1986 jaguar xj

The Jag: Diagnosing those new project car issues

Jeff Glucker

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My 1986 Jaguar XJ6 runs strong. It sounds good. The seats are supremely comfortable, and the ride is great. It also runs rich and chugs fuel, needs an updated radio since I have zero tapes, could use some tint on the glass so I don’t fry in there, and the AC needs a recharge. Also, if it idles too long it will overheat. I’ve fixed two of the above-mentioned issues, which are the least two important ones to fix. That’s how it goes with a drivable project car, right?

I’ll have updated pics and information soon, but just know the Jag now has darker windows and the Continental head-unit I purchased has been installed. The day prior to doing that, I was idling my way through a slow Starbucks drive-through when noticed the stock temp gauge climbing. Not the aftermarket one that I was told to keep an eye on, but the OG Jag gauge. It had shot past 90-degrees Celsius and was climbing. By the time I pulled away with my cold brew in my hand, I saw a wisp of smoke flow out the nose of the car.

Thankfully, however, I pulled onto the street, got that fan spinning, and the temp immediately came back down. No harm, no foul. The Jag overheated again though when I left it at the shop in the above photo for its tint and tunes upgrades. They had let the car sit for a minute with the engine running, noticed it starting to boil over, and immediately shut it down. I arrived to pick up my car with its hood up and a puddle of coolant beneath it. I assume this is a standard Jaguar operating procedure.

So I’m starting to go through and see why this might be happening. A 180-degree thermostat is on its way from RockAuto. I’ve already received a new O2 sensor to start checking on why it might be running rich. And I was thinking about trying one of those AC Recharge-In-A-Can setups to see if they work.

More on all of this, soon.

About

Jeff Glucker

Jeff Glucker

Jeff Glucker is the co-founder and Executive Editor of Hooniverse.com. He’s often seen getting passed as he hustles a 1991 Mitsubishi Montero up the 405 Freeway. IG: @HooniverseJeff

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17 responses to “The Jag: Diagnosing those new project car issues”

  1. danleym Avatar
    danleym

    Tried the recharge in a can once, had no success. Almost went down the road of buying all the proper AC tools, then decided I’m okay taking my cars to someone for A/C issues.

    Overheating sucks. So many possible issues. A new thermostat couldn’t hurt (and they’re cheap), but I wouldn’t expect the lower temp (assuming the current one isn’t a 180) to fix it, unless the current thermostat already isn’t opening all the way. Good luck figuring that one out!

    Reply
  2. Zentropy Avatar
    Zentropy

    Considering they close when parked and go cold, it’s not out of the question that the thermostat could have failed while sitting for a long period. And as danleym mentioned, it’s a cheap and easy place to start.

    Big engine swaps are notorious for cooling issues, though. I don’t recall whether the stock or an aftermarket radiator is being used, but either way, its capacity might not be sufficient, or you just need more air flowing past it. Regardless, at least in some way, shape, or form, it’s a fixable problem, and I’m sure a little troubleshooting will remedy it.

    I like this project. These are good looking cars, and there’s nothing wrong with an American V8 under the hood of anything.

    Reply
  3. nanoop Avatar
    nanoop

    Puddle of coolant is annoying. When the radiator is sufficient when rolling: is the fan electrical, and if yes, is the temperature switch ok? I.e. the fan is turned on, at an appropriate temperature?

    Reply
    1. OA5599 Avatar
      OA5599

      Dual electric fans. But Jeff’s comment “I pulled onto the street, got that fan spinning, and the temp immediately came back down” makes it sound like they weren’t running at idle, which is when it was needed.

      Reply
      1. Jeff Glucker Avatar
        Jeff Glucker

        I *assumed* I got the fan spinning.
        Today I let the car come to temp to see if the fans kicked on… they did not. Neither one.

        Reply
        1. nanoop Avatar
          nanoop

          Wow, that’s about the second time in my career as armchair mechanic that I was thinking in the right direction!

          So there is something to check: switches, motors, grounds, fuses – finally an electrical issue on a British car!

          Reply
  4. Batshitbox Avatar
    Batshitbox

    Woot! Progress!

    Overheating can be caused by poor timing, but I think you said you checked that already.

    On my Econoline some chiseler had gone so far as to make another timing mark to match how far off the timing had been set in order to get it past SMOG. When I hear your swapped SBC passed SMOG and is overheating red flags go up in my head.

    Timing that bad would rob some power, so if it seems peppy enough it’s probably something else, but have a peek around the timing marks to see if there aren’t any spurious data points.

    Reply
    1. Jeff Glucker Avatar
      Jeff Glucker

      Could that also be related to the whole running rich thing?

      Reply
      1. Batshitbox Avatar
        Batshitbox

        Inconclusive. I would speculate that if it passes SMOG then the fuel is burning completely, which would mean maybe running very lean, but not rich. I’m not sure how you can run really rich and still pass SMOG. (Well, the SMOG tests are done at idle and at an RPM above idle, but with no load on the engine, so maybe runs well at idle and gulps gas under load?)
        My van had a rich condition at the same time as the timing, but it was due to an unrelated fuel injection issue. (My engine thought it was under full load all the time due to a faulty connection, so it maxxed out the fuel flow all the time. My ’91 Ford probably doesn’t have the same fuel injection setup as your SBC.)

        Reply
  5. OA5599 Avatar
    OA5599

    Don’t forget what I said about refilling the coolant: that cylindrical filler is plumbed at the bottom, and won’t purge the air from the system. You’ll need to figure out a way to do that manually. https://d9p7j8e7.rocketcdn.me/wp-content/uploads/2021/04/The-Jag-First-Look-17-1200×675.jpg

    If you get someone to send you a recharge in a can for free, go for it. Otherwise, you can get a “good enough for occasional use” manifold gauge set online for about $30, shipped, and Walmart sells the 134a for $4.88 per can.

    Reply
    1. Jeff Glucker Avatar
      Jeff Glucker

      Yeah I need to figure out that dumb tank…

      Reply
  6. Bergeractoo Avatar
    Bergeractoo

    I am a long time, multiple XJ6 owner.
    It’s never just the thermostat so I would advise you to fix it right, right now

    Pressure check the system and find the leaks
    Take out the radiator and have it serviced by a radiator shop
    Replace the radiator cap and thermostat and coolant
    Replace any questionable hoses
    Make sure the aux fan and coolant temp switch work at about 190 degrees
    Pressure check the system again after reassembly
    Don’t use Bars Leak

    Reply
  7. Jeff Glucker Avatar
    Jeff Glucker

    Let the car come up to temp, idling in my garage today (garage door open, I’m not *that* dumb… I think). Fans never came on.
    The radiator cap started sounding like a pissed-off tea kettle once I shut the car off. Nothing boiled over though.

    Reply
    1. Jeff Glucker Avatar
      Jeff Glucker

      https://www.instagram.com/stories/hooniversejeff/2598419171245466855/

      Not sure if that link will work…

      Reply
    2. Scoutdude Avatar
      Scoutdude

      Yeah it wanted me to sign in so no I couldn’t see it. However the fact that it sounded like a tea kettle and fluid didn’t come out means that you were low on coolant. It could mean a weak cap too depending on exactly what else was going on. See below for other troubleshooting tips.

      Reply
  8. Scoutdude Avatar
    Scoutdude

    The cooling system is a simple math equation. Heat out = Heat in. OK in reality heat out needs to be equal to or greater than heat in. For most situations the radiator’s ability to dissipate heat exceeds the engine’s ability to generate heat. That is why the thermostat exists, to allow the engine to come up to a temp it operates well at and then stay at that temp by limiting the flow of heat to the radiator.

    Heat in is directly proportional to the position of the throttle pedal. The more fuel you burn the more heat you generate.

    Heat out is determined by the radiator itself, the air flow and the difference in temp between the air and coolant flowing through the radiator.

    Because it cools down very quickly once you get up to 25-30-35 miles per hour (ram air speed depending on car aero design) and continues to cool at higher speeds the problem is air flow through the radiator at low speed/idle. When you speed up you are putting more heat into the system so the fact that it still cools down means that the radiator is shedding that extra heat and more due to that air flow.

    So yeah the fan is a likely culprit. As I mentioned in a previous article about this GM frequently didn’t turn on fans until has high as 210-220 degrees so the other possibility is a bad radiator cap that isn’t holding sufficient pressure. However that will show itself by starting to vent after the vehicle has parked for a minute or two after a drive that has fully warmed up.

    The first test would be to figure out how the fan is supposed to be powered and make sure there isn’t a problem like a blown fuse, disconnected wire or bad ground. Then it is time to try powering the fan directly. temporarily jumper power to the fan and make sure it actually works. If it does then start up the vehicle and let it idle for a long time and see what happens. Again this is where the infrared thermometer is useful to take temps of the thermostat housing/radiator inlet and radiator outlet/water pump inlet.

    If that solves the problem then it is time to figure out what is supposed to be turning on the fan and if it is actually trying to turn on the fan at that temp.

    Reply
  9. Lokki Avatar
    Lokki

    I follow the rule of “try the simplest and cheapest fixes first”. However, I also have learned that you are better off just jumping on a problem like the overheating now and just fixing it right away than nibbling and nibbling for months at at time, always worried and afraid to drive the car. If you can at all afford it, just tackle it all now. It will get no cheaper by putting it off, and in the mean time you risk doing some potentially expensive damage.

    So, as suggested above:

    1. Replace the thermostat. If you are curious, afterwards, you can test the old one by dropping it in very hot water to see if it opens up. No need to since you already replaced it but it’s fun.

    2. Replace radiator cap. Make sure that the cap pressure is correct for your Chevy engine. Remember, as far as the engine and radiator are concerned, you have a Chevy, not a Jaguar so get the right one for your engine, not your car.

    3. Have the radiator flushed and cored. On a car and engine this old, it can’t hurt. It’s not that expensive: just do it.

    4. Afterwards, refill with a good 50/50 et al antifreeze/water mixture. A lot of people think that antifreeze is only necessary in cold places but glycol coolants are even more important in hot places as they work much better than mere water at extracting heat. Be sure that you put the nose of the car on blocks when you fill the radiator to ensure that the radiator cap is the highest place in the cooling system or otherwise you may have an air pocket trapped somewhere in the system. This can often happen in conversions where the radiator and cooling system are not designed to make sure that there are no air trap spots. Be sure to idle the car with the cap off while still on the blocks to make sure all trapped air gets out.

    5. Replace your fan(s). Those cheap aftermarket electric fan thermostats are often suspect. Some of them are adjustable, which always means the P.O. (P.O. = Previous Owner as in “The Curse of P.O. affects most old cars”), didn’t know how to adjust it and got it wrong. I have experience here. Just get rid of the fan set you have while you have the radiator out for the flush and recore. Get the biggest SHROUDED fan or pair of fans that you can fit. Don’t cheap out – get a Derale or other reputable brand, not a Chinesium knock-off.
    https://derale.com/product-footer/electric-fans/shrouded-powerpacks

    Ideally, until you get things sorted on the cooling front, you want a manual fan switch that allows you to override the thermostatic switch so if things aren’t adjusted right you, you can step in Deus ex machina and turn them on.

    I would do the whole cooling system before I would work on the running rich problem. You won’t be able to do a proper tune anyhow until the engine temps are stabilized in the correct range, and it is much safer to drive a rich running car with a cool engine, than a properly tuned car that wants to blow a head-gasket because it’s overheating all the time.

    Reply