Overheated 940 won't re-start

[typhoon]

You can dance around the issue all you like, you overheated an engine.
Water in oil or oil in coolant are not the only signs of a headgasket failure. I've had more than one go between cylinders. Not all head gasket failures are sudden, dramatic failures, I bet it's been going for ages, which is a much more typical failure mode. Overheating an engine with an aluminium head that has miles on it usually fails a gasket. Since you've been losing coolant externally for a long time, how would you know if the engine has been losing coolant elsewhere?
Did you know that even a small amount of coolant missing from the system leaves the cylinder head either partially or completely uncooled, causing perfect conditions for head warpage?
Have you performed a compression, leak down or coolant gas test? No? Then get to it and check for a blown gasket.
A sensor is not going to result in a no start, hard start, misfire/backfire maybe but not no start at all.
If you have spark and a starting aid will not fix it, you have either a valve problem or no compression.
As you overheated the engine I am leaning towards no compression. I don't buy the pure coincidence/just happened to blow something electrical same time as engine overheated theory at all.
Any EFI engine will start and run if a sensor has failed (except crank angle sensor) they go into a default mode and assume sensor is dead and substitute a default value.

Regards, Andrew.

[HenryM]

Gentlemen I have the answer!
Having checked through everything except doing a compression test I narrowed it down to the engine temperature sensor, the one used by the FI not the gauge sensor under the inlet manifold below pot No. 2. It's a bitch to get at and as others have written there is almost no slack on the wiring harness to be able to fiddle with it.
If this sensor goes faulty it will fool the engine into thinking the temperature is minus a zillion degrees and dump fuel into the cylinders like it's going out of fashion.
I took out the fuel pump fuse (No.1) and cranked the engine for about 20 seconds to expel any build up of vapour. While still cranking I put the fuse back in and hey presto she fired up. Gently easing my foot down on the throttle to bring the tick over up to about 1500 RPM kept it going and after about a minute I could let it tick over normally. Once warmed up the engine started first time every time.
I haven't replaced the sensor yet but of course will do ASAP.
At least now I can get back to changing that damned radiator with piece of mind and have banished all thoughts of having to change my beloved brick.
I hope this helps everyone else on here that seem to have dead engines on their hands at the moment.
Thank you everyone who has chipped into this link. This forum is invaluable.

[HenryM]

In the last week since getting her running again I have been using the car every day. I have not changed the suspected sensor and only checked the coolant since changing the radiator. I have had no reoccurence of the non starting problem and can detect no difference in the performance of the engine. Smooth tick over, same MPG as before..... weird.
Thank you for your comments Andrew. I agree with your thoughts about head gasket failure and in due course will do the deed but for now there is no sign of failure. I will get a compression test done and let you know what the results are. For now all I can say is...... Volvo have built a damned good car. This is one is certainly living up to the bullet proof reputation.

[oman5]

Hi, glad you have sorted it, I just wanted to add that if you have overheated an engine you should really wait until its cooled before adding fresh coolant. adding cold water to a smoking hot engine is a recipe for disaster (cracked block, rad etc)

[mv10tc]

I have similar problem, Volvo 940 1991/92 (petrol, no air-conditioning, non turbo) overheated, due to small hole appearing at the top of the radiator. Stopped as soon as I noticed, found hot water/steam shooting out. Waited for about 15 min, engine cooled a bit, topped out with about 3 litters of water. Started ok, and got back home about a mile and half away. filled hole with JB weld from outside and put half container of Radweld. run engine as RW recommends, and left until next day. It would not start again.
Code 121, so took MAF unit, I could see tiny wires inside the cage, sprayed it with electric contact cleaner. It started OK, did short sopping run, but for going back (engine still warm) it started only when I kept foot off accelerator pedal (as if engine is getting flooded). Idling very smooth, better than before the overheating incident.
Fine, but early next morning refused to start again. Tried 10 hours later, it started ok on the second try.
Battery is 5 yr guaranty but about 4-5 years old.

[TonyS9]

Topping up with water only will cause Al-Fe corrosion between the head and damage the head gasket, however it doesn't usually cause it not to start. Usually just one cyl will be leaking and be reluctant to start, the others should go ok and it will just run on 3 cyls until the other is dry enough.

Have you checked you are getting sparks and fuel?

[mv10tc]

Hi Tony. Thanks for replying, it is appreciated.
After turning key to II little red light inside fuel gauge comes on for about 4-5 sec and goes out. Turning key off to I and turning back to II it comes on but for less than second. On both occasions fuel pump/injector relay (far left, row two relay board clicks @ finger touch test), so I assume that pump works and fuel line pressure is OK.
Tomorrow I'm borrowing a spark test-lamp (to be connected between HT lead and a spark plug) so will report back, but according to previous experience it might start with not much of a problem, but never had start failure in past.
Put battery on charger, despite green colour in the round window on it , It's YUASA about 4 years old (with 5 yr guaranty).
Forgot to add: Engine oil on dipstick is deep black colour (assume no water entered that side) and no floating oil in the expansion tank. Looked at the exhaust gas with white piece of paper, looks clean with tiny bit of condensation/damp held for 15-20 sec next to tail pipe.

[kiloran]

It's the season for exhaust condensation so don't let that lead you down the path of worrying about water going where it shouldn't.
Have you tried pulling Fuse 1? When I had hot start issues (as a prelude to an HG failure several weeks later) I found that it usually resolved it and the car would start again.

[mv10tc]

thanks, the first thing I did check fuel pump fuse (1), it is Ok, will try for hot start, but keeping foot away from pedal does it.
Changing head gasket is expensive job, I had some preliminary over the phone quotes up to £800.

[TonyS9]

Originally Posted by mv10tc

Hi Tony. Thanks for replying, it is appreciated.
After turning key to II little red light inside fuel gauge comes on for about 4-5 sec and goes out. Turning key off to I and turning back to II it comes on but for less than second. On both occasions fuel pump/injector relay (far left, row two relay board clicks @ finger touch test), so I assume that pump works and fuel line pressure is OK.
Tomorrow I'm borrowing a spark test-lamp (to be connected between HT lead and a spark plug) so will report back, but according to previous experience it might start with not much of a problem, but never had start failure in past.
Put battery on charger, despite green colour in the round window on it , It's YUASA about 4 years old (with 5 yr guaranty).
Forgot to add: Engine oil on dipstick is deep black colour (assume no water entered that side) and no floating oil in the expansion tank. Looked at the exhaust gas with white piece of paper, looks clean with tiny bit of condensation/damp held for 15-20 sec next to tail pipe.

Red light in the fuel guage? This is a reserve indicator for the fuel level. as far as I am aware its not any indication of pump or pressure level. A pump voltage light that comes on when the pump voltage is low would be a useful fault indicator, using this and the rpm needle you can narrow down many faults.