Headgasket? Help needed

1991 4.0 XJ6 with 40k miles. Runs really well, was leaking coolant from the coolant expansion tank seams. Replaced it with a used one from local junk yard. That one lasted a good 100 miles and then blew a leak at the seams. I JB welded the used expansion together but haven’t put the car back together yet. Now I’m thinking I might have a blown head gasket causing excessive pressure?? I could drive the car, let it sit for a week+ and then open the coolant cap and you can still hear the pressure escape.

Evidence of no head gasket failure

  • Did a combustion engine block test on the coolant. The liquid did not change colors.
  • Have started and let the car idle with the cap open, no gushing of water or bubbling. I do notice very tiny particles that might resemble “oil balls” might just be dirt too?
  • Oil on dipstick looks fine.
  • Runs really well, doesn’t overheat. Will move between Normal and Hot at idle when I’ve lost too much coolant but will move back to normal after driving. (have only ran in this state for 10-20 minutes)

Evidence of head gasket failure

  • There is “milkshake” residue in the PCV pipes that connect into the Oil filler neck
  • Same looking residue in one of the skinny coolant hoses that connect to the expansion tank.
  • I noticed “smoking” coming from the oil filler neck with the cap open and the oil dip stick with the dip stick open

Here are some videos and pictures. Please help and advise. Its a very good condition car, does it sound/look like I will need to change the head gasket?

Please view the videos I took of the “smoking” at this link https://drive.google.com/drive/u/0/folders/1Fq0ceyPG4g8d4KSngq8Kwub11FOIEm_X

Steve …

Interesting, I dealt with this same issue on my '89 a year ago. It did in fact turn out to be a failed head gasket between the #1 cylinder and a water galley. You can see where the water actually steam cleaned the top of the piston.

IMG_0 copy

Obviously the first thing you need to do after you get your coolant reservoir problem fixed is to determine if you actually are losing coolant. If you are and there is no obvious external leak then a bad head gasket is certainly a possibility.

The good news is that your coolant test for combustible gas was negative. As I understand it they are very sensitive. Are you seeing white smoke out of the exhaust pipes? That could be a bad sign.

I’m sure that an over pressurization of the coolant system would open the coolant reservoir pressure relief cap before it would rupture the reservoir. You could also drain out a little engine oil and check it for obvious signs of water.

Milkshake in the oil filler cap and small hose is more indicative of short runs and not getting everything up to operating temperature for a reasonable period of time.

Okay that makes me me feel a lot better, when I first purchased the car at 38k miles the oil filler cap had a bunch of milkshake that I wiped off and since driving 1.5k miles it hasn’t returned. Would having a bad thermostat cause any of this? I placed an order for a new thermostat already. Was going to buy new coolant hoses and replace them but after much looking and feeling the hoses on the car are very good condition. Not trying to spend $500 on a new expansion tank either.

No white smoke from the exhaust tips at all. I know the coolant cap will leak if I don’t tighten it really hard, its a brand new oem cap too.

Steve …

Quite a few used tanks on EBay for around $100 …

The thing is I just bought a used tank from a local salvage yard for $8 and it busted from the seams after 100 miles, worse than my original tank actually, granted it was local so probably an Arizona car that has seen lots of sun. I wonder if someone can 3D print a new tank? or retrofit an 88-89 tank or the later x300 tanks.

I’m going to try to JB weld my tank and see how well that holds up for right now.

@grooveman what issues were you experiencing? losing coolant? or leaking from the seams? thanks

Split the tank open along the seam, liberally apply JBweld hi-heat, pop-rivet the seam together again with closely spaced rivets. That should do it!

Squeeze the hoses just after starting up and with the engine cold. If they pressurize immediately it might be a problem. A little milkshake with short distance drives is normal. Smoke too (like hot tea). Frequently check the oil and coolant, keep driving, have a towing service just to be sure.
It’s fine.

Steve …

$8 for a replacement tank that lasted for 100 miles ! It looks like you got your money’s worth :joy:

But seriously I wouldn’t condemn all the used tanks available because of that and usually they come with some kind of warranty. Of course you can TRY to repair the one you already have but honestly because it operates under pressure and one seam has already failed I wouldn’t trust it. On the other hand what do you have to lose other than being stranded by the side of the road.


Thanks everyone for the help, so my lazy experiment of just JB welding the sides (seams) of the tank failed. I tried applying it on two different occasions too, using it liberally but it leaks from the same hairline crack at the seams. Once I get the time I will try riveting and sealing from the inside on my other leaky expansion tank.