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2004 25 1.6
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45 Posts
Discussion Starter #1
Hi
Can anyone tells me what the heater bypass valve does on my year 2004 K series 25?
as in what is the point of it?
and what are the symptoms of failure?

When I got mine a few weeks ago there was no coolant in the header tank.
Once I refilled it and got it running the heater didn't work.
Eventually after getting the engine warm and then driving to round the block in 1st gear (5000rpm) the heater started working.
The current status is that after a long delay in traffic (hardly moved for 20 minutes) the heater stopped working again, but then once the traffic was moving the heater then worked again.
Specifically I mean that the blower motor was working, but that the air coming out of the vents goes cold.
Anyway I am wondering if this is symptomatic of a dodgy bypass valve, or whether the head gasket is on its way out and it's filling up the heater matrix with exhaust gasses until the engine is that revved up to purge them out.
The car doesn't overheat or push all the water out but I don't like the look of the oil as it looks very slightly emulsified to me.
thanks
 

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Now the coolant has been topped up and bled, is the level staying stable? Is the oil really emulsified? I’ve seen minor emulsification on cars that have stood for a while and had condensation form.

I bought my 25 a couple of years ago. The state of the coolant was disgusting and the heater didn’t get hot. I first suspected HGF, but got away with replacing the IMG, flushing the coolant throughly, and replacing the Saab valve. The messy coolant was probably because it was not properly flushed after previous work. If you are not losing coolant now, you probably don’t have HGF.

See link Regarding Saab valve:
 

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Registered
2004 25 1.6
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45 Posts
Discussion Starter #3
Now the coolant has been topped up and bled, is the level staying stable? Is the oil really emulsified? I’ve seen minor emulsification on cars that have stood for a while and had condensation form.

I bought my 25 a couple of years ago. The state of the coolant was disgusting and the heater didn’t get hot. I first suspected HGF, but got away with replacing the IMG, flushing the coolant throughly, and replacing the Saab valve. The messy coolant was probably because it was not properly flushed after previous work. If you are not losing coolant now, you probably don’t have HGF.

See link Regarding Saab valve:
It doesn't overheat
It doesn't push coolant out
I haven't driven it far enough to know how much coolant it does or doesn't use (and in any case coolant loss can be any number of places).
A mechanic friend looked at the oil and said it's okay, but I think it's a bit milky.
The oil level isn't rising though.
Thank you for the reply.
 

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'06 MG ZR +120 (HQM) '04 MG ZR 105 (IAB)
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9,212 Posts
.....or whether the head gasket is on its way out and it's filling up the heater matrix with exhaust gasses until the engine is that revved up to purge them out.
Not likely to be honest. If the gasket had failed at a fire ring and was allowing exhaust gases to be forced into the coolant, the gases would accumulate in the head and be discharged to the expansion tank via the jiggle valve at the dipstick end of the inlet manifold (that is basically what the jiggle valve is there for, to allow trapped gases to vent to the expansion tank when refilling/bleeding the coolant system whilst keeping the coolant in the head, and thus alleviate the possibility of an air lock). If you have exhaust gases being forced into the coolant, you will generally find that there is still noticeable residual pressure in the coolant system, even when the engine has be allowed to cool completely, and there is a small but distinct hiss when loosening the expansion tank cap.

The heater symptoms you describe could certainly be a failure of the Ranco/SAAB bypass valve. It was fitted to improve flow of hot coolant back to the thermostat when the cabin heater valve is set to cold/off - there was already a bypass passage between the heater inlet and outlet pipes through the thick rubber block between the heater inlet and outlet hoses, but MGR were getting increasingly desperate to find something; anything, that may help to reduce the incidence of HGF, and it was felt that perhaps the flow through the very narrow passage within the rubber block was insufficient to help the thermostat to open soon enough and the Ranco valve was added to the system to improve this flow. With the heater on cold, the plunger in the Ranco valve is forced open against its spring to allow hot coolant to circulate to the thermostat, and when the cabin heater valve is set hot, it relieves the pressure against the plunger and it closes under spring pressure to make the flow go through the heater matrix instead.

In practice, as with another knee-jerk modification that MGR fitted to K series installations in the TF and 75/ZT (the remote PRT thermostat) there is no particular evidence that it made any measurable difference to the incidence of HGF, and quite a few people have actually retro-fitted the earlier hoses from older 200/25/ZRs in breakers yards to delete the Ranco valve as these do seem to fail very easily - the big problem is that the failure happens because the centre of the plunger breaks away, and can float of down the pipe work and get into the thermostat where, if your lucky, it jams the thermostat permanently open. If your unlucky, it might get past the thermostat and possibly jam/damage the water pump or block a coolant passageway in the head (with obvious disastrous results).

It is fairly easy to repair however - the valve assembly can be removed from the hoses and the case dismantled (it is in two parts and clips together, although getting the parts unclipped can be a fiddle without causing damage). Once apart, you will see immediately if the middle part of the plunger is missing. Once you have found the missing part and removed what remains may still be in place, the valve can be repaired by simply using a 3/8" ball valve washer (same type as found in the ball valve of most toilet cisterns and household header tanks). I put one of these in the Ranco valve of my Mk2 ZR about 7 years ago as I had this problem (with very similar symptoms), and it has been no problem since.
 
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