Having a problem with my 90' v12 xjs that im trying to figure out. car normally has no problem starting however if its above 90F it wont start until later that night when it cools down abit. If I drive it in the morning it has no problem starting in the heat. I do have a cheap wallmart battery in it so my current theory is the alternator is heating up just enough that the resistance is increased to the point that it no longer has the voltage to get the engine spinning. Either that or the battery is just crap. Interestingly as I approach the temp where it doesn't like to start I often have to hold the key down for a few seconds before the engine starts cranking. Trying to jump it doesn't seam to help. Any other potentially heat sensitive systems involved in starting the car?

 

Any other potential heat sensative parts...hold on while I count grains of sand on a beach.

There are a number of hot start systems on these cars depending on year etc, does this occur after a hot drive aswell? Say you stop after being on highway then get up and go again? Does keying on the car a few times (to activate fuel pump) assist or no effect? Have you taken a can of contact cleaner and a toothbrush to every connector in the engine?

gotta narrow down the scope here lol

 

Sometimes the starter itself doesn't like heat.

Locate the starter relay and remove the large white/red wire (there will probably be at least one smaller gauge white/red wire. You want the big one) and apply 12v directly to that wire. Does the starter now spin normally? If so, you have a problem with the relay or upstream of the relay. If no change, you have a problem downstream of the relay...possibly the starter. But also possibly a connection at the starter or voltage supply to the starter via the RH "+" junction post on the firewall

Or.......

Begin with the basics: clean the battery posts and cable ends, clean the ground cable attach point at the body, clean the "+" junction posts on the firewall. Any change?

Get a known good battery. An iffy battery will confound your efforts at narrowing down the problem

Cheers
DD

 

lol when i say heat sensitive I mean the relative coolness of 90F compared to whatever temp the engine bay likes to reach. After the car is running for a bit and gets hot it does wonderfully and has 0 issues starting. It also has no issues if I get it running before its apparently too hot too start. messing with the fuel pump does nothing and upping the voltage to 14 or so (whatever the alternator of my truck is at) with some thick jumper cables does nothing at all. Im not ruling out wired electrical issue but im not sure what it would be other then just increased resistance in the probably tiered starter. besides whatever weirdness a wallmart battery is made of

 

IF you are jumping it with GOOD solid cables, and NO difference, then I doubt the battery is the root cause.

My V12 batteries are 640CCA or better.

Dirty terminal, all over the place, 30 years is a loooong time without some maintaining being done.

Ignition switch electrical section, also 30 years old, and well documented as cuasing starting issues, easily dismantled and cleaned.

The EFI resistor pack, in the engine bay, behind the RH headlamp basically. WELL documented to reek havoc with starting, clean the socket and the plug, and I mean CLEAN them.

Throttle bodies caked with Jaguar Black Death goo. Clean them thoroughly.

Its a Marelli car, you have 2 crankshaft sensors, front + rear. They die, the leads fail, the plugs fail, maintenance again.

Among the terminals already mentioned, there are teh Power side, and the Earth side. The latter is more important by whisker that the former. A good days work to dismantle and clean them all.

Read my sticky at the top, maybe there is a light bulb moment in there to help you.

Good luck.

 

To clarify, are we talking about a "Cranks but won't start" problem or a "No crank, no start" problem?

I assumed that latter

Cheers
DD

 

Thanks Doug, I forget the oddities of some descriptions some times.

Must remember to clarify.

HAHAHA.

 

its a no crank no start. what it almost feels like is the oil becomes viscous enough in the heat to flow off the cylinders making it that much harder for the hot and old starter to crank the engine. I say that as if I start it in the morning and even wait a few hours for the peak of the heat it has no problem. would a heavier weight oil help with this atall or am I just imagining this problem.

Im not completely discounting odd electrical problems, this car has plenty, im just not sure what part would be more bothered by the heat of sitting but not by driving. Also not discounting contact issues and it may be about time to go throw and clean abunch of contacts (im going to end up doing allot of wiring to fix all the othe problems if i end up going down that path)

 

I wouldn't go there just yet, personally

Not everything fails in a way that's predictable or makes sense. Sometimes you just gotta pick a stepping-off point, dig in, and see what happens. If no joy, move on to the next idea.

Cleaning all relevant connections and grounds is never a bad place to begin if you have a mystery on your hands.

Cheers
DD

 

alright so I had some errands to run today and figured why not try to take the jag. its 92 out and around 1pm here

I held down the key for about 3 sec and right when I let off the ignition it cranked a little. I tried starting it again and it started right up with almost no delay. I also stopped at a gas station and it started right up when I left. It really does feel like as soon as the engine is freed up abit it has no problem cranking which makes me think the starter is functioning properly electrically wise.

 

would i be able to pull a resistance number from the white/red wire as well? Do these starters get enough airflow driving to cool them down to below the temp they were resting at?

 

Another thing came to mind. As I had similar issues which didnt really corrospond to temp, not that I was paying attention to that at the time.

The positive battery cable is sliced in two and pushes together like a big, poorly designed plug about 6 inches down from the Battery + and apparently even looking at it the wrong way can cause it to act up, go push the cable together (firmly) and see if it helps.

In my case I think the connection was just loose enough that not all the cranking amps got to the starter making for a electrical crackling lazy start

 

Ill have to try that. the battery is not in the battery box in the truck and is pretty much just held in place by the heavy gauge wire so its definitely a possibility.

 

OK, only on coffee #5 without add ons, yet.

BUT

That hold at "start" and nothing happening, and then SOME activity JUST as you release the key from the start spring loaded position, kind of yells at me that the Electrical Section of that Ignition Switch is in need of a clean out, VERY common after about 20 years.

Probe the White/Yellow wire AT the starter relay, and look for odd readings as someone goes to the ":start" position. You are looking for 12v +/-, but below 10V is a no good switch.

 

lol. This is my new excuse at work.

 

Ill give that a shot but I should have clarified that if i had kept the key turned the car would have continued cranking over and started. As soon as the engine is free it starts great.

 

How’s the starter relay? Does it click when you turn the key to start? They can be quite intermittent apparently.

 

That is a very different explanation.

Sooooo, with the key is the "START" position, the engine turns/cranks/rotates on the starter motor, RIGHT???

Coffee #3 on the mat.

If so, then the issue may still be the switch section. I have had 3 over the years that when turned to the START position, they "lose" the Ignition ON signal. Soooo, again, the engine cranks fine, but since the IGN ON signal is lost, fails to FIRE. BUT, when the key is "released", that internal contact "JUST" makes circuit, and away it goes, sometimes.

As well as the switch, I would be doing all the other relating items. Checking the Marelli system, as in cap and rotor, crank sensors, Ignitors, coils etc etc.

 

Car has new cap, rotor, plugs, wires, coils. And when its "too hot" the car does not crank. I do hear the fuel pump going I haven't yet checked the starting relay. It feels like the starter engages and just sits at stall without being able to rotate the engine. As soon as the engine rotates any the starting system works perfectly regardless of temp.

 

OK.....

Key turned to start and the starter isn't cranking.....

When this occurs do your interior lights and/or dashboard warning lights go dim?

If so, it suggests the starter has indeed stalled. It's sucking up a ton of amps but isn't turning. It further suggests that everything in the starting circuit is OK....except the starter.

Cheers
DD