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mattrextravis

94 S4 - Overheating

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Hi all. I have a 94 S4, and it seems to be overheating. I say seems because I seem to have several semi-related problems...

I live in Texas, so it's quite hot out. High running temps aren't that unusual from what I've seen. However a few weeks back I started having a problem where oil temp would reach 90+deg, and oil pressure would read so low that the oil pressure telltale came on intermittently. After much panic about low oil pressure, it was determined that the oil pressure gauge is just reading low - a mechanical gauge showed pressure to be OK even at high operating temps. So a new oil pressure sender is in the mail, we'll see if that helps.

Now the car has started exhibiting signs of overheating, but the coolant temp gauge never reads more than 85 or 90C. The oil temp will reach 100+, and it even had the low coolant indicator come on before I was able to shut it down. After further testing, I know that the air exiting the radiator is >100C based on infared thermometer readings. I just got my ALDL cable Friday and freescan shows coolant temp at 100C+. I know the ECU uses a different coolant temp sender than the gauge, so am now convinced the car really is that hot...and that I have another sender/gauge issue with the coolant temp as it consistently appears to read 20deg low.

My question is what to check next for the overheating. Pressure testing showed no leaks in the coolant system. I know that hot coolant is flowing to the radiator, so that should eliminate a failed thermostat I think. I looked under the car to see that all 3 fans come on at 92C (per freescan/ecu coolant temp) and stay on continuously as long as the car is hot, so it's not the fans. The only other thing I know is that it won't overheat if you drive fast enough. Keep it moving at highway speeds and it will go 100+ miles without overheating, but slow down to city traffic speeds and it won't last 5 minutes. Normally I would think that's a fan problem, but like I said - checked those already.

Any suggestions are greatly appreciated! And sorry for the long message - but at this point I'm not sure what's important info and what's not, so would rather not omit anything.

Matt

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Hmmm, brain storming here...

1994 so i take it you've got the single header tank system not the seperate one for the changecooler.

Umm....pressure cap, (ONLY WHEN COLD !!) check the sprung pressure cap is on the bottle in the engine bay and the blanking cap is on the expansion bottle in the rear wheel arch....this often gets replaced wrong way round and the car wont develop system pressure and will tend to dump coolant when the engine gets hot (rings right with the low lvl).

If it's not that I would be looking at not a pressure test but a circulation test - it could be that the thermostatic valve in the pump is closed and stopping cirulation or any manner of blockages...but you've done that by looks of things...still if it was anythng like my GT3 when i got it the coolant was pretty dirty with rusty bits in it from the corrosion, you tried drainging and replacing the coolant, very easy to do you just need a big bucket !

Umm so the ECU is getting the signal to cut in (92degrees) and it's bringing the fans on but you're still over heating. Past the circulation test is the radiator obscured or has crap in the fins ? That will hamper cooling - seems like everything is working electrically.


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Good points Jonathon. Pressure cap only when cold is very good advice. :blink:

The cap on the engine bay tank is definitely a sprung cap. Have had that one off enough times this last weekend to know that for sure. I'm not familiar with the wheel arch expansion tank you refer to... Which side would this be on? And is access to this tank gained through the wheel arch?

Have had coolant in and out several times with the reservoir replacement, but that's not an entire system flush. Clearly some coolant is circulating since radiator is scorching hot, but how much circulation could be another matter. What's the best place to open the system up to drain it. Am sure I can find a big bucket around somewhere...

Radiator is not obscured in any obvious manner, however as I understand it the first thing you see when looking under the bumper is not the radiator but actually chargecooler or a/c related? What's the best way to get at the front side of the radiator?

Thanks,

Matt

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I had very similar problems with my s4 wen I first got it. But in my case the coolant temp would overheat and boil out. But oil temp was always perfect. Turned out in my case that the front rad (Aircon/CC) had alot of mashed fins preventing air flow through to the coolant rad behind. With your being the oil temp getting very hot I'd be tempted to check circulation and that the oil rad fins are clear. The oil rads are in front of the front wheels and take air from the lower front left and right air in takes.

Worth a quick check. Otherwise, echo everything Jon has already pointed out.

Other than the type of oil your using isnt up to the job.

Good Luck.

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Good point about the oil temp, although I suppose there's another bit of info along those lines that I hadn't really considered... Using the infared thermometer on the front face of the oil coolers I found them to be 80-85C when the gauge read 100C. I kind of blew it off at the time, thinking I would focus on the things that were actually hot. I wonder if the oil temp gauge is reading high? Given that every other gauge seems to be having issues I'm not sure that I can assume oil temp reading is correct either...

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The expansion bottle is inside the trunk, it should be in the drivers side back under the carpet behind the wheel arch. As the main tank gets hot the water expands, if it gets too much the valve opens and the water is allowed to expand into this tank. On cool down the system will syphon this fluid back so you dont lose it.

You can always open the radiator air bleed - very tricky to spot, sorry thats not much use, if you need to work on the car yourself you really need a manual. On my car (lhd) its is in the (your drivers) side wheel arch at the front - at the top of the radiator.

If you unscrew this and start the car, have someone top the level up and look at the arch - it should produce a flow of water the same rate as a house tap (ie a small jet, not a trickle). you never know it might clear it too if there is an air lock.

The drain plug is on the oposite side of the rad at the bottom (again no pics - not much use).

but the system holds a LOT of fluid so beware.


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Heat the themostat in a pot of water with a thermometer. It's an unusual device that also operates a bypass valve which diverts water flow through the pump. When fully open it closes a bypass passage in the pump. If that bypass isn't fully extended there could be no or only a minimal flow to the radiator.


DanR

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I agree with the thermostat going bad. unless you have an air in the system.

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Checked the overflow tank, confirmed it has the right cap. So caps are s/b OK all around (assuming the spung cap is holding the appropriate pressure).

Didn't have time to drain coolant or clean radiator fins last night so that's probably tonight's project.

I am fairly certain that the problem is not caused by air in the system. When coolant reservoir was replaced we went to great lengths to ensure no air in system, including tilting car, vacuum-bleeding, etc.

I'll see if I can pull thermostat tonight as well. Have had thermostats fail before in my daily drivers but in those instances it was complete failure, so the car overheating with a cold radiator was a dead giveaway. I follow your logic though - partial failure that is restricting flow could make sense in this situation... Is there a generic replacement to be found at the local auto parts store?

Thanks!

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OK, cleaned radiator fins last night. I am absolutely amazed at the amount of crap that came out. What a mess!

I took the car our for a good long loop on the highway, then around town for 15 minutes at low speed, then sitting at idle in my driveway for 20+ minutes. If it was going to overheat, I imagine that would do it. Coolant temp never read above about 92C (which shows as 72C on my gauge, since it reads 20 deg low...) Granted this was at 7pm so the outside temp wasn't at max, but my drive home generally occurs at 6:30pm and it certainly had no trouble overheating at that time previously.

Don't know if it's completely fixed or not, but it is quite a bit better than it was. Today's drive home will tell for sure - expecting 37C here in Houston today. Thanks for all the tips, and I'll report back on the results of today's testing.

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Just a quick one if it's any use,

Have you pressure tested the cap?, some kits have an adaptor that lets

you test the pressure cap, worth doing as it's not actually in the system

when you test it.

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OK, after much additional testing, I managed to get the coolant temp (per freescan) up to a max of 102C, but no higher. Took considerable effort to get that hot too - long hard driving followed by barely moving at all in traffic then idling in the driveway for the better part of a half hour. Given how hot it is outside right now I figure I'm in good shape @102C. Let me know if folks think differently.

Now I'm on to other issues, such as window seals improperly installed by dealer jamming up the window travel and why many of my gauges read the wrong values...

Thanks!

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