I have a internal coolant leak on my 1999 Ford F-150 v6. Where should I look for leak?

I have already ordered head gasket kit to replace all seals. However, with system 8 below, I was wondering if the leak could be on the intake manifold opposed to the head gasket or some where else I should be looking.

Symptoms are as follows:
1.Fail to fire on cylinder one error code
2. Only cylinder one shows signs of coolant leak.
3. No signs of coolant in oil; however, signs of oil in coolant compartment
4. All spark plugs show signs of over heating, but not bad.
5. Can hear cylinder one failing to fire when idling, with the occasional backfire.
6. Distinct sweet smelling, white cloud exhaust, that lingers in the air.
7. Once engine is to temperature engine runs smoothly even if idling.
8. Occasionally exhaust clears up when I'm driving at city speeds and after idling again it stays clear without overheating. I suspect that this is happening when the thermostat opens but not certain of that.
9. Have had the signs of the occasional air bubble in system, where heater stops working and then engine starts to over heat. I turn off truck and open reservoir cap to confirm coolant then close. I start truck up again and every time I do this everything starts working fine.
10. With driving truck about 200-350 miles a week I need to add a gallon of coolant about once a week.

You might have a crack in your number one cylinder or the head itself which would cause it to fill with coolant

You could do a compression test on the bank with number one cylinder and compare them. If #1 is low then head gasket is possible. If not then intake leak is likely. When the exhaust clears it may be because the level of coolant is not there any more. Dangerous situ for engine health.

It would be more cost effective to hire a shop to do the pressurization test so they can tell you where the leak is… Then you can take it home and fix it. You're going to spend a lot of time and money trying to figure it out yourself without the pressurization test.

Good question with lots of things covered. I have a hard time with these people that "think" you could have a bad head gasket or a crack in the head and only the anti freeze will enter the cylinder. They seem to fail to realize the pressure of compression will over power the little pressure of the cooling system. At least you have looked for air bubbles in the radiator. What you have seen is not a head gasket, you would have seen the cap blow off if it was the gasket. Ford's have water going through the throttle body and they are known for the aluminum to corrode causing the gasket to fail. I think there's where you should start. I'm thinking you have a cold water leak and the leak goes right to number one cylinder. As it heats you burn off the anti freeze and it also seals up the leak. You mentioned oil in the reservoir, that could be stop leak or anything that floats and you will never get it out as it will cling to the sides as anti freeze is pulled into the engine as it cools. You have also said the heater stops working and the truck over heats at that time, well that tells me the anti freeze is low as for why it cures its self after a restart I'm not sure. May have to do more with the viscus fan drive then anything else.

Leaking gaskets were the cause of a lot of 4.2 failures. Replace head and intake manifold gaskets both upper and lower and you should be ok.

At the Ford service department where they can give you info about service bulletins that pertain to coolant losses.

Engines may loose coolant internally or externally at any gasket, dye tests are the norm, with teardown, for cracked engine part diag.
Or gaskets and sealing methods being the cause, fasteners threads etc.

The thing about service bulletins, is engineers have already determined issues and parts or procedure for correction. They quote laughable labor amounts they pay for warranty on a new one, if that was what it was.

MOST of this is CAUSED by the use of the R+WRONG COOLANT TYPE! FOrd haas used many different TYPES of coolant over the years! They even used DEXCOOL< but used PROEPR GASKETING that does NOT dissolve over time like in GM cars! ANYWAYS! Your "SYMTOMS" sugest that the cyinder head with NUBMER ONE on it has a HEAD GASEKT or INTAKE GASEKT problem! ALSO as they age, the FUEL PRESSURE REGULATORS may go bad! Thsi causes the burngin of EXCESSIVE FUEL adn the plugs darken from this! ALSO remove the IDLE AIR CN{ONTROL VVALVE and clean it along weith the exposed THROTTLE BODY! The FINAL air misture is done here for stop and go driving! FI youa re using VALVOLINE< stop at once and use MOBIL ONE 0W-30 as AL FORDS love this oil! NO FACTROY wil use VALVOLINE ever, and you shodul NOT as well! Use BOSCH ro DESNO iridium pkgus fater the IAC is cleaned up! ALSO>>>You may NOT even need new HEAD GASKETS< but ONLY new head bolts! Get a set ( you will need them anyways) and replace them ONE by ONE with a proper TORQUE WRENCH!<<<IF YOU REMOVE A PLUG and it is discolored by COOLANT< then THAT is where the leak is to be found, either in HEAD or INTAKE gaskets! HIGH MILEAGE CARS do very well once they are CLEANED UPa and new gaskets installed! The FORD V6 is EXCEPTIONALLY well made followign the BENZ< and EUROPEAN way of engine making! GOOD LUCK! NEVER USE PRESTONE either use ONLY GENUINE FORD COOLANT! Get a MILD motor flush usign AMSOIL POWER FOAM< and use the FOAM to cleant he T body and the IAC valves to SHOWROOM! Also run it through the PCV and EGR vavles if you can do so! YORU FORD wil outlast most other engiens! It is UP to YOU to provide it with teh RIGHT COOLANTS and best OILS out there! AMSOIL gives a 25mile interval change, a dn PENNZOIL ULTRA gives a 15 year 500k mile OIL WARRANTY if you qualify! BYE NOW!

It might be either because of overheating your car or any internal damage in car's cylinder head.

if you don't came across any overheating case of your car then you must take a pressure test of your cylinder head which will ensure you the proper reason before installing new cylinder head gasket.