Cooling System Problem?

[Ncostello]

My 1991. 5 is suffering from a cooling system issue... in my opinion. Its not acting like any vehicle I've ever owned.



About a year ago I replaced the radiator, hoses (all of them), water pump, thermostat and filled with new coolant. I never really knew what the normal operating conditions of this 1st gen was like prior... cause when I bought it, it had a pinhole leak in the radiator.



Anyhow... the "pressure" in the upper hose never goes down. If I let it sit a week... the upper hose is still very firm and there is no way I can easily squeeze it together. This is different than any other vehicle (diesel or not) that I've owned. After I start it... the pressure seems to really increase and the upper hose gets even more firm. After the coolant reaches temp... the stat opens and the upper hose gets soft... although HOT!! I assume at this point everything is flowing normally and the temp gauge shows fine.



I thought I was careful to install the thermostat in the correct direction... but I don't specifically remember. From what I recall... it could only physically go in one way due to the depth and the shape of the water neck casting. And I thought it had an arrow saying "to engine" direction.



But after it cools down to room temp the upper hose gets firm again and remains as such for as long as I let it sit there. Now I can open the new lever cap and it always burps either a tiny amount of coolant or air in the the overflow tank... . which always shows pretty stable levels. I can do this every time I run it and let it cool down.



I installed a 7LB lever cap... after I thought maybe the 16LB cap was not allowing the air to escape the system. I know the 7LB cap isn't ideal... but its what I got for now until this is resolved.



Its like the block and radiator is completely filled with coolant after every run/cool cycle. As if it sucks in from the bottle whatever it needs to be completely full... and later it has no room for me squeezing the upper hose.



I can't see this being a head gasket leaking combustion gas into the system... but I'm starting to wonder. Its a stock truck with no mods... so its not as if I had too much boost to blow a gasket.



Does this seem typical of a head gasket failure or other type of failure?

 

[AH64ID]

Is the system still under pressure if you open the cap after a few day? i. e. does the hose go soft with the cap open?

 

[PBJ2500CTD]

I agree with the questions/test recommended by AH64ID. Your symptoms are definitely strange. Are you using the proper cap for a coolant recovery system? Even if it was the wrong cap it would seem that the pressure would reduce once the coolant cooled down.
The pressure rating of the cap will determine the boiling point of the coolant, along with water purity, antifreeze mixture, altitude, etc. . The higher the pressure the higher the boiling point. A healthy cooling system should easily handle a 15 - 16 pound rated cap.

 

[Bob Beauchaine]

I have tested literally hundreds of radiator caps and found that the OEM cap from Chrysler was by far the best out of all that I tested. They last the longest and perform properly throughout their life span, which exceeds the best of the after market replacements.



Bob

 

[PBJ2500CTD]

I have tested literally hundreds of radiator caps and found that the OEM cap from Chrysler was by far the best out of all that I tested. They last the longest and perform properly throughout their life span, which exceeds the best of the after market replacements.

Bob

100% in agreement with you Bob. The look, feel and longivity of the OEM caps far exceed most aftermarket choices.

Bruce

 

[tugboatphil]

Mine isn't OEM, but it's either a 13 or 14 psi cap and that's what I also had on my 90. I've had no problems with either.

 

[Ncostello]

If it goes through a heat/cooldown cycle... and sits for a day or three... I can crack the lever on the cap and it will release a very small amount of coolant/air/whatever in the the reservoir. I can hear it. But its a very small amout. If I had to guess from watching the liquid in the bottle while I do it... I'd say a table spoon at the very most (if its actually liquid and not air). The upper hose then is "soft" to where I can squeeze it together... keeping in mind the hose being new has some strength to it anyhow.



Today I ran the truck in the garage at elevated idle until it reached operating temp. The upper hose prior to starting was "full" feeling... more than it was pressurized feeling. I also noted that the bottle level was about 1" below the full mark. (FWIW everything is spotless clean including the bottle... and the cooling system is very clean with new coolant).



During the heat up... the upper hose gets fairly firm, as if the water pump is putting pressure to it but it can't go anywhere cause the stat is closed. No way your going to squeeze it together without really trying. Then the temp gets hot at a normal rate and the stat opens up where expected. The gauge shows the same place where it normally is when hot. And the laser temp gun shows like 160 ish on the stat housing. (I never really trust the gun with internal temps versus external metal temps).



Then the hose goes soft once the stat opens. Soft to where you can easily squeeze it together... although its darn hot!! The radiator gets got quick at this point and I know coolant is flowing normally. I should also note that the over flow tube gets hot as well at this time and the fluid level starts going up. So hot coolant is flowing to the bottle almost immediately after the radiator starts getting hot coolant going thru it.



The bottle level increases a little to where its at the full mark. After I run it like this for a while and then shut it down, the upper hose will remain soft for about 2 hours. I can squeeze it and everything seems normal. As it really cools down over several hours... the upper hose will get firmer. Overnight and after total cooldown... the upper hose is once again very firm and the bottle gets pulled down to 1" below the full mark... sometimes even 2". Just depends I guess on the attitude of the truck that day.



Nonetheless I think the cap could well be the problem although I've tried two different new Stants. One 15 and now a 7. Both lever caps.



At this point I'm guessing that somehow during cooldown the engine is "pulling" enough coolant back into the system to be completely full with basically NO room for air or any free space. Thus the next day or two later... squeezing the upper hose is to no avail as there is no room for any liquid to go from where its at. If I squeeze hard... I can pop the 7psi cap and hear it burp into the tank. This is while totally cold.



Thats where I'm at. The radiator is aftermarket. Again I'll try a new cap from Dodge. If that don't work... perhaps the fill neck on the radiator isn't made to proper dimensions and the cap can't perform properly. IE... the seat for the gasket is too low or high. I don't have the OEM radiator to check the measurements any longer.



Lastly... when I've drained the system I refill the radiator completely full... run it to temp... then fill there again until completely full to the neck. Then I top off the bottle to midway between cold/hot fill marks. Thats just how I always did things. I figured most of my vehicles have burped out what they didn't want... and pulled in more if they wanted more. Could I be filling too much??? Or would the system just pull in from the bottle until it ran the bottle dry anyhow?

 

[PBJ2500CTD]

Do you have the Stant part numbers for the caps you used? It almost sounds like everything is working as it should - but the pressure release when cold - which may mean an improper cap type. My memory cells of 40 plus years ago are a bit hazy but it sure sounds like an old pre-coolant recovery system operation. On the caps you use, how much tension is holding the return valve (vaccum release valve) in place? (The little disk in the middle of the cap's gasket) There should be very little if any tension holding against the gasket. Also, IIRC a non-recovery cap had a conical brass tension ring where it meets the top of the fill neck while a coolant recovery cap has the spring with a gasket/seal. Maybe it's just the nature of the beast.

Here are the Stant part numbers for a 91 D250

<TABLE id=search-list cellSpacing=5 summary="listing of query results" cellPadding=5><TBODY><TR><TH>Part #</TH><TH>Part Type</TH><TH>Application</TH><TH>Attributes</TH></TR><TR class=altRow><TD style="WHITE-SPACE: nowrap">10331</TD><TD style="WHITE-SPACE: nowrap">Radiator Cap</TD><TD style="WHITE-SPACE: nowrap">Dodge D250 (Truck) 1991</TD><TD>5. 9L L6 359 CID</TD></TR><TR><TD style="WHITE-SPACE: nowrap">10231</TD><TD style="WHITE-SPACE: nowrap">Radiator Cap</TD><TD style="WHITE-SPACE: nowrap">Dodge D250 (Truck) 1991</TD><TD>5. 9L L6 359 CID</TD></TR><TR class=altRow><TD style="WHITE-SPACE: nowrap">18331</TD><TD style="WHITE-SPACE: nowrap">Radiator Cap</TD><TD style="WHITE-SPACE: nowrap">Dodge D250 (Truck) 1991</TD><TD>5. 9L L6 359 CID</TD></TR><TR><TD style="WHITE-SPACE: nowrap">18231</TD><TD style="WHITE-SPACE: nowrap">Radiator Cap</TD><TD style="WHITE-SPACE: nowrap">Dodge D250 (Truck) 1991</TD></TR></TBODY></TABLE>

 

[PBJ2500CTD]

Now after re-reading your last post I almost want to say all is normal. If it is only a little pressure difference then no problem. What are your temperature swings from day to night? If your cap's vaccum spring is tight it can draw in fluid when very cold and cause a small pressure buildup when the ambient temp rises a bunch. Just more thoughts...

The big thing is - No overheating, No coolant loss, Nice sealed system. And, by the way - there should not be any air in the system once it has bled out from the initial fill. That helps prevent system oxidation (corrosion).

 

[Ncostello]

You guys helped alot!!! Prior to reading your posts Bruce... I had come to the same conclusion about the tension issue on the return valve. I had recently purchased a new Stant 10331 (thats what the computer said I needed for it) for my dads Ram 1500 360 V8 truck. He had the original Mopar 16psi cap on there and wanted a new one when we put in his new radiator (his was leaking). So I was lucky to have a know good (albeit used) Mopar cap on hand.



When I purchased my new radiator... I just picked up a 15psi Stant regular cap. I paid no attention to the cap model number but only to the 15psi rating. It looked the same. After having these issues I bought a 7psi lever lock... again not paying attention to the model number.



In comparing both Stants I have... I noticed that the return valve tension is much different. The Mopar cap has very little tension. Almost none. The Stants both have significantly more... one a little more than the other it feels.



After putting the Mopar cap on, ALL is normal once again. The hose feels normal now and when I squeeze it cold (and slowly) it will push coolant past the weaker return valve and into the tank. If I do it fast... you can feel the return valve seal up and shut flow off. Which is what it does once system pressure comes up I'm assuming.



So my problem was that the Stants were allowing the system to draw in coolant on cool down and completely fill up. Yet the Stant return valve was too strong and remained totally sealed for any "leakage" flow in the opposite direction after the system had totally cooled off.



I'm going to look at a new Mopar cap and compare. Otherwise my issue was the system was 100% full of coolant with no room for movement anywhere when dead cold. As if the Stant caps were working too well. I doubt now anything would be hurt from this... but I like the action of the Mopar cap better.



My fear of a head gasket failure or something else allowing pressure into the cooling system is now gone. Funny how something as simple as this makes you feel so much more at ease. Thanks again!!

 

[Ncostello]

Noticed something else. The Chrysler cap kinda allowed for a nice gradual warmup and release into the radiator. With the stant cap... when the engine reached operating temp it would quickly dump hot coolant into the radiator and the entire radiator would heat up real quick. Now you can feel the radiator get warmer and warmer as the thermostat opens more and more.



Anyhow... thanks again.

 

[Ncostello]

I finally bought a new Mopar 16psi cap... to replace the used Mopar cap I have been running. I did notice that the return valve of a new Mopar cap has no tension on it. No spring at all. It just floats up and down freely. The used cap is the same.



Any Stant cap called out for this truck has a spring holding the return valve normally closed snug. THIS seems to be the difference. If the return valve has a spring... my truck holds pressure always. And I've tried several Stant versions. Using a Mopar cap I get what I feel is normal operation with this cooling sytem... . which is no pressure on start-up, pressure when up to temp, pressure releases upon cooldown... all with the level of the overflow tank adjusting as needed.



Thus I suggest 1st gen owners ONLY use Mopar caps.

 

[DVolk]

You said you put a diffrent thermostat on ? was it a actual cummins one ? If not change it thats the problem ? I got aftermarket rad caps on mine . They work fine but the thermo stat ansd water pump best be a cummins one or it will act up !!!!!!!!!!!!!

 

[JLEONARD]

I agree with DV. I had an aftermarket cap and it worked as it was supposed to.

I never changed the pump or T stat but assumed it was OEM.

 

[Ncostello]

It was a genuine Cummins stat purchased from Scheid Diesel. Although it looked slightly different than the original. Honestly... the only stat I'll buy otherwise is a Murray and I get them from OReilly's. They are high quality looking and feeling and they work great in everything I've installed them in. And they are NOT made in communist country like Stants are nowadays.