I had the shop replace my waterpump on my 94 aerostar 4.0 he also flushed it out put a new thermostat etc etc. I noticed today while driving on the free way the guage was spiking from the n all the way over the the m in NORMAL and then it would go back just as fast. So I'm not sure what's happening there?? Also after we got it back I've been noticing a small knocking sound(just a little louder than a lifter, But seems to be a knock) I was getting worried it may be a rod. But with the temp guage going haywire I wonder if it could be the waterpump making noise???????????????

 

I don't know what's making all the noise but the erratic temp gauge might be the result of air trapped in the system. Here’s a thread that addresses that issue.
https://www.ford-trucks.com/forums/s...d.php?t=470737

 

Water pumps don't usually make a knocking noise. Is the noise there all the time? or does it go away when the engine is warmed up? rod bearing noise would get worse as engine gets hotter and lifter noise would decrese, usually anyway. Could be cavitation in water pump if there is air in the system.

 

I changed my tstat and wp out about 4 years ago. (96,4.0,2wd,ext) I noticed that I can "watch" it open and close as the water temp goes up, and then down again, after a pass thru the radiator. This is only when the temp outside is moderate to cool, say mid to lower 80's and below, or at night when it is cooler. When it is real hot outside it will open up one time and stay open. I guess the heat load from the engine is more balanced to the available cooling from ambient air/radiator/fan combination, at higher outside temps. Mine does not spike, per se, but it is obvious that it is opening as the fluid heats, and then closing as the fluid cools off. You can plainly see the needle move. As I recall, it cycles to about the "O" and down again to just under the "N". And it will do so repeatedly. My radiator runs full with no leaks. My tstat is cheap aftermarket, so maybe it does not modulate as well as OEM, but I have also noticed the same phenom in a Voyager I had years ago. I also just replaced my radiator, a few months back, and it seemed to be more noticeable. After the flush your cooling system may be working much better. Sudden rise in temp followed by a sudden cool down. Don't know if anyone else has seen this or not. One other thing I would hold suspect is the fan clutch may not be working just right. And just FYI, when I replaced my tstat I also examined the fan blades---visible cracks where the blade transitions to the hub. I replaced it (fan blade only) also.
The knock....hmmmm? I know I also replaced my tensioner/idler pulley at the same time. Just precautionary. Maybe noise could come from there.

 

I looked under the hood there is no knock around thefront of the engine. I can hear it when I bend down and listen under the van which makes me a little nervous, BUT it doesn't seem to be doing it constantly at times I can't hear the knock so I'm not sure what's going on

 

hi 9d4,

aftermarket import low cost tstats are notorious for uneven operation and sticking...most on forum only use FoMoCo Motorcraft original or Stants premium stainless steel....smooth reliable operation....
betcha the mechanic did not get all the air out of coolant system...gota belch the baby
park on slope with van nose up....let engine cool...fill coolant to full...make sure overflow tank coolant is up to cold line and line has no plugs and no leaks or cracks, us wire tie at neck connection to cinck up tube connection....

start with new radiator cap....put on to first click, half turn or so....run engine in place until warm, 20 minutes....goose throttle several times to 2,000 rpm....tighten cap to full lock...air should be cleared

is your upper radiator hose collapsing while engine running hot or when engine cools off?

rod knock can only be diagnosed by pan removal and plastigage of bearing to journal clearance of all six...
search forum for using section of garden hose to listen for engine noises

 

TStat is a cheap part and easy to replace, so if I have any suggestions, that my TStat is bad I replace it. But my TStat was replaced twice! I had to replace Tstats on other cars. Sometimes stuck fan clutch may cause similar problem, but you can hear loud sound of fan, like sound of taking off helicopter! Sometimes this problemes appeare together!

 

I noticed my temp guage going haywire when turning the key to the run position. It was reading a little cool when running. I changed the sending unit in about 10 mins and fixed it completely. Have the new sending unit ready when old one is removed. I got the Motorcraft one with the sealant already on it. Wouldn't hurt to change.

P.S. That knocking may be coming from the exhaust also.