The rig is a 84'CJ7. Rebuilt 258 block and 4.0 head only have a couple thousand miles on them. Running Mopar MPI and have been for nearly a decade. This is my 2nd engine rebuild on the same MPI kit. 1st rebuild was destroyed by detonation - that's my theory anyway. Pinging is gone with the 4.0 head conversion.

I just installed a VDO Mechanical Water Temp Gauage. Running uphill in 4th gear at about 2300RPM in 100+ degree weather today (Boise, ID), I was able to muster about 145 degrees on the new gauge. Something does not seem right. Here's what I've done:

I first tested the gauge itself. I put the guage's probe in a boiling pot of water along with a meat thermometer - they read within 4 degrees of each other so I'm assuming the VDO gauge is good.

I replaced the thermostat and it made no difference.

So...could the placement of my temp sender be an issue? (see picture). What you are seeing in the first pic. is the VDO temp sender on one side of the brass tee and the MPI temp sender on the other side. Because of my serpentine setup, MPI instructions say put the temp sender inline with the heater hose. It's been running in that spot (minus the brass tee and VDO temp sender) for 10 years...if I haven't been reaching proper operating temps I'm sure my MPI has not been running efficiently. I believe the computer needs to see 195 degrees before running optimaly.

Last winter (the first winter on the new rebuild), I noticed that my heater was not blowing very hot air; I even went to the length of covering up the front of the radiator with cardboard. This is telling me that the engine is NOT getting up to temp.

The stock water temp gauge works. The needle will swing about 1/3rd of it's range at the max but usually hangs out about 1/4 of the way when fully warmed.

So, any thoughts on the placement of my VDO water temp sender, should it work there? Is there coolant flowing through there -- I monitored temps with my heater core open and closed (thinking it would affect coolant flow through that heater hose), but it made no difference.

Any other ideas??? Gauge error or engine really not reaching normal operating temp?

Sorry, thought I'd be able to post a picture...can anyone help me out there, I can send via email.

Put your meat thermometer on your tstat housing - will be within 10 or 20 degrees of actual temperature... Drawing a blank as to why your temps would be so low though. What temp does your thermostat open at?

Basically if you have two gauges reading the same low temp it's a good bet that both gauges are correct...

The heater hose doesn't seem like a good spot for a temp gauge to me IF you have a control valve for your heater core.

Get a laser temp gun and read the t-stat housing. Maybe you t-stat is stuck open or something. Or you got a POS gauge.

Put your meat thermometer on your tstat housing - will be within 10 or 20 degrees of actual temperature... Drawing a blank as to why your temps would be so low though. What temp does your thermostat open at?

Basically if you have two gauges reading the same low temp it's a good bet that both gauges are correct...

The heater hose doesn't seem like a good spot for a temp gauge to me IF you have a control valve for your heater core.


I'll try reading the temp of the tstat housing with the meat thermometer; don't have a laser heat gun. The tstat is 195 degree and just replaced today for good measure (absolutely no change).

The stock 258 temp sender is threaded in a small hole in the back driver side of the head. I'd like to get the new sender in that spot but the VDO temp sender is HUGE. I'll have to see if I can find the right fittings...

He means an infared heat gun with laser targeting.:shaking:

But it sounds like to me your heater core is open and your coolant is bypassing your thermostat thru the heater core.

Even with the thermostat out and the heater on, at some point it should get close to the correct running temp with his outside temps being 100.

Are you by chance still running the cooler plugs from before your 4.0 head install? I know they were reccommended due to the detonation/ping witht he mopar kit and anything less than 92 octane for fuel.

Just something to check. But I don't know what is reccomended with the 4.0 head swap.

Even with the thermostat out and the heater on, at some point it should get close to the correct running temp with his outside temps being 100.

Are you by chance still running the cooler plugs from before your 4.0 head install? I know they were reccommended due to the detonation/ping witht he mopar kit and anything less than 92 octane for fuel.

Just something to check. But I don't know what is reccomended with the 4.0 head swap.

Yes, I'm certain I'm still running the colder plugs recommended for the MPI kit. Do you think that could make such a huge difference?

Looking at my tstat housing I saw the 'bypas' for the heater hose. I was wondering why the big square hole in the head where the tstat fits and then saw it lined up with heater hose outlet on the tstat housing.

So a clarification - I'm guessing coolant DOES NOT circulate out the heater hose outlet and back in the return unless I have my Temp Knob pulled out on the dash, is that correct? If so, it would seem that the coolant trapped in the section of heater hose where my temp senders are would get very hot, because it is not circulating through the radiator. It's just sitting there when my Temp knob is pushed in. Either way, I've read the temps with the heater core open and closed and saw no diffference in temp read outs.

Doesn't the target 195 degrees put the MPI computer in 'closed loop?' If the MPI temp sender is not seeing that temp. I may be robbing myself of a few MGP. I currently see 15 MPG around town (33's, 4:10's).

What spark plugs are folks running in the 4.0 motors??

Edit: I lied about my plugs....I forgot, with the new motor I installed Bosch Platinum +4 plugs. Fancy little suckers with a 5 year warranty.

Even with the thermostat out and the heater on, at some point it should get close to the correct running temp with his outside temps being 100.

Are you by chance still running the cooler plugs from before your 4.0 head install? I know they were reccommended due to the detonation/ping witht he mopar kit and anything less than 92 octane for fuel.

Just something to check. But I don't know what is reccomended with the 4.0 head swap.

With the thermostat out, but the thermostat is in. THINK about what is going on: IF coolant temp is not going up to opperating temps then it is a bad gauge, bad sender, or by-passing coolant thru the heater core to the radiator. THIS causes the coolant NOT to be up to opperating temp, making it easier for the radiator to cool down the coolant. I ask again are you SURE coolant is not "freeflowing" thru your heatercore?




With the thermostat OUT the coolant NEVER has a chance to cool down in the radiator, then will get hotter and hotter.

Fuck Bosch get some copper champions or the NGK oe spec plugs... Either way, a step colder plug isn't going to make your engine run cold.

Don't know about the Mopar EFI conversion, but you probably won't go closed loop until around 180. Newer cars will set a code for not warming up or not warming up in the expected amount of time.

15mpg around town with 33's and 4:10s... I get 15mpg around town with 31"s and 4:10s... and I'm at 210 degrees. I doubt both gauges are reading wrong, but you need to figure out what temp your coolant actually is to start. Whats your fan setup out of curiosity.

With the thermostat out, but the thermostat is in. THINK about what is going on: IF coolant temp is not going up to opperating temps then it is a bad gauge, bad sender, or by-passing coolant thru the heater core to the radiator. THIS causes the coolant NOT to be up to opperating temp, making it easier for the radiator to cool down the coolant. I ask again are you SURE coolant is not "freeflowing" thru your heatercore?

But even if coolant was just circulating through the heater core freely, how is this different than him running with the heat on... something that you can surely do and still reach operating temperature. Sure, the heater core will dissipate some heat, but the thermostat will open later because of this. Just clamp the heater hoses off and see if it heats up though, easy enough.

But even if coolant was just circulating through the heater core freely, how is this different than him running with the heat on... something that you can surely do and still reach operating temperature. Sure, the heater core will dissipate some heat, but the thermostat will open later because of this. Just clamp the heater hoses off and see if it heats up though, easy enough.

I can only assume coolant is not free flowing through my heater core with my Temp knop pushed in. Either way, I'm not running the fan, so there shouldn't be much heat dissapation if it is running through the core. (I'm pretty sure it's not, my heater turns on and off as expected.)

Running the stock radiator, mechanical fan and fan clutch.

I'm going to see if I can get my sender plumbed to the small hole at the back of the head where my stock temp sender is....

I can only assume coolant is not free flowing through my heater core with my Temp knop pushed in. Either way, I'm not running the fan, so there shouldn't be much heat dissapation if it is running through the core. (I'm pretty sure it's not, my heater turns on and off as expected.)

Running the stock radiator, mechanical fan and fan clutch.

I'm going to see if I can get my sender plumbed to the small hole at the back of the head where my stock temp sender is....

First, don't assume anything... just slap some vice grips on the heater hoses and see if your problem is solved... easy enough to do.

Moving your temp sender is a worthless endeavor if you engine is running cold.. Figure out the actual temp before going further.

You say you have a mech fan, is the clutch seized so it's drawing air through the rad constantly?

But even if coolant was just circulating through the heater core freely, how is this different than him running with the heat on... something that you can surely do and still reach operating temperature. Sure, the heater core will dissipate some heat, but the thermostat will open later because of this. Just clamp the heater hoses off and see if it heats up though, easy enough.

And what exactly is the path it takes? From Head to heater core to thermostat housing to radiator WHERE IT GETS COOLED DOWN. Since the thermostat has to feel a hot temp to open, it stays closed. BUT yes clamping off the hoses will tell you if this is the case. Now I don't have his Jeep in my Driverway. Because if I did it would take about 5 minutes to find out whats wrong. But then I own a Raytech infared heat gun.:D

And what exactly is the path it takes? From Head to heater core to thermostat housing to radiator WHERE IT GETS COOLED DOWN


Agreed, but when it gets to the thermostat housing, doesn't the stat need to be open for it to continue to the radiator?

Okay, first, clamping the heater hose off just above my temp senders did nothing. I tried it; temp rose to about 140 at idle, same as before.

I just got back from a mechanic's shop that's helped me out in the past, he had the niftly little infrared heat gun that quickly showed what was going on but didn't necessarily explain why. While idling outside his shop (fully warmed up), the heat gun showed 180+ at the bulge on the tstat housing where the tstat rests (normal). Then took a reading on heater outlet part of the tstat housing and the temp was 30 degrees cooler or so . My temp gauge is accurate; the coolant in the heater line is not getting hot. Which means my MPI computer is getting very bad information regarding engine temp, I'm hoping this will have implications on fuel economy, but we'll see.

What's really odd, is when I open my heater core (temp knob pulled out) my temp does not go up. I figured the flow through heater core would draw in the hot coolant.

So, maybe my heater core is blocked, this would also explain the not so hot heat last winter. Either way, I don't think the heater hose is a good spot for the temp senders.

My plan now is to move the MPI temp sensor to the back of the head where the stock temp sender currrently resides. The mechanic pointed out a 3/8th inch plug below my manifolds where I plan to put the VDO temp sender.

Cool - finally.

Cool - finally.

Totally.

Your temp cable DOES not alter coolant flow ,it is controlling a door inside the heater box ,not a flow valve in the heater hose.So it is not part of your diag.

My '84 cj stock gauge also only shows 1/4 warm (between the"C"and the 1st line ,so I call your gauge good.

Your stock gauge doesn't have numbers on it ,so it can see a different temp.ie heater feed temp ,and can still tell you what is normal.The vdo sender needs an accurate temp,so buy a cheap($40 )infrared gun and find a better place.And -screw in the sender with coolant running out ,so that no air is trapped between the gauge bulb and the coolant to change the reading.

I can't get my heater core temp up where I want it either ,but I haven't studied the prob yet.