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2006 Suzuki Forenza - Runs great around town, overheats on highway after about 5 miles

2006 Suzuki Forenza with (GM/Daewoo) 2.0 I-4
~150,000 mi - always maintained well - have owned since new.
Timing Belt broke last year (Fall 2014)
I installed new Timing Belt Kit, Water Pump, Idlers, Oil Pump, FelPro Head Gasket, Alternator (noisy bearing), serpentine belt, front exhaust pipe & gaskets, new exhaust and intake manifold gaskets, new valve cover gasket, rebuilt head, using NEW casting, thermostat, DexCool coolant. Have been working on cars for 35 years - did the work myself. Followed all procedure, and everything went together well, and parts all seemed fine at the time.

Has been running great for about 9 months and does NOT overheat ever around town. However, when I take it out on the highway, it starts to overheat after about 5 miles at 60mph or above. If I slow down slightly, or pull over, temp drops right down and it runs at normal temp for another 4-5 miles and then starts to creep up again. I then need to slow down or pull over for a minute, and repeat the process. The car will run all day long at 50mph without budging from normal temp, however, soon as I get above 60 for 5 miles straight, the problems start.

Sometimes it heats up to the 3/4 mark and stays there for a few minutes. If I drive a little farther, it will go back to “normal temp” “1/2 mark” on it’s own, with the gauge behaving just like a “stuck thermostat” that just opened and allowed engine to cool. Sometimes it doesn’t and creeps up higher, close to the red line. . . . then I need to pull over or slow down to perhaps 40mph to allow it to cool. When I watch the gauge, I am convinced it’s a stuck thermostat by the way it behaves. . . . . but there was a brand new thermostat installed when I replaced the head. This highway behavior started right after I installed the timing belt, new head, and new thermostat, etc, but lived with it the last 9 months, since I really haven’t done any highway driving with this car.

Today, tried installing a new pressure cap on the overflow tank to see of that was the problem. Did not affect problem - still overheats after 5 miles on hwy.

Just pulled the thermostat today and did a coolant flush with a garden hose. Coolant looked good (and looked clean in tank), and I didn’t see any obvious sediment come out. Flushed from radiator hoses, backflushed through the thermostat opening, and flushed through the overflow tank. Ran car today with a bottle of Prestone Radiator Cleaner/Flush and plain water. Still overheating after 5 miles on highway.

After I dump the radiator cleaner on Sunday or Monday, I am going to install another new thermostat, and see if that solves the problem.

Hoses look OK (they are still original hoses), but don’t seem to collapse when you rev the engine when warm. Perhaps I should replace these. . . . but it’s not obvious they are the problem.

One of the first times the car overheated on the highway (got close to the red mark), I pulled over and popped the hood to see what was going on. Both upper & lower hoses were rock hard and I could hear the coolant boiling in the overflow tank. I cracked the cap to relieve the pressure, screwed it back on, and turned around and drove home. Soon as I started off again, temp was back to “normal”, but crept back up after 5 miles or so on highway.

I haven’t popped the hood on the side of the highway since that day, and don’t ever hear coolant boiling when I pull over. . . . but just thought that incident might help with diagnosis.

I have also sprayed the radiator fins from both sides to clean out any debris that might impede airflow. Externally, radiator looks to be in good shape - fins are good, not bent, and no corrosion. Don’t see any external coolant leaks in radiator, hoses, engine, etc.

Both radiator fans work and come on as intended - however, even if fans weren’t working, it shouldn’t cause an issue at highways speeds anyway. If they weren’t working, I would have issues at idle, etc, when air wasn’t being forced through radiator core.

Since everything is “new” except radiator and radiator hoses. . . . any thoughts on what might be causing this issue? Any thoughts on what to look at or test?

I have NOT done a compression test to date.

Car never overheated before the broken timing belt issue. Could I have a faulty (new) head? Bad “new” water pump? Pump seems to circulate fine - when engine is warm and you rev engine, can see good flow in overflow tank.

Don’t see any obvious oil in coolant, and don’t see any coolant in oil.

Any suggestions anyone can provide would be GREATLY appreciated. Have been driving this car for 9-10 months, but can’t take it on any long trips and just use it around town because of the overheating issue. Let me know if you need any additional information.

It’s not at all unusual for a brand new thermostat to be defective right out of the box.

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With the radiator cap off and engine running, have someone rev the engine in neutral and look for bubbles in the coolant. You may have a head gasket leak, especially since you say the gasket was just replaced recently. You may need to retorque the head bolts.

If there is a head gasket leak and retorquing the head bolts doesn’t fix the leak, you may have to redo the head gasket job.

You know more than I do but I’d suspect a worn/coated/tired radiator if the other stuff checks out. Highway driving would put more load on the cooling system than maybe it can dissipate the heat even with the greater air flow. I also put new thermostats in a pan of water with a candy thermometer to make sure it opens and closes at the right temp. Been doing it for 50 years since my dad had a bad one. Never really found a bad one but its a lot of work to change one so better to make sure.

Your cooling system can’t stand up to the heat load at 60 mph (yeah, I know, that’s obvious). It can, however, cool the engine at 50 mph. That pretty much eliminates the thermostat.

Look for things like restrictions in the airflow through the radiator at speed, restrictions in the coolant flow (as in a plugged radiator), restrictions in the pumping ability of the water pump (maybe a bad impeller), and most of all, get rid of the lower radiator hose. I’d bet that it’s collapsing and blocking flow into the water pump.

I do not know if this model uses a lower radiator support airdam but if so and if that airdam is missing an engine can overheat at higher speeds only. Airdams usually come up missing due to curb strikes, critter hits, road debris, or sometimes just miles of vibration cracking the plastic which then causes them to fall off.

Are you certain the head gasket application was the correct one? Just wondering if the new gasket matched the original or if there’s a difference in a cooling jacket hole in the gasket which could be affecting coolant flow.

Dex-Cool is infamous for causing sludging problems and it could also be that the lower extremities of the radiator cores are clogged. Sometimes the OTC flushes and a garden hose just can’t do the job properly.

Have you solved the problem, I have the exact same problem on my 2007 forenza and changed everything like you did except the radiator.

Oh come on, this person has been gone for a year.

Believe it or not, it was the thermostat. Even though it was brand new, it was bad. Changing it cleared things up and car was perfect for a year, until last week. Now, it’s running hot again above 65mph - haven’t gotten around to trying a new thermostat again. Around town and under 60mph, the car NEVER goes above normal temps. Soon as it gets to 65mph and above, it goes to about 3/4 mark on temp gauge. I’m going to try a thermostat again, and see if it disappears or if I have another problem now.

As was mentioned already, bad new thermostats are not unheard of

I’ve run into it myself . . . in my case I replaced a thermostat as preventive maintenance, simply because it was really old, and it was time for a coolant drain and refill, by time

It was running fine BEFORE the thermostat was replaced

It started running hot immediately AFTER the thermostat was replaced

I went back with my receipt and the thermostat and demanded another. No problems since

Inactive members are unwelcomed?

Yes, and now I’m back!

OK so I am having the same problem as you are my Suzuki Forenza for some odd reason will overheat going over 60 mph when I go up a hill for some reason it goes back down to the half Mark and seems to be fine once I start going down a hill it starts to red line and overheat and it there’s no explanation for it because I have coolant in the system and there’s enough in there and for some reason is just being faulty and I don’t know if it’s the temperature gage or if it’s the the radiator I’m just not sure what is causing it but it is a problem and I need help to fix it

Had this same issue, replaced the thermostat twice and then my “old school” mechanic came up with the solution - drill 2 small 1/8" holes in the thermostat (not the housing but the actual thermostat). He popped out the thermostat from the housing, drilled the holes at the top and bottom, approx 180 degrees away from each other, replaced it back in the housing and installed it. This allows just enough pressure to be released before the thermostat pops open and keeps the engine from heating up unevenly. I live in FL and have not had a single overheating issue since he did this (knock wood).