hi5.0

OK, I've been chasing a problem for quite awhile with my car. I suspect the cooling system was neglected by the previous owner, but it ran fine with the factory thermostat, aluminum radiator, original water pump, and FAL S-blade fan/VSC combo I installed shortly afterward until the radiator started leaking. Replaced most of those parts. Noticed after having a new water pump installed - according to my factory temp. gauge the needle swings toward the "N" in "NORMAL" and then gradually settles back down between the "M" and "A" until it repeats again every 10+ min or so. Doesn't matter what kind of driving or speed - it will behave normally and then just spike. You wouldn't know something was amiss unless you kept an eye on the temp. gauge and listened to the fan - as the vehicle's behavior and the rest of the other gauges indicate nothing is wrong. Found signs the coolant appears to be boiling as the underside of the reservoir cap is wet and sometimes it pukes up enough to get into the battery tray. I can also hear the electric fan working hard idling in traffic when a temp. spike occurs. Replaced the temp. sender, already burped the system many times, changed radiator caps, tested for the presence of combustion gases in the cooling system, and coolant in the oil which came up negative. Exhaust out the tail pipes is normal as well. The system has been reversed flushed and every time the coolant has been changed (at least 3x in the past yr.), it came out clean. At this point, I'm stumped.

FWIW, the car has FRPP silicone hoses, a Fluidyne radiator (2nd one), Milodon water pump (for a 5.0 serpentine belt application), 180 degree thermostat, and a S-blade electric fan modified to fit the stock shroud controlled by a Delta Current Control FK35 set at the default activation setting (180 deg.). Any clues or insights would be appreciated.

jacostang

I know you replaced the thermostat but it sounds like its sticking.... I had one brand new that was doing that...

yugoboss

Sounds like a Head Gasket to me, My old mercury cougar did the exact same thing. I would still try changing
the thermostat first.

Red Jay

maybe something is blocking airflow to the engine in the air intake system? just a guess, thats what the problem was with this ladys dodge neon who works at the tech place i go to everyday.

825LTRGT

I've had several Fox bodies that have a similar issue..all of the gauges move at one time. Fuel, water temp, oil press. Is it one or all of the gauges? That seems to be an electrical issue.

As to the wet cap, did you purge the cooling system? I've noticed when I put a little too much coolant back in the pressure goes up and it pukes.

hi5.0

Thanks for the ideas so far. Hmm, as much as I hate to say it, the head gasket sounds like the most probable reason. Dang it. Even so, knowing my luck, getting 2 bad thermostats (different brands) in a row could not be out of the question. I did check the lower bumper area for air flow obstructions when changing radiators - it's clear. Regarding the gauges, only the temp. gauge shows the most activity. Voltage gauge responds normally to electrical load (lights on, fan running full blast, etc.). The fuel level gauge drops steadily as I drive it . And oil pressure seems constant. I've heard the gauge clusters on these are similar to the 1st gens with the IVR or something like that, where if the IVR fails all the gauges act funny. Doesn't seem to be the case here. But can always try just to be sure since I have a mechanical temp gauge sitting in the trunk. Gonna be funny routing the capillary tubing through the cowl hood opening and zip tying that gauge to the cowl grille for that ghetto-install look.
Absolute worst case would be cooling passage deposits/crud built up restricting coolant flow in the block as others I asked have suggested. Ugh. In any case, looks like my car will be getting a bunch of "presents" this X-mas...