1991 5.0 with HCI - started car, running for a few minutes, started pushing coolant into the overflow tank and then puking out from there. I'm lost

Hey all,

I'm having an issue with my 1991. The car has been sitting for a month or two with a leaking power steering high pressure line; I have not had time to fix it but I've been starting the car up every week and letting it get up to temp.

Last weekend, I started it up, everything seemed fine, aside from the leaking power steering line which I kept a close eye on. Car was running for maybe 3 minutes when it started barfing coolant out of the purge tank, which had become completely full. I shut if off immediately. No apparent milkshake in the coolant, didn't have time to mess with anything else. I don't know if it's a stuck thermostat, or if air somehow got in the system (bad radiator cap? Radiator is relatively new). Car had been driving fine before I parked it due to the issue with the power steering leak. It has had a "coolant low" light on intermittently for a while, but I had frequently checked the coolant and the level was fine.

To help in the diagnosis, below is a list of modifications done by the previous owner (according to him)...........sorry in advance for the wall of text……...

1991 Mustang LX 5.0 5-sp. hatchback.

Modifications per previous owner - all done within 10k miles ago (I have omitted suspension and interior mods for obvious reasons):

“Rebuilt” original 302 w/approx. 105k miles on car, 5k miles since rebuild; ARP bolts/hardware used
Ford Racing B303 Cam
SVE cylinder heads
Ford Racing hardened pushrods
Ford Racing hydraulic lifters
Trick Flow RR 1.6
Ported Ford GT40 upper and lower intakes with 1/2 inch phenolic intake plenum spacer
AEM adjustable FPR with liquid-filled pressure gauge
DUI Billet aluminum HD distributor with MSD coil
Summit Ceramic shorty headers
Cat deletes
Smog system deleted
H-pipe with Flowmaster 40 series mufflers
New brake master cylinder with brake booster
New aluminum radiator with overflow tank
New motor mounts (OEM)
Polished aluminum UDP’s (BBK)
Upgraded 3G alternator with wiring
New water pump
New timing gear kit with timing cover
New hoses for heater core and radiator
New heater core and AC evap
BBK fender well cold air intake
BBK 65MM TB with new IAC valve and Ford Racing adjustable IAC plate
New TPS sensor with adjustable plate
New IAT sensor
New oil pump with oil pressure sensor
New fuel pump (OEM) with fuel filter and fuel level sending unit
“Salt and pepper shakers reworked”
OEM computer checked and capacitors replaced
Chrome valve covers (not stock style)
New OEM harmonic balancer
New engine seals, including front and rear main seals
New OEM clutch fan

Since I have had the car, I have done the following:

New Duralast distributor with new TFI
New Accel coil
New AC compressor
New pushrods, guide plates, inspected lifters, and adjusted rockers - apparently there was an issue with the lash/preload adjustment and the
guide plates were gouged out a bit and there was some valve train noise. The entire valve train was inspected professionally, pushrods replaced in an abundance of caution, and rockers adjusted properly.

Car was tuned by Thunder Autosports.
New valve covers (old ones were leaking oil)
Rebuilt rear end again
Computer wiring checked, another new TFI put on
New power steering rack


I don't know what's going on. Should I run the car with the radiator cap off to try to burp air from the system? Stuck thermostat maybe? Head gasket? Cracked head or block?

Seems like just another random problem on a car that I've put a lot of money into and was "built properly" and then had been gone through by various mechanics and professional shops.
 
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KRUISR

5 Year Member
Apr 16, 2015
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I always run the car with the rad cap off when I drain and refill the rad. Basically fill the rad, start the car and let it warm up. Once it gets warm enough for the T-stat to open the coolant level will drop and you will start to see coolant come through the rad tubes into the driver side tank. Let it run this way for a few minutes keeping an eye on the coolant level. If the rad tank drops to almost empty, add more coolant. Keep idling and circulating the fluid and let the level in the rad tank stabilize (could take 15-20 minutes of idling). Once it appears stabilized I top up the rad and put the cap on immediately. Run it a little bit longer and shut off the car. Let everything cool off and then look at overflow and top up to proper level.

This is what I have always done with no issues.
 
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2000xp8

SN Certified Technician
Aug 8, 2003
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I always run the car with the rad cap off when I drain and refill the rad. Basically fill the rad, start the car and let it warm up. Once it gets warm enough for the T-stat to open the coolant level will drop and you will start to see coolant come through the rad tubes into the driver side tank. Let it run this way for a few minutes keeping an eye on the coolant level. If the rad tank drops to almost empty, add more coolant. Keep idling and circulating the fluid and let the level in the rad tank stabilize (could take 15-20 minutes of idling). Once it appears stabilized I top up the rad and put the cap on immediately. Run it a little bit longer and shut off the car. Let everything cool off and then look at overflow and top up to proper level.

This is what I have always done with no issues.
I was under the impression that this is how everyone has always done it.
Wonder if all the air issues with see around here are from people who don't do it this way.
I must have had my radiator out 20+ times over the years, never did i get any air.
I also always have run a mr gaskets 180 thermostat, it's a different design, wonder if it helps with stuck air.
 
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CAMTWO1070

Active Member
Dec 17, 2021
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I was under the impression that this is how everyone has always done it.
Wonder if all the air issues with see around here are from people who don't do it this way.
I must have had my radiator out 20+ times over the years, never did i get any air.
I also always have run a mr gaskets 180 thermostat, it's a different design, wonder if it helps with stuck air.
Letting a car sit will cause possible issues...I found out that I needed to retorque my intake not once,,not twice but 3 times and I used FelPro gaskets not racing gaskets as racing gaskets in a street driven vehicle turn to mush..... ....

Another 3x torquer was the timing chain cover and waterpump too........It seemed I was losing water and I thought my headgasket was bad..

I rented a pressure tester from AutoZone attached it to my radiator and pumped it up to 17lbs and also put a piece of cardboard under my vehicle and went to bed...Woke up looked under and found two small puddles.....One was the front left corner of the intake by the temp sensor for the gauge and the other was at the waterpump next to the PS pump....

I tightened up all of tthe intake and waterpump aswel as timing chain cover bolts which were all loose and were originally torqued with a 1/4" Snap-On Torque wrench to 20ftlbs...........The third time I torqued them with a 3/8" torque wrench to 28ftlbs....

The way I do my rad fills is with a radiator fill kit...

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The funnel attaches to the radiatorcap adapter and you fill the funnel halfway up put the cover on then start the car and turn heater knob to HOT then switch up to high then I rev the engine to 2000 rpm's for a few minutes after the thermostat opens then I release the throttle when the air bubbles are burped out then I take off the apparatus & cap the radiator off and put coolant to proper level in the overflow bottle after the engine and radiator completely cools because the radiator fluid will shrink and when it does the excess fluid in the overflow bottle will suck into the radiator without cracking the cap.

Also what most dont know is that a cap needs to hold its rated pressure ...Reason being is for 1 lb of pressure it holds back the pressure halps lower the boiling point of the solution by a degree or two each lb exerted on the cap,,,,,I run a 15lb pressure cap on my vehicle...

On my vehicle I have a coolant air burp out line that runs from the nipple that used to attach to my throttlebody and ran it directly back to an extra nipple i had on my radiator under the coolant overflow nipple...
 
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