In the past few weeks my car will stumble about once per trip. It is usually earlier in the 7 mile trip to school, but sometimes it is towards the end.

Sometimes it happens at constant RPM and sometimes it is when I try to accelerate. Today it happened twice, so I am growing concerned that I'm going to be stuck in the middle of the road calling a truck while I hold up traffic.

It only lasts around 1 second, RPMs will suddenly drop like from 2,000 RPM to 1,200 RPM and it will just as suddenly go back to normal.

As this is totally random and intermittent, I'm having difficulty pinpointing the problem, so I was looking to see if anyone has experienced something similar or recalls the same happening to someone else.


-A month or so ago the main wire came off the coil and cut power totally. I've poked at it while running to see if its loose or causes any change, but nothing.
-The fuel tank is historically dirty. I've replaced filters and pumps multiple times.
-Cap is vented so it can't be a vapor related fuel issue
-Voltage regulator has been replaced and since then it seems to output a bit above the middle of the gauge. The lights also tend to slightly flicker at idle only.
-Tranny leaks bad but I'm assuming issues here would result in revving with no movement, not stumbling or stalling.
-The car has yet to completely die from this issue. It happens so fast I never noticed if electrical power drops or just the engine.
-Wiggled around vacuum lines to regulators while running which did nothing.
-Plugs, cap, and rotor are 3 years old
-I'm guessing this is ignition or fuel but with it working normally 99% of the time, I can't really troubleshoot.

 

I'm currently thinking of spraying some starter fluid to check for vacuum leaks and go ahead and order a new fuel pump and filter. I've never had it do this in the past, it typically gets much louder and suddenly dies altogether.
Today it happened while driving at constant RPM through the neighborhood, and again (much worse) trying to take off out of the subdivision. This has me thinking its a fuel issue and not an electrical issue.

 

Hi Sidescrollin

Although I haven't had this happen on my XJS (Yet!)

I've Certainly had it happen on my Merc, where the Engine would die for a Second and then pick up again.

Exactly the same thing at Traffic Lights and sometimes when doing turns.

So to start with I was thinking the Same as yourself.

New Fuel Pump (Two on a Merc!)

New Filter.

New Fuel Pump Relay.

Which was looking to be a Very Expensive Day Out.

But as it turned out, it was None of those things! as on advice from 'Grant' I bought myself a £5 Tin of Electrical Spray Contact Cleaner.

Then I pulled out the Fuel Pump Relay and also the Main Relay and Cleaned the Pins with Wire Wool and then Sprayed the Pins and the Sockets

in Contact Cleaner.

Then when it had evaporated, I put the Relays back and Started her up and ever since She's been running like a 'Bird'

While I expect you already know this, just for those who don't.

The Fuel Pump Relay is in the 'Black Socket'

The Main Relay is in the 'Red Socket'

Both Relays are located in the Boot/Trunk, under a Plastic Cover on the Drivers Side by inner Rear Wing/Fender. on my 1990 XJS (UK) Car.

It's got to be worth a $5 dollar Shot, before you start splashing the Cash on all the Stuff that you may not need.



Fuel Pump Relay in Black Socket.
Main Relay in Red Socket.

 

Flakey / misadjusted / dead spotted TPS?

Easy to check at least.

 

Yeah, these things can be tough to trace. Sometimes you have to wait until the problem become worse before you can zero-in on the cause.

The tach dropping out suggests a problem with the primary ignition system. My money is on a loose/dirty connection or other wiring fault. Gotta start the search somewhere so I'd pull the under-dash trim and give the wires at the back of the ignition switch some gentle wiggles and tugs. If the engine responds to the jiggles, there ya go!

If not that....keep digging. Lot's of potential problems with the baked wiring in the area of the coil and ignition amp ...particularly the notorious "shielded wire"

Cheers
DD

 

Also.....

The two wires (and connector) from the distributor to the amp. These are the wires that lay down in the "Vee" and are sometimes known to fail.

Cheers
DD

 

Thanks man, I cleaned these off with some sandpaper and sprayed them down but if they are the issue I bet its just how worn down the contacts are at this age. I'll probably be ordering new ones regardless.

 

Is the only thing I can check the tolerance/gap on it? I did remove it fairly recently to replace the oil sensor.

 

Wouldn't the tach drop with rpm if fuel was cut out as well? Just wondering what makes you think this points strictly towards ignition.

I'll definitely start jiggling as I go, I tried that on the coil connections, amp, etc the other day when the hood was open.

I previously haven't dealt with any wiring in the vee or had an issue with it, so perhaps its time to start rooting around in there. What shielded wire are you talking about?

 

Broadly speaking, with a fuel problem, the tach will remain steady for a moment or two before dropping down as the engine falters.

However, if you have a problem with the primary ignition the tach will usually react instantly...because the tach operates off of the primary ignition system

(Primary ignition = the low voltage part of the ignition system. Ignition module/amplifier, ignition switch, low voltage side of the coil, and associated wiring)



It is a thin wire wrapped in a woven/braided metallic 'shield' (and an outer sheathing) from the ignition amp to the ECU. The woven shied is grounded at one end. It sends a trigger signal to the ECU. The inner wire can break, or partially break, and/or ground-out if it rubs against the woven shielding. Common issue.

Cheers
DD

 

Is your car a Lucas ignition, SS? If so, I would suspect the wires from it that go to Dizzy bottom or the ECU (Doug's shielded wire). The amps do go home, and often intermittently. In the end I bought a new ones earlier this year, and all my odd random no-starts and other oddities disappeared.
Greg

 

Yes, it is. I'll definitely check out that wiring later today and perhaps ask my dad to send me the amp off of my spare V12 that is in his backyard.

Is the shielded wire easy enough to reproduce/replicate or does its shielded construction make things difficult? Is it coax or some shielded wiring you can purchase online? Is there a terminal close by that makes it fairly modular or does it seriously run all the way to the trunk? (Thanks for the insight on the diagnostics doug)

Does the one to the dizzy tend to fry out and short or are you talking terminal issues? I also can't think of what wire you mean off the top of my head, I can only recall the ignition wires and vacuum line.

EDIT: Just glanced at it, it seems the shielded wire runs to the front of the dizzy, so its only a bout a 1.5' section of 2 wire shielded cable and it has a connector before it goes into the dizzy.
Are there typically any issues with the other wiring coming off the amp? Those seemed less supple than the shielded wire but obvious the issue/mystery is that it could be anywhere within that shielded wire, invisible.

 

Here's some info that might help

https://www.jaguarforums.com/forum/x...-grant-167093/

In a nutshell, repair/replace broken/damaged section of the wire...almost always just a section subjected to engine bay heat...and make sure the braided shield isn't contacting the wire, and make sure the braided shield is grounded at one end



The distributor wires (blue and orange as I recall) have a soft insulation and get heat baked as they're exposed to the heat in the "Vee". You'll have to root around a bit to get your fingers (or even eyeballs) on them

Cheers
DD

 

SS
The wires to the dizzy bottom exit the amp unit at the cabin end and plug into the amp with a flat black plastic connector and there is an earth there too. This bit of loom gives the rotor position signal to the amp from the star wheel in the dizzy. There is a joining plug 2/3rds of the way to the dizzy, which is a royal pain to undo and clean, and access is impossible with the HT leads in place.

The shielded wire runs towards the back of the car and the amp wire that feeds it is thin and white, it exits the amp unit through the white grommet thingy that the coil and rev counter wires also go through, and connects to the shielded wire with a connector about 3 inches from the amp. peel back the shield carefully to see of the very fine inner wire's insulation is cooked. If it is replace the cooked part. Keep the shield's earth wire attached (it usually looks like it is attached to the shield by a huge bodge dob of solder!)
Greg

 

The Throttle Position Sensor below the throttle capstan needs to be adjusted by pulling the capstan and loosening it's three retaining screws. It needs to be set to .32V-.36V when the ignition is in position II, on but not started.

The reading should be taken on the red and yellow leads without disconnecting the cable. Insert leads under insulation or small alligator clamps into a gap between connectors.

Re-reading your symptoms, it's not likely the TPS as it would not likely cause a stumble at higher revs.

 

To clarify this poorly written post.....

'Tis the thin inner wire that sends a signal to the ECU. Not the woven shield !

Cheers
DD

 

Shoutout to orangeblossom. (Touch wood, knock wood, punch wood, headbutt wood etc) I have been driving 5 days and this hasn't happened since cleaning the relays a bit. It was doing it pretty consistently, so I am hoping it really is gone.

I didn't even think to check the relays. It also felt as if I was simply going through the motions when cleaning them. My best guess would be the inside contact ( I removed the tops) because they actually seemed a little bit worn and dirty). The exterior connections look totally clean, but I hit them with some sand paper and MAF cleaner, which was the only electronics stuff I had laying around.

 

Hi SS

Cheers Bro!

It worked for me.