954thgen

So this post is to help those who have the all too common overheating issues associated with owning one of these mid 90s f body cars. For a little over a year (on and off) i have spent time troubleshooting my coolant temperature issue in my 95 z28 to no avail. Until today. First off id like to point out im my case the issue was that the engine would over heat at idle but would cool to 180-195 degress once air flowed through the air dam while moving. Did coolant flush, refilled and bled system COMPLETELY of air per shoeboxes instructions. Made sure thermostat opened at 180 degrees by placing in 180 degree water in a pot. Made sure fans were working. And heres where it got weird. The fans only kicked on once coolant temp guage got to the very last white mark. which i would guess is about right according to alot of people on these threads. BS! The car has a 180 degree thermostat from factory! Therefore, the engine coolant is meant to meant to stay right around 180! i have a late model 95 so it uses 3 relays for fans that both operate at the same time (high/low) then noticed that once i turned the AC on the fans never kicked on and they should. AC hadn't worked in 2 years and i really didnt care but once i found that the engine coolant fans and AC circuitry are connected i thought id better take a second look. When i selected AC the compressor would not kick on. Did low pressure test and, no suprise, the system was dead. Well in order to properly add 134a the compressor has to be running. So i jumped the AC compressor relay terminals under the hood (pin 30 to 87) to see if the compressor would start (and it did) and if so, refill the system. Once filled, i removed the jumper and put the relay back in but in order for ac compressor to run normally by using. AC selector switch inside the car, the low pressure trip in the pcm for AC system needs to be reset. Disconnect the positive lead from the battery for 5 min or so then reconnect. Once i did all this the ac worked, the fans came on at reasonable temps and was able to maintain 195 degree coolant temp for as long as i liked..... at idle..... in 90 degree weather! Now as far as temp goes if AC selector switch is turned off im guessing the only other option is to reprogram the pcm to turn the fans on sooner but with the AC on the car runs right smack in the sweetspot for normal op. temp (about 190). I hope this helps for those that have AC/ engine coolant temp issues. And if its already been dicussed i have not seen it, and ive been looking on here for AWHILE.

Injuneer

The thermostat only STARTS to open at its rated temperature. A 180°F ‘stat may not be fully open until ~195-200°F. The 1995 factory service manual indicates that “normal” engine operating temp is 185-203°F.

You also have to understand the differences between a conventional flow cooling system, vs. the LT1’s “reverse flow” system. In a conventional system, the ‘stat is controlling the temperature combining out of the engine. The coolant has first been heated by flowing into the block, then rising to cool the heads, picking up more heat. Conventional flow systems typically run a 195°F ‘stat. In the LT1’s reverse flow system, the ‘stat is controlling the temperature coming out of the radiator and into the engine, where the coolant first cools the heads, and then drops into the block. That’s why the stock thermostat on the LT1 is 15° colder than the earlier SBC engines. Doesn't mean the engine “runs” at 180°F.

The PCM starts to pull timing at high throttle openings when the coolant reaches ~220°F. Fans on low at 226°F, high at 235°F when the A/C is off. The change from 2 relay system (Fan 1/Fan 2) to 3 relay system (Low/High) was made on late 1994 models (as on my 3 relay late 1994). The 1994 factory service manual shows the two versions. Helm just copied the 1994 diagram into the 1995 factory manual, giving the erroneous impression the change was made in 1995 model year.

Most of this is right out of Shoebox's site., and factory service manual.

954thgen

When i say 180 i mean 180 degrees ballpark. Ive always thought 190 was ideal. As of now it sits and idles at 190-195 and never goes any higher IF the AC is on. All of that aside, If the engine is running at 220-230 degrees ON THE GUAGE and does not get any cooler unless moving 40+mph and there is heat waves coming from the hood i have a problem with that. Didn't do that when i bought it. But as far as when the ac is off i am not sure what to do to keep the whole system a little cooler, other than reprogramming the fan on/off temps.

shoebox

People may need reminding about the interaction of the a/c and the fans. In the cooling system operation section of my website, a paragraph states:

"The fans are programmed to come on when the a/c is turned on. A/c Pressure monitoring sensors feed the PCM info and depending on the situation, the PCM may command the fans off for brief periods. If your a/c is broken or operation inhibited because of low refrigerant, the fans will not come on when the a/c is turned on (but will still operate by engine temp). Also, when the car reaches sustained higher speeds, the fans may be commanded off so incoming air can flow through the radiator unimpeded and provide the cooling needed."

Injuneer

I agree that the engine idling at 220-230° is not good, particularly if it never ran that hot before. Something changed, and whatever caused the problem is still there. What you did “fixed” the problem, by putting a band-aid on it. You could also have used a manual fan switch (details available on Shoebox) and avoided the penalty of using the energy consuming/heat producing A/C system to turn the fans on.

Or, you could have tried the popular 160°F LT1 specific thermostat, with reprogrammed fan on/off temperatures. Still a band-aid. Have you scanned the PCM to compare the data the PCM is receiving from the coolant temp sensor in the water pump to the dash gauge reading?

I guess what rubbed me the wrong way is your comment that was basically calling out the advice we have given in the past as “BS”.

ACE1252

At idle, per stock, the cooling fans should be cycling on and off(independent of the AC system). Engine temps will climb to around 220-230, fans will turn on, cool it down to around 200, fans will turn off, then lather, rinse, repeat. It's not BS, it's the way the system was engineered.

I agree that the OP has band aided some other issue.

954thgen

Dont mean to offend..i know some of you guys really put in the time and know your stuff. Just seems a little odd to me that any v8 engine was designed to runnthat hot. i thought about the manual fan switch but personally id like it to just run at a nice temp without me having to flip a switch...and accidently forget or something. Everything on the cooling system is new besides the radiator and heater hoses. which are good and have been tested. and the guage sensor has not been replaced either. Guess could be the guage sensor off a little or something. But i have a new topic on the car and will bring it up later that really has me stumped.

Chimera96

the higher engine temps GM had the LT1 reverse cooling engine set at were more for emissions. By design it was set to run about 220 and will all day long

Many people had PCM tuning that lowered fan "on" temp and run a 160 T stat. Mostly so they could then run more timing & fuel = more HP. Operating temps for this set up is generally 180-185

1995_LR

IIRC, the secondary fan turns off and on according to cooling system load. The primary fan is always on with the air conditioning. Reason being is that the condenser does not function (pull heat out of the freon) unless air is moving through it. As a result, whenever the AC is on, a fan is on.

Vintage_SG

The BMW M62 V8 (as fitted in my 2004 Range Rover) is designed to run at 221 Fahrenheit. This was done to improve emissions and is the cause of exploding hoses and coolant expansion containers. A popular Mod is to fit the thermostat from the earlier generation M62 to keep things cooler.

954thgen

Yes that sounds about right...which in turn helps the engine run a bit cooler. Although the ac is consuming exta energy, its worth it to me at this point. With the ac off, my primary kicks on around 240 ish not 230 (right plumb next to the red) exactly at the very last white mark on the guage then once the temp is cooled to about 220 ish ( a little above the midway mark) the fans turn off. And stays consitanly in this range. Just seems like that range where the fans cycle in and off should be a little lower on the guage. Its beyond me why this would be programmed this way. And then programmed for the secondary fans to come on at what? 250...260? Call me crazy but that just seems a little high. I understand its got a reverse cooling system and all but temp is temp and heat kills power and warps sh**.

shoebox

Read the first paragraph here: 4th Gen LT1 F-body Tech Articles

It's been explained. The factory programmed temps are for emissions.

Based on your other thread, you have a possible head gasket issue, so that is going to affect your temps.

Injuneer

Programmed what way? I explained the fan on temps in post #2, and the fans are not programmed to turn on high speed at 230°F. And your 95 doesn’t have a “secondary” fan.... it has both on low or both on high. You told us that's the way your 3-relay system works.

If you need the stock fan high/low/off temps, it's covered in Shoebox's “how to page”.

4th Gen LT1 F-body Tech Articles

As the temps drop, the fans drop from high to low at 230°F, and low to off at 221°F. All of this is right out of the factory service manual.... we're not making it up. The GM engineers aren’t stupid. They set it up this way because it minimizes emissions, and helps fuel economy - the two standards they are required to meet under Federal law. The dual fan version of the V6 are set up exactly the same way. Every car/pickup I've driven in the past 15 years run at these temperatures.

If you feel the fans are coming on or shifting to high speed at a temp higher than 235°F, you need to verify the accuracy of the dash gauge. To do this, download Scan9495, get a cable and run a PCM data log. Look at what the temperature reading is per the PCM. What the PCM does is controlled by the sensor in the water pump housing. The log will tell you when the fan relays are actuated, and what the ECT reading was when they were activated. That’s the data you need to understand what is actually happening, and know whether the dash gauge is right. I've looked at hundreds of these data logs for people over the past 20+ years.

The dash gauge can be way off. To get values for the intermediate hash marks, you need to interpolate between 160°F and 260°F, assuming the gauge scale is linear. That gives you 160-185-210(center)-235-260. The Firebird version of the gauge actually has 210°F on the center mark, unlike the Camaro. Looking at the somewhat crude scale markings, you're probably lucky to get it right within +/-5°.

Then verify the accuracy of the sensor, and the reference voltage from the PCM, again all on Shoebox's “how to” page. Also has the test procedure for the gauge sensor circuit.

4th Gen LT1 F-body Tech Articles

You indicated your car did NOT have this “overheating” problem when you first got it. If it was fine (running cooler at idle) before, and the problem only occurred a year or so ago, something in your engine, cooling system, airflow, or control elements changed. To get it back to not “overheating” (in your view) like it was when you first got the car, you have to figure out what changed.

ACE1252

My '96 works differently(per Shoebox and Injuneer above), but my main point is no matter what the AC system is doing, the PCM will keep the temp below ~230 sitting in traffic or at idle by cycling the fans on and off. You should never need to have your AC on all the time on to maintain normal engine temps.

Looking at the off temps posted by the gents above, my off temp of around 200* must be too low....as such, I've got to be wrong on that.....but for some reason I was thinking that's where I would see the temps settle before it would it would start climbing again in traffic. Might be a gauge sensor vs water pump sensor difference...but more than likely is me not remembering it correctly....in fact, pretty sure I'm remembering it wrong.

ACE1252

Yep, that's where my normal cruise coolant temps are with my 160* stat. Closer to the 180* side.