Hello from the UK

I’m new to the forum, and fairly new to American vehicles, so please bear with me if I am naïve about forum protocols or about the car !
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I have an ’88 Firebird Sport Coupe, (350 V8, TH700 with Posi-axle & WS6, current mileage 69,000), which has recently started overheating at low speeds / idling. I know overheating is a common issue with 3<SUP>rd</SUP> gens, and that you discourage new questions on this topic, but I’ve tried to read the existing posts on the subject and not been able to decide if my particular problem is there or not. If you have already covered these exact symptoms, do please tell me and I’ll look again. Here are the details:
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All the relevant air dams, scoops etc are in place under the car. Two fans are fitted. The radiator is clean and free from obstructions or damage, as are the airways to and from it. It has the correct pressure radiator cap and correct temperature thermostat, and correct spark plugs. There have been no modifications of any sort to the car and following a recent inlet gasket replacement to cure a water leak, no problems showed on the diagnostics readout despite the fact that the overheating was happening before the repair. She runs fine on open roads, but overheats after even brief running in traffic or even while parking if it involves a lot of shunting about to get into tight spaces. Once the temperature starts to rise beyond 220 degrees, it climbs rapidly, though I have never allowed it to get too near to the red zone. Once the engine is switched off, she boils up and ejects from the coolant recovery tank roughly the same volume of coolant as the recovery tank normally holds between the “Hot Full” line and the bottom of the tank. The boil-over was much more spectacular during a recent emissions test when the test guy held her at 2000-3000 rpm for a few minutes to check the catalyser – she ejected her whole radiator contents through the recovery tank filler.
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Here’s what I have observed: both fans run for a short while – maybe thirty seconds, perhaps a bit more - when the engine is first switched on, so I know that they are serviceable in themselves, but neither of them appear to start up again when the engine is up to normal temperature or beyond. I’ve checked what result switching the air-con between normal and off has at below running temperature and once running temperature is reached and can’t see anything happen in either case – I think from reading the workshop manual that one of the fans at least should react to this. So I’m wondering if anyone can confirm my suspicion of a failure of the fan’s coolant temperature sensor, or is it liable more complicated that that ?
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Thanks for a great forum.

 

If you have 2 fans one should come on at ~222 deg and shut off at ~200 deg and the other should come on with the A/C comnpressor on(MAX,NORM,BILEV, and maybe DEF) or if the car gets to ~235 deg.
With one fan setups the fan should turn on at 222 or whenever the A/C compressor is on.
It sounds like neither of your fans are coming on. Check and see if one of your fans will turn on if you turn the AC on(trhis should happen at any temp). If neither of the fans turns on under the temp being high or AC on then you have something wrong with the electrical system to the fans or fan motors etc...

 

Thanks for this !

The fans do run on starting the engine for a short period, but don't cut in again even when the A/c is turned on, nor do they cut in again before the temp guage goes uncomfortably high and being worried, I switch the engine off before it does something nasty ! I can't recall exactly how hot the guage showed before I shut the engine down, but I'll check again. If the guage works, that presumably means the coolant temp sensor is working. So I assume it informs the ECU for fan control too, as well as driving the temp guage ?

The car ran just fine through last year in some atrociously hot conditions (for the UK) - my wife was taken ill with the extreme heat while we were at an American car event last June and we had to run the car stationary for about 90 minutes with the aircon running to cool my wife down - the temp guage never went up to more than about 225 and the car didn't boil up then, though it did probably cook the gasket under the inlet, which had to be replaced a few months later.

 

On my car I have a single fan because I have the 5.0L TBI engine and my fan is controlled by a separate fan switch that is in the pass side head. I think for your car with dual fans, your primary fan is controlled by the ECM and the secondary is controlled by the fan switch in the head or AC switch. I'm not sure of this, maybe a TPI guy can help out here. It's wierd that your fans go on and then stop running They should not go on when the car starts unless AC is on and in that case they should stay on.
Since both run(when car starts) then the motors are worrking.
Since they both dont turn on at high temps then it cant be the fan switch in the head or the ECM screwing up(cuz only one would be not working)
I guess it could be some type of relay that controls the fans but I'm not really sure, I don't know enough about the wiring setup

 

When that happens its because the coolant has begun boiling. When I had a leak a few months back, and I had been replacing coolant with plain water (just for a week on the trips to work) I could get about 3/4 of the way there before I'd spot the meteoric rise in temperature. It literally takes 15 seconds to rise to 260 and after the first time it happened I learned for my cylinder head and blocks sake to pull off and shut down the motor!

Essentially you've gone beyond the coolant and radiators ability to cool the motor.

As 91black5spd pointed out, the first (primary) fan doesn't come on until ~222 degrees. On my gauges this is just the tiniest bit past the "bottom" center of the gauge. Nearly vertical pointed downwards.

Your second fan should ALWAYS be on when the A/C is on. (MAX,NORM,BILEV, AND DEF). If your secondary fan isn't coming on with the A/C then it probably isn't going to come on when the temp reaches 235.

For whatever reason your car isn't cooling as it should. Normal operation (even on a hot day with traffic) should have the max temperature at like 240 (EXTREME) and it should cycle 180-225 with the primary fan, the secondary occasionally coming on.

So I think you have 2 problems:
1. Your car isn't cooling itself effectively. I was able to drive my car without any fan at all for 35 miles on the highway one day. I didn't realize there was a problem till I got off the highway and it immediately overheated at the light. When the car is moving at cruise speeds it probably won't overheat without the fan.
2. The secondary fan (which is kind of a backup) isn't coming on to help cool when it gets really hot.

Thus the moonshot temperatures.

Several ways to go here -

Incorrect. There are 2 coolant temperature sensors. One for the gauge, and one for the ECM (which controls the fan).

My first thought is a restriction in the radiator, build up of gunk could effectively reduce the size of your radiator! Failing bearings or slipping belt on the water pump. Stuck thermostat, bad radiator cap, bad ECM coolant temp sensor could do this too.

Are there any leaks at all? THere is a "weep hole" on the shaft on the bottom of the water pump pulley that leaks coolant when the seal is almost totally worn. It's an early indicator of a bad water pump and by design.

Have you replaced these easy items yet? You said they are correct, but no indication that they were replaced. That would be my first step.

Once its hot you can check for general water pump operation by squeezing the upper radiator hose. It should be very firm, but yield a bit under pressure. If you've got good water pump pressure then you could move to the radiator. Replace it if it has been in service for more than 5 years.

On to the second problem, the fans. The ECM sends the signal to a relay to turn on the primary fan. It gets its info from the second coolant temp sensor and at 222 degrees F it turns on. So your second coolant temp sensor could absolutely be bad, and the car could still run relatively OK, just the fan wouldn't turn on. The relays location depends on the particular engine, I'm not positive about yours, but I've heard they can be on the firewall almost directly in front of the driver, or other locations as well. There is one for each fan. So either or both of these relays could be bad, or more likely, the wiring in the connector for that relay could be bad, as in its old and corroded or the wire has pulled too far out of the connector, or the connector is flayed and intermittent. Be very careful taking ANY plastic connector out, after years of service they turn brittle and so far I've managed to break just about every single connector in my engine bay the first time I've messed about with them.

 

Thanks for the detailed analysis, guys. I have a better idea of what does what now, and where to look for things.

A friend of mine, a muscle car owner for 20 years or more, who runs a '70 Camaro and a '78 Camaro, (plus a Mustang 2 and a Caddy STS - some people just don't know when to draw the line ! :-) ) reckoned the initial running of both the Bird's fans on startup is probably the ECM testing the system.

The odd thing is that there's no SES light indication of a fault (though the lamp does light when the ignition is first switched on) and the ECM scanner didn't pick up the fault when the car went to have the inlet gasket replaced, despite the fact that the boiling up problem was already happening before that.

I will of course check all the things you've suggested, but as far as I am aware there are no signs of any water seeps (even slight ones) anywhere, the belt seems tensioned ok and in good condition and there's no bearing noise that I can hear from any of the pumps and stuff.

The engine is still very quiet and tight at 69,000 miles, (because it's American and not a European or Jap car, which would be due to start smoking around now - you guys build the coolest and the toughest cars on the planet !) and a specialist F-body service dealer here with a great reputation has serviced the car (he did the gasket work) and considered it was in very good condition throughout.

The overheating problem began suddenly, so I think it's a failure of a component or a wire rather than any collection of debris in the radiator fins or whatever - it all looks pretty clean there.

 

The car cooling normal while on the highway and then then heating up after slowing down, sounds like a thermostat. All of the above mentioned items sound logical, but if you say all those are in good shape, my suggestion is to spend the $10 and get a new thermostat. I'm really suggesting the simplest fix, but it won't cost much and you could even get a cooler thermo while your at it.

Eric

 

The thermostat and radiator cap have both been renewed since the cooling problem first made itself known, so I think it's most likely to be a sensor or some problem that's developed with the wiring that is causing the trouble.
But I'll check everything thoroughly.

 

By way of an update...

I located the main and auxiliary cooling fan relays and checked them out with a test meter - the coil shows normal resistance in each case and the contacts work when the relay is operated by pressure on the armature, so I am assuming the relays are working, but will check with 12 volts across the coils tomorrow to be completely certain. That kinda leaves the temperature sensor and fan switch or a wiring problem, I think. Fusible link is intact. Found one poor earth adjacent to the aux fan relay but improving the earth made no difference to the fan problems.

I originally said that both fans ran for a few seconds when the engine was first started, but today that wasn't happening. There is still no reaction from the aux fan when a/c is switched in, so it seems likely both fans are not running at all now, though nothing has happened to them as far as I am aware since the last time I checked a few days back.

I'm still a bit hazy on what the common factor might be here - does the coolant temp sensor (or any other component) affect both fans ? If there's one thing that would disable both, what might it be ?

I'm also still not clear about another thing. If the sensor for the coolant temperature is the (driver's side) one on the front of the block, below the air intake ducting to the inlet, and that sensor feeds the ECM for fan control, where does the temperature guage take its feed from ?

 

I believe that the fans operate independantly of each other. The ECM controls the fan on the driver side and the sensor (on the pass side of the block) controls the other fan. I don't know of anything that would kill them both at the same time, maybe one has tried to compensate for the other not working and blown itself in the process. Perhaps somethingis amiss with the switch for the AC which may not trigger the aux fan. They both could just be old and in need of replacement.


The sensors are as follows: driver side of the block is the sender for the guage, on the front of the manifold is the feed for the ECM (and to an extent, the driver side fan), and the sensor on the pass side is the 230* aux/AC fan.

I can't wait to finally see what the problem is.
Eric

 

All is being focused on the aux system.

The fans arent turning on when its hot.

Check relays (you said you were going too)
Check FUSES!!! (no one has mentioned it?)
To the fans work directly to the 12v?

 

Once again, many thanks for the advice to everyone who's offered suggestions so far.

The fuses in the system are intact - the one behind the battery and the one in the main fusebox. I have checked the main cooling fan relay and it works, still have to check the aux fan relay but so far that is looking like being functional as well.

I am working my way through the diagnostic tests listed in the workshop manual, and although I have omitted one or two tests so far on the grounds of awkward access (I'll do those on a day when I have more time and more daylight !) I suspect that the wiring is checking out ok as far as the main cooling fan. I have seen this fan run briefly at startup within the last three weeks, but not in the last week or so. I'll check the fan by disconnecting it from the harness and directly connecting it to the battery, but I think it's going to prove to be functional. It is looking more and more like the coolant temperature sensor may be out of range, I suspect, as the primary fan is not cutting in before the coolant boils. I will do all the checks on the aux fan system too.

 

what about the water pump? what kind of condition is the coolant in?

 

Coolant is clean - no froth, scum or oil, no particles, (and conversely, no contamination in the oil either, as somebody is bound to ask).

The water pump is not making any undue mechanical noises and is not weeping from the tell-tale hole used to alert owners to a problem. However, the pump hasn't actually been physically checked at this stage. As the problem occurred suddenly - all ok one time and then not the next time I used the car), I suspect it's more lilely to be an electrical fault, nut will check the pump if nothing comes to light with the electrics. But if the pump had failed, would the car stay cool and normal when moving at reasonable speeds, which it does ? Surely it would overheat then as well ?

 

Based on what is happening it's gotta be the fans. 1. it only happens when sitting still, 2. The fans don't turn on!

 

Well, it all turned out to be simpler and cheaper than I expected - the fans ran when I disconnected them from the main harness and put power straight across them from a spare battery, and I couldn't find any faults with the wiring or relays, so I started out by replacing the coolant temperature sensor, and that fixed the problem.

Both fans now operate normally again, with the auxiliary cutting in soonish after air con is turned on and the main fan cutting in just above 220 degrees. She's staying stable at 220 now even if kept idling for a long time, and no longer boiling up after switching off.

Thanks for all the helpful suggestions and encouragement ! I'm very grateful !

 

Whoops - I spoke too soon....

After the replacement of the sensor, I tested the car only by letting her idle for a long while, because that's when the problem was occurring. And she seemed to be absolutely fine, fans working, no overheating.

But I took the car out for the first time after that, just as far as the gas station - less than a mile from here - and she boiled again when I was parking up after returning.

Everything seemed completely normal right up to the time I switched off the engine. Then I heard that familar bubbling sound starting up. I released the hood and looked at the recovery tank. The level was a little above the Cold Full mark, but rose steadily until coolant began flowing out of the cap vent.

I was surprised enough by the overheating not to think about checking what the temperature guage said at that moment or switching the engine back on to see what the fans were doing.

So I guess I either I wasn't tough enough on the original test after replacing the temp sensor or I have an intermittent failure of the fans.

"Back to the drawing board"....

 

Try a new radiator cap,if it don`t keep adequate pressure the liquid will boil at 100*C.
When the circulation stops after shutdown,the temp can rise well above boiling at atmospheric pressure.

 

Thanks - the cap is new, but I guess it could be faulty.

I've also thought of another possible reason why she boiled up again after I replaced the sensor: I have a feeling there was simply too much coolant in the system. When she boiled up at her annual roadworthiness and emissions test (after the guy ran her fairly fast for a few minutes in a garage), she emptied her radiator via the expansion / recovery tank.

The garage guys then refilled the radiator quite enthusiastically, and the level in the rad was right up to the cap, not just up to the bottom of the filler neck. The level in the recovery tank appeared to be right when the engine was cold, but I noticed the fluid was still right up to the radiator cap when I checked in the radiator.

When she overflowed again following her first use after I replaced the coolant temperature sensor, I noticed she wasn't really bubbling and boiling like before, just expanding her coolant too far for the tank to contain it all. I let the car cool right down overnight before I attempted to replace the lost coolant, but I found it had settled back to the Cold Full level again in the recovery tank So I didn't add any and she didn't boil up next time I took her out.

So I think there were two problems: initially, the sensor had failed and then, coincidentally, because the garage guys had topped up the coolant in the radiator too far (and not done it via the recovery tank as they should have done) there was excessive fluid in the system and it expanded more than the tank could contain.

 

Seeing as how you have problems with it boiling, what coolant you are using and what percent mixture?

 

I don't know what the percentage mix is now, as the garage guys at the emissions test place only topped up with water after it ejected the radiator contents, and it could do with having some antifreeze added now. But when the gasket work was done a couple of months ago by a specialist dealer, it would have been refilled with the correct mix of antifreeze and water as recommended by GM. I'd have to check what that is, off the cuff I don't remember, but the guy is very conscientious and always insists on everything being done "by the book".

I really think that the basic problem is solved now the sensor has been replaced, and the car hasn't boiled again since the excess coolant came out. The levels in the recovery tank are now correct for hot and for cold, where before, the cold was right but the hot was going way over.

 

The UK Car Parks and Drives are notoriously not level. Sounds like you may have (or had, from all the work), some air trapped in the system. Find a level pavement, or better yet an area where the nose is slightly raised. Run the car at idle (parked safely) with the rad cap off, watching for coolant flow after the thermostat opens. You should see the coolant level drop in the radiator whenever an air pocket is cleared.

And have the heater on, it is simply another radiator.