I am new to this forum and new to Jags in general. Just picked up a 1993 XJS 4.0 convertible (about a month ago) and am slowly sorting out the issues.

There are a few I am having a hard time searching for because they are hard to describe and get results, so I am hoping that is where the forums come in!

If anyone has any suggestions or similar experiences, any help would be greatly appreciated.

Brakes:
1) when I first press the brake pedal after driving for a while, it feels like there is a "gap" before the pedal connects to the brake system (maybe a 1/2" of travel). Subsequent presses feel normal, then after a few minutes of driving the first press has that gap again.
2) more recently, after driving for a while and then stopping and holding the brakes (like at a red light) occasionally the pedal will stick. When I let up, the pedal does not and the brakes remain engaged until I touch the gas (or tap the brake again) and I can feel the brakes release and the pedal pops up.

Suspension and steering:
1) at highway speeds (70+) I get a bit of a shake and if I let go of the wheel I can see it moving back and forth. Car drives straight, no pull, and just had new tires on, balancing, and alignment.
- my plan of attack is steering rack bushings, then control arm bushing, then wheel hubs. Any other suggestions?
2) anyone know what replacement springs to use? I did the front shocks because they were shot and the rears are pretty bad, but I'd like to do those with new springs. 1993 XJS Conv 4.0 (not 1993.5).

I am also enjoying issues with a hot start problem, inaccurate fuel reading, relays that stop working when hot, etc. Usual Jag stuff, but info on all that is readily available and easy to search.

Again, any insight is always appreciated!

 

I have steering wheel jutter just like you at those speeds, question is does hard acceleration even it out? That will narrow it down.

I'm just doing ALL front bushings/subframe and eng mounts in polyurethane to cure it, I chose to do so after seeing another post here similar issue turned out to be the subframe mounts, was it Daim? I remember the pics but not who posted - might be worth jacking the car up and inspecting every mount/bushing?

 

Hi and welcome to the forum.

Brakes - I would suspect sticking pistons. If I was you I would pull the calipers off and rebuild/replace with changeover units, flush the brake fluid and replace it.

Steering shake, Greg has this problem and is in process of replacing subframe mounts to see if this cures it. The XJS front suspension and rack are fixed to the subframe so if the mounts are bad then you can get steering issues. I would replace rack bushes with Poly and depending on how you want the to handle rubber in everything else. If you put Poly in the suspension it will significantly improve steering response but at the expense of NVH.

 

Indeed I am. I will post about it once (if) I have definitively sorted it out. But the story so far is that the new, but OEM front subframe 6 shot bushes, those at the front of the subframe, may well be more compliant than they used to be. At certain speeds this sets up an oscillations (ie an undamped "bounce") that gives the shimmy. In my case this was apparent only at dead on 77kph to 83 kph, and there was perfect steering at all other speeds. This shimmy was about 4 beats a second on the steering wheel, which moved left and right about an inch. I got to this view about the six shots being the problem - which I did not think it could possibly be when I started - by a series of increasingly expensive steps, and ended up renewing every single part in the front suspension and subframe. Each step helped but did not eliminate the problem.
Most recently I have beefed up the six shots by filling the holes with acrylic glue-like stuff called Stixall, which is very durable and does not set 100% solid. This has cured the problem at 80kph, but has produced a tiny, repeat tiny, touch at about 95pkh. Just a touch mind, nothing I would probably concern myself with if I was not sensitised to it.
I have a hot country spec 3 row radiator and dual SPAL fans, all of which are mounted (in effect) on the front crossmember, which is about 18 inches in front of the axle line. I have a idea that this extra weight (as OEM it is a two-row rad, and the main fan is on the engine) has something to do with the inability of the six shots to control the oscillation. I am in the process of reducing the weight of this by (I hope) around 20 kilos, and will post with my findings.
But based on what I have so far, faced with a persistent shimmy at a certain speed range, and with no catastrophic wear present on the other components (rack bushes, wishbone bushes, bearings, subframe mounts) I would replace the front 6 shots with poly ones - which (eg) Manners sell for a bit less than OEM ones - as a first step. I have certainly proved in my case (by my beefing up the OEM ones) that too compliant six shot bushes, has been a major cause of my shimmy, and making them more solid has gone almost all the way to eliminate the problem. I suspect poly ones, rather than hone-made harder ones, would have done it completely.
More when my weight reduction tests are done.

 

I would think everything you're doing to eliminate the shimmy is actually just masking an issue that is there. Have you checked your front wheel bearings for excessive play?. Swapped your rear wheels to the front, to suspect wheel out of round or bent slightly? Wheel/tire balance?
Beefing up the mounts are all good but may just be a way to isolate what may still be there.

 

Yes, yes, yes and yes. Believe me, the most experienced minds on the forum have been helping me via PMs. When I post definitively with the answer, the list of things that have been done/checked/renewed etc makes the normal type of thing you mentioned, and which I certainly started by checking, seem just the tip of the iceberg.
I do follow your point about symptoms and causes though, which is why I mentioned the weight reduction. It remains true, however, an undamped oscillation at a given speed being allowed by the six shots (obviously in combination with other forces and movements allowed by the rubber components in the subframe/suspension system) is an underlying cause. Thus altering the spring rate of the system to eliminate it, is not just masking the problem, but eliminating it.

 

Sorry to appear dim but what's a six shot bush?

 

Steve
The front subframe (including therefore steering suspension, hubs and wheels, also taking the weight of the engine) is held to the body at four points, two at the rear and two at the front. The rear two called "V mounts" (2 below) take all the weight and are in compression. The front two are round rubber bushes with holes right through them, and look rather like a revolver's chamber. See 10 on the diagram

These do not take much weight but locate the subframe; fore and aft, mainly.
My problem is/was/seems to be, caused by these being too soft, although brand new OEM metalastics, and somehow allowing the subframe/body to move relative to one another (vibrate or oscillate) in a way that produced steering shimmy, at a certain speed only. I can only think the subframe somehow moved up one side and down the other (in effect rocking slightly side to side) in an unintended way. It might also be aggravated by a heavier radiator and fan package than OEM. But this problem appeared a few of years ago, very slightly, and got a great deal more noticeable a few months ago, hence my quest to track it down.
Making the bushes less compliant, by filling the holes, has had a huge effect - but sadly and expensively only after renewing everything else! Nobody thought the six shots could be a problem, including me. I reckon poly ones will fix it permanently, but I also wish to reduce the weight in that area ahead of the front axle line; as I am pretty sure I am getting some sort of unintended harmonic effect, and reducing weight will change it. We are talking some sort of sympathetic resonance-type effect, I am pretty sure, which a smallish weight change may well cure, by altering the frequency to eliminate the effect.

 

Thanks Greg, just unfamiliar with the term.
I have been chasing a squeak for ages that proved very difficult to trace; had the car up on a lift with no weight on the wheels a few times and pry barred everything but no movement anywhere.
Checked all exhaust hangers, everything rotational and replaced the front shock sponge washers all to no avail.
Eventually the local garage had me drive slowly over some bumps with the bonnet up whilst they peered in to the engine bay; the noise was definitely coming from the left front suspension area. (the whole cradle was replaced about 4 years ago with new bushes).
The put it up on the ramp again, this time with the wheels taking the weight of the car, and started trying to tighten things up.
The big bolts (number 16 in the diagram I think) were completely slack so the whole cradle was moving around; there was enough movement to scrape paint off!
After tightening them up the squeak has finally gone away.
Part of the problem over here is that the roads are so bloody awful that any suspension play is masked by the continual bouncing and jarring.
Anyway, I can hear the engine better now!

Aren't poly bushes more rigid than the OEM ones? I would have thought that any vibration would be transmitted even more?

 

Steve,
I expect having fitted the six shots, and tightened up the clamp bolts, someone forgot to tighten the big bolts. Very easily done! But isn't it great when at last you track something down?
As to the bush material, It is not a question of rigidity, I believe, but of the natural frequency of the bush, or the bush/subrame/body as combination. Just as the natural frequency of a glass will alter if you half-fill it with water, a less compliant bush (or a more compliant one for that matter) will alter the natural harmonic. In my case this odd harmonic has undoubtedly happened at 80kph, and hardening up the bushes has fixed it. I do not have an NVH problem of unwanted vibration being transmitted through the bush; rather what I have seems to be an unintended "bouncing" of the entire front end of the subframe, which is provoking a reaction in the steering. As I mentioned the "beat" is only 3 or 4 times a second, and it is only at a given speed, regardless of gear, acceleration or any drivetrain effects. So I am driven to the conclusion that it is the subframe/car front end vibrating at its natural frequency. As beefing up the six shots got rid of it at 80kph, I believe this is because the natural frequency of the "system" was disrupted by the change. What I am after now is weight reduction forward of the axle line, which will, or might, or should, have a similar effect in altering this undesirable harmonic.
Anyone who thinks this is unlikely might be right, obviously, but when I removed the mechanical fan, years and years ago, and replaced it (on what turned out to be poor advice) with a 12 inch electric, the turn-in of the car was really noticeably changed; the gyroscopic effect of the fan having been removed being the reason, I surmised. So if I get 20 kilos off the front end I reckon - famous last words obviously - it might well do the trick, particularly as I have a vague idea the problem might have started to show itself, however slightly to start with, when I fitted my 3 row radiator which is a great deal heavier than the normal one.

 

Hi Greg

Are you now thinking the Mechanical Fan might be better and wishing you'd stuck with that?

 

Well, good question. The electric reduces noise considerably; makes a slight but noticeable improvement in fuel economy; and gives far better access to the front of the engine. getting it controlled properly was a struggle, which only Grant really solved for me, to my satisfaction. Also, fixing them in properly was not that easy. Now, I have solved all those problems and have a reliable and easy to service system that I am very happy with.
However, looking back, if I was starting anew, and without a workshop and not so much time as I have now, or giving advice to a new-to-XJS person, I think I would counsel keeping the mechanical.
As far as my steering shimmy problem is concerned, for years I had a large 17 inch Pacet allied to the OEM two-row rad, and it was all brilliant. For some reason I thought a 3 row would be a good plan, and the Pacet gave up and was replaced by a huge SPAL. It could be, could be, the two changes, both of which increased weight well forward of the axle line, have tipped the system into an unintended harmonic. We shall see!

 

Hi Greg

Cheers!

In that case I'll keep what I've got for the time being

 

What you need to do is get the car up to about 140mph and see if the vibration has moved further up the speed range.
I'm not sure what it will prove but it'll be good fun!

 

I'm getting all superflex poly bushings done, and picking the car up tommorow complete with poly rack and new (supposedly uprated) front sub-frame V-mounts, engine mounts to that will go along nicely with the uprated and poly rear end bushings/mounts, I'm hoping it eliminates my juttering issue, otherwise this will have been a very expensive mistake.

As for your weight issue have you looked into the fiberglass hood replacement? that should shave off quite abit. Carbon canister and smog pump are like 10lb+ each to plus the associated junk, I ripped those off no problem.

 

Been there, done that!