Ran fine two days ago, parked it in the garage. Just tried to start it...cranks like a champ but that's it. Being very new to this car, I'm not sure where to start. Clearly it's either not getting fuel, spark, or compression. My repair manual still hasn't arrived, so if anyone could take a swing and helping me narrow this down I'd appreciate it.

Oh, it's for sure not out of gas.

Thanks,

John
1987 XJS V12, 61,000 miles.

 

At this stage of the game the basics ....not Jag V12 oddities....apply, as you mentioned.

Overnight loss of compression is unlikely. That leaves fuel and spark.

I'd say the easiest leaping off point is "spark". Pull a plug wire, insert a screwdriver into the end, position the metal part of the screwdriver 1/8"-1/4" away from a metal part of the engine, and have a helper crank 'er over. You want to see a bright blue spark to jump the gap.

Also, another quick-n-easy preliminary check: open the trunk and have your helper turn the key "on" but don't crank the engine. Do you hear the fuel pump run for 2-3 seconds?

Very old school at this point :-)

Post back

Cheers
DD

 

Go check the connection on the Intake Air Temperature Sensor on the LH (US Driver's side) air filter housing tube...

Dave
Atlanta

 

Got paged to work after posting, so didn't have a chance to look at it until now. Dave and Doug, thanks for the replies!

I listened for the fuel pump, and can hear it.

I checked every electrical connection I could see (I'm sure there are many I missed) and did find a few connectors loose. Reconnected them, but no help.

Got out the screwdriver, stuck it into a spark plug wire, held the other end close to a metal engine component and cranked it. Nothing. No spark.

Moved the screwdriver closer (maybe 1/8 inch) did the same thing, no spark.

Tried it again in the same location...it sparked, and the engine fired right off!

While I'm delighted it started, it clearly has some intermittent problem that I need to track down before I'll feel confident to drive it. Since I didn't do anything, other than re-position the screwdriver, between the first two failures to get a spark and the third success and engine start, I'm wondering how to proceed.

Dave, is the attached photo the sensor you are talking about? If so, are their particular tests I can do with it?

Thanks,

John

 

Lots of ways to approach this :-)

Do you have a 12v test light or test meter?

If you can get it back into "no start" mode check for 12v at the "+" coil post when cranking the engine. If no voltage, the ignition switch is one possible culprit....they've been known to give trouble.

Or......

Remove the driver's side under-dash trim panel and, with the engine running, gently jiggle the wiring at the back of the ignition switch. If the engine falters or dies, you know you're hitting paydirt.

(Note that passing the jiggle test doesn't positively prove that the igntion switch is good....but failing the test proves that there's a problem in the ignition switch area.)

Any of the engine bay wiring is a possible suspect as it all suffers from years of baking. One especially trouble prone area is the shielded wire at the ignition module. Give it a good eye-balling. The module is the square-ish black box bolted to the left intake manifold.

There are various possible faults but, you gotta begin somewhere, so I'd begin by checking the above.

Others will chime in.

Cheers
DD

 

BTW, yes, your picture does show the Air Temp Sensor. This is basically for fuel mixture "trimming" according to air temp.

Dave may have additional input regarding this sensor but I'd focus on the "no spark" situation first.

Cheers
DD

 

Ok, great! Every problem is manageable once one can find a place to start.

While I was doing my original hunting, I came across two disconnected connectors. I'm assuming that they've been disconnected since I've had the car as I've not really messed around in these areas.

One photo is from under the hood. The bellows device has three wires coming out of it, and you can see they were not attached to their respective connector. I attached them, but didn't notice that anything in particular changed. What does this bellows device do?

The other photo is from under the passenger (right) side dashboard. There are three wires that lead to this relay. Two of the wires are clipped together and are then clipped into the far left lug of the relay. The relay has two other lugs, labeled "B" (near the center) and "WL" (to the right). I would suspect that the loose wire clips to one of these lugs, but don't know which, or what this relay controls.

Oh...under the hood (sorry for jumping around) I noticed that the distributor vent line (the one that has the filter attached) is disconnected at the distributor. The other vent line on top of the distributor cap is loose. Would I be correct in guessing that this line is for running cool air into the distributor?

Thanks again, will post the results of the suggested tests soon.

John

 

That's the cruise control bellows in the center of the engine.

The widget under the dash with one wire unplugged looks like a "bulb failure unit"

FTR, the squarish part to the right is a circuit breaker....probably for the power windows

Cheers
DD

 

This is also a well known old school phenomenon
Starting off the spark by touching the metal and then "drawing the sparc" by moving the screwdriver away a bit again. This is a symptom of a marginal connection, usually in the high voltage circuit. What happens is that the spark is so weak it can barely jump the gap but once you get it going it "burns through" oxidation in the bad connection. However, it will re-appear and most likely when left to itself again for a while. So clean up the connections, and replace rotor and cap + plug wires (+ plugs) unless replaced recently and of good quality. And go over the low voltage side as well although it is less likely to be the main culprit.

 

The shielded wire or ignition switch is the best place to start. Although the ignition switches usually fail and don't let the vehicle crank either. However that is not 100% so a test is always good to do. If you can get it not to start again test for 12v as Doug said. If you have that connect the test light across the coil and crank. if it flashes you have a trigger if not then you have narrowed it down significantly.

With problems like this it is best to make a list of possible items. Then work down the list one item at a time. Things like cap and rotor should be checked and replaced periodically anyway. Put all the ideas on your list then one at a time do the tests or inspections to eliminate them. swapping parts is by far the worst way to approach a problem. You will spend more money this way and get extremely frustrated.

 

For better or worse, I've been unsuccessful in getting back to the 'no start' condition. I'll have more time to experiment with it this weekend.

Thanks for all the help! If I can get it to repeat I'll be able to get somewhere with what you've all shared...especially since the service manual still hasn't arrived.

John

 

My 1990 XJ-S spins but will not fire. It was fine two weeks ago.

1. The fuel pump is pressurizing
2. The battery is strong
3. I replaced the coil today
4. The wires underneath the dash look as new and seem all attached.

I’m not sure I know where the shielded ignition wires are. Any help locating those and any other tips will be appreciated. (I have a probe test light but no voltmeter on hand)

 

Please start a new thread rather than digging up one from 9 years ago. And when you start that thread, it would be helpful if you listed the engine.