Having too large of holes in the thermostat will have a similar result as a thermostat that is slightly stuck open...it won't allow the engine to reach the proper temperature in the proper amount of time. So, the code you received was probably the same one...P0128. Your mechanic was likely right to begin with...the thermostat was stuck open.

Wow! This thermostat thing is certainly a hot topic here. I didn’t mean to start the pot boiling, but it looks like maybe I did?

I see a lot of posts asking for “confirmed” info about cold stats setting a code and I don’t normally do this on-line, but I will this time just to tame your hunger.

This is taken right from a GM factory service manual on a 2003 Bonneville:

DTC P0128

Circuit Description
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.
The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, diagnostics that use ECT as enabling criteria may not run when expected.
This DTC will only run once per ignition cycle within the enabling condition. If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

Conditions for Running the DTC
• DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P0502, P0503 are not present.
• The coolant temperature is more than -39°C (-38°F), but less than 80°C (176°F).
• The intake air temperature is more than -7°C (19°F).
• The engine is running more than 4 minutes.
• The vehicle speed sensor (VSS) average is more than 24 km/h (15 mph).
• The mass air flow (MAF) average reading is more than 20 g/*.
• The vehicle is driven more than 5 km (3 miles).

Conditions for Setting the DTC
The PCM detects that:
• The calibrated amount of engine run time has been met.
• The calibrated amount of engine air flow has been met.
• The calibrated vehicle speed and distance have been met.
• The calibrated ECT of 80°C (176°F) has not been met.

Action Taken When the DTC Sets
• The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
• The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.


Does that help?

I know that car enthusiasts are always looking for more power, but not all members of a forum are car enthusiast. Or maybe I should say that not every car enthusiast is looking for more power?

I would be willing to bet that over 70% of the members here are looking for REPAIR help and not performance help. Maybe this topic should have been moved to the performance section right from the start because it was asking about modifying a Bonneville from its factory state. That is not a repair, that is a modification no matter how you look at it.


For future reference, I think it would be better for topics like this to be moved to the performance section. I think it would help keep the forum friendly.

Now to continue my opinion, those who like speed and power please raise your hand……….OK, I see a lot of hands. Now those who still have their hand up, keep your hand raised if you have children. I still see a lot of hands. Continue to hold your hand up if making your car go faster is more important than your Childs life………what happened? The hands disappeared?

Why did the hands disappear? Isn’t going fast very important to the fathers on this forum? Isn’t it more important than your children’* future? I mean, if you can gain a tenth of a second in the quarter, then that should be worth a few years off of you kids life….right?

Emit more pollutants into the atmosphere to make that car faster, it’* not like your going to have to deal with the circumstances.

Still think the mods are worth it?

Watch this movie…”An Inconvenient Truth”

So oldbuick'* problem with the 180 stat could have been a temperature sensor that'* off by as little as 5 degrees.

I suppose so, It could also be a faulty 180 T-stat. It only takes 5 F to set the code from a 180 F stat. That'* why my original post on this thread stated "You’re pushing it pretty close to setting a light with a 180 F stat".

Since I first started modding my engine on this forum back in 02, I have been very cautious.
When I first swapped to the 180 stat there was no problem.
Then I drilled a couple of 1/8" holes in it to delay warm up so the engine would run cooler to make more power. That is when the P0128 codes started to occur.
I only drive about 4 miles at a time. The engine never really warms up in the winter or cooler days.
It'* only a warning light. I used to turn it off every two starts with my code reader.

Now I have it disabled....a $10. option when I last had the PCM upgraded by INTENSE.

Bottom line is that I do not believe that any codes will be set running a 180 stat, as long as it isn't drilled.

I agree. I have yet to see any problem with running the 180, or even a slightly drilled 180 as far as temperature problems. The thermostat will open at 180, but the actual coolant termperatures within will be higher. I have 5 3800 engines, a '98, 2 '99s, and 2 00s running 180s, 3 out of 5 of them lightly drilled, without issues.

While a lower temperature will allow for more "go fast" mods, that is not the only purpose of it. To assume that everyone who has one has it for the purpose of flooring the engine, acclerating to unsafe speeds, putting their life, the of their children, as well as all drivers around them is flawed logic. Using something like that, everyone who drives a v6, or God forbid a V8, instead of a 4 banger is doing the same.

To keep the discussion friendly, please keep the pollution opinions in the Lounge. This one is for t-stats. Thank you.

bonneville, many members here change the thermostat as an early step towards other modifications. Both for reliablility, efficiency, and power.

Thank you for posting your source, but perhaps now you've gotten the impression that despite what your 2003 FSM says, we have YET to see this happen here? On ANY Bonneville?

The FSM is not the final answer. We literally have thousands of 3800'* on this Forum running great on the 180 thermostat. And that doesn't count the thousands on other forums like ClubGP.

Were you aware that we've never had a modified Bonneville (with go-fast mods) fail an emissions test? Are you aware that we are very adamant with all members about running catalytic converters?

This Forum isn't about a bunch of V-tec ricer boys. It'* about responsible ownership, and reliable performance.

I suggest you test a 180 in your car to convince yourself who'* right or wrong. Our Forum and experience, or that page in your FSM.

Is this documented on the number of cars running the 180? Is this a published source of information or just your word of mouth? Because you seem to change your mind by at least 1800 cars within two pages of posts. If your second number was referring to all years, then that is a pretty irrelevant number considering this code only applies to 2000+ model years.
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Then why did you want to know the source of my information? What would have been a valid source if not GM? If I said a magic pixie fairy told me that info, would that have been a satisfying source?

What I posted is facts, not opinions. The computer WILL set a code if the conditions stated are met. That doesn’t mean a 180 stat will make the car meet those conditions. It means that you are very close to the conditions, and if you read my first post in this thread you will see that that is exactly what I said.

If the 180 works on these car without problems, then great! Go ahead and use it. But please don’t ask me to prove my source of information and then discredit the source just because you were hoping to see something else.

Maybe it would be best to say that both the GM manual and this forum are correct in this case. The manual is correct in that the code will set if the temp in not met and the forum is correct in saying that the 180 stat is still hot enough to keep the temp above the code setting limit of 176.

Bonneville,

It seems like it'* pointless trying to convince you that the GM FSM is not always right.

.....I can't find anywhere in my manual where is shows this 00 has a PCV valve or how to change it. Learned that from Bill.

I spent 3 years playing with different stats and drilled hole sizes. I used temp tape, IR gun and ran my own controlled experiments.

Just to quantify my expertise.....I am a Professional Mechanical Engineer (P.Eng) with 28 years of Industrial experience. I'm the kind of guy who would probably write one of those manuals after designing the systems.

So believe us when we tell you that running a 180 stat will not set off codes or harm your engine in any way.

A long time ago I asked GM about transmission coolers and they told me that adding one was not necessary and in fact it would hurt my transmission.
Most of us on this Club have to run with one. In fact, I'm installing a larger one right now.