Temp sensors bad?

Stefano, he's already replaced the coolant (and the thermostat) and is still getting the same intermittent high temperature readings. Sensor lag would not account for that.

Barry

 

What I think is happening is there is an air pocket forming in the water outlet so the thermostat is seeing air and closing until the engine revs pick up enough to force coolant to the thermostat.

Personally I think a steam line at the thermostat housing would solve this problem but the cognoscenti I've spoken to all say that a properly bled 550 with good hoses will not overheat and that a steam line is not necessary. I suspect this is true but my concern is slow traffic and long stops where I can see the temperature needle moving up.

I may put on on anyway, after all the cognoscenti were all wrong about the Corvette shock actuator till I proved they will work.

You might try this since its free advice from Dave Helms on how to get rid of air bubble in rear of left cylinder head.

Warm up car to operating temp
Remove cap on header tank (carefully)
Jack up front of car as high as possible
Let sit that way overnight
Re-check coolant level, add as necessary, might take another gallon.

 

I have just recently had my temp sensors replaced and the symptoms were exactly as would be covered by the workshop manual.

the sensors work as a pair whereby the left manifold sensor works the fans normally kicking in at around 90c /194f, if however the right manifold temp sensor sees 105c / 221f or above, it then kicks the fans in as a safety backup.

Whilst there may be other factors, this seems to be the most common issue whereby a car works fine and keeps the temp below 190f when driving, but just idling etc the temp rises, no fans kick in and then just before coolant temp red line the fans come on and drop the temp back down slowly, they then ultimately often turn off if left long enough and if still idling then the process repeats as the temp will start rising again as soon as the fans turn off.

When mine first started happening it was for a short time a sometimes it happened, sometimes it didn't experience. Soon changed to always did it though.

It's worth getting fixed asap, as you are doing, as whilst there's a fail safe in the system, being ali things could get rather expensive and on mine I ended up with a small pressure leak on the rad which even re-coring the rad is none to cheap. Replacing the temp sensors, pipes and gaskets isn't too bad, about 4hrs as a rough guide I suspect and the parts are fairly reasonable in comparative terms.

ps for the cost of the temp sensor (only a few $), I would replace both personally.

 

Dave,

That's a possibility too especially if he didn't fill the cooling system as per the WSM.

Would cycling the HVAC to a heat setting after a long period of time at a cold setting cause an air pocket to form?

Barry

 

Jon,

That seems to sum up what's happening to Chistopher. And although the right "backup" sensor seems to be working, it's best that they both be changed along with the other items mentioned.

Thanks,
Barry

 

Barry, the water pump rotating and just the process of heating water causes the water to out gas air. That air is going to sit trapped in the highest place in the system. The only way for it to get out is with steam lines to the header tank. If you look at it the thermostat housing is the highest place and there is no steam line. Air also collects in the radiator but there is a steam line for that.

Yes, its important to get all the air out of the heating system, but air is constantly being introduced into the whole system by out gassing.

That's one of the reasons F1 cars run at around 50 PSI in the cooling system, higher pressure reduces out gassing caused by pump cavitation and heating.

The pressure shock of combustion in the cylinders also causes out gassing but to a lessor degree. This a problem with diesels with their higher combustion pressures. The out gassing in the water jacket causes erosion which can lead to eventual failure of the cylinder wall.

In short, you can get all the air out but you can't keep new air pockets from forming, you have to provide escape routes for that newly produced air.

 

Thank you for the explanation, Dave.

When I picked up my car, the previous owner briefed me completely. One of the things he advised was to periodically and frequently cycle heating and cooling all year long. He said it would help prevent air pocket formation in the system. Would stagnant coolant in any of the lines lead to out-gassing as you describe?

Barry

ps It's nice to hear you're happy with your new center straight pipes.

 

No, stagnant coolant only looses its anti freeze capacity as far as I know. and thanks for the thought about the center pipes.

In retrospect I probably should have gone Fatso's route, getting the resonator out was easy and it would have been a piece of cake to use its ends with a couple pieces of straight pipe thus keeping the original curved sections and hangers.

 

Can someone please clarify the bleeding process? The WSM say to warm up the car and then bleed . . .is this with the motor running or stopped? Should I expect a lot of air or just some bubbles? I just don't want to get burnt with coolant spray!

The reason I suspect a 'bubble' is that in any 'normal' operating parameter (under 70 f) the car runs @ 165. If I use a little more power it immediately goes to 195. Lighten up on the right foot and the temps come right down again. It never seems to operate in the middle of the stat (new) just at both ends.

Thanks

 

I'm happy to hear this, Cristopher.

Barry

 

Christopher- I love happy endings.

Taz
Terry Phillips

 

Hello,

Based on what I just read here and since my car continues to run hot (after changing all hoses, fluids, cleaning radiator, bleeding, verifying that the fans come on, etc...), I am planning to replace both of the temp sensors. Could somebody please tell me the part number or on what page in the Ricambiamerica catalog I can find these? This help would be greatly appreciated.

Best regards,
Angelo

 

Consider replacing the water temperature transmitter (#39 on that diagram) when replacing the temperature sensors.

 

Christopher & Barry,

Thank you very much.

Best regards,
Angelo

 

The replacement of all the hoses, the three temp sensors, and the intake gaskets (and new o-rings on cleaned injectors) has just been completed on my 1998 550. Upon road testing this work I found the temp gauge does not move off zero, or is in-op, whatever. The harness was absolutely correctly connected to the newly installed water temp (the pricier one) sensor. It's sad that one of the most anticipated things I wanted to see in the car is a temp gauge that isn't constantly seeking the right peg. Now, it is yet again, parked and unusable.

Any ideas? Some kind of reset? Something other than a disconnected connector to the sensor?

Thanks much in advance,

~Troyce

 

Troyce, IIRC, that temperature sensor needs to make a good electrical contact with the manifold, as its case acts as the return signal conductor.

So if you wrapped the threads in teflon tape or other type of thread sealer, you'll get a dodgy signal or none at all.

 

OK, thanks, John. That's exactly it. Damn.

Wonder if I could reach in there and put on a metal between?! Haha.

 

Sorry my friend, but I think you're going to be pulling that intake manifold again.....

Once the manifold is off, you can test my suggestion by putting a jumper from the exposed hex of the sensor/transmitter to the water manifold or block and seeing if you get a more believable reading on the temp gauge.

 

Argh.