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elbekko

Overheating after top end rebuild

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Very strange issue with my P38, 1999 with the Thor/Bosch engine.

Did a rebuild of the top end; new cam, new chain, changed the valve springs with the heads in situ, and pressurising the cylinder with a leakdown tester so the valves wouldn't drop out. Also fitted a new thermostat because the old one seemed to be stuck open.

Started back up smoothly, and kept it at 2k rpm to run in the new cam. After about the 10 minutes, the engine started overheating.

Figured the new thermostat was a dud, so swapped the old one back in (at least it stuck open), still the same.

Couldn't see any flow from the radiator bleed hose, so replaced the water pump today.

After a lot of fiddling to see some flow after that, the same thing still happens. Made sure to fill up the block and radiator, blow the overflow pipe out until it starts flowing, the works.

And still the same thing... slowly creeps up to temperature, then suddenly starts overheating. Top hose is got, rad outlet is cold, water pump inlet is warmish.

So... what the hell? What can be wrong? I'm out of ideas. Except that I killed a head gasket pressurising a cylinder, but I don't see how...

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No coolant in the oil or vice-versa that would indicate a head gasket issue?

Top hose hot, bottom hose cold, that suggests either no water flow or it's actually working fine and your temp gauge is wrong... got an IR thermometer?

I don't know if it's possible to have plugged the temp sensor plug into the wrong thing (perhaps fuel temp sensor?)

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First , I would take the thermostat out altogether & see what happens , then second , I would check for a blocked radiator & if you still have no joy , then third , I would check the water pump again , these I understand are the only things that can stop the water flow , good luck . Regards Paul .

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18 minutes ago, FridgeFreezer said:

No coolant in the oil or vice-versa that would indicate a head gasket issue?

Top hose hot, bottom hose cold, that suggests either no water flow or it's actually working fine and your temp gauge is wrong... got an IR thermometer?

I don't know if it's possible to have plugged the temp sensor plug into the wrong thing (perhaps fuel temp sensor?)

No mixing that I can tell.

Temp gauge could be, but seems to agree with the IR thermometer.

Temp sensor is a special plug that can only be plugged into one thing on the Bosch.

4 minutes ago, P38 Paul said:

First , I would take the thermostat out altogether & see what happens , then second , I would check for a blocked radiator & if you still have no joy , then third , I would check the water pump again , these I understand are the only things that can stop the water flow , good luck . Regards Paul .

Can't take the thermostat out on a P38, well, not easily anyway.

Radiator is free, and block seems to be too, flushed both when replacing the water pump.

What more could be wrong with the pump? It's a brand new item, looked fine, and the one that came out was old a rusty but still seemed to work fine.

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It may be a duff pump with the impeller just spinning on the shaft or the pulley spinning on the shaft , but my gut feeling points to the thermostat that's at fault .

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Well, it wasn't spinning when I tried it before I fit it, I doubt it's under much more load than I can put on it manually.

Thermostat was my gut feeling as well, but it's doing the same thing with a known "good" one...

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5 minutes ago, elbekko said:

Well, it wasn't spinning when I tried it before I fit it, I doubt it's under much more load than I can put on it manually.

Thermostat was my gut feeling as well, but it's doing the same thing with a known "good" one...

Its going to have to come out , its only a few hours work ūüė¨¬†take some sandwiches with you & a hot drink , good luck with this one , regards Paul .

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I agree everything point to the thermostat (including the troubleshooting section in Rave). Still, a new one being bad is one thing, an old one behaving exactly opposite as before to give the same symptoms as the new one, that does sound strange. I checked both the old and new waterpumps as well, we were hoping to find a lose impillor or something, but they seem perfectly fine. And everything has been flushed, so what could be preventing flow??

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34 minutes ago, Escape said:

I agree everything point to the thermostat (including the troubleshooting section in Rave). Still, a new one being bad is one thing, an old one behaving exactly opposite as before to give the same symptoms as the new one, that does sound strange. I checked both the old and new waterpumps as well, we were hoping to find a lose impillor or something, but they seem perfectly fine. And everything has been flushed, so what could be preventing flow??

You are going to have get the spanners out again my friend , good luck .

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Have you got the hoses connected to the right spot on the pig's heart? I think Rave has a diagram for that.... 

The only other thing apart from the suggestions above is perhaps an air lock?

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8 minutes ago, Bowie69 said:

Have you got the hoses connected to the right spot on the pig's heart? I think Rave has a diagram for that.... 

The only other thing apart from the suggestions above is perhaps an air lock?

Triple checked that...

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1 hour ago, P38 Paul said:

You are going to have get the spanners out again my friend , good luck .

Sure, but to do what? I don't think I have the correct hoses to fabricobble a working thermostat replacement, and taking out the water pump is a bit useless if I don't have a known better one.

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Is it really overheating  ie. boiling over, plus the AC fan switching on as it supposed to in an overheat condition, or is it that just the dash temp gauge going into the red ?

You have eliminated any issue with the thermostat by replacement of the old one which is faulty being permanently open, in normal conditions that would mean the the coolant would be slow to get up to temp.

If the water pump isn’t leaking,  plus the impeller and pump bearing are intact there’s no need to replace the pump.

 

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13 minutes ago, teabag said:

Is it really overheating  ie. boiling over, plus the AC fan switching on as it supposed to in an overheat condition, or is it that just the dash temp gauge going into the red ?

You have eliminated any issue with the thermostat by replacement of the old one which is faulty being permanently open, in normal conditions that would mean the the coolant would be slow to get up to temp.

If the water pump isn’t leaking,  plus the impeller and pump bearing are intact there’s no need to replace the pump.

 

Yes, I could hear it boiling in the tank. Didn't cross-check with the ECU, and didn't explicitly notice the aircon fans coming on, but I have no reason to believe the gauge is wrong.

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You could get a long drill bit and make some holes in the centre of the thermostat, that would allow you to discount that.

How are you filling the system?

I know you have the hoses in right place on pigs heart, but what about the front cover?

I had a really odd situation with mine with pigs heart in place, would overheat at 50mph.... I appreciated you can't swap out to a Hotwire style thermostat housing and plumb it up that way, but it solved it for me.

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Yes, I could indeed try to gut the old thermostat, not a bad idea.

Filled the system in multiple ways. First time the normal way through the tank, last times filled block and rad separately through the top hose.

Front cover only has the water pump inlet, rest is on the manifold. Can't really connect those wrong. Heater hoses are also correct.

The one thing I keep doubting about: could I have fit the valley gasket the wrong way around, and messed with the coolant flow that way?

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According to this, it is symmetrical, so no...

https://www.shop4autoparts.net/parts/Engine/ROVER-V8-COMPOSITE-INLET-MANIFOLD-VALLEY-GASKET-SEALS/

Did you use any kind of sealer?

Is there a chance a piece of cellophane is blocking something?

Tried back flushing the rad to see how freely it flows?

I seem to recall someone saying you can fit the serpentine belt so the water pump runs the wrong way, not sure if that is specific to your engine, but worth checking the flat pulleys are on back of belt and grooved on front - apologies if this is teaching you to suck eggs or seems a silly idea. :)

 

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All suggestions welcome :) I did check the belt, and it's on correctly. No sealer used, and can't imagine I left any rags anywhere... but you never know.

Both block and rad flowed freely when putting the hose in.

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Did you check the new gaskets have all the same waterway and oilway holes as the original?  Could be a variation in type, or a production fault, omitting a coolant passage.

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Not explicitly, but also nothing that sprang out. I like to think I would've noticed.

Other than the valley nothing that touched coolant, until I replaced the water pump, and that looked good.

Another route I've been wondering about: an already failing head gasket, disturbed by unbolting and refitting the intake, that now opens up when the engine gets up to temperature? How likely would that be?

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But then also seemingly blocks coolant flow? It is an odd one, I grant you.

Doing the thermostat will prove coolant flow or not I guess.

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I did check the valley gasket. It was flattened a bit during transport, and I worked it back into shape to align all the boltholes and passages before Ben fitted it.

Not convinced a blown head gasket could give the symptoms of low coolant flow. But the temperature sensor is very close to the suspected leak, so maybe a local hot spot before the thermostat gets warm enough to fully open?

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Right, I've been staring at the drawings some more.

P38_THOR_COOLING.jpg.211b51ddcb0c9a56f45

It's just impossible that the thermostat housing is cold, even if it isn't working correctly, because the bypass still goes through there. Which makes the expansion tank boiling also really strange. Either it's going through the heater matrix, or it's sending steam through the manifold bleed hose.

So everything very much still points to a flow issue, but what on earth could be causing it...? Or localized heating that causes stream into the bleed hose, and boiling the tank that way, still an option.

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My experience is more diesel than petrol, so I didn't think I could usefully contribute, but having now seen the diagram ...

I'm pretty sure there was a Tech Bulletin about this, it concerns the thin green radiator bleed hose (16). After draining, if coolant remains in the lowest section, with air in the higher sections at either end, the pipe is too small a bore to self bleed. An air lock then develops in the top of the radiator, inhibiting flow.

The expansion tank will get hot, via the purple expansion pipe (20), but there will still be no flow.

The in service work round was to remove the cap from the expansion tank, remove the bleed hose from the radiator, then blow (lung power only) along the pipe towards the expansion tank. When you heard air issuing from the tank you knew the pipe was clear.

The factory cure was to reroute the pipe along the top of the inner wing instead of dropping down to the 'chassis rail', thus eliminating the giant U bend.

Regards.

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1 hour ago, elbekko said:

Right, I've been staring at the drawings some more.

P38_THOR_COOLING.jpg.211b51ddcb0c9a56f45

It's just impossible that the thermostat housing is cold, even if it isn't working correctly, because the bypass still goes through there. Which makes the expansion tank boiling also really strange. Either it's going through the heater matrix, or it's sending steam through the manifold bleed hose.

So everything very much still points to a flow issue, but what on earth could be causing it...? Or localized heating that causes stream into the bleed hose, and boiling the tank that way, still an option.

Is the top hose hot all the way down to the thermostat ? if so , & the bottom hose is cold the only thing in the way is the thermostat .

If the top hose is cold at the thermostat it could be blocked or collapsed inside therefore restricting the flow , good luck , I feel your pain .

 

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