 is set to engage at 210 and fan 2 at 215. with 204,xxx miles on it and many of them have been neglected at that(especially the cooling system, where i think i'm running on nearly 10 year old antifreeze), i haven't seen fan #2 come on even idling in the summer. fan #1 is a rare occurence even.
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Old 11-09-2013, 10:00 AM   #17 (permalink)
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I have a '95 and '96 Century each. The '96 I bought new back in .....1996. The '95 came along later when we needed a 3rd car, and I thought "Man, we need something reliable like the Century. Hmm, maybe I can find one..." So I found the '95 with 62k on the clock, and it now has 126k on it. Yeah it was reliable.

In my experience with both cars, the quickest, and easiest mod for these cars to keep their cool with the 3100 is a replacement radiator. A new replacement radiator from Spectra Premium or another good rad will come with a core that about 75% thicker than OEM. This makes a very significant difference in the car's ability to keep cool, especially on the highway.

But, climbing flights in a parking garage on a nice day when the A/C is turned off, the fans won't run until the needle is one tick away from H, then the fan(s) come and the temp quickly drops.

My primary concern isn't the piston slap; it really isn't too bad. I've always made it my SOP to let the car idle a bit, and then drive gently for the first mile or so. After that they are very quiet.

My primary concern is the LIM gasket. I certainly don't want any more heat or pressure than the LIM can handle. The '95 is still holding with the factory LIM. The '96 is ready for it's second gasket (external leak thankfully). This time I will do that job myself with a Fel-Pro metal gasket.

Getting back to the ECM. I have a spare ECM I pulled from the junk yard while getting a ABS Controller module (Faulty ABS Controller was disabling the cruise control). I suppose I could use that a test mule. But did someone mention that you could get a new chip to program on? I think that would be preferable.

"Something you should do on your 95 is replace the vacuum "harness", part of which plugs into a T connection on top of the intake, next to the throttle body. That T fitting hardens and shrinks, allowing unmetered air in, which will lean out the fuel mixture, helping the engine to run hot."

This is very interesting. I've been chasing an uneven idle that afflicts the car once in a long while on dry cool days. If I didn't know better, it felt like a vacuum leak. I'll take a look!

On the thermostat issue, I think I'll stand pat with the factory setting. Run too cool, you can actually increase wear in the cylinder as these things are designed with certain tolerances. A good example of this is that you'll always see the most taper in the coolest cylinder. Ask anyone who rebuilds inline sixes, and they'll tell you #1 has significantly more, it's at the front, and gets the cold water charge from the water pump. Might be apples and oranges with the 3100, but I think with the larger radiator and fans set for a lower temp, I think I can get what I want.

...and that '95 can turn in a nice 31 mpg any time I put it on the road at 65 mph with A/C, and has done more up to 34 mpg on one tank (and yet I fill the tank FULL every time).

Thanks for the quick replies guys!

Ken T.
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Old 11-09-2013, 03:49 PM   #18 (permalink)
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Search for ACDelco 24503949. You'll come up with one of two things. One is a wiring connector and pigtail for the AC compressor. The other is a bundle of three vacuum lines. That's the one you want.

Due to it being called a "harness" it's been mis-filed by many sites under wiring. There are no aftermarket reproductions available. Apparently GM had a few million of the things made for the 1994 Century with 3100, then changed a bunch of stuff for 1996 and thus came nowhere near to using up the supply of a part made for a single model and engine combination.
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Old 11-10-2013, 07:36 PM   #19 (permalink)
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Thanks for the info! This could be the intermmittent rough idle that I am chasing. It just shows up once and a while, an sticks around. No DTCs on the computer.

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Ken T.
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Old 11-11-2013, 12:11 AM   #20 (permalink)
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Hopefully it works for you. Also check your oil fill cap and PCV connections to the valve covers for good sealing.

I dunno why GM has had a history of making completely unnecessarily different parts for single models or single model/engine/transmission combos when a part that does the exact same thing is used on many others with the same drivetrain - and those different parts do nothing different other than adding more expense to GM's bottom line.

I had a 1986 Cimarron with 2.8L V6. The right radiator tank cracked. Guess what? Unique tank for the Cimarron. Couldn't use the exact same tank as the "hotted up" Cavalier J-Body model with V6. I lucked out at the radiator shop. About a decade before he'd replaced the right tank on one of those Cavaliers and the place he got the tanks from had screwed up and sent a Cimarron tank, then didn't want it back. So instead of a week I got the radiator back the next day because he'd had that tank up in the shop's loft for 10 years.

Two different, non-interchangeable parts that do the same thing, in the same basic vehicle, with the same engine. No rhyme nor reason for it!

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