'interesting' temp gauge readings

[Romulus]

the engine temp gauge on my P38 is behaving rather 'interestingly' as of late.
After about 5-10 minutes running, with engine already warm when I take out, it starts to creep up and wanders a bit. With the A/C on, in wanders up a bit more and just goes past the last 'long' white marker on the gauge, but again tends to wander a bit, when stopped at a set of lights, it does go a bit higher.
As soon as I switch the A/C off, it immediately drops about one notch and wanders a bit, and I mean IMMEDIATELY. As soon as I switch the A/C on, it immediately goes up about a notch again and starts to wander.
After driving with the A/C off for a while, it does drop a bit but still wanders anything up to a notch or so.
Now, I have swapped temp sensor on the plenum, no change. Water level is good, there are no leaks. It does not feel any hotter under the bonnet than usual.
Could it be a dodgy header tank cap, I do not have a spare one to try it out but will get one I think, or could it be dodgy electronics, bad earth?
Any input would be most appreciated, thanks.

[Scouse]

I'd be checking/cleaning all the earth points you can find to start with, especially in the engine bay.

[Romulus]

I think I solved the problem....must have as it has not done it again....the higher than usually heat I think was due to an 'air lock' or similar, I did replace the temp sender and by doing so water did come out of course and I think that created an air pocket.
I removed the sender again, letting water out once again, also with the header tank cap off, then I squeezed the hoses to try and get rid of any possible air pocket, there was a lot of bubbling and gurgling going on when I was squeezing the hoses, I then let it settle for a while, topped up the header tank and its been fine since then, so here're a set of crossed fingers!!

[Scouse]

I too have found that they can be a right pain to bleed. I've found that having the car facing uphill helps (maybe that's just my car though).

However, that doesn't explain your rapid guage fluctuations.

[Romulus]

I too have found that they can be a right pain to bleed. I've found that having the car facing uphill helps (maybe that's just my car though).

However, that doesn't explain your rapid guage fluctuations.

yes very true...it could still be a bad earth problem that is intermittent....great!?!?!

[karlz]

Head gasket? Hopefully not.

[Romulus]

hhhmmmm....don't think so, oil looks clean, no traces of water.....

[Romulus]

just an update...after eliminating the 'air lock', things did work better, but I noticed that the gauge would sometimes still move erratically.....I then swapped the pressure cap on the header tank and it now seems to be all OK...I drove during the last weekend when the heat here in Melbourne was severe to say the least with aircon running, no more erratic movements and steady on the halfway mark....so it looks as though a combo of a possible air lock and a dodgy pressure cap

[rofosixone]

i had same problem once with temp gauge ,once outside temp showed 28 or higher on your climate control lcd display in cabin next to gearshift it would go into the red ,but i know use to sit between 1/4 and 1/2 .again like you had no heating isues (never had ),but had slow starting problem it was time for new battery .put in a new battery same slow turnover ,harry at bossley park auto eletrical put a new earth cable from battery to alternator bracket,but left original earth cables alone (have extra dual earth now)after that extra earth cable temp gauge sits between 1/4 (cold days ) under 1/2 (hot days , running frount twin fans when air con swiched on ,makes hell of a difference cooling ) so now back to normal and as a bonus starter motor spins faster and car feels faster ( computor not starved of electical current now ) so if you still have bad readings do what i did worked for me ,cheers.

[p38arover]

Ralph, you mention your electric fans run. Mine never do any more. I've only ever seen them on a few times.

I suspect the trinary switch in the air con but it's a lot of money to replace it and, if wrong, it would be a waste.

[rofosixone]

what i found out is those 2 fans come only on demand from ecm only when air is on and the head presure gets to a point that is high hence my fans never came on except when burst radiator (got the signal only then ,defeats the purpose to late then ),i too like you thought it was those 2 swiches went to mascot saw anthony got the two of them ,harry changed them no difference (unless the fans get signal from burst radiator again bah blah blah ) harry just crimped those two yellow wires nothing else (see my report else were here) now when i swich air fan on that criping of those 2 yellow wires joining them together makes the fans work by bypassing the ecm signal and those 2 swiches your talking about and they work half speed. if you unplug and pull out the lower swich and join wires together like george did at land vehicle spares you get fans working full speed and a book note signal on your lcd air con screen .i found leaveing plug in and criping makes for lower battery comsuption as fans work half speed quite good enoth i found , and no book note on your screen /why agravate the computor just trick it .as said did change those 2 ,wasted my money .just crimp and costs you only for that plastic with metal clip .and on cold days or going through deep water over the bonnet like last lroc trip chrismas at geehi just swich off air fan , then you get no splashing from fans to the motor ,good trick ah cheers .

[Romulus]

so far its been behaving itself no probs, but will keep that earth strap in mind if it should play up again.
I am getting my 4.6 rebuilt/recon, its done 318,000 or so, tappets are getting a bit noisy and oil pressure is a bit down, so decided to have a complete rebuild, bearings, rings, cam, heads etc., so this ought to clean up any bad earth joints as well as giving me a few more years of trouble free running.