Odd Heating/Cooling??

[mmltml]

Hi. I'm new on this site...my 1995 Monte Carlo Z34 has been doing something odd...

It will get very hot and then cool down during normal driving; My temperature gauge swings hot and cold constantly. This happens usually 5 or 6 times within 5 minutes.

It seems to be affected by the outside temperature and when I use my heat. When it is cold outside, and I am using heat on setting 2 or 3, it will heat up until the point where the HOT light comes on, but the light stays on for less than 10 seconds, and then the gauge swings back down to normal temperature, in which the gauge sits around a quarter of the way from cold. But when it is warm out, and I am not using heat, the highest my gauge will go would be about 3/4 of the way up.

I've been asking around for opinions and most people say that the gauge is bad, but once when it was getting hot, I used a laser thermometer to check how hot the engine actually was, and the gauge was correct.

Oh we also replaced the thermostat back in November because it was doing this, but it would just overheat (the gauge never swung back down.)

any opinions? I am a member of mymonte.com, and i posted this same thing on there but this site seems to get more traffic. thanks

[55trucker]

You mention that this anomaly was happening before you replaced the stat. Are you certain without a doubt that the cooling system was properly bled of all possible air before you got into it?....Air in the system will cause an erratic reading on the temp gauge via the sending unit. Now that you Have replaced the stat ...has all of the air been bled from the system?...are you familiar with the small brass bleeders located in the coolant piping to bleed off the air?

[mmltml]

You mention that this anomaly was happening before you replaced the stat. Are you certain without a doubt that the cooling system was properly bled of all possible air before you got into it?....Air in the system will cause an erratic reading on the temp gauge via the sending unit. Now that you Have replaced the stat ...has all of the air been bled from the system?...are you familiar with the small brass bleeders located in the coolant piping to bleed off the air?

yes. we bled the air from the system. we got as much air out as we could (until it stopped bubbling)...do you think that there is still some stubborn air in there? or what else possibly could it be?

[pocket-rocket]

I usually drill a 1/8" or so hole on the outside edge and place that spot at the top of the opening that the t-stat sits in to help aid in bleeding air out. It couldn't hurt to crack the bleeders open to see if there was some air trapped from before that has worked its way to the bleeders.

[55trucker]

Well, as stated above open the bleeders (cracking the rad cap at this point to see if there is any trapped air at the top of the rad couldn't hurt either). When your gauge needle pegs over on the *Hot* end of the gauge are your cooling fans cutting in? Do the fans work at all?...The fans are actuated by the ECM which takes info from a second temp sensor mounted in the intake manifold to start them if the engine temp rises above a measured point. At this point do you really have an overheating problem or is it an erratic gauge or sending unit?

[mmltml]

Well, as stated above open the bleeders (cracking the rad cap at this point to see if there is any trapped air at the top of the rad couldn't hurt either). When your gauge needle pegs over on the *Hot* end of the gauge are your cooling fans cutting in? Do the fans work at all?...The fans are actuated by the ECM which takes info from a second temp sensor mounted in the intake manifold to start them if the engine temp rises above a measured point. At this point do you really have an overheating problem or is it an erratic gauge or sending unit?

yes my fans seem to be turning on...and i don't think it is the gauge because like i said in my original post, i used a laser thermometer to see how hot the engine really was when the gauge said it was getting hot, and it was consistent to what the gauge was saying.

[pocket-rocket]

I made a mistake in my post. It made it sound like I drill a hole in my intake, and I don't do that. I drill it in the edge of the thermostat itself.

[55trucker]

Just trying to get some basics here.....are the fans cycling on & off (one or both) or are both the primary & then the secondary coming on & staying on consistently without shutting off when the temp gauge pegs? If the temp is going up and staying up then the ECM will bring on the fans (primary first and then the secondary will follow) and they should stay on if the temp does not drop to the point where the ECM will shut them off. Also ...has a code 14 (OBD1) been set in the ECM's memory? This would be a high engine temp (270+ degrees) Something else...I'm wondering if the water pump has failed? ..perhaps Ben or someone who has had to replace their water pump can inform us....is the impeller a plastic impeller? I don't know for certain...If it is they were known for disintegrating on the some of the pushrod engines.

[walterdude]

Glad someone mentioned waterpump.... I HAVE read of issues with rebuilt/aftermarket (and newer pushrod engine) pumps used on DOHCs... The rebuilts use stamped steel for the impellor,,, original style uses a cast impellor... Some people have "claimed" that the steel impellored pump doesn't work well with DOHC... Myself,, I can't prove or disprove, never did a pump on my DOHCs.
Just a thought,,, if you HAVE replaced the pump it might be worth it to take a look.....
On another note,,, it sure DOES sound like air in the cooling system..... After you KNOW the thermostat is open try bleeding again... I've actually gotten air in the system trying to bleed it when it wasn't hot enough, especially the bleeder by the thermostat..... I like the idea of drilling a small hole in the thermostat plate, next time I have it out I will do that...
Just a couple of thoughts,,,, if you've thought of them already,,, SORRY!!!!
If not,,, hope this helps!!!!
Tom....

[mmltml]

Glad someone mentioned waterpump.... I HAVE read of issues with rebuilt/aftermarket (and newer pushrod engine) pumps used on DOHCs... The rebuilts use stamped steel for the impellor,,, original style uses a cast impellor... Some people have "claimed" that the steel impellored pump doesn't work well with DOHC... Myself,, I can't prove or disprove, never did a pump on my DOHCs.
Just a thought,,, if you HAVE replaced the pump it might be worth it to take a look.....
On another note,,, it sure DOES sound like air in the cooling system..... After you KNOW the thermostat is open try bleeding again... I've actually gotten air in the system trying to bleed it when it wasn't hot enough, especially the bleeder by the thermostat..... I like the idea of drilling a small hole in the thermostat plate, next time I have it out I will do that...
Just a couple of thoughts,,,, if you've thought of them already,,, SORRY!!!!
If not,,, hope this helps!!!!
Tom....

now that you say that, my water pump WAS changes 3 years ago at 81k miles by the previous owner...i wonder if he did use a rebuilt or aftermarket one... but i will try to bleed the system again this week and keep this updated

[walterdude]

3 years??? I'd say an issue of pump styles would show up right away,, like as soon as it got hot... If you search enough there are threads here about the pumps,,, stamped steel vs cast impellor.... Don't think that's your prob tho.... I'd make good and sure there is no air in the system. They can be tricky if you're not used to it.... After it should be good (you've bled it) to go I go for a good blast and crack the bleeders for any leftover air. I do that a few times just making sure the overflow res has coolant.
If you KNOW there is no air and it still does it maybe the radiator is plugged??? If not then sensors (I believe they are easy to test with a meter) or ECM... I think it was 55trucker that was helping,,, he has good points you could check out easy enough.... It's even possible you got a bad thermostat...
Still sounds like air.....
Good luck!!
Tom...

[mmltml]

3 years??? I'd say an issue of pump styles would show up right away,, like as soon as it got hot... If you search enough there are threads here about the pumps,,, stamped steel vs cast impellor.... Don't think that's your prob tho.... I'd make good and sure there is no air in the system. They can be tricky if you're not used to it.... After it should be good (you've bled it) to go I go for a good blast and crack the bleeders for any leftover air. I do that a few times just making sure the overflow res has coolant.
If you KNOW there is no air and it still does it maybe the radiator is plugged??? If not then sensors (I believe they are easy to test with a meter) or ECM... I think it was 55trucker that was helping,,, he has good points you could check out easy enough.... It's even possible you got a bad thermostat...
Still sounds like air.....
Good luck!!
Tom...

yeah...and it never did this until we replace the tstat so air in the lines is probably it. i appreciate everyones help...thanks much haha

[55trucker]

I suppose that *when push comes to shove* ...remove the stat altogether....just leave it out of the system...if the overheating condition persists then you definitely have a problem elsewhere. Whether it be a bad pump, seriously clogged coolant system, or on the outside possibility ...a head gasket that has started to leak between a cylinder and a coolant gallery. If indeed one of the gaskets has developed a leak into a coolant gallery and only a coolant gallery you might see considerably more white smoke out the tail pipes as the engine is literally turning into a *steam engine* (I doooo hope that the latter is not the case).

[pocket-rocket]

You know... speaking of thermostat, was it by chance an Autozone jobbie? With the way they send bad parts out the door all the time, I wouldn't be shocked if the problem WAS the thermostat. Still have the old one? Toss it in and see what happens. I would also get a coolant system pressure checker from the same mentioned above abomination of a parts store and see if you don't have a coolant system like from say, Mr. Headgaskets. Is there steam coming out of the tail pipe at all? Does it use any coolant?

[mmltml]

You know... speaking of thermostat, was it by chance an Autozone jobbie? With the way they send bad parts out the door all the time, I wouldn't be shocked if the problem WAS the thermostat. Still have the old one? Toss it in and see what happens. I would also get a coolant system pressure checker from the same mentioned above abomination of a parts store and see if you don't have a coolant system like from say, Mr. Headgaskets. Is there steam coming out of the tail pipe at all? Does it use any coolant?

actually it was from autozone...hmmm. unfortunately i don't have the old one anymore. I haven't noticed any steam from the exhaust. It doesn't use any coolant either. Actually, take a look at the picture in my garage of my engine. Notice the coolant hose running from the coolant pickup on the front of the engine over the holding tank to the back coolant pickup. We did this because one time on my way to school my car was SEVERELY overheating and afterschool I came out and there was a big green puddle under my car (i lost all of my coolant). We found that the metal piping running down under the engine along the chassis rusted out or burst, so we just ran a new hose around the bad section, and ever since then it hasn't used a drop of coolant.

how much $$$ do you think a coolant system pressure checker would be?