91 VN strange overheating???

[ice_cold17]

I have a 1991 VN series 2 executive,standard 3.8,car usually runs at perfect temps,however randomly the temp guage will rise past the "H" at a very fast rate...too fast,when i check it though there is nothing bubbling,no steam,idling fine,all checks out,wierd right?

then for no reason the guage goes back to normal as quickly as it went up,ive checked the radiator,the cap,it has coolant (slight leak at times but not enough to cause this),the fans come on at optimum temp,new thermostat,bled the line,had the sensor checked and its fine,no signs of a blown head or gasket,its got me and 3 other people stumped,the 3 others are mechanics,any ideas? im worried that one day im just gona think its just being stupid then it actually cooks it

[E-T-H-A-N]

I HAVE THE EXACT SAME PROBLEM!!!!! with my vp. i have no fricken idea what the hell it is, ive asked a few radiator specialist's and they are stumped to. if someone knows what the problem is could you please help us out!

[ice_cold17]

thank god im not the only one thought my car was just a retard,im thinking its the guage but cant be sure without ripping it apart,but if u check the sensor by grounding it that should basically tell u if the guage is responding properly

[E-T-H-A-N]

ohh alright, i was thinking the sendor unit might be to blame aswell. i have to remove my alt to acess the sendor dont i ?

[mr calais 5.0]

I would put a new temp sensor in and see how that goes.. Mine would not move then I took the thermostat in and gets to about a 3rd.. Old was stuck 1/4 open all the time..

[poyter]

i would take out the thermostat and then try it again

[Pickle']

i'd be trying the thermostat, but thats just me. even if you just take it out and run for a few days.

You dont want to cook your engine because you thought it was electrical.

[ice_cold17]

ive got a new thermostat in does that make a difference,and i know it opens coz when its running normally coolant flows :S

[Pickle']

ive got a new thermostat in does that make a difference,and i know it opens coz when its running normally coolant flows :S

Did you bleed it? (dont know if this would cause overheating)

[Wombles]

But a thermostat can come faulty..
Could the sensor be screwed? Or the radiators blocked?
Just go over it.
Could the punp be playing up?

[mr calais 5.0]

A new temp sensor is only 15 buck I think from memory, that would be my first try and you can take out the thermostat and put it in a pot and boil it to see if it works or not.. Does the heater work quick as well?

[ice_cold17]

its been bled,yea might have to just go over everything,heater works fine,thing is the thermostat opens coz there is flow and the guage operates correctly most of the time,can a thermostat not work at times,or is it a "either it works or dosent" situation?

[mr calais 5.0]

sometimes they dont open all the way.. have you tried it in a pot of boiling water to see if it opens all the way? How long since a cooling system flush and collant change as well..

[ice_cold17]

no i never tried that,i might have to take it out and test it,well ive only had the car a few months so i havent done it,ive topped up coolant when it was needed,think a good flush would help?

[92 VP SS]

Yes flush it good when you take out the thermostat and test it. Flush back wards against the direction of the flow, with the heater in the on position. Just boil it in a pot, before it boils the thermostat should open about 10mm.

Note when you put it all back together and refill the system it takes a few goes to ensure there are no air locks, so keep checking your system each time it cools down for a few days. Also always ensure the resivoir tank is full.

**Its a good idea to check your thermostat even if its new just to make sure. Peace of mind.

[mr calais 5.0]

yes a good idea to have the heater tap on and clean the res bottle as well.. Oh and make sure you use the same coolant in the bottles and dont mix it.. just cos its green its not all the same..

[ice_cold17]

yea thanks,think this could really be the problem? im not seeing anything going on in the engine bay when the guage goes up though,like no steam or bubbling or nothing,which is what made me think that its the sensor in the first place

[bcw61]

yea thanks,think this could really be the problem? im not seeing anything going on in the engine bay when the guage goes up though,like no steam or bubbling or nothing,which is what made me think that its the sensor in the first place

Almost tempted to say that it could be a sticking or intermittent thermostat. Had similar issues in my old VN. I would replace the thermostat on spec. If nothing else, it would eliminate one item to chase.

BTW, I would NEVER EVER run an engine without a thermostat - full stop. Been there, done that, with a 'professionally' reconditioned engine that I didn't know was running without a thermostat & subsequently overheated.

[mr calais 5.0]

I too alway have a thermostat in the motor and when i get a new car to me the first thing i do is change it and the coolant and then i know im good for a while..JUst thinking here but are the temp guage earthed or the sensor in the motor?

[Morton]

Sudden inexplicable overheating is not all that uncommon.

1) air bubbles. If you get a rapidly expanding pocket of air in the top of the manifold, it can cause an interruption of coolant flow and subsequently a spike in temperature.
- short term, leave the car running (do NOT turn it off) and squeeze the top radiator hose near the thermostat. Give it a series of hard pumps like an air pump on those blood pressure bands, and try to move the water around enough to get it flowing again
- long term, have the system bled properly (either with a pressure fitting like a hose or bottle on the radiator cap, or through a bleeder screw)

2) thermostat sticking. Even new thermostats can stick, don't discount the thermostat just because you replaced it last week.
- short term, again squeezing the top hose to get water moving through the thermostat can help unstick it.
- long term, try a thermostat from a different auto store (hope for a different batch of thermostats) or go to another retail chain for a different brand.

[VNVacationer]

Hows the radiator hoses condition, maybe one is collapsing with the suction and restricting the flow and when you stop to look the revs are down and it opens back up so everything looks good. There cheap enough to change to be on the safe side.

[Circlotron]

Flush back wards against the direction of the flow, with the heater in the on position.

With the engine not running is there enough vacuum left in the system to hold the heater tap open while you flush it?