While I was in Afghanistan, my wife was kind enough to go out and purchase a 1987 H&E convertible on my behalf. I'm just getting home, and after taking my new baby out of the garage, I was surprised to find that it has a pretty rough idle (sputters, a little bit of white smoke). It's at about 900 RPMs, which I thought was a little high for a V-12.

I also noticed that if I get the car above 70mph (our freeway speed limit is 75 in a lot of places where I live), it shakes VIOLENTLY. I was a bit surprised to hear that my wife didn't take it above 65 whenever she test drove it, but y'know, made do with what I had.

Anyways, where do I start for troubleshooting? My biggest fear is that the rough idle is caused by a cylinder not firing. I can't even begin to guess why it's shaking. It's kinda scary, like the car's going to fall apart.

 

Many possibilities, including, almost as a certainty, a stack-up of several different problems.

Has the car sat unused for a long period? If so, stale fuel and a rusty tank should be considered. Remove the fuel filter and empty the contents into a clean container. Your next steps will be dictated by what you see.

Other considerations would be the ignition system: plugs, cap, rotor, and plug wires and/or faulty/burnt/loose wiring on the primary side of the ignition system (that is, wiring to the coils, ignition module, and such)

Fuel injector harnesses get baked by engine heat and can result in injectors not firing. Most of us make our own replacment harnesses.

A faulty coolant temp sensor can result in a too-lean or too rich mixture.....and misfiring/sputtering.

I'm afraid there's no magic to diagnosing running problems on these cars. Pick a place to start, fix whatever needs fixing as you happen across it, and then move to the next item. If you're comitted to the car and wanna really jump in with both feet then many would suggest an all-at-once approach that will address all the common fault areas which, unless you're blessed with a very unusual example, your car will certainly have. It's all within the realm of a decent DIY guy but the car will be down for a bit as "slow and methodical" is the order of the day with these cars :-) .

Is the massive shaking at 70 mph more of an engine thing or a wheel/tire/suspension type of thing?

Cheers
DD

 

Are you mad? You let your wife buy a car for you?
Is she mad? She bought you a V12 Jaguar.
Good girl.
Anyway, keep calm and carry on.
How long has the car been stood before you (or your obviously smart and beautiful wife) drove the car?
Are we talking months or years?
Talking from experience, massive vibration is usually really bad tyres on out of balance wheels. Another possibilty (apart from totally shot suspension) is badly worn universal joints on the prop shaft but that would show throughout the speed range.
As for a misfiring cylinder; I challenge anybody to notice if there is one cylinder missing on a Jag V12 - you can only really notice if you are down to 9 cylinders in normal driving.
Open the bonnet (hood), start the engine (this is best done in a garage with the lights off) and see if there is a fabulous display of blue sparks running across the distrubutor cap.
I have that on my car and every time that the sparks appear the engine coughs.
And so do I, all that wasted petrol. Then I just drive faster and it's fine.
Basically, I think that is just a cracked distributor cap.
Anyway, don't panic, everything is fixable.
Steve

 

I'm going to dig in to it when it gets a little bit warmer today, but I drove it across the state yesterday afternoon, and found that it only shakes when I'm coasting or slowing down, not when I'm accelerating...Oh well, I'm sure I'll find it sooner or later.

 

1st of all, thanks for doing what you did in Afgan. I tip my hat to you.
2nd, a tip of the hat to your wife for such a thoughtful, and awesome gift!
3rd, we can help you sort it all out! Usually shaking like that at particular speeds is caused by tires being out of balance or worn. So it might be an easy fix. The skipping could be nothing more than a desperate need for a tune up, especially the infamous front cylinders that nobody wants to change the spark plugs.

 

thanks for the advice. After driving it around a bit today, it started behaving a bit better. It hassent a lot of it's life sitting (only 39,900 miles on the odometer). Sadly, I think the seller told a lot of lies to my wife, because I've been discovering a lot of the car's little secrets in the past couple of days. However, it will be fun to work on...

Here's the car's "secrets" that I have discovered (for those who are curious)

Driver's ash tray had ash caked in the bottom (seller claimed to be a nonsmoker)
Seller claimed the car has been garaged it's entire life, but there's a nice bit of clear coat peeling on the trunk that looks more like sun damage (attributed to cleaning)
Minor cigarette burn on the top (again, nonsmoking?)
Battery is a decade old (and without enough charge to start the engine as of this afternoon)
Engine idles rough
Car Shakes
A/C Runs hot
Cruise doesn't engage
Driver's window motor is going out
Electric locks don't fully engage

Still finding stuff that wasn't disclosed to my wife, however, I still love this car. Good stuff is that it doesn't run hot, starts just fine when it's got enough juice to do so, and it isn't destroyed on the inside.