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P38 Intermittent Overheating


Tex Gore
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There's a number of posts on this I know which I have read, but none seem to quite suit my specific situation. Here's what happens to my 1995 P38 4.6:

  • RR has never overheated before. Not had it long though, and not done anything other than long(ish) runs, maintaining at least 30Mph for the most part - no stop/start traffic
  • Got into stop/start traffic today, and after 15 minutes noticed the temp gauge had climbed into the red (between the last white mark and first red one) and the warning light was on
  • No 'hot' smell inside the car or under the bonnet particularly, did not seem any hotter than usual
  • Fan looked to be operating Ok although a slight squeaking noise (not quite like a belt slipping)
  • Nothing unusual under the oil filler cap (no gunk of any kind, no white/yellow etc.)
  • Stopped car - after 5 minutes tried to remove expansion tank cap and lots of pink fluid started coming out so abandoned that idea
  • Temperature dropped, set off back home
  • Was showing signs of nudging back up the gauge in stop/start traffic
  • On motorway, back to 12o'clock on temp gauge as normal
  • Got home and ran it stationary - tried it in drive with footbrake applied (to similar being stopped at a junction) - temp gauge stayed at 12'oclock for around 10 minutes (was going up much faster before)
  • No white smoke
  • No water loss from the system
  • Appears to be running and starting fine, although sometimes a bit rough when idling
  • After 30 minutes removed expansion tank cap and started car. Constant stream of water from the thin hose that comes from the top of the radiator, bubbles in the water (but could just have been the stream of water hitting it)

I'm not overly experienced with P38s (Series 3s and Disco 1s are more my thing) so am wondering all kinds of things - silted up radiator? Failed head gasket? Or (shudder) slipped liner? I have a CO tester - not sure if I can use that somehow to test?

Any thoughts very much appreciated,

Thanks

TG

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I didn't want to read this thread, as the causes can be many, but you have included enough detail for me to feel instinctively 'viscous fan coupling'.

They can be tested mechanically, comparing how stiff it is to turn after being left overnight (stiff), to how stiff it is after 5 minutes idling (much less stiff), to how stiff it is when the temperature is normal (not stiff), to how stiff it is when the temperature is nudging the red (stiff as after overnight).

BUT, this depends of you judging how stiff is stiff, without any previous experience.

When stiff, you will hear a lot of air movement. Even in the car, with windows and bonnet shut, lift the engine to 2000 rpm and you should hear the wind roar from a fan driven by a stiff coupling.

Simpler, perhaps, to say wait until it gets hotter than normal, then wind the HEVAC temperatures and fans up to maximum, when you hope to see the temperature drop.

Oh, open at least the trailing edge of the sunroof, and direct all the air at the windscreen. This ensures the best throughput of hot air.

HTH, and that I'm right :-)

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Overheating at low revs seems to point to either insufficient coolant flow or a weak viscofan.

I find the easiest way to test the fan is to get it the engine to operating temperature and let a piece of carboard touch the fan (DO BE CAREFULL!). A good fan will slap away the cardboard, a worn one will slow down or even stop.

As for flow: (partially) blocked radiator, bad waterpump (worn blades can cause low flow at idle) or bad/blocked thermostat.

I've had a similar experience with my GF's 4.0, it would run fine on normal runs, but would occasionally overheat rapidly when stuck in traffic. Increasing revs to fast idle (1500rpm or so) would see the temperature drop again. When testing at the shop, it would happily idle for hours on end (even just after returning). Changed the thermostat, noticed the operating temperature was a bit lower and more 'lively', i.e. it would rise or fall about half a mark depinding on the conditions.

No overheating since, hope it will stay that way during summer as well.

Greetz,

Filip

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All,

Some further thought and conversations last night made me wonder about the fan VCU as well - so very glad to see it suggested here also. As soon as any overheating presents itself the inexperienced mind starts fretting over HGs (which are at least affordable and DIYable) and slippy liners (which are not) but then that's because the P in P38 ends up standing for Paranoid doesn't it.

So thanks all for the info - will check out the fan today, then move on to sensors, 'stat, pump and rad.

I'm assuming then that the small constant flow of water from that thin top pipe back into the expansion tank is normal?

Cheers

TG

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Tex..

The small pipe flow you talk of is normal... It's there to help balance out around the closed stat when cold, get air out of the system and back to the header etc. It always has a flow of water.

Neil

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Having had a mechanic take a look at it and suggest the fan VCU was too loose we replaced it yesterday. Unfortunately that wasn't the cause, as today on the motorway at a steady 70Mph the temperature needle climbed up and the warning light illuminated. I stopped and let it cool, as doing this on Friday seemed to be enough to stop it happening again on the trip, however this time it didn't and I had to stop twice more before it settled down. I had to maintain a steady 50 as it was past 12 o'clock for most of the way back home.

Next we flushed out the rad with radflush stuff and plenty of clean water, and replaced the thermostat. Took it out for a run and it started overheating again, once it got to temperature and on a fast, constant run.

We removed all the spark plugs and they were all the right colour - chocolate brown with no evidence at all of steam cleaning or rust.

I have noticed that the exhaust smokes a little all the time - difficult to say what colour, but appears a greyish white. I note also that the exhaust drips water every 10 seconds or so (or at least a liquid that has no colour or smell).

Starting to worry about HGs and slippy liners now..... Any more thoughts anyone?

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Addendum - hoping we have found the cause. Still a bit worried about the water dripping from the exhaust and the constant white/grey smoke from the exhaust (not much of it so hard to say what colour), however after a good run (where the needle started climbing) we found that whereas the edges of the rad were hot, the middle was stone cold. So there is certainly a rad issue (lots of flushing then filling with hot water made no difference) and I'm told this typically results in limited impact around town but overheating under load/ragging.

So I guess, unless anyone has a neat trick it's time for a new rad. Shame I've already been through a new VCU and 'stat, but you live and learn don't you.

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