It isn't only about boost. Higher boost doesn't always mean higher cylinder pressures.

 

 

Have you done this?

 

I haven't yet. It seemed to me that if you vented the pressure that would lower the boiling point and possibly cause the coolant to boil. Wasn't sure if that would be a good test or not. If that will help narrow it down though I will do it.

 

That is the prescribed test by Ford (page 4 of TSB 09-8-3). You need to be careful when you vent the pressure off. I wrap a heavy towel around and over the cap.

Yes operating the truck with no pressure in the degas will lower the boiling point somewhat, but it is still the best way to tell if you have combustion gas getting into the system.

 

Makes sense. I will give it a shot tomorrow after work and see.

 

Well I brought the truck back to work today, after sitting all day yesterday after my hard trailer run that showed 19 PSI on Saturday this morning the pressure was at .4 PSI. This morning after 25 miles I took the cap off and vented all of the pressure, before I vented the pressure I was at 13 PSI. Once I put the cap back on it almost immediately rose to about 1.5 PSI which I assume would be the steam coming off the hot coolant adding a little pressure back in. Then I got back on the freeway and over the next 20 miles it would be between 1.3 PSI and 2.3 PSI depending on coolant temp, for example after climbing a hill ECT would be at 204ish and that would be up at the 2.3 PSI level, then going down the hill on the other side ECT would drop to 198-200 and pressure would drop to 1.3-1.5 PSI. I have a switch in my truck that I installed a while back that sets the cooling fan at full duty cycle, I have never needed this in practice but I installed it none the less. So for the last 5 miles I flipped the switch to try and keep ECT as cool as possible. This locked it in at 1.3-1.7 PSI after a hard run it would be 1.7 PSI and ECT would be up at around 202 after cooling off it would drop to 198-200 and be 1.3-1.5 and this would happen quickly with the cooling fan at 100%. When I parked it it was at 1.3 PSI. This seems to suggest that it is in fact not getting combustion gas in the cooling system at least for this test correct? What is strange is the 13 PSI I was showing before venting seems a bit high. I think I will have to do this test again with my trailer for the added load. I might also load my SRL+ tunes for the drive home and do the test again with the added cylinder pressure of the hotter tunes. Also I assume the system will have a vacuum in it once the coolant cools correct?

 

Personally I would let it cool down with the cap off and then re-install the cap.

If you really feel the need to do the test with the SRL+ and/or towing, I would go through the entire cycle again (get everything hot and up to pressure, vent, then run test again under the conditions you choose).

 

Good idea, I pulled the cap off and will reinstall it later.

I just can't understand my cooling system pressures. If under the current condition we have determined that my head gaskets aren't leaking then what else could cause my cooling system pressure to be 13 PSI earlier this morning before venting? That TSB 09-08-03 states that a cap that holds between 12-18 PSI is in range so technically if my pressure is even 13 a cap that is in range could vent.

 

Caps and thermostats are cheap enough that I'd replace it again if I was in doubt of it working right. You might have a combination of things going on and ruling out one for good would help.

Did you ever try pressurizing the system

 

My testing has been with a mix of 4 different caps and they all seem to act about the same so I don't think the cap is questionable. The thermostat seems to open at exactly the right temp and hold the temps pretty close to that so I don't think there is much of a question there unless there is something I am missing. Thermostat either opens and closes at the correct temp or not and I don't see anything to suggest mine isn't doing that.

As as far as the pressure tests goes I didn't because I have seen pressure over 16 PSI while testing so that tells me it must be holding enough pressure. Unless there is something else a pressure test would show I couldn't think of any reason to do it.

 

I had to go run an errand so I decided to do the same test with SRL+ and it was essentially the same. Never saw over 2.4 PSI and when I turned it off it was 1.9 PSI. I had given it a pretty good run right before I pulled in the parking lot so I think that explains why when I turned it off the pressure was slightly higher because the coolant had just got a load of heat. I will leave SRL+ on it for the drive home and do the test one more time for longer to verify. I haven't ran that tune in a long time and I had forgotten the grin that it will put on your face!

 

I don't want to confuse things here, can you post your EOT and ECT as you are posting your Coolant PSI? So they are all synchronized with your observation point.

 

I can start. ECT in all of these data points has been between 198 and 208(truck has a 200* tstat). EOT is always less than 8 degrees higher.

 

Those sound like good ranges.