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Engine temp goes up and down

(2004 Suzuki Forenza)
in Charlotte, NC on June 18, 2009
when driving in the city the engine temp goes up and down... i mean from normal to H in abt 20-30 sec and the back to normal in abt the same time.
when driving on the interstate it jus goes high .

Displaying 9 answers & 12 comments
Popular Answer
on July 31, 2009
Hello, Have you resolved the problem? If so, I'd really like to know what was causing the problem. Thanks
on October 19, 2014
SOLVED! OK here is what it took to resolve this rising and falling temp reading. As I posted earlier I had already replaced the complete cooling system and done a top end rebuild as well. Well as it turns out when I had the engine evaluated by my local mechanic he told me that the ECM was reading the temp sensor at 170 degrees when the car was dead cold at 70 degrees. As the temp would rise it showed an overheat condition just about the time the thermostat finally opened. This also caused the ECM to try and make adjustments to the timing and fuel mix which resulted in stalling and poor idle while the car was still warming up. So he reset the ECM so that the beginning engine temp was accurate from a cold start and like magic the problem was solved! I never could have resolved this myself because I don't have the ECM interface to read the live data on the car. So in short, it was a calibration issue between the ECM and the new temp sensor. Frigging computer was at fault yet again! I hate modern cars with all the sensors and on-board computers always causing an issue with an other wise perfect engine >:[ ..........I hope this helps someone out there in Suzuki hell. I sure could have used this answer for the past year..
on October 23, 2014
So now your forenza operates normal? You dont experience the temperature rising and falling. Because I've been going through this h*ll changing parts that did nothing. I didnt chamge as much as you did. But I changed the reservoir cap. The coolant temperature sensor. The radiator. And the thermostat. And I drained the coolant and flushed it literally 20 times. After reading yout post I really hope this solves the problem because I brought my car to the mechanic and I was told the car operates perfectly and there isn't any problem and he charged me $140 for his diagnoses which I know I got robbed haha anyways im excited to try this as soon as possible and of it works your a life saver :) how did you reset your ecm? Please get back to me as soon as u can. Thanks!!
on October 23, 2014
UPDATE PART 2: Well the problem was only solved for about 3 days and then the temp gauge did it's usual rise and fall again. So it appears that my pals at Brakes Plus made a quick $100 off me for a temporary fix. So here's what I did next that DID resolve it. Let me preference this by telling you that I'm an old school hot rod guy and self taught mechanic with about 35 years experience building and fixing vintage cars. These new computerized cars really drive me nuts because most of the time there's nothing actually wrong with the engine and your fighting the sensors and control module that are the real problem. In this case it comes down to bad engineering of the engine in terms of placement of the temp sensor. The temp sensor can't get accurate readings until the thermostat actually opens. This is why the sensor shoots up to hot and then dives right down to normal once the thermostat finally pops open and allows the water to flow into the EGR manifold. As it turns out this a common problem across many car makers and the solution is an old school one that hot rod builders have been using for nearly 100 years. The thermostat needs two 1/8" bleed holes drilled into it at the 12:00 and 6:00 position. This is so the water can flow gradually and any trapped hot air pockets can be released without any big procedures. In fact, the bleed holes that I added to my thermostat are standard on many engines right from the factory today (Dodge and Jaguar to name a few). Now I know this not something you will be able to get the average mechanic to do for you so hopefully you can do it yourself or you know another hot rod guy who can do it for you. The only change I saw in engine operation after the modification was the engine took 6-8 minutes to warm up rather than the usual 4-5 minutes to warm-up, indicate an overheat condition, and then drop right back to normal operating temp. My temp gauge rise and fall issue is now resolved and truly I hope this solution helps someone else resolve their temp gauge issue too... Craig, Avondale AZ
on August 13, 2015
Hello Forenza dilema... i have 2005 Forenza with an overheating issue. The vehicle went through a repair to fix a blown head gasket causing mix of coolant and oil...aldo got a new thermostat installed.
Now, the thing is that persist the overheating issue... the vehicle drives fine with normal temp gauge for about 6-7 miles then start going up few seconds, then back to normal.... then up and so on.... so my final try is to do the two bleeding holes in the thermostat as you recommends....
Now the million question is:..since the thermostat is sold with the housing, is possible to remove it from the housing in order to drill the holes??... otherwise , it looks more complicated...
Could you Please advise and let me know how you drilled the two holes...
Thank you so much!
on August 13, 2015
Yes the thermostat is made of stainless steel and it just presses into the aluminum housing with very little pressure. I popped mine out, taking note first of the orientation so I can put it back the same way. Then I marked the 2 locations I intended to drill with a Sharpie pen making sure that the drill will not run into the mechanism on the opposite side of the thermostat. I clamped the thermostat into my vise and drilled the 2 holes with a hand held cordless drill. I deburred the 2 holes to be sure no hanging metal shards were going to fall off into the engine and put it all back together. The car has not had the phantom temp rise and fall for over a year since I made this repair. Now go and fix that car!
on November 18, 2010
If you have resolved this problem can you please post the answer, I am having the same issue
on August 27, 2011
OK, well I am having this same issue. I have spent over a thousand dollars SO FAR. Both fans were replaced, the reservoir was replaced, the thermostat/housing/gasket were all replaced with NEW parts. So NOW the mechanic tells me that it is "something electrical" causing it to go to HOT before it even has time to really heat up. (no kidding) - That they tested the HEAT of the engine, it is not running hot, but but by simply messing with the ignition (I guess jiggling the key is what he was saying), he could cause the temperature gauge to go up and down, BUT like I told him, if it is not overheating, WHY when it goes to H in 30 seconds, does my A/C start blowing warm air suddenly, then it goes back to normal on the gauge, and air blows cold again. This is BS and I would really like some answers without having to replace MORE parts that apparently do not solve anything.
on February 25, 2012
I have had a truck (chevy) and a car do this. It is your HEAD GASKET. It is blown. You will screw up your engine if you let it keep over heating. Your temp gauge is going from normal to "Oh Crap" in seconds, then if your heater is on it will blow cold air and if your a/c is on it will blow hot air.
on April 12, 2016
I had a similar problem. Temp gauge went to full hot about 5 minutes after starting engine then went to cold and stayed there. I put an ODB on the car and took the coolant readings. Found it went to 127 degrees normally then jumped to 261 degrees and stayed there even though temp gauge went to full cold. Who knows why. I was also getting engine problem codes 117 and 118.
Looking at the electrical schematic figured is was probably the ECM. I got a rebuilt one off of E Bay for about $150. Real easy to install.
Problem solved as the gauge started working normally. Hope this helps someone.
on November 17, 2012
change thermostat do a coolant flush and fill will new coolant if that doesnt fix the problem then head gasket,valve cover gaskets egr gasket, check spark plugs,check for vacuum leaks in hoses behind motor just did a job on one of these and this was everything i found that had to be replaced the coolant journeys on the head gasket tend t crack on the factory chevy gasket you want to get the complete felpro hgasket kit and doing the other things i mentioned should fix the problem
on October 15, 2014
Well I've been down this path for about a year now and still have not gotten it solved. Here is what has been done: Head professionally rebuilt (valve job, surfaced, new guides and seals, etc), new Felpro top end gasket set and head gaskets, new radiator, new hoses, new water pump, new cam position sensor, new crank position sensor, new thermostat, new temp sensor, new fan relays, new coolant, new coolant tank and cap, new timing belt & tensioner & idlers, burped any air from the cooling system. The engine runs and drives great. But regardless of all the work the has been done the thing still does the same thing every day. Starts right up, temp rises normally to just below midway on the temp gauge then after about 10-15 minutes the gauge suddenly rises up to the 3/4 mark or more toward the "H" and then just floats right back down to below midway again in about 20-30 seconds. During this little exercise on the temp gauge the A/C stops blowing cold air and starts blowing hot air until the temp gauge drops back down below the midway point. There are no coolant leaks and the coolant level in the reservoir tank stays constant. I know it is impossible for the water to heat up and cool down as suddenly as it is showing on the gauge. So it seems like the hot water in the head is pooling at the back of the motor in the EGR manifold and telling the temp gauge that the engine is overheating and then the ECU try's to protect the engine by shutting off the A/C and then suddenly whatever is preventing the water from flowing freely in the head opens up and allows the trapped hot water to circulate. Is there some other valving in the cooling system other than the mechanical thermostat that constricts or allows the coolant to flow? I've already swapped thermostats and temp sensors just in case my replacements were bad but still the same result. One mechanic recently told me that he heard of this problem and it being a calibration issue with the gauge cluster and wanted to charge me $600 to send it out for calibration. I asked him if he would guarantee that the rising and falling temp gauge problem would be solved or my money back and he said no so I didn't try the calibration. Anyone actually solve this rising and falling temp issue for good?
on August 18, 2015
Hi Forenza_dilema - I'm not sure who you are or where you came from, but after about $5000 of trying to figure this mystery out, I'd love to buy you a beer. It's just comforting to know I'm not alone. I've been through everything you have and I am going to get the magic thermostat fix today!

My question for you is this... Since getting the top of my engine rebuilt and radiator replaced after the radiator hose blew off 3 times, a mechanic yesterday noticed that the mechanic before him (post head gasket repair last month) didn't bother to put a thermostat in. He said given my issues, it would probably be best to go without it.

This morning, the gauge jumped for 20 seconds to the 3/4 line again... with no thermostat in place. Does this still fit your solution? Would it not have happened if there had been a thermostat in place with the holes in it? THANKS!

on August 18, 2015
NO, It is critical that you run the thermostat and you should find a new mechanic. The rising and falling gauge can come from one or several things.
- Bad or missing thermostat
- Failed head gasket
- Clogged radiator/cooling system
- Failing water pump
- Running old antifreeze or with no antifreeze
- Faulty engine temp sensor
Please let me know if EACH of these things have been addressed and then I can tell you more about what may solve the issue...
on August 18, 2015
All of these things have been checked/replaced. Do you think if I install a new thermostat with the holes drilled it will finally solve the intermittent rising of the temp gauge which has proceeded the radiator hose blowing off?
on August 18, 2015
It's hard to say without knowing a detailed history of the car and all the things that were fixed vs "checked" and whether the replacement parts were Suzuki OEM quality or cheap KO. I don't even know whether the head was completely rebuilt, including hot-tank, valve job, seals, surfacing, etc. So given my lack of knowledge of your particular vehicle repair history here's what I can tell you... Assuming your vehicle is running and driving just fine, both with and without the AC operating, both in rush hour traffic bumper to bumper and in normal driving, and the ONLY issue is the occasional rise and fall of the temp gauge.. Then and only then, would I say YES the old-school thermostat treatment should work fine. But if your car has had questionable repairs done to it by some hack would-be mechanic then I can't say what will happen other than maybe nothing, no improvement. I shouldn't get worse unless the head was not fully reconditioned and there is warpage due to not being resurfaced or cracks in the head allowing water to travel where it does not belong, etc. LMK how you make out... Craig
on June 21, 2017
Hi I've been dealing with same problem and was wondering its been 2 years since you made they adjustment to the thermostat has your temp behaving.
on June 21, 2017
Never had any issues again... and what better place to test the cooling system than Phoenix AZ right? It's been 120 degrees every day for 3 days in a row this week!!!!

For those who are jumping in here at the end.. here's the Readers Digest version:
Professionally rebuilt the head and of course replaced the head gasket and complete cooling system, including heater core, radiator, cooling fans, relays, hoses, all. Still had the rise and fall phantom temp gauge! Solution was to go old-school and drill two 1/8" bleeder holes in the outer ring of the thermostat to relieve the hot air vapor that was building up and locked behind the thermostat on the same side as the temp sensor. Once the vapor was no longer trapped while the engine came up to temp the faulty temp sensor readings and ECM compensating adjustments were no longer an issue. The A/C stays cold, the gauge is now accurate, the engine never overheats, and life is good.

You can see more detail about why and how it came to this if you read back through my comments in thread. In my case, it was the only way to resolve the problem because everything else had already been done..

Get out there and fix that 'Zuk!.. Craig
on June 28, 2017
Hello, would you guys please help me out! Had the misfortune to buy Forenza used, head gasket blew before I even got home. Long story short, spent $2 grand on another used engine with about 76k miles. Assured all is well, no head problems. Started driving around, gauge jumps to H. Took to my local mechanic, bled radiator. Many repairs later still creeping up to 3/4, sometimes to H. My guy pulling his hair out. So far, had new water pump put in (when engine replaced), coolant sensor (twice), new temperature gauge, bled it multiple time in case of air pocket. Does the same thing everyone else is saying, with a/c on, temp creeps up, a/c blows hot air, temp gauge goes down to just below 1/2 and stays there. Usually happens about 10 minutes into drive time. Worried that car is overheating or may blow. Is it safe to drive it like this? Is the engine really overheating or is it the hot temp in thermostat causing the flux? Should my mechanic attempt to drill holes? Should I just drive this car into the ocean and forget about it? Thanks.
on December 20, 2010
I had the same problem the dealer replace my themerstat free
on February 05, 2011
slounds like head gasket
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