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Galane 11-29-2012 03:08 AM

What works with 1995 Buick Century OBD1?
The ECM in the 1994 and 1995 Buick Century with 3100 V6 is apparently unique to that application. It's OEM number is 16196387

The inventory software that salvage yards use claims that the ECM from the 94 and 95 Oldsmobile Cutlass Ciera will work, but that's the only place I've seen that so I really don't trust it. (It has plenty of errors, such as claiming major body parts from the Chevrolet Corsica are the same as the Oldsmobile Acheiva.)

I was looking into getting one of Innova's OBD1&2 scanners, downloaded the manuals and guess what? The 94 and 95 Century is NOT supported.

What also doesn't work is any sort of jumper at the ALDL to make it blink out codes.

There must be something I can plug into this thing and at least get a trouble code number!

Digging around various web fora, the difference seems to be that other applications of the 3100 in 94 and 95 had an O2 sensor for each bank along with a MAP sensor and MAF, where the Century has only one O2 and no MAP, plus its MAF is combined with the throttle body instead of being a separate piece like other GMs of the 90's. Why would GM make a different setup for the Century instead of using the same as all the other uses of the 3100? Perhaps they were compensating for never changing the design of the dash between 1982 and 1996? Logic and automotive engineers seem to part ways quite often!

Since changing the engine (which thanks to GM's hinky engineering got collapsed piston skirts at the slightest hint of overheating) with another that has only 66,000 miles (1,000 less than the failed one) it wants to run hot too.

So I changed the 195F thermostat for a 180F one. Runs great but in traffic, even when it's cool and raining, the temp gauge will go up almost to the red, the fan kicks on then it drops down to just below halfway. Wash, rinse and repeat.

I'd like to reprogram the fan turn on temperature down a ways to work better with the lower temp thermostat. I don't care about it supposedly being more efficient running hotter, I care about the engine not self destructing because it simply cannot tolerate running near the limits of a 16 PSI cooling system, thanks to GM's too tight piston tolerances. It has a second fan in front of the radiator that is supposed to come on when the temp hits the bleeding edge of blowing the radiator hoses off, but it didn't work when it overheated and ruined the engine. I'd like to program that one to come on at the temp the main fan is set for. The fans will work, I've tested the relays and there's power to them, jumper the relay plugs and they run. I've also installed new ECT and temp gauge sensors.

Another thing it needs is reprogramming the inaccurate from the factory speedometer. It reads up to 3MPH faster than the actual speed, checked with GPS. Yes, it has the correct size tires, with only 5,000 miles.

meanjoe75fan 12-01-2012 01:28 AM

The simple (albeit inelegant) solution to getting the fan to come on sooner is to just wire it into a "hot when key in run" source of 12VDC.

If you want the second fan to operate at the same temp as the first, the simple (and reasonably elegant) solution is to splice and solder so that both fans share a common source of electricity.

5% optimistic speedometers seems pretty "industry standard" to me...most vehicles I've owned were in the 3-6‰ range.

Galane 12-01-2012 03:48 AM

Simple, and would make it run too cool in winter.

Going by the inaccurate odometer it gets around 26 MPG in highway/freeway driving, but since it's reading less miles than actually traveled, it's getting better than 26 MPG. How much better? Dunno. Would be nice to have it closer to reality. ;)

meanjoe75fan 12-02-2012 11:40 AM

Constant-on fan should'nt "cause the car to run too cool in winter": that's why cars have thermostats. Constant-on fans driven off the serpentine belt have been around for darn near a century and constitute "proven technology."

(Obviously not the most efficient approach, but you did say you were primarily interested in saving the motor here...)

A slightly more elegant solution would be to adjust the temp sensor to tell the computer the coolant is slightly warmer than it actually is: 195F when its actually 180. IIRC, the sensor is a variable resistor, and you'd need to slightly tweak the Ohm rating.

As for odometer issues, a 5% optimistic speedometers does NOT necessarily correspond to a 5% optimistic odometer. (Usually sppedos read slightly fast, on purpose, to discourage speeding.)

To calibrate your odometer, "zero" it on an interstate, right at a mile marker (MM 120, for example). Then drive a fixed number of miles (say 20, to MM 140). If your odometer reads 20.3, you know it's 1.5% optimistic.

(A little math seems much easier than hacking an OBDI system, but hey, if that's what snaps your girdle, who am I to tell you not to? Just trying to be practical.)

nemo 12-02-2012 01:17 PM


Originally Posted by Galane (Post 342487)
So I changed the 195F thermostat for a 180F one. Runs great but in traffic, even when it's cool and raining, the temp gauge will go up almost to the red, the fan kicks on then it drops down to just below halfway. Wash, rinse and repeat.

On my car (06 GP) the operating range 194 to 225. The cooling fans with the AC off, won't come on until the temp reaches 220 degrees then turn back off at 197degrees, via the u gauge. The factory temp gauge acts very similar to your description, this appears to be normal operation as the temperature stays within the specified operating range.

Read this for some insight on factory instrumentation:


An alternate method of checking the engine temperature may be in order.

RobertISaar 12-03-2012 06:28 PM

yay, something i'm an expert on....

the 94-95 3100 A-bodies have an ECM that is unique to them, so any other 94-95 3100 A-body ECM will work.

you can build or buy an ALDL cable to communicate with the ECM via a laptop using tunerpro RT. i've built the definition for it and it's been tested by a few people to work correctly.

the A-body is unique in that the 94-95 3100s use an integrated MAF and no MAP sensor. the other 94-95 3100s don't use a MAF sensor. they still all use 1 O2 sensor. the A-body also uses an older style MEMCAL ECM, the other platforms used a module that is equipped with flash ROM.

i could reprogram the MEMCAL from your ECM to whatever thermostat you want to run, but it would probably take around a week to ship it from you to me, me reprogram it, then send it back to you.

the speedometer being off could be one of two things: goofy calibration from GM (would not be the first time) due to one calibration having to fit multiple scenarios, such as different wheel/tire options or it could be that the speedometer itself is the issue. if the odometer is reading correctly and the speedo seems off, then the calibration is fine. if the odometer reads incorrectly, then the calibration is at least one problem.

i actually have the calibration used on the 94-95 A-bodies listed as having fan 1 and 2 on at:

106 *C (223*F)
112 *C (234*F)

then, they turn off when they reach:

102 *C (216*F)
108 *C (227*F)

Galane 12-03-2012 08:58 PM


Originally Posted by RobertISaar (Post 343315)
yay, something i'm an expert on....

you can build or buy an ALDL cable to communicate with the ECM via a laptop using tunerpro RT. i've built the definition for it and it's been tested by a few people to work correctly.
i actually have the calibration used on the 94-95 A-bodies listed as having fan 1 and 2 on at:

106 *C (223*F)
112 *C (234*F)

then, they turn off when they reach:

102 *C (216*F)
108 *C (227*F)

Got a link to software and plans for the cable?

234F is too @#%%# hot, especially for the pre-1997 3100. That's right at the limit for a 16 PSI system, and the early 3100 doesn't have the polymer coated piston skirts. By the time the 2nd fan comes on the engine is eating itself, if not before the 2nd fan comes on.

GM knows they screwed this up*. Sometime in 1996 they started putting the coating on pistons 5 and 6, by 1997 they put it on all the pistons. All new 3100 pistons are polymer coated, and rather expensive.

I've checked the odometer against mile markers, it's off as much as the speedometer.

I assume the MEMCAL chip has to be removed, erased (UV lamp or is it EPROM?) then the edited code loaded?

*If you think the too tight pistons in the 3100 are screwy, how about some of the newer GM V8's? They're designed so the pistons just hit the heads at operating temperature. A knocking sound during warm up is carbon deposits getting knocked off the edges of the pistons and combustion chambers. Let one of those get too hot and it's broken pistons and heads. A GM dealer service manager (who's been such for near 40 years) told me that and about the 3100's issues. "That sound that sounds like the engine is beating itself to bits? That's normal."

RobertISaar 12-03-2012 09:41 PM

software: TunerPro and TunerPro RT - Professional Automobile Tuning Software i prefer V5, though if your computer can't support it, V4 will still work. then you'll need the ADX (V5) or ADS (V4) for your application, i can email or put them on a file server for you.

cable will depend on if you want to make a USB or serial version, depending on what your computer has. if you have the option, go USB, it's much simpler. the serial version would require a MAX232 circuit to convert the TTL signals to RS-232 specs so that the computer will play nicely. a USB version only requires a USB to TTL converter, which takes all of 2 wires to connect (i'll expand upon this later, if necessary).

yes, the stock temps are way too high... running a 195*F thermostat, i run either a 205 or 210 *F on temp for fan 1, depending on how sticky the thermostat tends to be. then slightly higher for fan 2. i rarely see/notice the fans on, but i never see the temp guage creep into uncomfortable territory.

i know all about 3100s and their tendacy to make noises implying immediate death... lifters and piston skirts are responsible for that, but i've had it happening for over 60,000 miles now and it still runs great otherwise.

since the odometer and speedo seem to be off by the same amount, i'd say the calibration is compromised due to GM not wanting to support another calibration.... so with your wheel/tire combo, it reads slightly too fast, but with another offered combo, it reads slightly too slow. not too big of a deal to compensate for, though if i were you, i would check the speedometer against what the ECM is commanding it to be in the datastream.

while the stock EPROM can be erased via UV, they're nearing their end-of-life date, so i replace them with EEPROMs that have a 100 year rating. that and 1.5 second erase times compared to ~10 minutes is a no-brainer.

Galane 12-04-2012 12:15 AM

If you could e-mail the info to I'd appreciate it. I can find the EEPROM easily if I know the chip's part number.

I used an inspection camera to have a look in the cylinders on the original engine after I got it out. It wasn't very nice inside anymore. Parts of the piston tops looked like they may have melted a bit.

The "new" engine's piston tops were nearly completely buildup free and the cylinder walls were shiny. Still makes a bit of noise until it warms up. So did the original until after it got hot enough to blow coolant out (and the 2nd fan never came on) it clattered all the time and had no power and would overheat going up the slightest grade. Most likely the knock sensor was making it always retard the timing.

It overheated going up a 4,200+ foot high mountain pass. I don't want that to happen to it again.

RobertISaar 12-04-2012 02:32 PM

is that email without or without the underscores?

knock sensor being oversensitive is a possibility, i've certainly ran into that before as well.

Galane 12-05-2012 04:15 AM

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.

Century7667 11-08-2013 07:38 PM

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.

Ken T.

Galane 11-08-2013 09:20 PM

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.

RobertISaar 11-08-2013 09:28 PM

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.

Galane 11-08-2013 11:36 PM

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.

RobertISaar 11-08-2013 11:43 PM

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.

Century7667 11-09-2013 10:00 AM

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.

Galane 11-09-2013 03:49 PM

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.

Century7667 11-10-2013 07:36 PM

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.

Ken T.

Galane 11-11-2013 12:11 AM

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!

Century7667 11-12-2013 07:50 PM

Hopefully. Runs ok right now. It just gets into a snit once in a while.

The engine, the entire car has been a faithful daily driver for me me for 5 years now, and the '96 has done so many more years.

Ken T.

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