Ptipon
(Ptipon, Sonora, CA, USA, 1990 XJS, Conv.)
1
Hi Norman,
I have had two seats drop, one on each head. One aft and one fore. The first one 10yrs ago. It happened one night in LA. Temperature in the high 50s. I went out about 9pm and then got back to the Jag about 11pm and ‘clik’, the valve dropped at start up. No over heating occurred at all. Left bank.
This last one occurred one morning with a temperature of -15 deg F. Did a short fun to the dentist and an hour later starting the car, ‘clik’, dropped valve seat again. I thought it might have been a lifter and drove it for a while thinking the lifter would loosen up. It never did and did some damage so a new head altogether this time.
Both beginning runs were under a half hour and of course no over-heating occurred. Both instances were not in summer nor daytime, runs were short and no prior overheating.
Any ideas why the valves dropped? The machine shop guy said he has seen both drops due to cold or no heat and due to over-heating and in many different makes and manufacturers. Key to all is aluminum heads with steel valve seat inserts.
All dropped valve seats are a result of overheating, but not necessarily at the moment the seat drops. Prior overheating can cause the seat to move allowing the final movement to occur without a heat effect.
It is unusual for a front seat to drop as most overheating occurs in the rear of the head due to inadequate water flow at low speed, such as occurs in slow moving traffic on a hot day.
TO prevent over heating in the rear you need to modify the water rails as per my mod instructions. (See Lutz water rail mod).
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
3
Like Norman says, always due to overheating, but not necessarily recent overheating. And it’s not due to the coolant reaching 220F. It’s due to the coolant turning to steam inside the heads, allowing the local areas around the valve seats to reach temperatures hundreds of degrees hotter than they should see.
I do disagree with Norman slightly on the rears being the usual. I dropped one in 2B cylinder in my car.
While flow to the rear is no doubt an issue, there’s also a possibility that the steam will rise into the forward part of the head, I guess.
There’s an argument to be made that the B bank is more liable to dropped seats than the A bank due to the issue of the upper 1/3 of the radiator getting plugged up, but even that doesn’t hold all the time – people have dropped valve seats in the A bank as well.
Many, many, many have recommended “staking” the valve seats so they don’t fall out. Unless you’ve machined a chamfer on the upper OD of the seats to have something to peen the metal over, I can’t see how that would accomplish anything at all. Of course, most people who have dropped valve seats never drop them again – staked or not – which may mean that they have learned how to avoid the overheating issue or the local machine shop does a better job of installing those seats than the factory did.
More than 90% of dropped valves I have had experience with have been in the 2 rear cylinders, but unfortunately Quality Control after the Leyland “takeover” wasn’t the best so it is possible to get seats in other positions to drop.
I know my Head Guy makes them a much tighter fit (no staking needed).
i have heard after Leyland takeover , save money, reduce quality control personal!
finances were tight around that era, also with Aston Martin, UK was in a major change!
on the dropped seat thing, some say that when engine was last run and possibly HOT, a seat may have microwelded to the valve , so next time opens it takes the seat with it!(or loosens it and things go down from there).
but one thing is for SURE , the Jag V12s did drop more than most other Alloy head engines !
i collected 4 V12s in about 5 yrs!, all had some valve train problems! so i junked 3 of them,got $75. dollars for the bunch, saved the best for my car(full rebuild ++ some performance mods).
ron
Valve seats drop because the area where the seat is either too big or the aluminum head expands quicker than the steel.
Once you have dropped a seal, its then up to the machinist to get the fit correct, hopefully the seat didn’t do too much damage, enlarging the recess.
There is only one size seat for the valve size. I have seen some knurl in an attempt to assure it doesn’t fall out if the fit is “questionable”. Not a fan. I’ve had oversize seats made and hardened, but there better be a good reason for the expense.
A V12 head cut…crap! Kirbert, I would have thought you of all people had one of those?
The heads I’ve seen with dropped valve inserts have been overheated. I usually junk them as they have gone soft. Heads are heat treated LM25.
Any S/H V12 head I used is brinall tested first for hardness, inserts are locked in by a tool that peens the alloy around the inserts.
I was lucky at one stage the local dealer had 6 sets of pre HE heads in stock & over time I used all of them.
Where can I find the Lutz water rail mod? I searched the archive and Bing with no luck.
I am currently replacing all hoses and water pump after having a right top hose burst last week. It lost 2 gallons and pegged the temp gauge after I shut it off. I have started the car a couple of times just to move it and I hear no unusual noises but reading this thread puts me on edge!
I also plan to install an electric fan but have not selected one yet.
Tony
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
12
I dunno why you’re replacing the water pump. Sounds like it was working fine. You might want to have your radiator serviced, though; if you built up enough pressure to blow a hose, it must be plugged up big time.
Really on the fence about replacing WP. I’m in there already and this one is likely the factory one with 70000 miles. I think I’ll loosen all the belts and see if the bearing is smooth turning. If so, I’ll probably keep it. New aluminum radiator was installed by PO.
I think the hose ruptured due to age. It may also be factory. It is stamped along its length by repeating -DEV 952- does that look familiar? Anyway, I never owned a car with a fan clutch and I think mine’s shot. It has the same stiffness(not much) hot or cold. That’s one reason I’m going electric. Hearing that fan cycle on will remove some doubt and hopefully some post running heat soak!
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
14
You should also check that pesky reducer in the line between the header tank and the atmospheric catchment tank. If plugged, pressure in the entire system could skyrocket.
for my V12 after all head machining,porting etc. i had the flat gasket surface thermo ceramic coated ,along with the exhaust port, trying to reduce combustion heat from getting into the aluminum,reduce expansion!
ron
The thermostats and water pump were replaced Tony. Did you check the electric fan switch up by the pump inlet, especially if you got the AC running. I’m pretty sure I replaced the thermostat hoses so Kirby’s comment may have merit, although I don’t fully understand it since the expansion tsnk has a 15lb relief cap on it?
Pete, I checked the Otter switch and it works. The fan and its supply circuit also check out OK. However, the connector at the switch is corroded and I suspect that’s why I have never seen/heard it operate.The hose that failed has numerous deep longitudinal cracks in the inside surface -very soft, seems to have died of rot.
16lb cap -I will test that and system integrity after installing new hoses(with filter screens).
I see the new radiator has top inlets on each side like the factory one. Is it’s construction similar -with divided left side tank and single right side tank? Doesn’t matter, just curious.
-Tony
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
19
Doesn’t matter what pressure rating the cap has if the relief passage is blocked.
FYI, I tested my fan switch and when it turns on based on my temp gauges. With standard thermostat for aux fan, mine wouldn’t come on until the coolant temp hit 210F. If you have switched main thermostats from 190F to 180F, that may only happen during really hard driving or tons of stop and go traffic in hot conditions.
I switched to a cooler aux fan thermostat, which I see now comes on when coolant temp hits 200F. I have yet to see temps above 200F while driving my XJS, although the hardest I’ve pushed it is 20 minutes stop and go traffic in 85F ambient temps.
I’ve also tested heat soak, and it usually goes up 10F. Highest temp I turned off engine was 198F (aux fan didn’t come on), and came back 15 minutes later, coolant temps were just shy of 210F. This keeps me hopeful that valve seats will stay put.
