• Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech

Water pump issue at idle

  • Thread starter Thread starter FirstFox89LX
  • Start date Start date Jan 8, 2020
F

FirstFox89LX

Member
Oct 20, 2019
9
1
13
Military, so constantly changing
Jan 8, 2020
#1
  • Jan 8, 2020
  • #1
Hey guys,
So yesterday I was trying to see how well my factory coolant temp gauge was working. I fired the car up and had it sitting out in the driveway idling for a while. After maybe 8-10 minutes of mostly idling, a few quick revs in that time as well.
I noticed a nice puddle reading out from my front tire, so I open the hood and see it’s coolant, coming from the cap of my makeshift coolant overflow bottle.
Turn the car off and the coolant level in the “overflow” bottle goes down fairly quickly, back up the tube and into the radiator. Open the radiator cap and the fluid is barely lukewarm, cap isn’t even warm to the touch.
Fire the car up, cap off, fluid does not seem to be moving around, after a minute if idling fluid level just rises back up and out the cap(or into the overflow if the cap is put back on)
If you rev the car the fluid get sucked out of the overflow and through the system, but at idle it starts to push into the overflow again.
If held above idle, probably 1700-2k rpm the overflow bottle is emptied (as far down as the hose going into it) and seems to be pumping the fluid without an issue.
The temp gauge, I do not think is working accurately, but the motor didn’t get real hot through the whole time of messing with it.
The water pump went out toward the end of august 2019, it got a new waterpump,190* thermostat, and radiator since I was already in it changing the water pump. Did not notice any issue between end of august and yesterday with the cooling system. But also never had it sitting just idling for an extended period of time.
Should the car be able to idle for a while and not push the coolant out of the overflow?
I have an 89 lx, 302/t5 car. Supposed to be a mild cam, short tube/exhaust 3.73 car.
 

Wayne Waldrep

Before I post a pic, do you have one of yours?
20+ Year Stangneter
Apr 14, 2003
1,338
659
143
Cuba, AL
Jan 8, 2020
#2
  • Jan 8, 2020
  • #2
Do you have the exact part number for the waterpump?
 
F

FirstFox89LX

Member
Oct 20, 2019
9
1
13
Military, so constantly changing
Jan 9, 2020
#3
  • Jan 9, 2020
  • #3
Wayne Waldrep said:
Do you have the exact part number for the waterpump?
Click to expand...
I do not right now, I will get one when I get back home to the car
 
F

FirstFox89LX

Member
Oct 20, 2019
9
1
13
Military, so constantly changing
Jan 9, 2020
#4
  • Jan 9, 2020
  • #4
Wayne Waldrep said:
Do you have the exact part number for the waterpump?
Click to expand...

MTC CP4038 is what the receipt from oriellys says.
 

Wayne Waldrep

Before I post a pic, do you have one of yours?
20+ Year Stangneter
Apr 14, 2003
1,338
659
143
Cuba, AL
Jan 9, 2020
#5
  • Jan 9, 2020
  • #5
Ok. I just checked to make sure it's reverse rotation and it is. So that blows my idea.
 
Reactions: FirstFox89LX
F

FirstFox89LX

Member
Oct 20, 2019
9
1
13
Military, so constantly changing
Jan 9, 2020
#6
  • Jan 9, 2020
  • #6
Wayne Waldrep said:
Ok. I just checked to make sure it's reverse rotation and it is. So that blows my idea.
Click to expand...
I thought of that as well, but for it to work fine for a few months in the summer as my daily driver, and all of a sudden have when it hasn’t been driven a lot in the last two months; and the fact that once it leaves idle it sucks back into the radiator and cooling system, and seems to cycle normally has me thinking it wouldn’t be a wrong rotation thing
 

Wayne Waldrep

Before I post a pic, do you have one of yours?
20+ Year Stangneter
Apr 14, 2003
1,338
659
143
Cuba, AL
Jan 9, 2020
#7
  • Jan 9, 2020
  • #7
Yep but I always check the obvious......Occam's Razor.
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,871
10,542
203
polk county florida
Jan 10, 2020
#8
  • Jan 10, 2020
  • #8
I had to look up the meaning of occam's razor, seems to apply to car building/repair.
 

Wayne Waldrep

Before I post a pic, do you have one of yours?
20+ Year Stangneter
Apr 14, 2003
1,338
659
143
Cuba, AL
Jan 10, 2020
#9
  • Jan 10, 2020
  • #9
General karthief said:
I had to look up the meaning of occam's razor, seems to apply to car building/repair.
Click to expand...
Who knew!
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,387
2,745
194
Kearney, NE
Jan 11, 2020
#10
  • Jan 11, 2020
  • #10
Have you replaced the radiator cap, or at least made sure the rubber seals are clean and in good shape?
you can pressure test it (and the whole system), but a new cap is called for with all your other new parts, (a Stant with a pressure relief lever is my preference). If that does not do it, you need a system pressure test, and hope it is not a gasket.
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,871
10,542
203
polk county florida
Jan 11, 2020
#11
  • Jan 11, 2020
  • #11
Some of this is normal operation of the cooling system. You should have a 16lb radiator cap (if I remember) and the overflow tank should only fluctuate during warm up and cool down, about 4-5 inches if you have the stock overflow tank.
i would install a manual temp gauge temporarily to get an idea of what your actual temp is at idle and driving around.
Is your overflow tank stock?
how is your system set up? Maybe some pics?
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,387
2,745
194
Kearney, NE
Jan 11, 2020
#12
  • Jan 11, 2020
  • #12
General, sir!, I think he said the tank was temporary. And you are right on about this sounding largely normal. That’s why I suspect the cap is not working.there is gunk (stop leak etc.) in the way of the brass center piece, it will not build pressure.
A more accurate set of gauges is always a good idea.
I was also thinking he needed to borrow a temp gun to see how hot it runs.
Fox body cars are notorious for having gauges that randomly max out, starting with the temp gauge, usually with an already full tank of gas. A friend’s 78 Fairmont, my 79 5.0 LX, and an 87 Cougar would do this regularly, and my 91 has done it twice. But they never spilled coolant while pretending to overheat. A “Fonzie tap” to the dash by the driver’s right knee would send them back to normal.
 
F

FirstFox89LX

Member
Oct 20, 2019
9
1
13
Military, so constantly changing
Jan 13, 2020
#13
  • Jan 13, 2020
  • #13
7991LXnSHO said:
Have you replaced the radiator cap, or at least made sure the rubber seals are clean and in good shape?
you can pressure test it (and the whole system), but a new cap is called for with all your other new parts, (a Stant with a pressure relief lever is my preference). If that does not do it, you need a system pressure test, and hope it is not a gasket.
Click to expand...
I have not replaced the radiator cap, and didn’t really inspect it. But when I had it off the other day it didn’t seem to have any obvious damage that I recall.
I will have to look and actually check it out better tomorrow.

the new cooling parts have been on the car for 5-6 months now I believe, most which the car was my daily driver and never gave me any issues with overflowing the tank.

my overflow tank right now is a smaller glass jack bottle that has sat right about 1/3rd full from the time I refilled the system, up until the other day when it decided to fill and overflow, but then sucked most back into the system with a light rev/holding above idle.

I doubt my temp gauge has been working since I replaced everything. The gauge got a little hot the last time I drove it before replacing everything.
My gauges started with the gas gauge jumping to above full when the tank was super low, then stuck at above full for probably 6-7 months now, temp gauge hasn’t see anything above a 1/4 way up since I replaced everything I believe, never really paid a lot of attention to it honestly. When have looked, it tends to be around the bottom line, if not pointed straight to the floorboard.

what gasket would you be referring to when you say “and hope it’s not a gasket”?
 
F

FirstFox89LX

Member
Oct 20, 2019
9
1
13
Military, so constantly changing
Jan 14, 2020
#14
  • Jan 14, 2020
  • #14
My overflow bottle is a jack Daniels bottle that sits on my battery tray.
I bought a new radiator cap, threw it on, and the battery in the car is dead. So hopefully the weather is good and tomorrow I can see if that’s the problem or not
 

Attachments

  • 159DF75D-47C4-4387-AF14-6BF6BCF61004.webp
    168.6 KB · Views: 591
  • B11660B1-05C8-4997-B419-8DCBDABD338E.webp
    294.7 KB · Views: 209

nickyb

I gotta say i never painted my nuts, Never Ever
15 Year Member
Apr 3, 2009
2,979
1,626
153
nevada
Jan 15, 2020
#15
  • Jan 15, 2020
  • #15
I hope the cap solves your issue.on another note or two a fan shroud will be a good idea,also the stock overflow tank and move the air filter out of the hot engine bay.Good luck and keep on rocking baby got to keep on foxin.
 

General karthief

wonder how much it would cost to ship you a pair
5 Year Member
Aug 25, 2016
27,871
10,542
203
polk county florida
Jan 16, 2020
#16
  • Jan 16, 2020
  • #16
Don't forget to check the air pressure in that left front tire.
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,387
2,745
194
Kearney, NE
Jan 18, 2020
#17
  • Jan 18, 2020
  • #17
FirstFox89LX said:
I have not replaced the radiator cap, and didn’t really inspect it. But when I had it off the other day it didn’t seem to have any obvious damage that I recall.
I will have to look and actually check it out better tomorrow.

the new cooling parts have been on the car for 5-6 months now I believe, most which the car was my daily driver and never gave me any issues with overflowing the tank.

my overflow tank right now is a smaller glass jack bottle that has sat right about 1/3rd full from the time I refilled the system, up until the other day when it decided to fill and overflow, but then sucked most back into the system with a light rev/holding above idle.

I doubt my temp gauge has been working since I replaced everything. The gauge got a little hot the last time I drove it before replacing everything.
My gauges started with the gas gauge jumping to above full when the tank was super low, then stuck at above full for probably 6-7 months now, temp gauge hasn’t see anything above a 1/4 way up since I replaced everything I believe, never really paid a lot of attention to it honestly. When have looked, it tends to be around the bottom line, if not pointed straight to the floorboard.

what gasket would you be referring to when you say “and hope it’s not a gasket”?
Click to expand...
A water pump or thermostat gasket would be OK, but a leaky head gasket is a bigger project.
 
F

FirstFox89LX

Member
Oct 20, 2019
9
1
13
Military, so constantly changing
Jan 20, 2020
#18
  • Jan 20, 2020
  • #18
So I put the new radiator cap on it, fired it up and had it sitting out front idling for a a little bit while I was moving some stuff around the garage. It was idling fine and didn’t have a puddle under it when I went back out to it. Opened the hood and could see where the coolant had been dripping into my overflow again and watched a few drips come from the tube. Gave it a quick rev and it came down, but didn’t not drop back into the same small-choppy idle it was at before (car has a small cam in it). I waited a couple minutes and it didn’t seem to be pushing into the overflow again, so I opened the radiator cap, and it instantly pushed out of the cap, so I closed it back down quickly.
Let the car sit there and idle for another 5-10 minutes while I finished the stuff I was doing in the garage and checked again, there was no more coolant dripping into my overflow, and no more coolant in the overflow than when I had walked out.
So hopefully it’s fixed. Haven’t really had another chance to take it out and cruise around and see if there is any problems. And the car is now wrapped up and I’m attempting to fix some of the ups I made rattle-can painting the blue door and black roof/hatch and sun spots on the car.
 
Reactions: General karthief
You must log in or register to reply here.

Similar threads

K
86 GT Water Pump
  • Kade5114
  • May 11, 2026
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
Replies
3
Views
81
1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- May 11, 2026
nickyb
L
Car has been overheating. Not sure what is causing it. Any help would be appreciated.
  • Luckysgirl
  • Jun 2, 2026
  • 1996 - 2004 SN95 Mustang -General/Talk-
Replies
2
Views
74
1996 - 2004 SN95 Mustang -General/Talk- Jun 2, 2026
Luckysgirl
L
Engine 1986 GT Overheating issues
  • 86 5O GT
  • Jan 25, 2026
  • Fox 5.0 Mustang Tech
Replies
18
Views
548
Fox 5.0 Mustang Tech Jan 29, 2026
86 5O GT
R
coolant
  • Rudy10thsvt
  • Sep 28, 2025
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
Replies
15
Views
417
1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- Oct 1, 2025
General karthief
Drivetrain 83’ Glx convertible C5 Trans help!
  • JacksonJared2113
  • Dec 25, 2025
  • Fox 5.0 Mustang Tech
Replies
2
Views
354
Fox 5.0 Mustang Tech Dec 26, 2025
JacksonJared2113
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?