Temp Gauge Sending Unit and Aluminum Heads - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 02-29-2012, 12:26 AM
406cu.in. of tire smokin' fun
 

Last journal entry: 406 Build Thread
Last photo:
Join Date: Jun 2008
Location: Tacoma, WA
Posts: 232
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Temp Gauge Sending Unit and Aluminum Heads

I have a street 406 small block with aluminum AFR 195 heads and I am having some temp gauge problems. When the truck is idling, the temp gauge moves up into the 220-260 area. When I start driving again, the gauge goes back down. It's clear to me that the engine is not actually overheating and I have verified this with an infrared thermometer at the water neck. At first I thought it was a bad sending unit, so I replaced it. It was old anyway, so I didn't bother testing it. The problem was completely unchanged. I also used the thermometer on the sending unit case and it also shows an increase in temperature when this happens.

The heads do have the steam holes drilled, but I am starting to think that the gauge is just picking up hot spots when the coolant flow is minimal. Does this sound correct? Should I move the sending unit to the intake? Not worry about it?

Any suggestions?

Thanks

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 02-29-2012, 01:51 AM
blwn31's Avatar
Registered User
 
Last photo:
Join Date: Mar 2011
Location: Placerville,CA
Posts: 142
Wiki Edits: 0

Thanks: 0
Thanked 9 Times in 9 Posts
Since you replaced the sending unit I would tend to believe the temp gauge. The heads will run hotter than the intake, that's why I like to take the temp from the intake near the thermostat. One thing I have done to my engine, is drill a small hole (1/16th) in the thermostat to keep a good supply of hot water moving up to the thermostat. I've found this keeps the engine temps more stable instead of the big swings I used to see. Difference for me is, the engine being all aluminum, it was sucking so much heat out of the cylinders, I felt the engine was getting to hot before the thermostat would open. Once I put the bypass hole in, everything became normal.

Keith
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 02-29-2012, 02:33 AM
406cu.in. of tire smokin' fun
 

Last journal entry: 406 Build Thread
Last photo:
Join Date: Jun 2008
Location: Tacoma, WA
Posts: 232
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
I thought about the gauge being faulty also, but the resistance of the sending unit goes up with gauge movement. If the sending unit is telling the gauge to go higher, and it is, I think I can rule that out.

As for moving the sending unit, is there any problem with moving it to the back of the intake? I have an Edelbrock RPM Air Gap intake and there are ports at all four corners.

I will try the thermostat hole though. I have heard of that before but I didn't think it would change this problem.

Thanks
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 02-29-2012, 04:38 AM
67Mustang Al.'s Avatar
Registered User
 
Last photo:
Join Date: Jan 2009
Location: Tasmania, Australia.
Posts: 432
Wiki Edits: 0

Thanks: 2
Thanked 6 Times in 6 Posts
Just a thought . If you have any question about how good the earth is between the sender unit and the chassis , rig a second earth from the brass casing on the sender unit with a clip to the chassis and watch for any variation on the gauge once again. I have al edelbrock heads and rpm intake on my 347. The temp sender is close to the thermostat, is new and works fine. A victor water pump is also used. I run two earth straps from the engine, one 8mm to the firewall from the rear of one head and a 10mm from next to the engine mount to the chassis rail. Also the gauge itself maybe having an earthing issue and could be proved out by using a second temp earth.
Good luck
Al
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 02-29-2012, 09:35 AM
blwn31's Avatar
Registered User
 
Last photo:
Join Date: Mar 2011
Location: Placerville,CA
Posts: 142
Wiki Edits: 0

Thanks: 0
Thanked 9 Times in 9 Posts
I'm pretty sure you will see a drop in temp just by moving the sender from the head to the intake. As far as location goes, give it a try at one of the rear intake positions.

Keith
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 02-29-2012, 10:22 AM
Registered User
 
Last photo:
Join Date: Oct 2007
Location: Portland,Or
Posts: 838
Wiki Edits: 0

Thanks: 0
Thanked 43 Times in 37 Posts
Just in case anyone is a little confused by 67Mustang AL, "earth" is Australian for ground.
He does make a good point. make sure there is a good ground between the engine, and the gauges. Do not just assume the engine is grounded to the chassis, and the gauge itself is also.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 02-29-2012, 03:10 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,656
Wiki Edits: 0

Thanks: 3
Thanked 399 Times in 346 Posts
Quote:
Originally Posted by Pre-Tuner
I have a street 406 small block with aluminum AFR 195 heads and I am having some temp gauge problems. When the truck is idling, the temp gauge moves up into the 220-260 area. When I start driving again, the gauge goes back down. It's clear to me that the engine is not actually overheating and I have verified this with an infrared thermometer at the water neck. At first I thought it was a bad sending unit, so I replaced it. It was old anyway, so I didn't bother testing it. The problem was completely unchanged. I also used the thermometer on the sending unit case and it also shows an increase in temperature when this happens.

The heads do have the steam holes drilled, but I am starting to think that the gauge is just picking up hot spots when the coolant flow is minimal. Does this sound correct? Should I move the sending unit to the intake? Not worry about it?

Any suggestions?

Thanks
X2 on blwn31's comments. Chevy mounts the temp sender close to the adjacent exhaust valves on the head as this is the hottest place on the engine, so it's the first place to look for signs of trouble. Aluminum transfers heat from the metal into the coolant faster than cast iron so when idling the overall coolant temp goes up some but by these adjacent exhaust valves it goes up a lot and quickly, then drops when getting into motion and with higher coolant flows. As has been suggested if this really bugs you just move the sender by the thermostat housing where the temps average out because of thermal dilution into the larger coolant flow.

You might want to raise the overall coolant temp and increase the pressure limit from the cap. With aluminum heads this will improve combustion efficiency by putting more heat into pushing the pistons instead of warming the atmosphere and the higher pressure will help prevent localized boiling around the exhaust seats. It's important to keep them wet, when steam forms the local spot cooling in this area is greatly reduced. Many guys doing serious racing with the SBC tap a coolant source off the pump horns and deliver it into the head under the header and right between the exhaust ports as a means to keep this area cooler. Others enlarge the coolant transfer holes in the head and/or gasket at this point to facilitate the same end of getting some extra coolant flow on these adjacent valve seats.

In simple terms the faster heat transfer rate of aluminum will get your attention, as it has and yes your shot about the gauge just picking hot spots is right on.

Bogie
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 03-01-2012, 04:09 PM
406cu.in. of tire smokin' fun
 

Last journal entry: 406 Build Thread
Last photo:
Join Date: Jun 2008
Location: Tacoma, WA
Posts: 232
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
I have a separate, 4-gauge ground from the alternator to the frame, so the ground shouldn't be a problem.

OldBogie, by raising the temperature you mean running a hotter thermostat? I have a 195 in it now, but the cap is just a Stant replacement cap, so maybe it's 15-16lbs.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 03-01-2012, 04:49 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,656
Wiki Edits: 0

Thanks: 3
Thanked 399 Times in 346 Posts
Quote:
Originally Posted by Pre-Tuner
I have a separate, 4-gauge ground from the alternator to the frame, so the ground shouldn't be a problem.

OldBogie, by raising the temperature you mean running a hotter thermostat? I have a 195 in it now, but the cap is just a Stant replacement cap, so maybe it's 15-16lbs.
Yeah 195 to 210 is what I'm thinking. For pressure something like an 18 to 22 pound unit if you have a brazed aluminum radiator. 16-18 if an epoxied aluminum, aluminum with plastic tanks or a soldered copper brass.

Bogie
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 03-01-2012, 05:07 PM
boothboy's Avatar
More bucks, go faster!
 
Last photo:
Join Date: Nov 2011
Location: Grass Valley, CA.
Posts: 580
Wiki Edits: 0

Thanks: 134
Thanked 236 Times in 212 Posts
Drilling a 1/8" hole in the thermostat allows the air to escape into the radiator when your filling you system. That way you eliminate trapped air. Usually when I'm tracking down a problem with a questionable temp reading I install a tee so I can add a known mechanical temp gauge. Both read the same, it ain't the gauge. Two diff temps, you have a gauge problem.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 03-01-2012, 06:51 PM
406cu.in. of tire smokin' fun
 

Last journal entry: 406 Build Thread
Last photo:
Join Date: Jun 2008
Location: Tacoma, WA
Posts: 232
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
It's a new AC Delco radiator, but it does have the plastic tanks. I'll have to check the cap rating to be sure of what I have on there now.

Also, I belived I checked the coolant passages in the gaskets when I built it and they matched the holes in the block and head. I'm not sure if I've seen a 400 head gasket that didn't match, but I could be wrong.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 03-01-2012, 10:12 PM
bentwings's Avatar
bentwings
 
Last photo:
Join Date: May 2002
Location: St.Paul, Minn
Age: 72
Posts: 1,791
Wiki Edits: 0

Thanks: 0
Thanked 16 Times in 15 Posts
I have my temp sending unit on the rear of the Edelbrock RPM heads. I have alway felt that the temp spikes were because the alum soaks up the heat faster than iron. Also at the rear of the heads is supposed to be the hottest part of cooling system.

I also have a sender at the thermostat housing but it turns out it is wired differently so I have not used it.

The thought that there is steam forming around the exhaust is interesting. I have a regular coolant loss problem that I have been chasing for 3 years. All the minor leaks have been fixed and I have a larger coolant recovery system. I think the higher pressure cap may really be the answer. Ceartainly worth a try as crusing season is knocking on the door.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 03-02-2012, 06:40 AM
Registered User
 
Join Date: Feb 2010
Location: Ohio
Posts: 112
Wiki Edits: 0

Thanks: 0
Thanked 5 Times in 4 Posts
Not sure how electric senders are made, but if this is a mechanical gauge-make sure the bulb that sticks into the head is not contacting the actual aluminum casting inside the water jacket. That will make it read all kinds of funny. If so, get a longer guage fitting to keep that from happening...
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 03-04-2012, 08:53 PM
406cu.in. of tire smokin' fun
 

Last journal entry: 406 Build Thread
Last photo:
Join Date: Jun 2008
Location: Tacoma, WA
Posts: 232
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Did some work today. I removed the thermostat, drilled two 1/8" holes, and reassembled. I also confirmed that it is a 195 thermostat and that the cap is 16lbs.

I also attempted to move the sending unit into the manifold. The sending unit I pulled out of the head is 3/8" pipe thread, but the holes in the manifold are 1/2". My Dad said to get a sending unit for a car from the 60's or 70's. Can anyone confirm this is correct or that the resistance is the same?
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 03-04-2012, 10:56 PM
blwn31's Avatar
Registered User
 
Last photo:
Join Date: Mar 2011
Location: Placerville,CA
Posts: 142
Wiki Edits: 0

Thanks: 0
Thanked 9 Times in 9 Posts
You don't need two 1/8" holes. You will move a ton of water through a quarter inch combined hole. The thermostat probably won't ever open. Get another thermostat, drill one 1/16th to 1/8th inch hole. When I originally did mine I drilled two 1/8th inch holes 180 degrees apart. I have since downsized to one 3/32nd inch hole. My temps are nice and stable now with no spikes.

Keith
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Tranny temp gauge sending unit location yragat Transmission - Rearend 12 12-28-2008 04:12 PM
Temp sending unit busterwivell Hotrodding Basics 5 09-16-2008 07:26 PM
Temp. gauge sending unit parameters? Thinking Man Electrical 0 11-20-2007 06:12 PM
Temp sending unit GMac Electrical 11 10-27-2004 12:46 PM
Temp Sending Unit? Seon Engine 2 06-23-2003 03:01 PM


All times are GMT -6. The time now is 07:06 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.